Musculoskeletal Imaging Principles
John A. deVries, MD, MS
Narayan Sundaram, MD, MBA
Rex Haydon, MD, PhD, FAAOS
Dr. Haydon or an immediate family member serves as a board member, owner, officer, or committee member of the American Orthopaedic Association and the OMeGA Medical Grants Association. Neither of the following authors nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter: Dr. deVries and Dr. Sundaram.
ABSTRACT
Imaging is indispensable for diagnosing most musculoskeletal pathologies, including trauma, arthropathies, inflammatory conditions, and neo-plasms. Imaging is useful not only for diagnostic but also prognostic and follow-up applications. It is important to review indications and applications for radiography, CT, MRI, ultrasonography, and nuclear medicine as well as recent advances in the field.
Keywords: CT; imaging; MRI; radiography; ultrasonography
Introduction
Musculoskeletal imaging represents one of the most important tools used by orthopaedic surgeons in patient care. The full range of imaging techniques includes simple radiography, CT, MRI, and ultrasonography as well as more advanced nuclear medicine tests such as bone scans or positron emission tomography (PET) CT. Strict indications for imaging should be followed when ordering tests to minimize cost and risks to the patient, such as unnecessary radiation or contrast agents. Reviewing each of these imaging approaches will provide clinicians with a thorough understanding of each radiographic modality to ensure accurate, safe, and cost-effective diagnostic testing.
Conventional Radiography
Physics and Digital Format
Conventional radiography uses x-rays to generate images. X-rays are transmitted through the body part of interest and are absorbed/scattered (attenuated) to varying degrees by different tissues to generate a two-dimensional radiographic image. The contrast between high-attenuation bone and low-attenuation soft tissues makes conventional radiography an excellent method for imaging of skeletal structures. Advances in technology have rendered traditional cassette-based film radiographs (computed radiography) obsolete. Digital radiography uses a digital x-ray detector to acquire images and a phosphor imaging plate to generate a digital image. The ability to manipulate windowing levels on digital radiography versus computed radiography does allow for some discrimination of soft tissues, air, and fluid from bone.1
Technique
As a general rule, orthogonal views are obtained for most long bones; however, certain anatomic locations may require more than two images per site of interest. For example, three views are routinely used for the evaluation of more distal joints, such as the wrist/hand or ankle/foot. Oblique views are often added to standard approaches depending on the nature of the pathology that is being evaluated, such as Judet views of the pelvis for the evaluation of acetabular fractures. Additionally, a wide range of specialized radiographic techniques exists that require special positioning of the patient and the x-ray beam to evaluate for specific conditions. Table 1 presents a list of many of these specialized examinations.
Screening Techniques
Trauma Evaluation
Plain radiographs are an effective and easy modality to use. They are very quick to obtain and of low cost. For evaluation of musculoskeletal trauma, radiography should generally include the joint above and below the area of interest to rule out concomitant or occult injuries. Fractures can be readily identified, and further imaging may not be needed to diagnose and treat many injuries. Guidelines have been developed in less
straightforward scenarios. For example, Ottawa rules have been developed to help determine when plain radiographs are indicated for the evaluation of ankle or foot pain in the emergency department. If the patient has tenderness directly over the midfoot or malleoli and is unable to bear weight, radiographs are generally indicated.2 Specific clinical contexts will dictate when imaging is needed and where. In the trauma bay for the multiply injured patient, plain portable radiographs can serve as a quick screening tool (Figure 1). High-speed trauma with a femoral fracture, for example, requires dedicated imaging of the ipsilateral femoral neck to evaluate for a concomitant femoral neck fracture3 (Figure 2). Criteria such as National Emergency X-Radiography Utilization Study Group indicate when it is appropriate to obtain cervical spine films.4 If there is any suspicion for pelvic ring injury, a simple pelvis film should be obtained in the trauma bay because these injuries can be life-threatening and results can be obtained in minutes. For most plain radiographs, the entire bone should be included, and orthogonal views should be obtained when appropriate.
straightforward scenarios. For example, Ottawa rules have been developed to help determine when plain radiographs are indicated for the evaluation of ankle or foot pain in the emergency department. If the patient has tenderness directly over the midfoot or malleoli and is unable to bear weight, radiographs are generally indicated.2 Specific clinical contexts will dictate when imaging is needed and where. In the trauma bay for the multiply injured patient, plain portable radiographs can serve as a quick screening tool (Figure 1). High-speed trauma with a femoral fracture, for example, requires dedicated imaging of the ipsilateral femoral neck to evaluate for a concomitant femoral neck fracture3 (Figure 2). Criteria such as National Emergency X-Radiography Utilization Study Group indicate when it is appropriate to obtain cervical spine films.4 If there is any suspicion for pelvic ring injury, a simple pelvis film should be obtained in the trauma bay because these injuries can be life-threatening and results can be obtained in minutes. For most plain radiographs, the entire bone should be included, and orthogonal views should be obtained when appropriate.
Table 1 Special Radiographic Views, Techniques, and Pathology Being Assessed With Each View | ||||||||||||||||||||||||||||||||||||||||||||||||
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Atraumatic Evaluation
In patients with atraumatic musculoskeletal pain, plain radiographs are usually indicated before any advanced imaging is to be ordered. Degenerative conditions, insufficiency/stress fractures, osteomyelitis, impingement syndromes, and many arthropathies can often be diagnosed on plain films alone. In the spine, alongside static AP and lateral views, dynamic flexion and
extension films can help detect segment instability that may contribute to a patient’s symptoms; however, advanced imaging is often needed for further evaluation.
extension films can help detect segment instability that may contribute to a patient’s symptoms; however, advanced imaging is often needed for further evaluation.
 Figure 1 Portable AP radiograph obtained in the trauma bay showing the pelvis of a young adult following a motor vehicle accident. Left obturator ring and sacral fractures indicate pelvic ring disruption and high-energy mechanism. Additional imaging and close monitoring of vital signs are necessary. |
 Figure 2 AP portable radiographs of the proximal femur (A) and distal femur (B) of a patient with trauma. Periprosthetic fracture, fracture of intramedullary device and distal interlocking screws, distal diaphyseal and metaphyseal femoral fracture with concomitant femoral neck fracture are shown. |
Arthritis and joint-based pathologies are readily studied with plain radiographs. If osteoarthritis is suspected, weight-bearing radiographs represent the gold standard for diagnosis (Figure 3). Other inflammatory arthropathies can be diagnosed and followed with radiographs, alongside laboratory tests and clinical examination.5 Plain radiographs are extremely accurate and valuable in the evaluation of the joint space, alignment, osteophytes, and other sequelae of joint degeneration. Other disease processes such as gout or pseudogout, tumoral calcinosis, myositis ossificans (Figure 4), or heterotopic ossification routinely result in soft-tissue mineralization and can be diagnosed and followed with plain radiographs.
Neoplastic Evaluation and Considerations
The workup of benign and malignant bone and soft-tissue pathology generally starts with plain radiographs. Especially in the pediatric population, there are many benign bone lesions that are characterized by radiographs and are sufficient for diagnosis and follow-up.6 Malignant features can also be seen, such as periosteal reaction and bony destruction or growth. A wide zone of transition versus well-marginated lesions may give clues as to the benign or neoplastic nature of the pathology. Osteosarcoma as well as chondrosarcoma may be diagnosed with radiographs.7 Enchondromas are usually incidentally found but must be differentiated from malignant pathology. Soft-tissue masses can occasionally be evaluated with radiographs but, besides looking for calcification patterns, they are better evaluated by advanced imaging.
Fluoroscopy
Fluoroscopy refers to the use of radiographs in real time, where the clinician uses the information during live manipulation of the structure of interest. This can be used to evaluate unstable fracture patterns by evaluating the joint or structure of interest under stress, such as loading the hip joint under live x-ray to assess for stability of the joint after an acetabular fracture8 or stressing the distal tibia-fibula syndesmosis after fixation of an ankle fracture to test the integrity of
the syndesmotic ligament complex. However, the most frequently used application of fluoroscopy is for intraoperative guidance. This gives real-time feedback during spine or trauma surgery for screw placement and fracture reduction, allowing for better outcomes without direct visualization, thereby making less invasive and percutaneous surgery possible. Fluoroscopy is also used for image-guided joint aspirations and injections.
the syndesmotic ligament complex. However, the most frequently used application of fluoroscopy is for intraoperative guidance. This gives real-time feedback during spine or trauma surgery for screw placement and fracture reduction, allowing for better outcomes without direct visualization, thereby making less invasive and percutaneous surgery possible. Fluoroscopy is also used for image-guided joint aspirations and injections.
 Figure 3 Standing AP radiograph of bilateral knees (A) and lateral plain radiograph of the knee (B). Significant end-stage osteoarthritic changes include bilateral varus alignment, joint space narrowing, joint subluxation, subchondral sclerosis, and osteophyte formation. |
Dual-Energy X-ray Absorptiometry
Imaging modalities to assess bone density have been developed over the years, much of which has been adapted from x-ray-based techniques. Dual-energy x-ray absorptiometry is currently used to calculate bone mineral density and thereby infer risk of sustaining an osteoporosis-related fracture such as vertebral compression fracture, hip fracture, or distal radial fracture,9 although more complex and accurate modalities are being developed using techniques that will be discussed in the next paragraphs.
Computed Tomography
CT uses a rotating x-ray beam to create cross-sectional images of the body. The x-ray tube is placed in a circular gantry, which in turn surrounds the CT table. The patient lies on the CT table, which slowly moves through the gantry as the x-ray tube rotates around the patient. The x-rays that transmit through the body part of interest are detected by multiple detectors located opposite the x-ray tube in the gantry. The input from all detectors surrounding the patient is analyzed and then images are reconstructed by computer.10
Current Advances in CT Technology
Dual-energy CT is an imaging technique that has many applications in orthopaedic imaging. Dual-energy CT uses two different x-ray energies to acquire images, which serves to demonstrate different tissue contrasts. This is accomplished by using two x-ray tubes and detectors mounted at 90° to each other (dual-source dual-energy CT), or by using a single x-ray source that rapidly changes energy levels and acquires data for each set during image acquisition (rapid kilovoltage switching). Dual-energy CT has many evolving clinical applications in orthopaedic imaging, including metal artifact reduction, detection of monosodium urate (MSU) crystals in gout, analysis of ligaments and tendons, detection of bone lesions, and detection of bone marrow edema.11
Conventional CT, despite using artifact reduction techniques, is limited by metal artifact that degrades image quality. Dual-energy CT can reduce artifact from metallic implants using postprocessing techniques that combine the datasets from the two different x-ray energies to generate an image that reduces metallic artifact from orthopaedic prostheses.12 This can help in identifying bone and soft-tissue pathology adjacent to joint arthroplasties and other metallic implants.
 Figure 4 AP radiograph of the distal femur of a 31-year-old woman demonstrating the characteristic calcification pattern of myositis ossificans. |
Dual-energy CT can be used to noninvasively diagnose gout (Figure 5). It has been shown to accurately identify MSU crystals in gout, taking advantage of the distinct attenuation characteristics of calcium from bone and MSU crystals. Gout traditionally has been diagnosed through joint fluid analysis for MSU crystals obtained through arthrocentesis.
Dual-energy CT is an evolving technique in the detection of bone marrow edema by removing bone from marrow and using postprocessing techniques to generate bone marrow edema maps. Other applications of dual-energy CT include analysis of ligaments and tendons by taking advantage of the unique attenuation of these collagenous structures; dual-energy CT also shows promise in detection of bone metastases by measuring water content, bone composition, and enhancement characteristics of bone metastases compared with trabecular bone and benign bone lesions.
Another emerging CT technique is cone-beam CT. This uses a cone-shaped x-ray beam and a flat-panel detector rather than a fan-shaped beam and linear detectors used in conventional CT. In cone-beam CT, the x-ray tube and detector rotate around the patient. Images are then generated, which can be reconstructed in three planes. This modality allows for weight-bearing CT images of extremities with high spatial resolution but at a lower dose than conventional CT. However, cone-beam CT has a limited assessment of the soft tissues and uses a small field of view. It is used in orthopaedics for identification of radiographically occult fractures, weight-bearing assessment of joint osteoarthritis, and assessment of sequestrum in chronic osteomyelitis.
Roles in Preoperative Planning and Surgical Guidance
CT structures have had a significant effect on preoperative planning of surgical interventions, most importantly in the areas of evaluating and treating complex fractures, and in procedures that incorporate various forms of surgical guidance technology. Axial CT images can be converted into three-dimensional reconstructed images for improved fracture reduction planning.13 Preoperative CT is generally required for robot-assisted surgeries. Briefly, proprietary CT protocols are used to scan the bone or joint of interest preoperatively (or in some cases intraoperatively). These scans are then referenced intraoperatively relative to specific anatomic landmarks using motion capture and additional reference devices placed on the patient. This then allows real-time navigation of saws, drills, or screws without the need for cutting guides or live fluoroscopy. This approach can be used for robot-assisted surgery or navigated procedures performed by the surgeon. A 2021 study has shown that this approach increases accuracy of surgical procedures across a number of applications.14 In addition to dynamic real-time intraoperative guidance, preoperative CT scans can be used to create a three-dimensional surgical plan, complete with three-dimensional printed models of the patient’s anatomy or three-dimensional printed cutting guides for complicated osteotomies or resections. Three-dimensional printed patient-specific implants have been developed, but the cost and time involved in manufacturing limits its use to complex and one-of-a-kind cases. CT is also useful for evaluating implanted hardware and is able to assess loosening, osteolysis, ingrowth, and nonunion.
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