Imaging of Back Pain

Advanced Imaging: Computed Tomography, Magnetic Resonance Imaging, and Nuclear Medicine

After radiographs, the best imaging technique for the evaluation of back pain in children is variable. The choice of computed tomography (CT), magnetic resonance imaging (MRI), or bone scintigraphy depends on the clinical presentation, suspected underlying pathology, and the child’s age. Another important consideration when imaging children is the amount of radiation exposure with each technique. Ionizing radiation places children at risk for development of neoplasms and therefore should be kept to a minimum. Pediatric patients are more susceptible to the mutagenic effects of radiation, and their lifetime risk is greater because they have more years to live and therefore accumulate more radiation exposure throughout their life. The ALARA (As Low As Reasonably Achievable) principles must always be applied, and whenever possible, imaging modalities that do not use ionizing radiation, such as MRI, should be considered.

Computed Tomography

The use of CT in children should be limited because of its ionizing radiation, and when indicated, CT must be restricted to the smallest field of view possible. CT is considered the imaging technique of choice for characterizing fractures and bony lesions. Two-dimensional reformatted images and three-dimensional reconstruction models are useful in the preoperative evaluation of spinal trauma and scoliosis and for presurgical orthopedic planning.

Magnetic Resonance Imaging

MRI is the imaging technique of choice in diagnosing intraspinal or paraspinal pathology and is indicated in children with back pain and one or more clinical red flags. In patients with abnormal neurological findings, initial evaluation with MRI of the entire spine may be indicated. When there is suspicion of infection, inflammation, or neoplasm, precontrast and postcontrast MR sequences should be performed to assess patterns of enhancement. Generally, noncontrast MRI is done in the child with low back pain with suspected spondylolysis or disk pathology. The decision of using MRI must be weighed carefully against its relatively high cost and the need for sedation in younger children.

Spondylolysis and Spondylolisthesis

Spondylolysis refers to a defect or fracture of the pars interarticularis, which is the weakest portion of the neural arch. The pars interarticularis is the bony segment formed by the junction of the vertebral pedicle, the superior and inferior articular processes, and the lamina. Although etiology remains unclear, risk factors include developmental defects and repetitive trauma from excessive hyperextension. A prevalence rate of 4.4% has been reported in children and 6% in the general population. The incidence of spondylolysis increases with the practice of sports such as gymnastics, soccer, volleyball, diving, and ballet. Bilateral involvement is more frequent than unilateral, and the most commonly affected vertebra is L5, followed by L4 and L3. Multilevel involvement can occur, and atypical involvement at a higher level, such as L2, tends to be more painful.

There is no general consensus for the imaging workup to evaluate suspected spondylolysis. However, evaluation typically begins with plain radiographs and can be further assessed with any of the following imaging modalities: MRI, CT, ^99m Tc bone scan whole body with SPECT spine, or SPECT/CT. Radiographs are considered a poor screening test with low sensitivity for spondylolysis in children; only two-view (anteroposterior and lateral) radiographs are recommended. It has been demonstrated that oblique views do not increase sensitivity and specificity, but rather add radiation exposure. A pars fracture can be seen as an oblique lucency at the base of the laminae on the lateral view ( Fig. 30.1 ). Radiographs are useful for the evaluation of spondylolisthesis , defined as anterior slippage of a given vertebra over the one below it, in the presence of bilateral pars fractures. The degree of anterior vertebral displacement is graded as follows: grade 1, less than 25% displacement; grade 2, 25% to 50% displacement; grade 3, 50% to 75% displacement; and grade 4, 75% to 100% displacement. Grade 5 (spondyloptosis) refers to complete displacement of the vertebral body anteriorly ( Fig. 30.1A ).

MRI is currently a preferred imaging modality to diagnose spondylolysis due to the lack of radiation and its ability to assess soft tissue and bony structures. The following spectrum of abnormalities of the pars interarticularis can be demonstrated with MRI: focal marrow edema without a visible fracture consistent with stress reaction, incomplete or complete fractures with marrow edema (in the acute/subacute phase), or chronic (nonunion) fractures with no marrow edema. Paraspinal soft tissue edema can be seen occasionally in the acute setting. Incomplete fractures extend from inferior to superior. Marrow edema will show hypointense signal intensity on T1-weighted images (T1WIs), hyperintense signal on T2-weighted images (T2WIs) with fat saturation, or STIR (short-tau inversion recovery) pulse sequence. Sagittal multiple-echo recombined gradient-echo images are also recommended because they provide greater sensitivity for visualization of pars fractures ( Fig. 30.1B ). On axial images, pars fractures have a transverse orientation and should not be mistaken with the adjacent facet joints, which have an oblique orientation. The facet joints are seen one or two slices above and below the pars fractures ( Fig. 30.2C,D ).

Healing response of incomplete fractures is not well seen with MRI or CT; neither modality can assess whether an incomplete fracture is in an evolutionary or reparative phase. CT scans can best characterize the fracture and can show associated findings, such as bony fragmentation or surrounding sclerosis ( Fig. 30.3 ).

SPECT bone scans are sensitive for detecting spondylolysis and reveal areas of bone turnover; findings are generally positive for a prolonged period (~6 weeks). Spondylolysis presents as increased radiotracer uptake in the posterior elements (pars interarticularis, lamina, or pedicle) ( Fig. 30.4 ) ( ). These findings may be consistent with stress reaction, stress fracture, or a symptomatic spondylolytic defect. Coregistered CT is now a standard component of modern SPECT systems. The advantages of SPECT/CT were mentioned earlier in this review.

The differential diagnosis of spondylolysis includes fractures of the vertebral pedicles, which are significantly less common, but can also show bilateral uptake in the posterior elements with SPECT. Fractures of the pedicle are most often unilateral but can be seen in association with contralateral spondylolysis. Bilateral pedicle fractures tend to occur in skeletally mature patients and are difficult to differentiate from pars fractures with SPECT. Coregistered CT can make this distinction as well as MRI ( Fig. 30.5 ).

Nonsurgical treatment of spondylolysis includes restriction of activity that exacerbated symptoms, activity modification limited to specific forms of exercise (e.g., stationary biking and modified swimming), bracing, NSAIDs, and physical therapy.

Disk Calcification

Calcification of the intervertebral disk is a rare disorder in children of unknown etiology. Proposed causes include inflammation or infection (i.e., diskitis, osteomyelitis), trauma, or metabolic disorders (i.e., hyperparathyroidism). It also has been described in association with syndromes such as Morquio syndrome, I cell disease, and Patau syndrome. Disk calcification occurs more often in the cervical spine, followed by the thoracic and lumbar spine, and has a slight predilection for boys. This entity usually occurs during the first two decades of life and is self-limiting, with most symptoms resolving within 6 months. Disk calcification can completely resolve with time.

Disk calcifications can be seen incidentally or can present with symptoms such as pain, stiffness, decreased range of motion, muscle spasm, tenderness, and torticollis. Fever and increased erythrocyte sedimentation rate (ESR) may also be present. Symptoms tend to be brief and rarely last for more than several weeks. Associated disk herniation may occur as well.

On radiographs and CT, disk calcification is seen as a round, oval, flattened, or fragmented calcification in the nucleus pulposus. On MRI the calcified disk will appear dark on T2WI and bright on T1WI. Associated calcification of the posterior ligament can occur. MRI can best evaluate an associated disk herniation ( Fig. 30.6 ).

Treatment is conservative; however, severe myelopathy or radiculopathy will require surgical intervention.