Abstract
Diagnostic imaging plays a central role in the evaluation of most types of orthopaedic conditions. Radiography has been a mainstay in orthopaedics since its inception, and advanced imaging modalities such as computed tomography, ultrasound, radionuclide scanning, and magnetic resonance imaging have become valuable adjuncts in the work-up of orthopaedic patients. This chapter will provide a brief overview of each of these modalities, including their strengths and weaknesses, and conclude with imaging algorithms for some common orthopaedic conditions.
Keywords
Musculoskeletal imaging, radiography, computed tomography, radionuclide scanning, ultrasound, magnetic resonance imaging
Key Concepts
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Imaging studies should be used as an adjunct to the history and physical examination.
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Obtain the least number of imaging studies needed to arrive at a diagnosis (or reasonable differential diagnosis).
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Each imaging modality has specific strengths and weaknesses that must be taken into account when considering which test to perform.
Imaging
Radiography
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Technique: A beam of x-rays is projected through the body to a detector that constructs a two-dimensional image based on the differential attenuation of the beam by various tissues.
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The primary modality for investigating the musculoskeletal system; it should be the first imaging study ordered for most indications.
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Four basic tissues are recognizable on a radiograph: metals, which are the densest structures on a film (this category includes bone because of its calcium content); air, which is the most lucent (black) ; fat, which is dark gray ; and soft tissue, which appears as intermediate gray (this category includes fluid that cannot be differentiated from muscle, etc.) ( Fig. 3.1 ).
Fig 3.1
Radiography: Soft-tissue contrast.
Lateral radiograph of the knee demonstrates dark, lucent air (A) ; dark gray fat in Hoffa fat pad (arrow) ; intermediate gray fluid in the suprapatellar bursa (F) related to a large joint effusion (note the similarity in density between the fluid and the hamstring muscles [M] posteriorly); and the relatively dense bones (related to their calcium content).
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At least two views are usually obtained, most often in the frontal and lateral projections ( Fig. 3.2 ).
Fig 3.2
Radiography: Importance of obtaining more than one view.
(A) Posteroanterior radiograph of the finger demonstrates a transverse fracture of the distal phalanx that does not appear to involve its articular surface (arrow) . (B) Corresponding lateral view reveals intra-articular extension and mild distraction along the fracture line.
Strengths
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Relatively inexpensive
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Widely available
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Evaluation of bone pathology (fracture, tumor, arthritis, osteomyelitis, metabolic bone disease) ( Fig. 3.3 )
Fig 3.3
Radiography: Tumor and arthritis.
(A) Frontal view of the shoulder reveals a coarse, sclerotic intramedullary lesion within the proximal humerus, compatible with a chondroid neoplasm, most likely an enchondroma. (B) Posteroanterior radiograph of the foot demonstrates classic findings of gout involving the first metatarsophalangeal joint including large marginal and para-articular erosions, calcific densities in the adjacent soft-tissue tophus, and relative sparing of the joint space.
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Assessment of orthopaedic hardware and fracture healing ( Fig. 3.4 )
Fig 3.4
Radiography: Joint prosthesis.
Frontal radiograph of the left hip shows prosthetic discontinuity of the femoral component at the junction of its head and neck with resulting superolateral migration of the proximal femur.
Weaknesses
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Pathology of the medullary cavity (bone contusion, occult fracture, medullary tumor) ( Fig. 3.5 )
Fig 3.5
Radiography: occult fracture.
(A) No discrete fracture is evident on this posteroanterior view of the wrist obtained after injury. (B) Coronal T1-weighted magnetic resonance image reveals numerous nondisplaced, low-signal-intensity fracture lines within the distal radius.
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Soft-tissue pathology
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Uses ionizing radiation
Computed Tomography
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Technique: An x-ray source is rotated around the patient, who is lying on a moving gantry, resulting in image “slices” in the transaxial plane.
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The data from these slices can then be viewed as axial images or used to create reformatted images in any plane (typically sagittal and coronal planes).
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Can be combined with intravenous (IV) contrast, which results in increased density (enhancement) in vessels and hypervascular tissues owing to its iodine content
Strengths
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Tomographic depiction of anatomy allowing for two- and three-dimensional reformatted images ( Fig. 3.6 )
Fig 3.6
Computed tomography: Reformatted images.
(A) Thin-slice computed tomography images obtained in the axial plane were combined to create this two-dimensional sagittal reconstructed image of the cervical spine. (B) A three-dimensional reformatted image of the pelvis depicts prominent diastasis of the symphysis pubis and less prominent widening of the right sacroiliac joint.
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Depiction of complex fractures, especially those involving the spine and flat bones (pelvis and scapula) ( Fig. 3.7 )
Fig 3.7
Computed tomography: Complex fractures.
Coronal, two-dimensional reformatted image from a computed tomography scan of the pelvis demonstrates an essentially nondisplaced, comminuted right acetabular fracture (arrows) .
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Evaluation of fracture healing
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Postoperative evaluation of the degree of fusion or hardware complications ( Fig. 3.8 )
Fig 3.8
Computed tomography: Postoperative assessment.
(A) and (B) Adjacent coronal reformatted images of the wrist reveal a nondisplaced scaphoid fracture transfixed with a surgical screw. Note the lack of metal-related artifact.
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Can be combined with intrathecal or intra-articular contrast (computed tomography [CT] myelography and CT arthrography, respectively) ( Fig. 3.9 )
Fig 3.9
Computed tomography arthrogram.
Coronal reformatted image from a computed tomography arthrogram of the left hip reveals a small cartilage flap along the medial femoral head (arrow) .
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Accurate demonstration of urate acid crystals using dual-energy CT allowing for a specific diagnosis of gout ( Fig. 3.10 )
Fig 3.10
Dual energy computed tomography (CT): Gout.
Color-coded coronal reformatted image from a dual energy CT examination demonstrates extensive monosodium urate deposition (green foci) throughout the wrist.
Weaknesses
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Fracture detection in the setting of significant osteopenia ( Fig. 3.11 )
Fig 3.11
Computed tomography versus magnetic resonance imaging for a tibial plateau fracture.
(A) Coronal reformatted computed tomography image of the knee reveals a very small cortical lucency (arrowhead) in the tibial plateau at the site of a nondisplaced fracture that is much better demonstrated using MRI as indicated by the arrow in (B) , a coronal T1-weighted image.
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Although CT produces much better soft-tissue contrast than radiographs, it is not as good as that obtained with magnetic resonance imaging (MRI).
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Uses ionizing radiation (unlike ultrasonography and MRI)
Radionuclide Scanning
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Technique: A bone-seeking radioactive material is injected intravenously (typically technetium-99m diphosphonate, a phosphorous analog that is taken up in areas of increased bone turnover such as tumor, infection, and fracture), and the patient is scanned 4 to 6 hours later, at which time whole-body images may be obtained.
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More localized, “spot” images may also be acquired in areas of specific clinical concern, and the use of single-photon emission tomography technology can produce tomographic images in the axial, sagittal, and coronal planes.
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Positron emission tomography scanning uses a metabolically active tracer, typically 18 F-fluorodeoxyglucose, a glucose analog that is taken up in tissues proportional to glucose use.
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Pathologic processes typically show increased metabolic activity and increased 18 F-fluorodeoxyglucose uptake.
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This modality also has theoretical value for the evaluation of a variety of neoplastic, infectious, and inflammatory conditions of the musculoskeletal system. Although promising results have been reported for some indications, the number of studies has been limited to date, and further investigation is needed.
Strengths
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Whole-body imaging allows rapid assessment of the entire skeleton; this is the study of choice to evaluate possible skeletal metastases.
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Provides physiologic information regarding the activity of a bone lesion ( Fig. 3.12 )
