Radiography

Radiography remains the initial tool for the evaluation of acute trauma, inflammatory arthritis, infection, suspected primary bone neoplasms, and skeletal dysplasias.

In the setting of acute trauma, nonarticular long bones should be imaged with at least two views (frontal and lateral). Osteoarticular regions should be imaged with three views (frontal, lateral, and oblique). Dedicated imaging of the digits is preferred rather than general imaging of an entire hand or foot when a patient has a single symptomatic digit. The radiograph should be the initial screening tool before computed tomography (CT) or magnetic resonance imaging (MRI) is performed in the setting of acute injuries. For alignment disorders, including scoliosis and foot deformities, weight-bearing views should be obtained routinely. In cases of suspected child abuse, a dedicated skeletal survey should be performed. Bone scintigraphy and MRI are complementary tools in the evaluation of child abuse, but they may miss the classic metaphyseal corner fracture.¹

In the setting of infection, radiography should be performed before advanced imaging, although a normal radiograph should not preclude referral for MRI for suspected acute osteomyelitis. Radiographs are helpful for initial screening before the use of MRI to ensure that symptoms of suspected infection are not a result of an underlying fracture or primary bone neoplasm.

For primary bone neoplasms, radiographs are key to determining coverage by MRI; they also complement the MRI diagnosis. Radiographs may help with the final diagnosis because the matrix and pattern of bone destruction are better delineated by radiography than by MRI.

Ultrasonography

Musculoskeletal ultrasound has three primary roles: evaluation of dysplasias, of soft tissue masses, and of pyogenic and nonpyogenic arthritis.

Sonographic evaluation of dysplasias includes developmental dysplasia of the hip, glenohumeral dysplasia related to brachial plexopathy, and selected congenital foot deformities. For developmental dysplasia of the hip, the optimal time of imaging is before the capital femoral epiphyseal ossification center appears, which usually is at 4 to 6 months of age and younger. Sonography can image through cartilage but not bone. Sonography can evaluate the relationship of the humeral head with respect to the glenoid in the setting of suspected glenohumeral dysplasia and subsequently can be used to calculate a glenoid version angle. Sonography also is useful for certain congenital foot disorders and can determine the relationship between the nonossified navicular and talus in the setting of congenital vertical talus.

Sonography is useful in determining solid or cystic soft tissue lesions. However, a positive or negative sonographic result does not preclude the need for additional imaging. Sonography may be the end point for imaging when a ganglion cyst is identified, and the tail can be definitively followed to a tendon sheath or joint space. However, sonography is nonspecific when a solid soft tissue mass is seen and should be used with caution in differentiating benign and malignant etiologies.

Identification of the presence or absence of a joint effusion of any region is straightforward with sonography. However, the etiology is predicated on clinical history. It is not possible to differentiate pyogenic, nonpyogenic, and posttraumatic causes based on sonography alone.

Computed Tomography

The three primary roles for musculoskeletal CT include fracture assessment, including orthopedic hardware failure; assessment of alignment disorders, including physeal bar assessment; and evaluation of tumor matrix and tumor recurrence in the setting of an existing neoplasm or a resected neoplasm with hardware in place.

A low kVp technique may be used because of the inherent contrast of bone. However, when orthopedic hardware is present, a high mAs and kVp technique is required to image through the hardware to minimize artifact. Multiplanar and volume-rendered reformats should be performed routinely for all musculoskeletal CT applications.

Fracture assessment by CT should be performed when radiographs do not fully define the fracture. Alternatively, CT should be performed when radiographs are negative and a high clinical concern exists for fracture and relative contraindications exist for MRI. Indications for CT when radiographs already demonstrate a fracture include fully defining the intraarticular component of the fracture, identifying additional fractures, determining exact measurements of fracture diastasis, which is particularly important at the articular surfaces, and identifying intraarticular loose bodies that may not be visible by radiography.

Alignment disorders that can be evaluated by CT include determination of acetabular and femoral version, tibial torsion, glenohumeral dysplasia, and patellofemoral tracking disorders. Patellofemoral tracking disorders usually are dynamic studies in which the knee is placed in varying degrees of flexion to evaluate lateral patellar dislocation. CT leg-length surveys have limited utility because they are performed in the supine position, and thus alignment cannot be assessed.

Magnetic Resonance Imaging

The volume and varying applications of musculoskeletal MRI in the pediatric population has grown because of the popularity of youth sports and subsequent injuries. MRI plays a useful role in oncologic, metabolic, and sports medicine imaging. For oncologic imaging, the child should be referred for CT imaging if orthopedic hardware related to tumor resection and graft placement is in place.

Four basic sequence types may be used for musculoskeletal MRI. These types include an anatomy sequence (usually T1, proton density); a fluid-sensitive sequence (proton density with fat saturation, T2 with fat saturation, short tau inversion recovery, and a fluid-weighted gradient echo sequence); a contrast sequence (postgadolinium T1 with or without fat saturation); and a susceptibility sequence (any gradient echo sequence).

The purpose of the anatomy sequence is to evaluate marrow replacement, trabecular anatomy, and ligamentous abnormality. In general, a T1W sequence usually is required to evaluate the marrow in children younger than 10 years. When children are older than 10 years, marrow abnormalities can be evaluated with a fluid-sensitive sequence. However, a T1W sequence still should be included in all oncologic or metabolic imaging, including children who are older than 10 years. Proton density sequences without fat saturation are useful for evaluating trabecular anatomy for fractures and to evaluate ligamentous, labral, and cartilaginous anatomy.

Fluid-sensitive sequences are used to evaluate marrow edema, ligamentous anatomy, and cartilage. Spin echo fluid-sensitive sequences are superior to gradient echo sequences in the evaluation of pediatric cartilage. Spherical growth plate, epiphyseal cartilage, articular cartilage, and physis can be differentiated on a spin echo sequence, whereas all four types of cartilage have the same signal intensity on a gradient echo sequence.

A susceptibility sequence is an optional sequence for most musculoskeletal protocols. Its role is to evaluate for anything that will cause a susceptibility artifact, including loose bodies and blood. When conventional anatomy and fluid-sensitive sequences show no abnormality, the susceptibility sequence adds little additional information.

Postcontrast sequences are useful for determining tumor vascularity, defining granulation tissue versus abscess in the setting of infection, and evaluating the synovium. Normal noninflamed synovium eventually enhances and should not be mistaken for pathology.² The relative thickness of the synovium as depicted by contrasted sequences is what is important in the setting of a joint effusion if pyogenic or nonpyogenic inflammatory arthritis is suspected. When initial precontrast images are entirely normal, postcontrast images have limited additional value.

In general, girls who are older than 6 years and boys who are older than 8 years may be able to undergo the magnetic resonance (MR) study without the need for sedation. MR sedation should be handled by a dedicated and qualified team.

Nuclear Imaging

Skeletal scintigraphy has a higher sensitivity relative to skeletal radiography for bone pathology that exhibits osteoblastic activity including fractures, tumors, and infection. The radiopharmaceutical agent that is most commonly used is technetium-99m methylenediphosphonate. Images obtained after injection of this compound reflect a combination of pathophysiologic functions (blood flow and bone turnover) but are nonspecific and do not provide exceptional anatomic detail. Images therefore should be interpreted in the context of the patient’s clinical presentation and relevant radiologic findings. The anatomic areas that are most difficult to image and interpret accurately are the growth centers. Before closure of the physes, these rapidly growing areas display especially high uptake that may mask the activity associated with an adjacent pathologic lesion. Single-photon emission CT helps in localization of such lesions because it improves the spatial resolution of activity in contiguous and overlapping structures.

Indications for skeletal scintigraphy are varied and include investigation of bone pain and diagnosis of early and chronic osteomyelitis, osteonecrosis, and occult trauma, including nonaccidental trauma and stress fractures. Improvements in the diagnostic accuracy of CT and MRI have usurped skeletal scintigraphy in many clinical settings; however, bone scanning remains a very useful imaging tool.

Positron emission tomography (PET) imaging with fluorine-18-deoxyglucose (FDG) offers improved spatial resolution and greater sensitivity for detection of aggressive bone neoplasms. FDG-PET is particularly effective in detecting an active tumor. Because of its high sensitivity, increased signal on PET is nonspecific. The fusion of PET with CT or MRI allows for better spatial localization of lesions and improves the characterization of lesions. PET also is an effective method of determining response of malignant bone tumors to therapy.

1. Offiah, A, van Rijn, RR, Perez-Rosello, JM, et al. Skeletal imaging of child abuse (non-accidental injury). Pediatr Radiol. 2009;39:461–470.

2. Ejbjerg, B, Narvestad, E, Rostrup, E, et al. Magnetic resonance imaging of wrist and finger joints in healthy subjects occasionally shows changes resembling erosions and synovitis as seen in rheumatoid arthritis. Arthritis Rheum. 2004;50(4):1097–1106.