Pelvis, Hips, and Thigh

Thomas H. Berquist

INTRODUCTION

Magnetic resonance imaging of the pelvis and hips has achieved wide acceptance as an imaging technique. This was initially due to the sensitivity and specificity of MRI for early detection of avascular necrosis (AVN).¹^,²^,³^,⁴^,⁵^,⁶ However, as with other musculoskeletal regions, the applications for MR imaging of the pelvis, hips, and thighs continue to expand. New pulse sequences, intravenous and intra-articular contrast enhancement, and other technological improvements have contributed significantly to the growth of MR imaging. Imaging is more commonly performed at higher field strengths (0.3 T).

TECHNIQUE

The image planes and pulse sequences chosen for evaluation of the pelvis, hips, and thighs depend upon the clinical indication. We use both 1.5 and 3.0 T units in our practice. The patient’s age and clinical status is also important. Sedation may be required for certain adults (pain, claustrophobia, etc.) and is commonly required in young children (see Chapter 3).¹^,⁷^,⁸ Specific techniques will be discussed later; however, there are certain standard techniques that can be used for screening examinations.

Pelvis and Sacroiliac Joints

Examinations for suspected pathology of the pelvis and/or sacroiliac joints can be accomplished using the torso coil. Smaller coils may be used for infants and children.⁷^,⁸ Scout images are obtained in three planes with the patient in the supine position (Fig. 6.1). Every effort should be made to be certain that the patient is properly positioned (Fig. 6.2) with the hips at the same level. The legs should be internally rotated (toes touching) to assure a symmetric appearance of the trochanters and soft tissues to allow easy comparison. A bolster can be positioned under the knees for patient comfort. This results in flexion of the hips (˜15°). Therefore, we try to avoid using this approach when hip pathology is suspected. If a soft tissue abnormality is suspected posteriorly, the patient should be placed in the prone position. This reduces compression of the soft tissues and anatomic distortion.⁹^,¹⁰ After the initial scout images, axial images are obtained using spin-echo T1-weighted and turbo spin-echo T2-weighted sequences (Table 6.1).¹⁰ The same sections are selected for both sequences. This allows more accurate comparison of T1- and T2-weighted sections. In the same fashion, coronal images are obtained using short TI inversion recovery (STIR), or fat suppressed turbo spin-echo T2-weighted and T1-weighted sequences (Table 6.1) (Figs. 6.3 and 6.4). Some institutions advocate a single coronal STIR sequence for a limited screening examination in patients with hip pain.¹⁰ When the coronal or sagittal plane is used, the slice thickness varies depending on the tissue volume of the area to be studied. These sequences and image planes generally provide an adequate screening examination for the pelvis (Table 6.1). In certain circumstances, which will be discussed later, further pulse sequences and image planes may be indicated.¹⁰^,¹¹^,¹² The field of view (FOV) selected varies with patient size, but generally 34 to 40 cm is adequate for adults. A matrix of 256 × 256 or 512 × 512 with 1 or 2 acquisitions is obtained. Respiratory compensation
techniques should be used to reduce motion artifact in the upper pelvis. Thinner slices can be used if a more defined area of the anatomy is to be studied.

Figure 6.1 Coronal T1-weighted image of the pelvis demonstrating rotation of the patient such that the lesser trochanter on the right (arrow) is seen but cannot be compared on the section with the left femur. The pelvis is also tilted (black line across the iliac crests).

When pathology of the sacrum or sacroiliac joints is suspected, a different approach may provide additional information.¹³^,¹⁴ This is especially true in patients with suspected sacroiliac arthropathy. Additional oblique coronal images in the plane of the sacrum (Fig. 6.5) and sacroiliac joints are performed using T1-, T2-weighted, and STIR sequences.¹⁰^,¹⁵ Fat suppression techniques are useful, especially when intravenous gadolinium is injected. Gadolinium is useful for identification of early or acute inflammatory changes in patients with sacroiliitis.⁸

Figure 6.2 A: Coronal T1-weighted image demonstrates pelvic tilt (black line) and patient rotation with the trochanters (arrows) evident on the left, but at a different level on the right. B: Fat suppressed fast spin-echo T2-weighted axial image demonstrates that the femoral heads are at different levels. The greater trochanter on the right (arrow) is visible and the upper portion of the contralateral femoral head is partially visualized.

Hips

Evaluation of patients with symptoms more defined to the hip region is performed with a slightly different technique. Using a three-plane scout sequence as a guide, a T1-weighted coronal sequence (SE 540/15) is performed using 4-mm-thick sections with no interslice gap. This technique usually defines osteonecrosis and other obvious marrow abnormalities in the hips. When there is no evidence of osteonecrosis or if other abnormalities are suspected, a second turbo spin-echo or fat suppressed T2-weighted axial sequence is performed similar to above (pelvis and sacroiliac joints) (Table 6.1). In certain situations such as osteonecrosis or subtle osteochondral abnormalities of the femoral heads, it may be of benefit to use a surface coil or dual coupled coils to improve image quality.¹⁰^,¹⁶^,¹⁷ Sagittal images are useful to quantify surface area involvement of the femoral head (Fig. 6.6). In this setting, a small FOV (12 to 16 cm) should be used.⁴^,¹⁷^,¹⁸

Evaluation of subtle femoral head changes, articular cartilage, synovial or labral pathology may require additional sequences and intravenous or intra-articular gadolinium.⁴^,¹⁹^,²⁰^,²¹^,²²^,²³ In these clinical settings, a surface coil, or coupled coils and smaller FOV (12 to 16 cm) is also required.⁴^,¹⁰^,¹⁷ Early changes in articular cartilage may be more evident with fat-suppressed T2-weighted fast spin-echo sequences. Three-dimensional spoiled gradient-echo sequences have also been advocated, with accuracy reported at over 90% for detection of cartilage lesions (TR 60, TE 5,
flip angle 40°).²² Thin section fast scan three-dimensional techniques can be reformatted in multiple planes to more easily assess subtle changes in articular cartilage.²³ Axial and coronal or sagittal T1-weighted spin-echo and T2-weighted turbo spin-echo sequences provide initial screening of the thighs (Table 6.1).

Table 6.1 MR Examinations of the Pelvis, Hips, and Thighs (based upon 1.5 T units)

	Pulse Sequence	Slice Thickness	Matrix	Field of View^a cm	Acquisitions (NEX)	Image Time
Pelvis-SI joints
Scout-axial	15/5 FA 40°	3-5 1 cm slices	256	40	1	26 s
Coronal sagittal axial	SE 410/17	6 mm	512	34-40^a	2	5 min 20 s
Axial	TSE 4,000/102	6 mm	512	34-40^a	2	4 min 36 s
Coronal	SE 580/13	5 mm	512	34-40^a	1	5 min 2 s
Coronal	STIR 5,600/109/165	5 mm	256	34-40^a	2	4 min 34 s
Hips
Coronal	SE 536/15	4 mm	320	20-24	1	4 min 38 s
Axial	TSE 4,000/102	6 mm	512	20-24	2	4 min 36 s
Sagittal	SE 536/15	4 mm	320	20-24	1	4 min 38 s
Thighs
Coronal scout	SE 400/15-20	3 1-cm slices	128×256	42	1	51 s
Axial	TSE 4,000/102	6 mm	512	30-42^a	2	4 min 36 s
Coronal or sagittal	SE 536/15	4 mm	320	30-42^a	1	4 min 38 s
Arthrography
Axial	SE 568/15	4 mm	256	18	1	4 min 55 s
Sagittal	SE 568/15	4 mm	256	18	1	4 min 55 s
Coronal	SE 420/15	4 mm	256	18	1	3 min 36 s
Coronal	TSE 4,000/92 (fat suppressed)	4 mm	256	18	1	3 min 39 s
Oblique coronal	SE 420/15	4 mm	256	18	1	3 min 39 s
Oblique sagittal	SE 420/15	4 mm	256	18	1	3 min 39 s
TSE, turbo spin echo; FA, flip angle; SI, sacroiliac; SE, spin echo; STIR, short inversion time recovery.
^aVaries with patient size and area of interest.

Figure 6.3 3.0 T coronal images of the pelvis and hips. A: Coronal 650/10 image with 1 echo train. B: Coronal turbo spin-echo 3800/62 with 7 ET.

Intravenous gadolinium is useful for detection of subtle ischemic disease in adults (osteonecrosis) and children, and
changes in articular cartilage and synovium.¹⁹^,²¹^,²⁴^,²⁵ Conventional sequences are usually performed followed by fat-suppressed T1-weighted sequences after contrast injection (Fig. 6.7).

Figure 6.4 1.5 T coronal images of the pelvis and hips. Coronal SE 500/10 (A) and fat-suppressed SE 450/10 (B) images. Coronal fast T2 (4800/102) (C) and fat-suppressed fast spin-echo T2 (4800/102) (D) sequences. There are contrast differences and there is less fat suppression with fast spin-echo T2-weighted sequences compared to conventional spin-echo T2-weighted sequences. Note the uneven suppression of fat signal and signal intensity loss on the left compared to right on these images. Uniform fat suppression is important to prevent errors in image interpretation.

Magnetic Resonance Arthrography

Magnetic resonance arthrography is now routinely performed to better define specific hip disorders (Fig. 6.7).²⁰^,²¹^,²²^,²³^,²⁴^,²⁵^,²⁶^,²⁷^,²⁸^,²⁹ MR arthrograms can more readily identify intra-articular abnormalities such as cartilage defects, loose bodies, and labral tears compared to conventional MR studies.²⁰^,²⁷ Though an arthrographic effect can also be achieved by delayed imaging (>15 minutes after injection) after intravenous contrast, there is no control of contrast volumes and joint fluid cannot be obtained for study. Therefore, direct MR arthrography (Fig. 6.7) is preferred in most cases. Contrast (8 to 20 mL of 1 mmol solution of gadolinium) is injected using fluoroscopic guidance. We mix 50% iodinated contrast and 50% Marcaine to dilute the gadolinium. This assists in confirming that the hip is the source of symptoms, specifically pain. The patient is exercised and transferred to the MR gantry. MR arthrography requires a surface coil with a small field of view (12 to 18 cm). Axial, coronal, sagittal, oblique, or radial images can be obtained depending upon the suspected pathology (Table 6.1).²⁸^,²⁹^,³⁰ Recent studies have demonstrated
that radial imaging does not add significant new information to the study.³⁰ Therefore, we do not perform radial imaging at our institution. T1-weighted fat-suppressed spin-echo and turbo spin-echo T2-weighted sequences are most commonly used with MR arthrography.²⁰^,²⁷ Three-dimensional gradient-echo sequences (30/9, 45° flip angle) can also be utilized.²⁶^,³¹

Figure 6.5 Scout sagittal image (A) to select image planes and sections for evaluation of the sacrum and sacroiliac joints. Oblique coronal images of the sacrum and sacroiliac joints using T1-weighted (B, C) images and fat-suppressed, T2-weighted (D, E) images at similar levels.

Figure 6.6 Osteonecrosis of the right femoral head. A: Coronal T1-weighted image of both hips using the torso coil and a large field of view (42 cm). Small field of view (16 cm) surface coiled images using T1-weighted coronal (B) and sagittal (C) and fast spin-echo T2-weighted coronal (D) and sagittal (E) sequences show the necrotic interface (arrows) to better detail.

Figure 6.7 Coronal post-contrast fat-suppressed T1-weighted image demonstrates a femoral neck fracture with no enhancement of the femoral head due to vascular injury.

Figure 6.8 Normal MR arthrogram. Scout axial (A), sagittal (B), coronal (C), oblique coronal (D), and oblique sagittal (E) images. Normal axial (F), coronal (G), and sagittal (H) arthrogram images. The joint capsule attaches several millimeters proximal to the superior labrum (arrow in G) creating the superior acetabular recess.

MR arthrograms of the hip are invasive and carry the same minimal risks associated with conventional arthrography. Examination time and cost also exceed conventional MR imaging studies.¹⁰^,²⁶^,³¹

Thighs

Generally, both T1- and T2-weighted sequences are required to fully evaluate the soft tissues of the thighs and the upper femurs. Again, a coronal scout through the hip and thigh region is generally obtained, from which conventional or fat-suppressed T2-weighted axial images are selected. Depending upon the findings noted with the T2-weighted images, a second series of T1-weighted images is performed in either the coronal or sagittal plane. The second sequence is important in characterizing the nature of the lesion. Two different planes are essential if the extent of a lesion is important to define, such as for a neoplasm, abscess, or hematoma. Short TI inversion recovery (STIR) or gadolinium enhanced
T1-weighted images are useful for evaluating subtle changes.²^,¹⁰

Figure 6.8 (Continued)

New angiographic sequences (see Chapters 1 and 3) are useful when vascular abnormalities are suspected in the pelvis or thighs.³² Variations in techniques will be discussed in detail later with specific pathological conditions.

ANATOMY

It is essential that the anatomy of the bone and soft tissue structures be clearly understood in all image planes typically used with MR imaging (Figs. 6.9,5.10,6.11).²^,¹⁰^,³³^,³⁴^,³⁵

Osseous Anatomy

The pelvis is formed by the two innominate bones that articulate posteriorly with the sacrum at the sacroiliac joints and anteriorly at the pubic symphysis. Each innominate bone is composed of an ilium, ischium, and pubis. The acetabulum is formed by the junction of these osseous structures. The posterior acetabulum is stronger and along with the dome comprises the weight-bearing portion of the acetabulum.¹⁰^,³⁴ The margin of the acetabulum is surrounded by a fibrocartilaginous labrum (Figs. 6.8 and 6.13). Identification of the shape of this labrum, its relationship to the femoral head, and whether the structure is intact is important in infants, children, and adults.¹⁰^,²⁸^,²⁹^,³⁶ Special image planes and intra-articular gadolinium may be required to demonstrate the entire labrum. These techniques are discussed in this chapter.

Further discussion of the articulations in the pelvis is important, as these areas are easily assessed with MR imaging. Anteriorly, the pubic symphysis is formed by the two pubic bones. The joint is supported by the superior
pubic and arcuate ligaments that encase a fibrocartilaginous disc. The disc lies between the two pubic bones (Figs. 6.9 and 6.12).³⁷^,³⁸ The sacroiliac joint is a synovial joint with posterior and anterior ligamentous support (Fig. 6.12). The posterior ligaments are stronger than the anterior ligaments, which allow some anterior motion. In addition, there are several accessory ligaments that assist in the support of the sacroiliac joint. These include the sacrotuberous ligament, which extends from the inferolateral margin of the sacrum to the ischial tuberosity; the sacrospinous ligament, which extends from the lower margin of the sacrum to the ischial spine; and the iliolumbar ligament, which extends from the anterior inferior transverse process of L5 and passes inferiorly to blend with the anterior sacroiliac ligament along the base of the sacrum. These ligamentous structures appear dark or have no signal intensity on MR images. The synovial cavity of the sacroiliac joint contains only a small amount of fluid. Joint fluid is most easily identified on axial T2-weighted or contrast-enhanced MR images (Fig. 6.7).³⁷^,³⁸

Figure 6.9 Axial MR images (SE 405/16) of the pelvis, hips, and upper thighs with accompanying illustration of the section levels to demonstrate MR anatomy of the pelvis and hips. A: Axial image through the upper pelvis. B: Axial image through the upper sacrum. C: Axial image through the lower sacroiliac joint. D: Axial image through the sciatic notch. E: Axial image through the anterior inferior iliac spine. F: Axial image through the upper femoral head. G: Axial image through the femoral head and greater trochanter. H: Axial image through the femoral neck. I: Axial image through the lesser trochanter. J: Axial image through the upper femur. K: Axial image through the upper thigh.

Figure 6.9 (Continued)

The hip is a ball and socket joint. The fibrous capsule of the hip joint is lined with synovial membrane and the hyaline cartilage covers the articular surfaces of the acetabulum and femoral head (Figs. 6.8 and 6.13). There are several important intra-articular structures that should be identified on MR images. Ligamentum teres is a firm ligament extending from the fovea of the femoral head to the acetabulum. The ligament enters a small notch in the medial acetabular wall where it is surrounded by fat (Figs. 6.10E and 6.13). A fibrocartilaginous acetabular labrum surrounds the acetabular rim. This structure is more clearly defined on MR arthrograms than conventional MR images (Figs. 6.8 and 6.10).²⁸^,³⁹^,⁴⁰ Axial, coronal, oblique (Fig. 6.8), or radial images (Fig. 6.14) may be necessary to completely define this structure.¹⁰^,²⁸ Inferiorly the labrum is incomplete but connected by the transverse ligament (Figs. 6.8G and 6.13). This ligament has an elliptical configuration and should not be confused with an acetabular labral tear. The ligaments that surround the capsule of the hip blend with the capsule and are not easily defined by MR imaging. Major ligaments (Fig. 6.15) include the pubofemoral ligament that arises from the body of the pubis close to the acetabulum and passes anterior to the lower part of the femoral head, blending with the lower limb of the iliofemoral ligament as it attaches to the lower margin of the femoral neck. The iliofemoral ligament is thicker and probably the strongest of the supporting ligaments of the hip. It is more triangular with its apex attached
to the lower part of the anterior inferior iliac spine and body of the ilium. The base of this triangular ligament attaches to the intertrochanteric lines (Fig. 6.15). The ischiofemoral ligament, the thinnest of the three major ligaments, arises from the ischium behind and below the acetabulum. Its upper fibers are horizontal; its lower fibers extend upward and laterally and attach to the upper posterior neck at the junction of the greater trochanter. The vascular supply of the hip and femoral head is important, especially in the etiology of osteonecrosis. This is discussed in depth in the section on osteonecrosis.³⁴^,³⁸^,⁴¹^,⁴²

Figure 6.10 Coronal MR images (SE 450/15) and accompanying illustrations of section levels of the pelvis, hips, and thighs. A: Coronal image through the pubic symphysis. B: Coronal image through the anterior aspect of the hips. C: Coronal image through the midjoint space of the hip. D: Coronal image through the greater trochanteric level of the hips. E: Coronal image through the ischium.

Figure 6.10 (Continued)

Muscular Anatomy

The anatomy of the muscles acting on the pelvis, hips, and thighs in axial, coronal, sagittal, and even oblique planes must be thoroughly understood to interpret MR images and evaluate symptoms related to these structures. The muscles acting on the hip joint per se are numerous. Therefore, it is simplest to discuss them based upon their function.²^,¹⁰^,³³^,³⁴^,³⁶^,³⁷^,³⁸

The chief extensors of the hip include the gluteus maximus and posterior portion of the adductor magnus (Fig. 6.16). Extension is also accomplished to some degree by assistance from the semimembranosus, semitendinosus, biceps femoris, gluteus medius, and gluteus minimus (Table 6.2).³⁴^,⁴³^,⁴⁴

The primary flexor of the hip is the iliopsoas muscle (Fig. 6.17). However, the pectineus, tensor fasciae latae, adductor brevis, and sartorius also function in this regard. Accessory flexors include the adductor longus, adductor magnus, gracilis, and gluteus minimus.³⁴^,³⁸ The iliacus and psoas muscle anatomy is important for accurate interpretation of MR
images.⁴⁴^,⁴⁵ The bulk of the iliacus muscle run parallel to the iliopsoas tendon and attach to the proximal femur. In some cases, a small iliacus tendon runs parallel to the iliopsoas tendon as it attaches to the lesser trochanter. The iliopsoas tendon is separated from the iliacus muscle and tendon by a small amount of fatty tissue (Fig. 6.18).⁴⁴^,⁴⁵

Figure 6.11 Sagittal MR images (SE 450/15) and accompanying anatomic illustrations section levels of the pelvis, hips, and thighs progressing from medial to lateral. A: Sagittal image through the medial ilium near the sciatic notch. B: Sagittal image through the level of the iliopectineal imminence. C: Sagittal image through the medial joint space. D: Sagittal image through the medial femoral head. E: Sagittal image through the femur. F: Sagittal image through the level of the greater trochanter.

Figure 6.11 (Continued)

Adduction of the femur is primarily accomplished by the adductor muscle group and gracilis (Figs. 6.9, 6.11, and 6.17). Adductor group includes the adductor brevis, adductor longus, and adductor magnus. Accessory adductors include the gluteus maximus, pectineus, and obturator externus. The hamstring muscles also assist slightly in this regard. Medial or internal rotation of the hip is primarily accomplished by the gluteus medius and minimus and the tensor fasciae latae. Semitendinosus, semimembranosus, and to some degree the gracilis also participate in
internal rotation. External rotation of the hip is accomplished by the gluteus maximus and the short rotators of the gluteal region including the piriformis, obturator internus, and the gemelli muscles (Fig. 6.19).³⁴^,³⁸^,⁴⁴

Figure 6.12 Illustration of the ligaments of the posterior sacroiliac region (A) and anterior sacroiliac region (B) and symphysis.

Table 6.2 summarizes the muscles of the pelvis and hips along with their origins, insertions, and functions. Identification of these muscles in all planes is essential in properly evaluating the pelvis, hips, and thighs with MR imaging. It is also important to know the relationships of these muscles to the various neurovascular structures.

The gluteus maximus (Table 6.2) is a large oblique muscle that largely contributes to the shape of the buttock (Figs. 6.9, 6.11, 6.16, and 6.19). This muscle arises from the posterior ilium and dorsal surface of the sacrum and coccyx. The gluteus maximus has both superficial and deep insertions. Portions of the insertion blend with the tendinous fibers of the fasciae latae and form a portion of the iliotibial tract. The deeper part of the muscle inserts into the gluteal tuberosity of the femur. This lies below the level of the trochanters along the posterior margin of the femur. A subcutaneous bursa frequently lies over the superficial portion of the tendon at the level of the greater trochanter. There is a larger bursa (Fig. 6.20) that typically lies between the tendon and the greater trochanter.¹⁰^,³⁴^,⁴⁶ When inflamed,
these bursae are seen as well defined, high intensity lesions near the insertions (T2-weighted sequences).¹⁰^,⁴⁷

Figure 6.13 A: Coronal illustration of the hip demonstrating the major articular components and capsule and the plane (line) of the labrum. B: Enface illustration of the acetabular fossa and labrum.

The gluteus medius (Figs. 6.9, 6.10, 6.11, and 6.19) arises from a large area on the posterior wing of the ilium, below the iliac crest. It extends inferiorly and laterally to insert on the greater trochanter along its posterolateral surface. An additional bursa typically lies between the anterior fibers and the adjacent trochanter (Fig. 6.20). Major fibers of the superior gluteal nerve and vessels lie between the gluteus medius and minimus (Fig. 6.9). The gluteus minimus is also a fan-shaped muscle arising from a more inferior aspect of the posterior ilium and extending along a similar course to insert in the upper anterior surface of the greater trochanter. There is also a bursa between this muscle and the trochanter. These two muscles combine to form the major abductors of the hip. The gluteus medius and minimus muscles are innervated by the superior gluteal nerve.¹⁰^,³⁴^,³⁸

Figure 6.14 Illustration of radial planes and labral configurations.

The tensor fasciae latae (Fig. 6.17) arises from the anterior-most aspect of the iliac crest and passes posteriorly and inferiorly, inserting in the anterior part of the iliotibial tract. It is innervated by the superior gluteal nerve and serves primarily as a flexor, internal rotator, and abductor of the thigh.³⁸

The piriformis (Figs. 6.9 and 6.19) is the most superior of the small muscles in the gluteal region and plays a significant role in vascular disorders in the gluteal region. This muscle largely fills the greater sciatic notch (Fig. 6.9), through which the sacral plexus and associated neurovascular structures pass. The superior gluteal nerve and vessels typically lie along the upper border of this muscle, whereas the pudendal nerve and vessels and inferior gluteal nerve and vessels along with the sciatic nerve typically lie along its lower margin (see Fig. 6.28). In up to 10% of patients, the piriformis is actually perforated by the sciatic nerve or its branches. The muscle arises from the lateral aspect of the sacrum at the level of the second through fourth sacral segments and extends laterally and posterior to the hip joint to insert on the posterior aspect of the greater trochanter.³⁷^,³⁸

Figure 6.15 Illustration of the supporting ligaments of the hip from anterior (A) and posterior (B).

The obturator internus takes its origin from the internal pelvic wall of the bones forming the obturator foramen (Figs. 6.9 and 6.11). The muscle passes laterally through the lesser sciatic foramen, posterior to the lesser sciatic notch of the ischium, running posterior to the hip joint to insert on the medial surface of the greater trochanter just above the trochanteric fossa. Again, a bursa typically separates the tendon from the bone in this region.³⁴^,³⁷^,³⁸ This bursa is typically not identified on MR images unless inflamed and distended with fluid.¹⁰

Figure 6.16 Illustration of the extensors of the thigh.

The superior and inferior gemelli muscles (Figs. 6.9 and 6.19) lieabove and below the obturator internus respectively. The superior gemellus originates from the posterior ischial spine and the inferior gemellus from the upper part of the ischial tuberosity. Both muscle bundles converge to insert with the obturator internus tendon and assist this muscle in external rotation of the hip.³⁴^,³⁸

The final muscle in the external rotator group is the quadratus femoris (Fig. 6.19). This muscle takes its origin from the lateral aspect of the ischial tuberosity and inserts on the posterior aspect of the femur just below the intertrochanteric line (Figs. 6.10 and 6.19).

The anterior muscles of the thigh (Fig. 6.21), with which the iliopsoas are included, are the sartorius and the four segments of the quadriceps muscle. The pectineus muscle is also included in this muscle group.³⁴^,³⁸

The iliopsoas as it appears in the thigh is a combination of two muscles, the iliacus and the psoas major (Figs. 6.9, 6.18 and 6.21). The psoas major arises in the retroperitoneum at the lateral margin of the T12 through L5 lumbar vertebrae. This muscle passes inferiorly and slightly laterally where it joins the iliacus to insert on the lesser trochanter.³⁴ The psoas minor is an inconsistent muscle that typically

arises from the adjacent borders of the last thoracic and first lumbarvertebrae and extends along the anterolateral margin of the psoas major to insert on the iliopectineal eminence of the innominate bone. The iliacus takes its origin from the internal surface of the ilium below the iliac crest and passes slightly obliquely anterior to the hip joint, inserting with the psoas muscle in the lesser trochanter (Fig. 6.21). An important bursa (can cause clinical symptoms and local hip pain) is the iliopsoas bursa, which lies beneath the iliopsoas muscle just as it crosses the anterior surface of the hip joint. This bursa may approach 3 to 7 cm in length and 2 to 4 cm in width. The bursa communicates with the hip joint in up to 15% of patients (Fig. 6.22).³⁴^,³⁸^,⁴⁴^,⁴⁶

Table 6.2 Muscles of the Pelvis, Hips, and Thigh

Muscle	Origin	Insertion	Function	Innervation
Gluteus maximus	Posterior ilium Dorsolateral sacrum and coccyx	Gluteal tuberosity of femur Iliotibial tract	Hip extensor External rotator	Inferior gluteal nerve (L5-S1)
Gluteus medius	Posterior ilium below crest	Posterolateral greater trochanter	Hip abductor	Superior gluteal nerve (L4-S1)
Gluteus minimus	Posterior mid ilium	Anterior greater trochanter	Hip abductor	Superior gluteal nerve (L4-S1)
Tensor fasciae latae	Anterior iliac crest	Iliotibial tract	Flexor Internal rotator Abductor	Superior gluteal nerve (L4-S1)
Piriformis	Anterolateral sacrum S2-S4	Superior border greater trochanter	External rotator Abductor	S1, S2
Obturator internus	Pubic and ischial margins	Medial greater trochanter	External rotator	L5-S2
Superior gemelli	Posterior ischial spine	Obturator internus tendon	External rotator	L5-S2
Inferior gemelli	Ischial tuberosity	Obturator internus tendon	External rotator	L5-S2
Quadratus femoris	Lateral ischial tuberosity	Posterior femur below intertro-chanteric line	External rotator Abductor	L4-S1
Iliopsoas
Psoas major	Lateral margin T12-L5	Lesser trochanter	Thigh flexor Accessory adductor	L2-L4
Psoas minor	Lateral margin T12-L5	Iliopectineal eminence	Tilt pelvis upward	T12-L2
Iliacus	Inner iliac surface below crest	Lesser trochanter	Thigh flexor	L2-L4
Sartorius	Anterior superior iliac spine	Proximal antero-medial tibia	Thigh flexor Accessory external rotator	Femoral nerve (L2-L3)
Quadriceps femoris
Rectus femoris	Anterior inferior iliac spine	Upper patella	Extensor of knee Accessory thigh flexor	Femoral nerve (L3-L4)
Vastus lateralis	Upper lateral and posterolateral femur	Upper lateral patella	Extensor of knee	L3-L4
Vastus medialis	Medial posterior femur	Medial tendon of rectus femoris	Extensor of knee	L3-L4
Vastus intermedius	Anterior mid femur	Upper posterior patella	Extensor of knee	L3-L4
Pectineus	Superior pubic ramus	Pectineal line femur	Thigh flexor	Femoral nerve (L2, L3)
Adductor longus	Anterior pubic bone	Medial linea aspera	Thigh adductor	Obturator nerve (L2-L3)
Adductor brevis	Pubic bone and inferior ramus	Upper linea aspera	Thigh adductor	Obturator nerve (L2-L3)
Adductor magnus	Ischium and inferior pubic ramus	Linea aspera Adductor tubercle	Thigh adductor	Obturator and tibial nerve (L3-L5)
Gracilis	Inferior pubic ramus near symphysis	Upper anterior tibia	Thigh adductor Medial rotator thigh	Obturator nerve (L3-L4)
Obturator externus	Outer margins obturator foramen	Intertrochanteric fossa	Lateral rotator	Obturator nerve (L3-L4)
Semitendinosus	Posteromedial ischial tuberosity	Upper anterior tibia	Thigh extensor Knee flexor	Tibial side of sciatic (L5-S1)
Biceps femoris	Long head: posteromedial ischial tuberosity	Fibular head	Extend thigh	Long head: tibial side of sciatic nerve (L5-S1)
	Short head: lateral lip linea aspera	Fibular head	Flex knee	Short head: peroneal side of sciatic nerve (L5-S2)
Semimembranosus	Posterolateral ischial tuberosity	Posteromedial upper tibia	Thigh extensor Accessory medial rotator and thigh adductor	Tibial side of sciatic nerve (L5-S2)
From Baum PA, Matsumoto AH, Teitelbaum GP, et al. Anatomic relationship between the common femoral artery and vein. CT evaluation and clinical significance. Radiology 1989;173:775-777 and Rosse C, Rosse PC. Hollinshead’s textbook of anatomy. Philadelphia, PA: Lippincott-Raven; 1997.

Figure 6.17 Illustration of the flexors of the thigh.

The sartorius is a long, straplike muscle that originates from the anterior superior iliac spine and passes obliquely and medially to run along the inner thigh. Below the level of the knee joint it inserts on the anteromedial aspect of the upper tibia (Fig. 6.21). At its insertion it lies largely above the gracilis and semitendinosus. The combination of these three tendons forms the pes anserinus tendon. A bursa typically lies deep to the insertion of the sartorius. This bursa tends to separate it from the insertions of the gracilis and semitendinosus.³⁴^,³⁸

The quadriceps femoris is composed of four muscles. These include the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis, all of which blend at their patellar insertions (Figs. 6.9, 6.10, and 6.21). The most anterior muscle of the group is the rectus femoris that arises from the anterior inferior iliac spine. This muscle has the distinction of being the only member of the quadriceps group that crosses the hip joint as it passes along its anterior superficial course to insert with the other muscles in the quadriceps tendon, which inserts in the upper border of the patella. A second, more inconsistent origin is also noted from the upper margin of the acetabulum. This is important in that a bursa typically lies deep to this reflected tendon of the rectus femoris. The quadriceps tendon, with the patella, continues to form the patellar ligament that inserts on the tibial tuberosity.³⁴

The vastus lateralis is a large member of the quadriceps group that is covered anteriorly by the rectus femoris and tensor fasciae latae (Figs. 6.9 and 6.21). The muscle itself lies anterior and lateral to the vastus intermedius. Its origin
is from the femur just below the greater trochanter and posterolaterally along the margin of the linea aspera and intermuscular septum. The vastus intermedius inserts along the upper lateral margin of the patella forming a portion of the quadriceps tendon. The vastus medialis makes its origin anteriorly from just below the lesser trochanter and the medial and posterior aspect of the femur. It extends inferiorly and inserts into the medial aspect of the rectus femoris tendon. The vastus intermedius covers a major portion of the front and medial and lateral aspects of the femur (Fig. 6.21).³⁴^,³⁷^,⁴⁸ It is completely covered superficially by the vastus lateralis and medialis on its sides and the rectus femoris anteriorly. Distally, its fibers fuse with the vastus medialis and lateralis to insert in the posterior upper surface of the patella.³³^,³⁴^,³⁷^,³⁸

Figure 6.18 Axial T1-weighted MR images (A and B) demonstrating the relationship of the iliacus intermuscular tendon (arrowhead) and the iliopsoas tendon (arrow) medially as they attach to the lesser trochanter.

Figure 6.19 Illustration of the external rotators of the hip.

The pectineus arises from the superior anterior aspect of the superior pubic ramus and extends obliquely in an inferolateral direction to insert along the pectineal line of the
femur that extends inferiorly from the lesser trochanter to the linea aspera (Fig. 6.21).³⁷^,³⁸ Anatomists often include the adductor brevis, adductor longus, adductor magnus, gracilis, and obturator externus in the anterior medial muscle group, as all of these muscles are innervated by the obturator nerve (Table 6.2).³⁸ In the axial plane, the adductor longus is the most anterior of the adductor muscles in the upper thigh (Fig. 6.9). The adductor longus takes its origin from the pubic bone superiorly near the pubic symphysis and extends in a triangular fashion inferiorly and laterally to insert along the medial aspect of the linea aspera at the level of the mid femur. This muscle forms the floor of the femoral triangle along with the pectineus and iliopsoas muscle. The adductor brevis arises from a broad tendon from the body and inferior ramus of the pubis and expands in a triangular fashion to insert in the upper half of the linea aspera. It is typically seen between the adductor longus and magnus on the axial views with the gracilis running along its medial aspect (Figs. 6.9 and 6.21).¹⁰^,³³^,³⁴^,³⁷^,³⁸

Figure 6.20 A: Illustration of bursae near the greater trochanter. Axial T1- (B) and T2-weighted (C) images demonstrating trochanteric bursitis (arrows).

The adductor magnus, like the other adductor muscles, is triangular but much larger (Fig. 6.21). It arises from the lower part of the inferior pubic ramus and the entire length of the ramus of the ischium. It courses inferiorly and laterally with the upper portion, inserting on the linea aspera with lower fibers inserting in the adductor tubercle just above the medial femoral condyle.³⁴^,³⁸

The gracilis is a long, thin muscle that arises from the inferior aspect of the pubic bone close to the symphysis
(Figs. 6.9, 6.10, and 6.21). It passes medially along the thigh, superficial to the adductor muscles (Fig. 6.10). Near the knee, it lies first between the sartorius and semimembranosus and then between the sartorius and semitendinosus. Below the knee, the tendon curves anteriorly and inserts in the upper medial anterior tibia with a bursa termed the anserine bursa intervening between gracilis, sartorius, and semitendinosus tendons and the tibia.¹⁰^,³⁴

Figure 6.21 Illustration of the anterior musculature (A) and neurovascular structures of the thigh (B).

The obturator externus (Figs. 6.9, 6.10, and 6.21) is the most deeply placed muscle in the adductor group. This muscle arises from the external margins of the obturator foramen, namely the superior pubic ramus, inferior pubic ramus, and upper margin of the ischium, and extends laterally and may be separated from the capsule proper by a bursa posterior to the hip capsule.³⁸^,⁴⁹ The obturator internus functions primarily as a lateral rotator.³⁸

The posterior muscles of the thigh, commonly referred to as the hamstring muscles, include the semitendinosus, the biceps femoris, the semimembranosus, and a portion of the adductor magnus that arises from the ischial tuberosity (Fig. 6.23). All of the hamstring muscles take their origin from the ischial tuberosity except for the short head of the biceps femoris. The semitendinosus arises from the posteromedial aspect of the ischial tuberosity. Its origin is fused with the long head of the biceps. Just above the medial condyle, the tendinous portion of the muscle continues posterior to the knee and forward along the medial aspect to insert in the upper anterior tibia, just posterior to the insertions of the gracilis and sartorius. As noted previously, this is, along with the above tendons, a part of the pes anserinus complex.³⁴^,³⁸^,⁴⁴

The biceps femoris has two heads, a short and long head (Fig. 6.23). The long head of the biceps arises from the posteromedial aspect of the inferior aspect of the ischial tuberosity in common with the semitendinosus tendon. The short head of the biceps originates from the lower lateral aspect of the linea aspera. This muscle lies lateral to the belly of the long head as it extends inferiorly, and at the level of the knee joint, a common tendon forms that crosses the knee and inserts into the head of the fibula.³⁴^,³⁸^,⁴⁴

The semimembranosus arises from a long, flat tendon at the posterolateral aspect of the ischial tuberosity that is
lateral to the common origin of the biceps femoris and semitendinosus (Fig. 6.23). The muscle becomes tendinous posterior to the medial meniscus as it crosses the knee. At the level of the joint, it gives off an oblique expansion that also attaches to the medial collateral ligament. Its insertion is the posterior medial aspect of the upper tibia just below the knee.³³^,³⁴^,³⁷^,⁴³^,⁴⁴

Figure 6.22 Bi-lobed iliopsoas bursa in a patient with advanced degenerative arthritis of the left hip. Coronal (A) and axial (B) T2-weighted images and post-contrast fat suppressed T1-weighted image (C) demonstrate a bursa (arrows) extending on both sides of the iliopsoas tendon.

NEUROVASCULAR STRUCTURES

The neurovascular anatomy of the pelvis, hips, and thigh is complex and the relationships of these structures to the sacrum and the above muscles is important in evaluating pathology along the course of the neurovascular bundles (Figs. 6.24 and 6.25).²^,³²^,³⁴^,³⁸^,⁴⁸ Demonstration of these structures can be accomplished using axial planes (Fig. 6.9) but may be difficult with coronal and sagittal planes because of the difficulty in following the changes in the course of the neurovascular structures (Figs. 6.10 and 6.11). The lumbosacral plexus can be identified on coronal (Fig. 6.26), sagittal, and oblique coronal planes. The last is preferred for evaluating neural structures in the sacral foramina (Fig. 6.27).⁵⁰^,⁵¹

The abdominal aorta usually bifurcates at the L-4 level, forming the iliac arteries. The iliac arteries divide at the level of the sacroiliac joints to form the internal and external iliac vessels (Fig. 6.24). The internal iliac artery gives off the superior and inferior gluteal arteries that supply the posterior or buttock muscles (Fig. 6.28). These vessels and their branches are difficult to define on conventional orthogonal MR images. The common femoral artery is a continuation of the external iliac below the inguinal ligament. The
circumflex arteries and branches of the obturator artery (via ligamentum teres) supply the hip. The superficial femoral and deep femoral (profunda) arteries form slightly distal to this level (Fig. 6.24).²^,³⁸^,⁴¹; they branch at about the same level as the superficial femoral and saphenous veins (Fig. 6.24). The course of the major arteries and companion veins is easily followed on contiguous axial MR images (Fig. 6.9). In the upper thigh (Fig. 6.24), the superficial femoral artery lies anterior to the adductor longus and deep to the sartorius. The profunda femoris artery and vein lie more laterally between the adductor longus and magnus near the linea aspera of the femur (Fig. 6.9). Perforating branches are usually identifiable between the adductor magnus and hamstring muscles just posterior and slightly lateral to the linea aspera of the femur.³⁴^,³⁸ Today, MR angiography is capable of demonstrating all major vessels (Fig. 6.27).

Figure 6.23 Illustration of the hamstring muscle group.

Figure 6.24 Illustration of the major vessels to the pelvis and thighs.

Figure 6.25 Illustration of the major neural structures of the pelvis and hips.

Figure 6.26 Coronal SE 450/15 image of the pelvis demonstrating the course of the sacral plexus and sciatic nerve. Note the relationship of the sciatic nerve to the ischium. SN, sciatic nerve; IT, ischial tuberosity; LA, levator ani.

Figure 6.27 Oblique T1-weighted coronal images demonstrating the sacral nerve roots (arrows) surrounded by fat as they exit the ventral foramina. The sacroiliac joints are also well demonstrated.

The major neural structures are derived from the lumbosacral plexus (L1 through S2) (Fig. 6.25).²^,⁵⁰^,⁵¹ Portions of the T12 and S3 ventral rami also contribute to this plexus. The sacral branches exit the ventral foramina along with the L4 through L5 segments for the sciatic nerve (L4 through S3) (Figs. 6.26 and 6.27). The sciatic nerve exits the pelvis posteriorly and lies just posterior to the hip (Fig. 6.26). This is best seen on axial MR images (Figs. 6.9 and 6.25).²^,³⁴^,³⁸

The sciatic nerve enters the buttock below the piriformis (Fig. 6.28) in about 85% of patients. In some cases, the sciatic nerve may pass through the piriformis. Alternatively, the peroneal and tibial segments of the sciatic nerve may be separated by the piriformis.³⁸ The nerve lies posterior to the obturator externus, gemelli, and quadriceps femoris, and it passes distally (Figs. 6.9 and 6.28). The posterior femoral cutaneous branch also exits below the piriformis but is less easily identified on MR images. The superior gluteal nerve lies between the gluteus minimus and medius above the level of the piriformis (Fig. 6.28). In the posterior thigh, the sciatic nerve lies posterior to the adductor magnus and deep to the biceps femoris (Fig. 6.8). The nerve branches innervate the hamstring group. Just above the knee, the sciatic nerve generally divides into common peroneal and tibial branches (Fig. 6.23).³⁴^,³⁸

The anterior musculature of the pelvis and thighs are innervated by the obturator and femoral nerve (Table 6.2). Both are derived from the L2 through L4 segments. The femoral nerve lies just lateral to the artery and vein in the femoral triangle (Fig. 6.21). In the upper portion of the femoral triangle it divides into muscular and cutaneous branches.³⁸ The largest branches may be identified on MR images. These branches (saphenous nerve and nerve to the vastus medialis) lie anterior to the adductor group and deep to the sartorius (Fig. 6.9).³⁴^,³⁷^,³⁸ The obturator nerve enters the thigh through the obturator canal, deep to the pectineus (Fig. 6.21). Its anterior branch lies between the adductor longus and brevis and the posterior branch posterior to the adductor brevis.³⁴^,³⁸

Figure 6.28 Neurovascular anatomy of the gluteal region.

PITFALLS

Certain errors in interpreting MR images can be reduced by reviewing radiographic findings. Pitfalls in MR imaging of the pelvis, hips, and thighs do not differ significantly from other areas in regard to hardware and software artifacts (Figs. 6.29,6.30,6.31). Metal artifact from orthopedic implants is common in the pelvis and hips due to the increasing number of patients with arthroplasty or fracture fixation devices. (Fig. 6.31) The role of MR imaging in this patient group is discussed later in this chapter. However, artifact from orthopedic implants can be reduced by orienting implants parallel to the magnetic field, modifying pulse sequences to reduce frequency shift and increasing the bandwidth along with
other parameters. There are new work-in-progress processing techniques that have dramatically improved the utility of MR imaging in the presence of orthopedic implants (Fig. 6.32). Titanium implants produce less artifacts than cobaltchromium alloys due to reduced ferromagnetic content.⁵²^,⁵³ This section focuses on bone and soft tissue variants, which require further discussion.⁵⁴^,⁵⁵

Figure 6.29 Coronal T2-weighted, fat-suppressed MR image of the pelvis demonstrating motion artifact (upper arrows) and asymmetric fat suppression (large white arrow).

Figure 6.30 Respiratory motion artifact in the phase encoding direction. Axial T1-weighted images of the upper pelvis (A) and more inferiorly at the level of the ischial tuberosities (B). There is considerable artifact in A (arrows) due to respiratory and bowel motion.

Soft tissue variants are important when evaluating patients with suspected soft tissue masses (Fig. 6.33) or sciatic nerve pathology.¹⁰^,³⁸^,⁵⁵^,⁵⁶^,⁵⁷ Knowledge of the normal variants in the sciatic-piriformis muscle anatomy is important in patients with suspected piriformis syndrome. As noted above, the sciatic nerve typically passes deep to the piriformis muscle, but it may pass through the muscle or separate the tibial and peroneal tracts.³⁴^,³⁸^,⁵¹ The psoas minor is absent in up to 51% of patients. Other muscle anomalies in the pelvis and thighs typically result in fusion of muscles (i.e., quadratus femoris and adductor magnus) or separate muscle bellies. The smaller muscles such as the gemelli and quadratus femoris may be absent, resulting in muscle mass asymmetry.³⁴^,³⁸ In young, active patients, one can identify changes in muscle signal intensity related to exercise. These
changes vary with the extent, eccentric nature, and time since the activity occurred.⁵⁸ Typically, the entire muscle is involved and changes usually are not similar to neoplasms or other soft tissue pathology.

Figure 6.31 A: Anteroposterior view of the pelvis in a patient with a bipolar implant on the right and three cannulated screws on the left. Coronal (B) T1-weighted image shows artifact bilaterally, greater on the right. Axial T1-weighted image of the left hip (C) shows minimal artifact around the hip pins. The marrow is clearly demonstrated.

Figure 6.32 Work in progress sequence demonstrating minimal artifact about the total hip implants. Coronal T1-weighted (A) and turbo STIR with echo train length of 17 (B) images demonstrating osteolysis about the femoral and acetabular components (arrows).

Figure 6.33 Hypertrophy of the tensor fascia lata. Axial (A) and coronal (B) T1-weighted images demonstrate marked enlargement of the muscle that was felt to be a soft tissue mass clinically.

Inflamed bursae (Figs. 6.18 and 6.20) should not be confused with neoplasms. Most of these bursae were described in the anatomy section of this chapter. The obturator externus bursa is seen less frequently than iliopsoas or trochanteric bursae.⁴⁷^,⁵⁹ This bursa communicates with the posterior inferior hip joint and, when enlarged, displaces the obturator externus inferiorly (Fig. 6.34).⁴⁹ Infrequently, bursae may develop between the piriformis and the femur, between the gluteal muscles, and over the ischial tuberosities. When present, these bursae appear as well-marginated, high-intensity lesions on T2-weighted sequences and are low intensity on T1-weighted sequences (Fig. 6.35). Size of the bursae may vary significantly.⁴⁷^,⁵⁷^,⁵⁹

Variations in marrow patterns in the pelvis, hips, and femurs and focal areas of abnormality in the femoral head can be very confusing.²^,¹⁰^,⁶⁰^,⁶¹ A more thorough discussion of marrow imaging is presented in Chapter 14. However, certain common problems and variants deserve mention here.

The transition from hemopoietic to fatty marrow occurs normally with aging. This may be partially related to diminished medullary blood flow.⁶¹^,⁶²^,⁶³ In younger patients, the capital epiphysis and greater trochanter are typically composed of fatty marrow, resulting in high signal intensity on T1-weighted sequences (Fig. 6.36). Also, partial volume effects can cause confusion on axial images (Fig. 6.36) in the physeal region. Changes should not be confused with pathology and can usually be clarified on coronal or sagittal images. With age, the hemopoietic marrow in the intertrochanteric region becomes replaced with fatty marrow so that after age 50 years most of the marrow in this region is fatty (Fig. 6.37).⁶² The compressive and tensile trabeculae in the femoral head and neck are seen as linear areas of low signal intensity on both T1-and T2-weighted sequences (Fig. 6.38).²^,⁶¹ Lack of familiarity with the changes normally seen in the marrow of the acetabulum
and femur can lead to a false-positive diagnosis such as osteonecrosis or metastasis (Fig. 6.39).⁶⁴

Figure 6.34 Sagittal T2-weighted fat-suppressed fast spin-echo images (A, B) in a patient with osteonecrosis demonstrate a joint effusion with an obturator externus bursa. A: Posterior communication with the joint (open arrows). B: The bursa extending medially. (From Robinson P, White LM, Agur A, et al. Obturator externus bursa: Anatomic origin and MR imaging features of pathologic involvement. Radiology 2003;228:230-234.)

Figure 6.35 Enlarged iliopsoas bursa (arrowhead) mistaken for a mass. CT (A) and T2-weighted (B) MR images demonstrate a characteristic enlarged bursa (arrowhead). The bursa was injected with contrast medial (C) and aspirated for diagnosis and treatment. Aspiration is usually not successful for long-term treatment.

Figure 6.36 Axial T1-weighted images (A, B) through the normal physeal region of the femoral head. These bizarre signal intensity changes should not be confused with pathology. Coronal t1-weighted image (C) shows the irregular course of the growth plate. Lines demonstrate the axial image planes.

Figure 6.37 Adult hips. T1-weighted image of the pelvis and hips demonstrating predominantly fatty marrow in the femoral heads and necks. The physeal scars (arrows) should not be confused with osteonecrosis.

Figure 6.38 Coronal T1-weighted image of the hip demonstrating linear low signal intensity in the femoral neck (arrow) due to the normal trabecular pattern. There is also a physeal scar (arrowhead).

Well-defined areas of abnormal signal intensity may be noted in the femoral neck. These herniation pits are due to mechanical effects of the anterior capsule. The result is a focal cortical defect that allows soft tissue erosion into the femoral neck. These defects are noted in about 5% of the adult population.⁵⁴^,⁶⁵^,⁶⁶ Radiographically, these defects are nearly always in the anterior outer quadrant of the femoral neck (Fig. 6.40). They are generally 1 cm in diameter with sclerotic, well-defined margins. Multilobulated herniation pits have also been described.⁵⁴^,⁶⁰^,⁶¹^,⁶⁵ Herniation pits may enlarge over time but maintain their characteristic appearance on imaging studies (Fig. 6.41). In this setting, fractures may occur.⁶⁶ The appearance is also typical on MR images. The location and typical low intensity on T1-weighted images and high intensity with a low intensity margin on T2-weighted images should not be confused with osteonecrosis, metastasis, or interosseous ganglia.⁶⁰ Comparison with radiographs will confirm the diagnosis when MR features seem atypical.

A simple but common mistake is the failure to compare other studies, especially routine radiographs, with MR images.¹⁰^,⁶⁷^,⁶⁸ This is especially important in the pelvis and hips, where marrow patterns can be confusing (Fig. 6.42).¹⁰ Also, areas of heterotopic ossification and periarticular calcification, common about the hips, may be confused with other pathology if radiographs or computed tomography (CT) are not available for review (Fig. 6.43).⁶⁹

Figure 6.39 Coronal T1-weighted image of the left hip and femur shows a target-like area in the marrow (arrow) of the femur in the subtrochanteric regions. This is a normal area of marrow transition. Note also the trabecular pattern in the femoral neck (arrowhead).

APPLICATIONS

Most patients referred for musculoskeletal MR examinations of the pelvis, hips, and thighs present with pain, a history of trauma, or suspected soft tissue or skeletal neoplasms.⁷⁰^,⁷¹^,⁷² There are numerous causes of hip pain.⁷² Applications for MR imaging of the pelvis and hips have evolved in adults and children. The following sections will review applications for MR imaging of the pelvis and hips. Pediatric disorders are discussed separately in the last section of this chapter.

Figure 6.40 Coronal T1- (A) and axial T2-weighted (B) images of the pelvis demonstrating a typical herniation pit (arrow).

Marrow Edema

Osteonecrosis of the femoral head can be diagnosed early with MR imaging, but is comparable to radiographs in the later stages. However, confusion can occur in the early phases prior to development of the typical geographic abnormality, double-line sign, and other features commonly noted on MR images.⁷³^,⁷⁴^,⁷⁵^,⁷⁶^,⁷⁷^,⁷⁸ Diffuse marrow edema syndrome is typically the term used for transient clinical conditions often of unknown etiology.⁷⁸ Bone marrow edema syndrome most commonly occurs in the hip, knee, and ankles. The condition most commonly presents in middle-aged males.⁷⁸ The transient nature of the process requires that transient osteoporosis of the hip and regional migratory osteoporosis must be excluded.⁷⁸

Figure 6.41 Large herniation pit. Coronal T2- (A) and axial T-weighted (B) images demonstrating a large herniation pit (arrow) on the left.

Figure 6.42 Benign bone island. Coronal T1- (A) and axial T2-weighted (B) images demonstrate an area of low signal intensity in the right femoral neck (arrow). Anteroposterior (C) radiograph demonstrates faint chondrocalcinosis, degenerative arthritis, and a sclerotic bone island (arrow), which created the signal abnormality on the MR images.

Figure 6.43 Coronal T1-weighted image (A) demonstrates a low signal intensity area in the perilabral region. Anteroposterior radiograph (B) shows obvious dense calcification (arrow) due to hydroxyapatite deposition disease.

Diffuse marrow edema may also be the initial feature in evolving osteonecrosis. Patients present with MR features demonstrating low signal intensity changes in the femoral head and/or neck on T1-weighted images, and high signal intensity on T2-weighted images.⁷³^,⁷⁴^,⁷⁵^,⁷⁶^,⁷⁷^,⁷⁸ Iida et al.⁷⁴ reported that high-risk patients with bone marrow edema (steroids, renal transplants, etc.) progressed to advanced osteonecrosis in 85% of cases. However, marrow edema can be identified with numerous conditions including transient bone marrow edema, migratory osteoporosis, transient osteoporosis, infection, trauma, neoplasms, and altered weight bearing.⁷³^,⁷⁶^,⁷⁹^,⁸⁰^,⁸¹^,⁸² The MR features of transient bone marrow edema, transient osteoporosis, and early osteonecrosis, though controversial, have been clarified in recent years.⁷⁶^,⁸³^,⁸⁴^,⁸⁵ Though not always clear-cut, the data from radiographs, radionuclide scans, MR images, and clinical data, specifically risk factors, are useful in differentiating these conditions and selecting conservative treatment or surgical intervention (i.e., core decompression).⁷³^,⁸⁶^,⁸⁷^,⁸⁸^,⁸⁹^,⁹⁰

Osteonecrosis

Osteonecrosis is a general term applied to conditions resulting in cellular necrosis of bone and marrow elements.⁸⁵^,⁹¹^,⁹² The term osteonecrosis is most commonly applied when the epiphysis or subchondral bone is involved. Osteonecrosis of the metaphyseal or diaphyseal bone is commonly referred to as a bone infarct.⁹³

Bone necrosis occurs when flow is disrupted by thrombosis, external compression, vessel wall disease, or traumatic disruption of vessels. Jiang and Shih⁹⁴ evaluated physeal scars and their association with osteonecrosis of the femoral head (Figs. 6.37A and 6.38). Many physeal scars (seen as a linear low signal intensity structure) are incomplete. Complete physeal scars that extend from cortex to cortex were associated with osteonecrosis in 32 of 72 (44%) femoral heads.⁹⁴ The numerous causes of osteonecrosis are summarized in Table 6.3. Though osteonecrosis can occur in any area of the skeleton, it is a frequent and significant problem in the femoral head. Therefore, the major discussion of pathophysiology and MR features are discussed here.

Susceptibility to osteonecrosis in the hip is due in part to the vascular anatomy. The foveal artery supplies only the area of the femoral head immediately adjacent to the fovea (Fig. 6.44). Distally, the medial and lateral femoral circumflex arteries supply the remainder of the femoral head and neck (Fig. 6.44). Their location makes them particularly susceptible to damage with femoral neck fractures and dislocations.³⁵^,⁹¹^,⁹²^,⁹⁵^,⁹⁶^,⁹⁷^,⁹⁸

Nontraumatic cell death may occur from direct effects (chemotherapy, radiation therapy, or thermal injury) or intraosseous extravascular changes such as edema or marrow infiltration diseases such as Gaucher disease.⁹⁰^,⁹¹^,⁹⁹

Table 6.3 Etiology of Osteonecrosis

Trauma

Corticosteroids

Sickle cell disease

Alcoholism

Gaucher disease

Nitrogen narcosis

Radiation

Collagen disease

Pancreatitis

Idiopathic

From references 91, 92 and 95,96,97.

Fat embolism and/or disorders in fat metabolism are frequently implicated in nontraumatic osteonecrosis. Jones⁹¹ described three potential mechanisms and four phases that evolve due to lipid disorders. Fat embolism may develop due to fatty liver, destabilization and coalescence of plasma lipoproteins, or disruption of fatty marrow or fatty tissues in nonosseous regions. Fat embolism (Stage 0) leads to interosseous vascular occlusion (phase I) that increases lipase, which in turn increases free fatty acids and prostaglandins (phase II). These changes can lead to focal intravascular coagulation, platelet aggregation, and thrombosis, which result in osteonecrosis.⁹¹

Early diagnosis and selection of the most appropriate therapy has been particularly challenging for patients with osteonecrosis of the femoral head.²⁸^,⁹⁰^,¹⁰⁰^,¹⁰¹^,¹⁰²^,¹⁰³^,¹⁰⁴^,¹⁰⁵ Early detection permits more conservative therapy such as non-weight bearing with crutches, core decompression, and vascularized grafts.⁵^,¹⁰³^,¹⁰⁶^,¹⁰⁷^,¹⁰⁸

Early detection of osteonecrosis with imaging techniques has been challenging.²^,⁸³^,¹⁰⁹^,¹¹⁰ Radiographs are often normal in early stages of osteonecrosis. Changes may be subtle even during stage II disease. Detection at this time (Table 6.4) may require more sophisticated studies than anteroposterior and oblique radiographs. Isotope studies have been very useful at this stage.²^,¹⁰⁴ However, early changes can be difficult to evaluate with radionuclide scans, as well. Comparison with the opposite hip may not be useful since the disease is bilateral in up to 81% of patients.²^,⁸¹^,⁸³^,¹⁰⁹^,¹¹⁰^,¹¹¹^,¹¹²^,¹¹³

Figure 6.44 Illustration of the vascular anatomy of the hip.

Table 6.4 Staging of Osteonecrosis of the Femoral Head

Stage	Clinical	Radiograph	MRI	Pathology
0	No symptoms	Normal	Normal to uniform edema (↓ signal T1WI, ↑ signal T2WI) (Fig. 6.45) Subchondral zone of nonenhancement or ↑ enhancement due to edema with Gd. (Fig. 6.48)	Hematopoietic cell necrosis followed by fat cell necrosis and osteocytes
I	May have symptoms	Normal or may have patchy osteoporosis (Fig. 6.45A)	Normal to uniform edema (↓ signal T1WI, ↑ signal T2WI) or low intensity zone on T1WI (Fig. 6.42) Subchondral zone of nonenhancement or ↑ enhancement due to edema with Gd. (Fig. 6.45)	Sinus congestion, fibroblastic, hypoplastic marrow, empty lacunae
II	Pain, stiffness	Osteopenia mixed, osteopenia and sclerosis, cystic changes (Figs. 6.45C and 6.52)	Wedge-shaped crescent sign (X-ray stage III)	Necrotic central tissue, margin fibrous with revascularization and new bone on dead trabeculae
III	Stiffness, groin and knee pain	Crescent sign, sequestra, cortical collapse, joint preserved (Fig. 6.45D and E)	Crescent sign sequestra cortical collapse joint preserved (Fig. 6.53)	Necrosis surrounded by granulation tissue
IV	Pain and limp, may be severe	III plus degenerative changes with narrowed joint space (Fig. 6.45F)	III plus degenerative changes with narrowed joint space (Fig. 6.54)	Changes of stage III exaggerated
T1WI, T1-weighted image; T2WI, T2-weighted image; Gd, Gadolinium. From references 10, 96 and 111,112,113.

Ficat¹¹⁴ and Ficat and Arlet¹¹⁵ described the stages of osteonecrosis of the femoral head based on radiographic and clinical features (Fig. 6.45). This staging is also useful in evaluating MR images (Table 6.4). Stages 0 and I have no radiographic findings and clinical symptoms are subtle. Stage II changes are often overlooked on routine radiographs but consist of focal areas of subchondral lucency or sclerosis (Fig. 6.45C). Stages III (Fig. 6.45D and E) and IV (Fig. 6.45F) are usually easily detected on radiographs due to the crescent sign (Stage III) and progressive subchondral collapse and degenerative joint disease (Stages IV-V) (Table 6.4).¹¹⁴^,¹¹⁵

Due to the increased use of core decompression, vascularized grafts and more conservative treatment approaches for early osteonecrosis, the Ficat and Arlet classification has been modified by multiple centers.¹¹⁶^,¹¹⁷^,¹¹⁸^,¹¹⁹^,¹²⁰^,¹²¹ Steinberg et al.¹¹⁶ (Table 6.5) reported the University of Pennsylvania classification and staging system, which incorporates imaging features that will be of significance when we discuss management approaches for femoral head osteonecrosis.⁹⁶^,¹²⁰^,¹²¹ Stage 0 is normal imaging studies, including MR imaging. Stage I disease has normal radiographs with abnormal radionuclide scans and MR features. Stage II osteonecrosis demonstrates lytic and/or sclerotic changes in the femoral head and stage III subchondral collapse (crescent sign). Stage III disease progresses to flattening of the femoral head, stage V joint space narrowing and acetabular changes, and stage VI advanced degenerative joint disease. There are subcategories based on image features for stages I-V (Table 6.5).

MR imaging approaches should be selected to fit the classification system and provide the surgeon with the extent of femoral head involvement, acetabular involvement, and the accurate stage of osteonecrosis.¹⁰³^,¹¹⁶^,¹¹⁷^,¹¹⁸^,¹¹⁹^,¹²⁰^,¹²¹^,¹²²

Before discussing the corresponding MR features, we will review specific aspects of the technique that were not fully discussed in the introductory technique section of this chapter. When positive, coronal T1-weighted images are adequate for diagnosis of femoral head osteonecrosis (Fig. 6.46A).¹⁰^,²¹^,⁶²^,¹²³^,¹²⁴^,¹²⁵ This sequence can be performed
quickly (2 to 4 minutes) using a 30- to 42-cm FOV, 4-mm-thick sections, 1 NEX, and a 256 × 256 or 192 × 256 matrix. This approach may provide a simple screening technique for high-risk patients.¹⁰ Tervonen et al.¹²⁶ detected occult osteonecrosis in 6% of asymptomatic high-risk patients. More recently, Iida et al.⁷⁵ demonstrated that 85% of high-risk patients (steroid, transplant, etc.) progressed from marrow edema to advanced osteonecrosis.

Figure 6.45 Radiographic features of osteonecrosis of the femoral head. A: Normal anteroposterior view of the hip. B: Specimen demonstrating normal femoral head contour and trabecular pattern. C: Stage II osteonecrosis: AP radiograph demonstrating mixed lucent and sclerotic areas (arrows) in the femoral head without articular collapse. Stage III osteonecrosis: Anteroposterior (D) and frogleg oblique (E) radiographs demonstrating lucent area (arrowheads) with articular collapse (arrows). F: Ficat stage IV or modified stage V osteonecrosis: Anteroposterior radiograph of the right hip demonstrating advanced osteonecrosis (arrows) with articular collapse and complete loss of joint space.

Figure 6.45 (Continued)

Table 6.5 Classification of Femoral Head Osteonecrosis

Stage	Criteria
Stage 0	Normal or nondiagnostic radiograph, bone scan, and MRI
Stage 1	Normal radiograph; abnormal bone scan, and/or MRI
A	Mild (<15% of head affected)
B	Moderate (15%-30% of head affected)
C	Severe (>30% of head affected)
Stage II	Lucent and sclerotic changes in femoral head
A	Mild (<15% of head affected)
B	Moderate (15%-30% of head affected)
C	Severe (>15% of head affected)
Stage III	Subchondral collapse (crescent sign) without flattening
A	Mild (<15% of articular surface)
B	Moderate (15%-30% of articular surface)
C	Severe (>30% of articular surface)
Stage IV	Flattening of femoral head
A	Mild (<15% of surface and <2-mm depression)
B	Moderate (15%-30% of surface or 2-to 4-mm depression)
C	Severe (>30% of surface or >4-mm depression)
Stage V	Joint narrowing and/or acetabular changes
A	Mild
B	Moderate
C	Severe
Stage VI	Advanced degenerative changes
From Lieberman JR, Berry DB, Mont MA, et al. Osteonecrosis of the hip: Management in the twenty-first century. J Bone Joint Surg Am 2002;84A:834-853; Cherian SF, Laorr A, Saleh KJ, et al. Quantifying the extent of femoral head involvement in osteonecrosis. J Bone Joint Surg Am 2003;85A:309-314; and Steinberg ME, Hayken GD, Steinberg DR. A quantitative system for staging avascular necrosis. J Bone Joint Surg Am 1995;77B:34-41.

Figure 6.46 T1-weighted images of the hips in a patient with early osteonecrosis of the right femoral head. Radiographs were normal. A: Coronal image demonstrating a small linear subchondral defect (arrow). B: Sagittal image of the right hip more clearly defines the extent of involvement (arrow).

We also perform small FOV surface coil sagittal images of both hips in abnormal cases to define the extent of articular involvement and improve mapping of the femoral head for treatment planning (Fig. 6.46B).¹⁰^,¹²⁰^,¹²⁷ T2-weighted or fat-suppressed, fast spin-echo T2-weighted sequences in the sagittal and/or coronal planes are important to define joint anatomy and articular cartilage changes. It is also important to assess the acetabular side of the joint for cartilage loss, geode formation, and ischemic changes (Fig. 6.47).⁹⁶^,¹²⁰ These findings may lead to a different surgical approach. In most cases resurfacing or bipolar implants are used for osteonecrosis of the femoral head. However, when there are acetabular abnormalities (Fig. 6.47), a total hip arthroplasty may provide more optimal results.⁹⁶^,¹¹⁴ Fink et al.¹²⁸ detected osteonecrosis of the acetabulum in 9.5% of patients with femoral head necrosis.

Figure 6.47 A: Sagittal gradient echo T2*-weighted image demonstrating an acetabular geode (arrow). B: Coronal fat suppressed fast spin-echo T2-weighted image demonstrates marrow edema, osteonecrosis (arrowheads) and an acetabular insufficiency fracture (arrow).

Figure 6.48 Patient with systemic disease on steroids. A: Coronal T1-weighted image is normal. B: Post-gadolinium fat-suppressed T1-weighted image shows no increased signal in the femoral heads bilaterally, suggesting decreased flow. Compare to the signal intensity of the intertrochanteric and acetabular regions.

In high-risk patients (systemic disease, steroids, renal transplant, etc.), more aggressive studies using fat suppressed T1-weighted images after gadolinium injection may detect ischemic changes earlier (Fig. 6.48).¹⁰^,⁸⁹

During the earliest phases of osteonecrosis, conventional pulse sequences can appear normal.¹²⁹^,¹³⁰^,¹³¹ Early findings are more easily appreciated using gadolinium (Fig. 6.48) and fat-suppressed T1-weighted sequences.¹²⁹ Red marrow enhances more than yellow marrow. Gadolinium is freely distributed into the extracellular space, so enhancement may be related to flow, increased capillary permeability, or both.¹³²^,¹³³ Normal marrow enhances rapidly, with increase in signal intensity of more than 80% typically within 36 seconds of injection. There is no enhancement in regions of ischemia (Fig. 6.48). In later phases (7 days), an enhancing zone is usually identified around the ischemic zone (Fig. 6.49). Li et al.¹³³ described three patterns using gadolinium-enhanced fat-suppression MR techniques. Stage I changes demonstrate focal low intensity in the femoral head surrounded with a high-intensity margin or hyperemic zone. Stage II changes are typical of bone marrow edema with diffuse enhancement in the femoral head and neck (Fig. 6.47B). Combined features of stages I and II were considered stage III. Though marrow edema may not progress to osteonecrosis, many believe that this is the earliest phase.⁷⁶^,⁸³^,⁸⁵^,¹³²^,¹³³^,¹³⁴ Differentiation of marrow edema from osteonecrosis is discussed later; however, follow-up imaging is the only way to be sure the transient bone marrow edema has resolved.⁷²^,⁷⁴^,¹³⁵

Figure 6.49 Coronal post-contrast fat-suppressed T1-weighted image demonstrates a focal area of necroses in the femoral head with surrounding enhancement or hyperemia (arrows).

Regardless of the technique used, the MR features can be correlated to some degree with histologic changes. Though not required for diagnosis in later stages, gadolinium clearly helps understand the ischemic and reactive changes that occur around the necrotic bone (Figs. 6.48 and 6.49).⁷⁶^,⁸²^,¹³¹

The MR signal intensity in the femoral head and neck depends upon the presence of fat cells, hemopoietic cells, and trabecular bone (Fig. 6.50).²^,³^,¹³¹^,¹³⁶ Marrow patterns are age-related. These variations must be considered when interpreting MR images. During fetal development, bone marrow in the hip is entirely hemopoietic in nature.⁶² After birth and during early childhood, the marrow in the epiphysis and trochanters is fatty (Fig. 6.51A and B).¹²⁵^,¹³⁷ Fatty marrow in the epiphysis is normally evident by age 2 years.⁶²^,¹³⁸ In children, red marrow predominates in the metaphysis, intertrochanteric regions, and pelvis (Fig. 6.51C). In young adults (aged 20 to 40 years), hemopoietic or red marrow is present in the femoral neck and intertrochanteric region, and proximal shaft is evident in 94% of patients. Fatty marrow is present in the femoral head and trochanters.⁶² These regions are composed of fatty marrow in 88% of adults over 50 years of age. (Fig. 6.51D).⁶²

Osteonecrosis of the femoral head can result from numerous conditions (Table 6.3).⁶²^,⁷⁵ All osseous elements become necrotic at different times after an ischemic event. Hemopoietic cells are most sensitive and become necrotic
prior to fat cell necrosis.⁹¹^,⁹² Osteocytes show necrotic changes shortly after hemopoietic cells.⁸⁵ It is not unusual to demonstrate mixed necrosis and survival if changes are early. Early animal studies show MR images remain normal until about the seventh day. Beginning on the seventh day, an inhomogeneous loss of signal intensity can be demonstrated onT1-weightedimages. These changes correspondhistologically to lymphocytic infiltration.⁹⁶^,¹¹¹ This inhomogeneity progresses over the first 16 days until day 20, when a more homogeneous loss of signal intensity in the femoral head becomes evident (Fig. 6.52). This correlates with increased lymphocyte infiltration and early fibrosis. As expected, radiographs remain normal during this time period (Table 6.4).¹⁰^,¹¹⁴^,¹¹⁵ Early uniform loss of signal intensity in the femoral head and neck on T1-weighted images that is similar to transient bone marrow edema has also been reported (Fig. 6.52).¹⁰^,⁷⁴^,¹²²^,¹²⁴^,¹³⁴^,¹³⁸^,¹³⁹ Conservative management is employed with either condition. However, follow-up studies are important in clarifying which disorder is present and to exclude other inflammatory diseases, specifically infection.⁷⁴^,¹³¹^,¹³⁴

Figure 6.50 Gross (A) and coronal cut section (B) in a patient with osteonecrosis and articular collapse (white arrows). Axial section (C) shows the articular defect (white arrows) with reactive changes (arrowheads) at the necrotic interface and in the adjacent subchondral bone.

Figure 6.51 Marrow patterns in the hip at different ages: T1- (A) and T2- (B) weighted coronal images of the hip on a young child showing fat signal in the developing epiphysis during ossification (arrow). Coronal T1-weighted image (C) in an adolescent with fatty marrow in the femoral head and greater trochanter. Coronal T1-weighted image (D) of the hips in a 60-year-old with fatty marrow in both upper femurs.

Figure 6.52 Coronal T1- (A) and T2- (B) weighted images demonstrating abnormal signal intensity in the femoral head and upper neck due to marrow edema.

Also during the early phase, a low intensity line of demarcation may be evident at the margin of the necrotic zone. This is easily appreciated on T1-weighted coronal images (Fig. 6.53). The signal intensity of the necrotic zone may be indistinguishable from normal fatty marrow.¹⁰^,¹²⁵^,¹²⁷ This low intensity zone surrounding the necrotic bone is most likely due to hyperemia. During this phase of osteonecrosis the radiographs are typically normal, though slight sclerosis or lucency may be evident with early stage II osteonecrosis (Tables 6.4 and 6.5).¹^,²^,¹²³ Photopenia (cold spots) may be evident on radionuclide scans at this point.¹²⁴

Gradually (>2 weeks), the cells around the necrotic zone modulate into fibroblasts that have low signal intensity on T1-weighted sequences. Hyperemia causes mixed low and high signal intensity margins on T2-weighted images.¹⁰^,⁶²^,¹²⁴^,¹²⁵ Hyperemic changes may be more clearly defined on contrast-enhanced images (Fig. 6.54). Little progression is evident on radiographs or isotope scans at this point.⁷⁴

Reinforcement of the trabeculae and persistent hyperemia lead to widening of the low-intensity margins on T1-weighted, T2-weighted, and postcontrast sequences (Figs. 6.54 and 6.55). The hyperemia remains high intensity on T2-weighted sequences. At this stage, a clear stage II radiographic picture (Fig. 6.56) is usually evident, namely, a lucent or sclerotic area surrounded by a geographic pattern.¹⁰^,⁶²^,¹²⁵

With progression, the subchondral bone collapses resulting in stage III osteonecrosis (Tables 6.4 and 6.5) or the crescent sign on radiographs (Fig. 6.45D). This can be seen as a subchondral low intensity line on T1-weighted sequences or a high intensity area on T2-weighted sequences with deformity of the femoral contour (Fig. 6.57). The latter may be more easily appreciated due to the high contrast between the dark cortical bone and high signal seen with T2-weighted sequences.⁹¹^,⁹²^,⁹⁵^,⁹⁶ Early subchondral collapse may only be seen with coronal and sagittal surface coil images. In fact, subchondral fracture may be most easily appreciated on coronal reformatted CT images.¹⁴⁰

Once stage IV osteonecrosis (Tables 6.4 and 6.6) has been reached, the appearance of the MR images and radiographs are similar (Fig. 6.58). However, effusions are more obvious on MR images at this stage.¹⁴⁶^,¹⁴⁷ Subtle joint
space changes are definitely more easily assessed with radiographs (Fig. 6.45D-F

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