Nuclear MedicineFree Access

Spondylolysis and Beyond: Value of SPECT/CT in Evaluation of Low Back Pain in Children and Young Adults

Published Online:https://doi.org/10.1148/rg.2015140092

Abstract

Single photon emission computed tomography (SPECT)/computed tomography (CT) is ideally suited for assessment of low back pain in children and young adults. Spondylolysis is one of the most common structural causes of low back pain and is readily identified and characterized in terms of its chronicity and likelihood to heal. The value of SPECT/CT extends to identification and characterization of other causes of low back pain, including abnormalities of the posterior elements, developing vertebral endplate, transverse processes, and sacrum and sacroiliac joint. Some of the disease processes that are identifiable at SPECT/CT are similar to those that occur in adults (eg, facet hypertrophy) but may be accelerated in young patients by high-level athletic activities. Other processes (eg, limbus vertebrae) are more unique to children, related to injury of the developing spine. The authors review the spectrum of pars interarticularis abnormalities with emphasis on the imaging features of causes of pediatric low back pain other than spondylolysis.

©RSNA, 2015

SA-CME LEARNING OBJECTIVES

After completing this journal-based SA-CME activity, participants will be able to:

  • ■ Recognize the spectrum of pars abnormalities defined by SPECT/CT.

  • ■ Describe unique features of the immature spine that predispose children and young adults to a distinct pattern of lumbar spine injury.

  • ■ Identify common alternative causes of pediatric/young adult low back pain related to the posterior elements, endplate and disk, transverse processes, and sacroiliac region.

Introduction

Low back pain is relatively common in the pediatric and adolescent population. Although structural causes for low back pain are uncommon, spondylolysis is one of the more common structural causes (1). Single photon emission computed tomography (SPECT)/computed tomography (CT) plays an important role in assessment of spondylolysis in the pediatric and young adult population. In this article, we review SPECT/CT assessment of spondylolysis and focus on other causes of low back pain in children and young adults that can be identified at SPECT/CT. To maximize the value of SPECT/CT in assessment of pediatric low back pain, it is important that the interpreting radiologist or nuclear medicine physician is aware of other entities that might be encountered.

Low Back Pain in Children and Adolescents

Causes

Low back pain occurs in 40% of children and adolescents at some point before adulthood (2). The prevalence of low back pain increases with age, becoming similar to that of adults by adolescence (2). Although some studies have suggested a female predilection, this has not held up in meta-analyses (2). Some investigators suggest that high-level exercise and athletic activities may predispose to low back pain; however, a definitive connection remains elusive (3,4). Despite the frequency of low back pain, a structural cause is identified in only 12%–26% of cases (1).

When a structural cause for low back pain is identified, spondylolysis is commonly implicated, since the pars interarticularis is the weakest portion of the immature neural arch
(57). However, it is not uncommon to identify an abnormality other than spondylolysis at SPECT (8). In a review of 209 patients who underwent SPECT only, Connolly et al (8) detected abnormalities other than spondylolysis in 17% (35% of positive cases). Similarly, in a review of patients who underwent SPECT/CT at our institution, abnormalities other than spondylolysis were identified in 40% of positive cases (9). The frequency of occurrence of various abnormalities in these studies is shown in Table 1.

Table 1: Sites of Abnormality Other than Spondylolysis in the Spine per Connolly et al and Gelfand and Sharp

Table 1:

Note.—Differences in the distribution of abnormalities are likely related in part to sample differences, but they also reflect differences in terms of which abnormalities are detectable at SPECT alone (8) and which require SPECT/CT for detection (9).

*Total number of cases = 36 (one case had two sites of uptake).

Total number of cases = 25.

The unique biomechanics and structure of the growing spine account for an injury pattern different from that seen in adults. Children lack mature muscle strength and neuromuscular coordination, which changes the distribution of forces over the spine (10). Joint laxity also contributes to the altered distribution of forces (10). The developing vertebra is formed from three ossification centers, one accounting for the vertebral body and one each for the paired posterior elements (3). Fusion of these centers is usually complete by 6 years of age, with the midline posterior elements fusing last (3). Longitudinal growth of the vertebra is a result of enchondral ossification of physeal plates along the cranial and caudal surfaces of the vertebral body (Fig 1) (11). These physes begin to ossify around 17 years of age (11). In addition to the endplate physes, there is a ring apophysis that encircles the developing endplate (Fig 1). This apophysis does not account for longitudinal growth but anchors longitudinal ligamentous fibers (12). The ring apophysis calcifies at around 6 years of age and ossifies at around 13 years (12). Fusion with the vertebral body begins around 17 years of age (12). The maturing disk is also somewhat different from the intervertebral disk in adults. Specifically, the nucleus pulposus in children contains more water, which directs applied forces more centrally onto the developing endplate (3).

Figure 1a

Figure 1a Normal pediatric vertebral anatomy in sagittal (a) and axial (b) projections. Physeal plates (1) along the cranial and caudal surfaces of the developing vertebral body (2) provide for longitudinal growth through enchondral ossification. A ring apophysis (3) within the developing endplate fuses with the vertebral body around 17 years of age. The pars interarticularis (4) is a bony bridge formed by junction of the superior (5) and inferior (6) articular facets and the vertebral pedicle (7) and lamina. The spinous process (8) and transverse processes (9) are also shown. The intervertebral disk consists of the annulus fibrosus (10) and nucleus pulposus (11). (Courtesy of Glenn Miñano, BSFA, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio.)

Figure 1b

Figure 1b Normal pediatric vertebral anatomy in sagittal (a) and axial (b) projections. Physeal plates (1) along the cranial and caudal surfaces of the developing vertebral body (2) provide for longitudinal growth through enchondral ossification. A ring apophysis (3) within the developing endplate fuses with the vertebral body around 17 years of age. The pars interarticularis (4) is a bony bridge formed by junction of the superior (5) and inferior (6) articular facets and the vertebral pedicle (7) and lamina. The spinous process (8) and transverse processes (9) are also shown. The intervertebral disk consists of the annulus fibrosus (10) and nucleus pulposus (11). (Courtesy of Glenn Miñano, BSFA, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio.)

Workup

The workup of a child or adolescent with low back pain is largely clinical, depending on a careful patient history and physical examination findings (3). Neurologic symptoms are considered “red flags” and necessitate more urgent imaging assessment, generally with magnetic resonance (MR) imaging. In the absence of red flags, imaging is performed at the discretion of the referring physician. Radiography, CT, and MR imaging all play a role in assessment of low back pain in pediatric patients.

Technetium-99m (99mTc) methylene diphosphonate (MDP) bone scintigraphy is an important tool in imaging workup of pediatric low back pain, particularly because of its utility in assessment of spondylolysis
(13,14).

Imaging with 99mTc MDP demonstrates local osteoblastic activity, with the radiolabeled MDP taken up in hydroxyapatite at sites of bone deposition. Developing bone, healing fractures, and reactive and malignant bone deposition all result in accumulation of the radiotracer. Images are typically obtained 1½–2 hours after administration of the radiopharmaceutical in preteenage and teenage patients and after a 3-hour delay in older patients. This interval allows distribution of the radiotracer throughout the body and its incorporation into developing bone, as well as renal clearance of soft-tissue activity. The North American Consensus Guidelines recommend administration of 9.3 MBq/kg (0.2 mCi/kg) of Tc-99m MDP (15). At our institution, we administer a dose of 6.8 MBq/kg (0.185 mCi/kg) and acquire anterior and posterior planar whole-body images, planar spot images (optional), and SPECT images.

Anterior and Posterior Planar Whole-Body Images.—Anterior and posterior planar whole-body images are obtained with a large-field-of-view (FOV) camera with a low-energy high-resolution collimator. These images are acquired with a camera speed of 12 cm/min, generating contiguous whole-body images. They allow a global assessment of radiotracer distribution and, in children, provide a sense of skeletal maturity based on physeal activity. In addition, whole-body images allow identification of extraspinal abnormalities.

Planar Spot Images (Optional).—Spot images are acquired with a large-FOV camera with a low-energy high-resolution collimator for 3 minutes per projection. Spot images may be acquired in varying projections (eg, oblique, lateral) and can be used to clarify findings identified on the whole-body images that will not be included in the SPECT FOV. Generally, these are foci of abnormal uptake outside the spine.

SPECT Images.—At our institution, the SPECT FOV is positioned based on (a) abnormalities identified on the planar images and (b) clinical history. Images are acquired with a Symbia T SPECT/CT scanner (Siemens Medical Solutions, Malvern, Pa) in step-and-shoot mode with 64 angular increments and 20 seconds per stop. A Butterworth filter is used to filter the data, with a cutoff of 0.5 cycles per centimeter and a power of 5.

SPECT has been shown to be of particular value in detection and localization of sites of abnormal radiotracer uptake in the spine, helping identify sites of uptake in up to 55% of patients (13). Coregistered CT is increasingly becoming a standard component of modern SPECT systems.

SPECT/CT has several advantages over SPECT alone, including more precise localization of sites of abnormal uptake in bone, identification of causes of abnormal uptake, and identification of osseous abnormalities without associated abnormal radiotracer uptake
(16). However, these benefits come with the cost of a small added radiation dose (Table 2). At our institution, coregistered CT is not uniformly performed in children undergoing SPECT of the lumbar spine, since the yield of CT in the absence of abnormal radiographic or SPECT findings has been shown to be low (19). At our institution, coregistered CT is performed when any of the following conditions are met:
  • 1. Request made by the referring physician (generally an orthopedist).

  • 2. Abnormality visible at SPECT.

  • 3. Abnormality visible at previously performed correlative imaging (eg, radiography, MR imaging).

Table 2: Typical Dose Ranges for SPECT and SPECT/CT of the Pediatric Spine

Table 2:

Source.—References 17,18.

Note.—The SPECT dose depends on the amount of radiopharmaceutical administered. The CT dose depends on the CT parameters (kilovolt peak, milliampere-seconds) and scan length. At our institution, CT is limited to the area of interest to minimize the CT component of the total examination dose.

When localization CT is performed, the z-axis FOV is limited to the area of clinical interest—typically L3–L5 or L4–S1, given the preponderance of spondylolysis at these levels. CT exposure parameters used for the cases described in this article were as follows: 110 kVp, 22 reference mAs (resulting in an average of 13–20 mAs), pitch of 1.65, and 0.8-second rotation speed. CT images were processed into 2-mm sections for review of bone findings and fusion with bone SPECT images, and into 5-mm sections for review of soft-tissue CT findings.

Imaging Findings

Spondylolysis

Strictly speaking, spondylolysis is a break in the pars interarticularis (Latin for “interarticular part”) of the neural arch (Fig 2). The pars interarticularis is a bony bridge formed by the junction of the vertebral pedicle, superior and inferior articular facets, and lamina (21). Pars interarticularis injuries are believed to indicate stress on the pars interarticularis related to either developmental deficiency or chronic low-grade trauma (21). Lesions have a predilection for the lumbar spine and are most commonly bilateral (21,22). Unilateral spondylolysis may also be accompanied by contralateral pars interarticularis stress, manifesting as sclerosis and buttressing. Many injuries of the pars interarticularis are asymptomatic (21). However, symptoms (when present) include activity-related low back pain and hamstring tightness without radiculopathy (20).

Figure 2a

Figure 2a Spectrum of pars interarticularis injury. (a) Axial SPECT image obtained in a patient with pars interarticularis stress but without spondylolysis shows focal radiotracer uptake at the level of the pars interarticularis (more on the left side than on the right) (arrows). (b) Coregistered CT image shows no spondylolysis, but subtle bilateral sclerosis is present (arrows). Pars interarticularis stress without lysis is seen in 50% of scintigraphically positive cases (20). (c) Axial SPECT image obtained in a patient with bilateral spondylolysis demonstrates focal symmetric radiotracer uptake at the level of the pars interarticularis (arrows). (d) Fused SPECT/CT image shows bilateral breaks (spondylolyses) in the pars interarticularis (arrows), findings that correspond to the uptake depicted in c. (e) Axial SPECT image obtained in a patient with pars interarticularis nonunion shows no focal abnormal radiotracer uptake. (f) Fused SPECT/CT image shows a clear break in the left pars interarticularis (arrow) with sclerosis along the margins of the defect, findings that reflect a chronic nonunited lesion (7).

Figure 2b

Figure 2b Spectrum of pars interarticularis injury. (a) Axial SPECT image obtained in a patient with pars interarticularis stress but without spondylolysis shows focal radiotracer uptake at the level of the pars interarticularis (more on the left side than on the right) (arrows). (b) Coregistered CT image shows no spondylolysis, but subtle bilateral sclerosis is present (arrows). Pars interarticularis stress without lysis is seen in 50% of scintigraphically positive cases (20). (c) Axial SPECT image obtained in a patient with bilateral spondylolysis demonstrates focal symmetric radiotracer uptake at the level of the pars interarticularis (arrows). (d) Fused SPECT/CT image shows bilateral breaks (spondylolyses) in the pars interarticularis (arrows), findings that correspond to the uptake depicted in c. (e) Axial SPECT image obtained in a patient with pars interarticularis nonunion shows no focal abnormal radiotracer uptake. (f) Fused SPECT/CT image shows a clear break in the left pars interarticularis (arrow) with sclerosis along the margins of the defect, findings that reflect a chronic nonunited lesion (7).

Figure 2c

Figure 2c Spectrum of pars interarticularis injury. (a) Axial SPECT image obtained in a patient with pars interarticularis stress but without spondylolysis shows focal radiotracer uptake at the level of the pars interarticularis (more on the left side than on the right) (arrows). (b) Coregistered CT image shows no spondylolysis, but subtle bilateral sclerosis is present (arrows). Pars interarticularis stress without lysis is seen in 50% of scintigraphically positive cases (20). (c) Axial SPECT image obtained in a patient with bilateral spondylolysis demonstrates focal symmetric radiotracer uptake at the level of the pars interarticularis (arrows). (d) Fused SPECT/CT image shows bilateral breaks (spondylolyses) in the pars interarticularis (arrows), findings that correspond to the uptake depicted in c. (e) Axial SPECT image obtained in a patient with pars interarticularis nonunion shows no focal abnormal radiotracer uptake. (f) Fused SPECT/CT image shows a clear break in the left pars interarticularis (arrow) with sclerosis along the margins of the defect, findings that reflect a chronic nonunited lesion (7).

Figure 2d

Figure 2d Spectrum of pars interarticularis injury. (a) Axial SPECT image obtained in a patient with pars interarticularis stress but without spondylolysis shows focal radiotracer uptake at the level of the pars interarticularis (more on the left side than on the right) (arrows). (b) Coregistered CT image shows no spondylolysis, but subtle bilateral sclerosis is present (arrows). Pars interarticularis stress without lysis is seen in 50% of scintigraphically positive cases (20). (c) Axial SPECT image obtained in a patient with bilateral spondylolysis demonstrates focal symmetric radiotracer uptake at the level of the pars interarticularis (arrows). (d) Fused SPECT/CT image shows bilateral breaks (spondylolyses) in the pars interarticularis (arrows), findings that correspond to the uptake depicted in c. (e) Axial SPECT image obtained in a patient with pars interarticularis nonunion shows no focal abnormal radiotracer uptake. (f) Fused SPECT/CT image shows a clear break in the left pars interarticularis (arrow) with sclerosis along the margins of the defect, findings that reflect a chronic nonunited lesion (7).

Figure 2e

Figure 2e Spectrum of pars interarticularis injury. (a) Axial SPECT image obtained in a patient with pars interarticularis stress but without spondylolysis shows focal radiotracer uptake at the level of the pars interarticularis (more on the left side than on the right) (arrows). (b) Coregistered CT image shows no spondylolysis, but subtle bilateral sclerosis is present (arrows). Pars interarticularis stress without lysis is seen in 50% of scintigraphically positive cases (20). (c) Axial SPECT image obtained in a patient with bilateral spondylolysis demonstrates focal symmetric radiotracer uptake at the level of the pars interarticularis (arrows). (d) Fused SPECT/CT image shows bilateral breaks (spondylolyses) in the pars interarticularis (arrows), findings that correspond to the uptake depicted in c. (e) Axial SPECT image obtained in a patient with pars interarticularis nonunion shows no focal abnormal radiotracer uptake. (f) Fused SPECT/CT image shows a clear break in the left pars interarticularis (arrow) with sclerosis along the margins of the defect, findings that reflect a chronic nonunited lesion (7).

Figure 2f

Figure 2f Spectrum of pars interarticularis injury. (a) Axial SPECT image obtained in a patient with pars interarticularis stress but without spondylolysis shows focal radiotracer uptake at the level of the pars interarticularis (more on the left side than on the right) (arrows). (b) Coregistered CT image shows no spondylolysis, but subtle bilateral sclerosis is present (arrows). Pars interarticularis stress without lysis is seen in 50% of scintigraphically positive cases (20). (c) Axial SPECT image obtained in a patient with bilateral spondylolysis demonstrates focal symmetric radiotracer uptake at the level of the pars interarticularis (arrows). (d) Fused SPECT/CT image shows bilateral breaks (spondylolyses) in the pars interarticularis (arrows), findings that correspond to the uptake depicted in c. (e) Axial SPECT image obtained in a patient with pars interarticularis nonunion shows no focal abnormal radiotracer uptake. (f) Fused SPECT/CT image shows a clear break in the left pars interarticularis (arrow) with sclerosis along the margins of the defect, findings that reflect a chronic nonunited lesion (7).

Abnormalities of the pars interarticularis cover a spectrum that includes stress without spondylolysis, spondylolysis, and nonunion
(Fig 2). It is important to recognize and initiate treatment of abnormalities of the pars interarticularis, since untreated lesions can progress and lesions that become chronic (nonunion) often fail to heal (7,22). Spondylolisthesis, in which the vertebral body moves forward relative to the caudal adjacent vertebral body, is an uncommon complication of spondylolysis. Progression to spondylolisthesis is most likely before 16 years of age, with the risk decreasing with age (21). Spondylolisthesis is not just the result of bilateral spondylolysis but is believed to reflect a physeal stress fracture along the vertebral endplate, which frees the vertebral body to displace anteriorly (21).

Radiography has a limited positive predictive value (57%) for spondylolysis and is even more limited in identifying pars interarticularis stress, which is generally radiographically occult (17). Scintigraphy has a higher sensitivity than radiography, and SPECT is more sensitive than planar imaging (7,20). Planar imaging and SPECT can help identify pars interarticularis stress in the absence of spondylolysis and can help distinguish acute spondylolysis from chronic nonunion (18,23). CT in isolation is highly specific but may not help identify pars interarticularis stress without lysis (7,18,20). CT can be used for follow-up to assess healing and osseous bridging of spondylolyses (22). SPECT/CT provides a high level of both sensitivity and specificity in identification and classification of abnormalities of the pars interarticularis.

The spectrum of abnormalities of the pars interarticularis seen at SPECT/CT is illustrated in Figure 2. At SPECT, pars interarticularis stress and active spondylolysis manifest as unilateral or bilateral radiotracer uptake in the posterior elements at the level of the pars interarticularis. The two entities can be distinguished at coregistered CT, which demonstrates no abnormality or isolated sclerosis of the pars interarticularis in the setting of stress injury and depicts a linear defect in the pars interarticularis in the setting of spondylolysis (20). Nonunion typically demonstrates no radiotracer uptake at SPECT but manifests as a pars interarticularis defect at CT. The margins of the defect are typically sclerotic (7).

An important alternative in the differential diagnosis in the setting of bilateral posterior element uptake at SPECT is vertebral pedicle fracture (Fig 3). These fractures are substantially less common than pars interarticularis injuries. Typically, they are unilateral and are seen in association with contralateral spondylolysis (5). Rare cases of bilateral pedicle fractures (Fig 3) have been described and are believed to occur in more skeletally mature patients (5). At SPECT alone, pedicle fractures can be difficult to distinguish from pars interarticularis defects because both entities manifest with focal radiotracer uptake in the posterior elements. Coregistered CT is particularly important for making this distinction, since it allows localization of uptake to the pedicles rather than the pars interarticularis and depicts the fracture lucencies.

Figure 3a

Figure 3a Vertebral pedicle fractures. (a) Axial fused SPECT/CT image shows symmetric radiotracer uptake at the level of the vertebral pedicles with associated transversely oriented fracture lines through the pedicles (arrows). (b) Sagittal reformatted CT image shows a vertically oriented fracture through the vertebral pedicles (black arrows). There is no abnormality of the more posterior pars interarticularis (white arrow).

Figure 3b

Figure 3b Vertebral pedicle fractures. (a) Axial fused SPECT/CT image shows symmetric radiotracer uptake at the level of the vertebral pedicles with associated transversely oriented fracture lines through the pedicles (arrows). (b) Sagittal reformatted CT image shows a vertically oriented fracture through the vertebral pedicles (black arrows). There is no abnormality of the more posterior pars interarticularis (white arrow).

Posterior Element Abnormalities

Radiotracer uptake in the posterior elements is normally uniform throughout the spine, with less uptake than that associated with the vertebral endplates. Focal asymmetrically increased uptake in the posterior elements suggests the presence of an underlying abnormality. When this uptake is related to the spinous processes, it can easily be distinguished from abnormalities of the pars interarticularis. However, facet abnormalities can be difficult to distinguish from abnormalities of the pars interarticularis at planar imaging.

Lumbar Interspinous Bursitis.—Lumbar interspinous bursitis, also known as Baastrup disease, is believed to be related to excessive lordosis, resulting in mechanical impingement on adjacent spinous processes (24). This impingement may occur at one or more spinal levels. Patients tend to be older, with disk bulges, central canal stenosis, and anterolisthesis; however, the process may be accelerated in young athletes due to repetitive hyperextension and hyperflexion (25,26).

SPECT demonstrates focal radiotracer uptake in opposing spinous processes (Fig 4). The coregistered CT image shows close approximation of the involved spinous processes (“kissing spine”) with spinous process enlargement, flattening, and reactive sclerosis of the opposing surfaces (25). The resultant bursitis is visible at MR imaging, which demonstrates interspinous fluid, most commonly at the L4-L5 level (25).

Figure 4a

Figure 4a Lumbar interspinous bursitis (Baastrup disease). (a) Sagittal SPECT image shows focal radiotracer uptake at the level of the spinous processes (arrow). (b) Fused sagittal SPECT/CT image reveals that the uptake is localized to opposing surfaces of the L4 and L5 spinous processes (arrow). The CT component of the image shows that the L4–L5 interspinous distance is narrowed and the opposing surfaces of the spinous processes are flattened with adjacent sclerosis.

Figure 4b

Figure 4b Lumbar interspinous bursitis (Baastrup disease). (a) Sagittal SPECT image shows focal radiotracer uptake at the level of the spinous processes (arrow). (b) Fused sagittal SPECT/CT image reveals that the uptake is localized to opposing surfaces of the L4 and L5 spinous processes (arrow). The CT component of the image shows that the L4–L5 interspinous distance is narrowed and the opposing surfaces of the spinous processes are flattened with adjacent sclerosis.

Spinous Process Avulsion.—Lumbar spinous process fracture is rare and may not be identified in the acute setting. This fracture is most commonly the result of acute hyperflexion of the spine with avulsion of the inferior margin of the spinous process.

Distinguishing an acute avulsion injury of the spinous process from an accessory ossification center can be difficult on the basis of anatomic imaging alone, except when callus formation is present. At scintigraphy, only avulsion injuries should demonstrate abnormal radiotracer uptake. At SPECT, avulsion injuries appear as focally increased uptake at the tip of the spinous process (Fig 5). Coregistered CT reveals that this uptake corresponds to a fracture lucency and, if healing has begun, adjacent callus. Avulsion fragments are typically less smoothly marginated and corticated than accessory ossification centers. Clinical examination can help make the distinction by localizing pain that intensifies with flexion to the spinous process (27).

Figure 5a

Figure 5a Spinous process avulsion. (a) Sagittal CT image shows a bone fragment (arrow) at the tip of the L4 spinous process. The facing margins of the fragment and the spinous process are irregular, findings that are compatible with fracture rather than an accessory ossification center. (b) Sagittal fused SPECT/CT image shows radiotracer uptake localized to the fragmented spinous process (arrow). (c) Sagittal CT image obtained 8 months later shows healing of the fracture with fusion of the bone fragment to the spinous process (arrow).

Figure 5b

Figure 5b Spinous process avulsion. (a) Sagittal CT image shows a bone fragment (arrow) at the tip of the L4 spinous process. The facing margins of the fragment and the spinous process are irregular, findings that are compatible with fracture rather than an accessory ossification center. (b) Sagittal fused SPECT/CT image shows radiotracer uptake localized to the fragmented spinous process (arrow). (c) Sagittal CT image obtained 8 months later shows healing of the fracture with fusion of the bone fragment to the spinous process (arrow).

Figure 5c

Figure 5c Spinous process avulsion. (a) Sagittal CT image shows a bone fragment (arrow) at the tip of the L4 spinous process. The facing margins of the fragment and the spinous process are irregular, findings that are compatible with fracture rather than an accessory ossification center. (b) Sagittal fused SPECT/CT image shows radiotracer uptake localized to the fragmented spinous process (arrow). (c) Sagittal CT image obtained 8 months later shows healing of the fracture with fusion of the bone fragment to the spinous process (arrow).

Facet Hypertrophy.—Adjacent lumbar vertebrae articulate via the articular facets. These synovium-lined joints are formed by the inferior articular facet of the cranial vertebral body and the superior articular facet of the caudal vertebral body. Chronic stress at the facet joint leads to laxity of the joint capsule and joint instability, which stimulate hypertrophic changes (28).

Degenerative changes at the facet joints are common in older adults but can be accelerated in younger patients who engage in activities involving excessive spinal motion. At planar scintigraphy, facet hypertrophy manifests as focal radiotracer uptake in the posterior elements, a finding that is off midline and is best appreciated on posterior planar images. At SPECT/CT, the abnormal radiotracer uptake is localized to the facet joint, with CT showing joint space narrowing with sclerosis, subchondral cyst formation, and osseous hypertrophy (overgrowth) (Fig 6).

Figure 6a

Figure 6a Facet hypertrophy. (a) Axial SPECT image shows focal radiotracer uptake in the posterior elements on the right side (arrow). On the basis of SPECT findings alone, determination of the specific location and cause of the uptake is difficult. (b) On an axial fused SPECT/CT image, the uptake is localized to the facet joint (arrow), which is hypertrophic with narrowing and irregularity. (c) Coronal fused SPECT/CT image better demonstrates the hypertrophy of the facet joint (arrow). Note the joint space narrowing and irregularity and the osseous hypertrophy of both articular facets.

Figure 6b

Figure 6b Facet hypertrophy. (a) Axial SPECT image shows focal radiotracer uptake in the posterior elements on the right side (arrow). On the basis of SPECT findings alone, determination of the specific location and cause of the uptake is difficult. (b) On an axial fused SPECT/CT image, the uptake is localized to the facet joint (arrow), which is hypertrophic with narrowing and irregularity. (c) Coronal fused SPECT/CT image better demonstrates the hypertrophy of the facet joint (arrow). Note the joint space narrowing and irregularity and the osseous hypertrophy of both articular facets.

Figure 6c

Figure 6c Facet hypertrophy. (a) Axial SPECT image shows focal radiotracer uptake in the posterior elements on the right side (arrow). On the basis of SPECT findings alone, determination of the specific location and cause of the uptake is difficult. (b) On an axial fused SPECT/CT image, the uptake is localized to the facet joint (arrow), which is hypertrophic with narrowing and irregularity. (c) Coronal fused SPECT/CT image better demonstrates the hypertrophy of the facet joint (arrow). Note the joint space narrowing and irregularity and the osseous hypertrophy of both articular facets.

An osteochondroma (osteocartilaginous exostosis) of the spine can mimic facet hypertrophy at both SPECT and CT, manifesting as radiotracer uptake localized to an osseous mass (Fig 7). Osteochondromas are benign bone tumors that, when occurring in the spine (1%–4% of cases), generally involve the transverse and spinous processes and are most commonly encountered in the cervical spine (29). The level of uptake at bone scintigraphy is variable, generally paralleling physeal uptake (30). Increased uptake in an osteochondroma can also be the result of acute or chronic trauma (Fig 7). CT classically depicts an osseous excrescence with cortical and medullary continuity with the adjacent bone. Within the confines of the posterior elements, however, this entity can be difficult to distinguish from the less organized osseous hypertrophy associated with facet degeneration. MR imaging and ultrasonography can be of diagnostic value by helping identify a cartilaginous cap, which is diagnostic for osteochondroma.

Figure 7a

Figure 7a Osteochondroma. (a) Axial fused SPECT/CT image shows osseous overgrowth near the facet joint with associated abnormal radiotracer uptake (arrow). (b) Coronal fused SPECT/CT image reveals that the radiotracer uptake corresponds to a large osseous projection from the inferior articular facet (arrows). The facet joint (arrowhead) appears normal without narrowing or sclerosis, and there is no osseous hypertrophy of the superior articular facet of the adjacent vertebra. (c) On a coronal CT image, the osseous projection (white arrows) demonstrates cortical and medullary continuity with the inferior articular facet, findings typical of an osteochondroma. A transverse low-attenuation area (black arrow) through the osseous projection indicates fracture, which likely accounts for the radiotracer uptake.

Figure 7b

Figure 7b Osteochondroma. (a) Axial fused SPECT/CT image shows osseous overgrowth near the facet joint with associated abnormal radiotracer uptake (arrow). (b) Coronal fused SPECT/CT image reveals that the radiotracer uptake corresponds to a large osseous projection from the inferior articular facet (arrows). The facet joint (arrowhead) appears normal without narrowing or sclerosis, and there is no osseous hypertrophy of the superior articular facet of the adjacent vertebra. (c) On a coronal CT image, the osseous projection (white arrows) demonstrates cortical and medullary continuity with the inferior articular facet, findings typical of an osteochondroma. A transverse low-attenuation area (black arrow) through the osseous projection indicates fracture, which likely accounts for the radiotracer uptake.

Figure 7c

Figure 7c Osteochondroma. (a) Axial fused SPECT/CT image shows osseous overgrowth near the facet joint with associated abnormal radiotracer uptake (arrow). (b) Coronal fused SPECT/CT image reveals that the radiotracer uptake corresponds to a large osseous projection from the inferior articular facet (arrows). The facet joint (arrowhead) appears normal without narrowing or sclerosis, and there is no osseous hypertrophy of the superior articular facet of the adjacent vertebra. (c) On a coronal CT image, the osseous projection (white arrows) demonstrates cortical and medullary continuity with the inferior articular facet, findings typical of an osteochondroma. A transverse low-attenuation area (black arrow) through the osseous projection indicates fracture, which likely accounts for the radiotracer uptake.

Endplate and Disk Abnormalities

The lumbar vertebral body of a child normally demonstrates relatively increased radiotracer uptake in the cortical bone of the vertebral endplates, with less uptake in the central less dense cancellous (trabecular) bone. Abnormalities of the vertebral endplate and, occasionally, of the intervertebral disk will manifest with disproportionately increased uptake at the endplate. This finding may be present at only one level or at multiple levels simultaneously and on planar images is often better seen on the anterior image. Common entities that result in endplate uptake at SPECT include endplate-apophyseal injuries, degenerative disk disease, and endplate compression fractures.

Endplate-Apophyseal Injuries.—In the developing spine, cartilaginous physeal plates are present along the cranial and caudal surfaces of the ossified vertebral body (Figs 1, 8) (11,12). In addition, there is a ring apophysis at the periphery of the developing endplate that receives fibers of the longitudinal spinal ligaments. When excessive compressive forces are applied to the pediatric spine, the nucleus pulposus of the intervertebral disk can herniate through the cartilaginous endplate. This herniation can (a) occur centrally, resulting in a Schmorl node; or (b) extend more peripherally and undercut the ring apophysis, resulting in a limbus vertebra (Fig 8). Both Schmorl nodes and limbus vertebrae can occur at either the superior or inferior endplate, but recent epidemiologic studies have shown Schmorl nodes to have a slight predominance of inferior endplate involvement (31).

Figure 8a

Figure 8a Ring apophyseal injuries. (a) Drawing illustrates the developing vertebra, with herniations of the nucleus pulposus (1) resulting in a limbus vertebra and Schmorl node. When the nucleus herniates anteroinferiorly (2) through the cartilaginous endplate (3), it undercuts the ring apophysis (4) and results in a limbus vertebra. When the nucleus herniates centrally (5) through the cartilaginous endplate, the result is a Schmorl node. (b) Sagittal CT image obtained in a patient with Schmorl nodes shows depressions in the superior endplates of three contiguous vertebral bodies (white arrowheads) with surrounding sclerosis. A similar depression is seen in the inferior endplate of the middle vertebral body (black arrowhead). These depressions are characteristic of Schmorl nodes. (c) Sagittal fused SPECT/CT image shows focal radiotracer uptake corresponding to the superior endplate depressions at multiple vertebral levels (arrows). (d) Sagittal SPECT image obtained in a patient with a limbus vertebra shows focal abnormal radiotracer uptake at the anterior aspect of a lumbar vertebra (arrow). (e) Coregistered CT image shows that the uptake corresponds to a depression in the anterior superior vertebral endplate with surrounding sclerosis (black arrow) and isolation of the ossified ring apophysis (white arrow).

Figure 8b

Figure 8b Ring apophyseal injuries. (a) Drawing illustrates the developing vertebra, with herniations of the nucleus pulposus (1) resulting in a limbus vertebra and Schmorl node. When the nucleus herniates anteroinferiorly (2) through the cartilaginous endplate (3), it undercuts the ring apophysis (4) and results in a limbus vertebra. When the nucleus herniates centrally (5) through the cartilaginous endplate, the result is a Schmorl node. (b) Sagittal CT image obtained in a patient with Schmorl nodes shows depressions in the superior endplates of three contiguous vertebral bodies (white arrowheads) with surrounding sclerosis. A similar depression is seen in the inferior endplate of the middle vertebral body (black arrowhead). These depressions are characteristic of Schmorl nodes. (c) Sagittal fused SPECT/CT image shows focal radiotracer uptake corresponding to the superior endplate depressions at multiple vertebral levels (arrows). (d) Sagittal SPECT image obtained in a patient with a limbus vertebra shows focal abnormal radiotracer uptake at the anterior aspect of a lumbar vertebra (arrow). (e) Coregistered CT image shows that the uptake corresponds to a depression in the anterior superior vertebral endplate with surrounding sclerosis (black arrow) and isolation of the ossified ring apophysis (white arrow).

Figure 8c

Figure 8c Ring apophyseal injuries. (a) Drawing illustrates the developing vertebra, with herniations of the nucleus pulposus (1) resulting in a limbus vertebra and Schmorl node. When the nucleus herniates anteroinferiorly (2) through the cartilaginous endplate (3), it undercuts the ring apophysis (4) and results in a limbus vertebra. When the nucleus herniates centrally (5) through the cartilaginous endplate, the result is a Schmorl node. (b) Sagittal CT image obtained in a patient with Schmorl nodes shows depressions in the superior endplates of three contiguous vertebral bodies (white arrowheads) with surrounding sclerosis. A similar depression is seen in the inferior endplate of the middle vertebral body (black arrowhead). These depressions are characteristic of Schmorl nodes. (c) Sagittal fused SPECT/CT image shows focal radiotracer uptake corresponding to the superior endplate depressions at multiple vertebral levels (arrows). (d) Sagittal SPECT image obtained in a patient with a limbus vertebra shows focal abnormal radiotracer uptake at the anterior aspect of a lumbar vertebra (arrow). (e) Coregistered CT image shows that the uptake corresponds to a depression in the anterior superior vertebral endplate with surrounding sclerosis (black arrow) and isolation of the ossified ring apophysis (white arrow).

Figure 8d

Figure 8d Ring apophyseal injuries. (a) Drawing illustrates the developing vertebra, with herniations of the nucleus pulposus (1) resulting in a limbus vertebra and Schmorl node. When the nucleus herniates anteroinferiorly (2) through the cartilaginous endplate (3), it undercuts the ring apophysis (4) and results in a limbus vertebra. When the nucleus herniates centrally (5) through the cartilaginous endplate, the result is a Schmorl node. (b) Sagittal CT image obtained in a patient with Schmorl nodes shows depressions in the superior endplates of three contiguous vertebral bodies (white arrowheads) with surrounding sclerosis. A similar depression is seen in the inferior endplate of the middle vertebral body (black arrowhead). These depressions are characteristic of Schmorl nodes. (c) Sagittal fused SPECT/CT image shows focal radiotracer uptake corresponding to the superior endplate depressions at multiple vertebral levels (arrows). (d) Sagittal SPECT image obtained in a patient with a limbus vertebra shows focal abnormal radiotracer uptake at the anterior aspect of a lumbar vertebra (arrow). (e) Coregistered CT image shows that the uptake corresponds to a depression in the anterior superior vertebral endplate with surrounding sclerosis (black arrow) and isolation of the ossified ring apophysis (white arrow).

Figure 8e

Figure 8e Ring apophyseal injuries. (a) Drawing illustrates the developing vertebra, with herniations of the nucleus pulposus (1) resulting in a limbus vertebra and Schmorl node. When the nucleus herniates anteroinferiorly (2) through the cartilaginous endplate (3), it undercuts the ring apophysis (4) and results in a limbus vertebra. When the nucleus herniates centrally (5) through the cartilaginous endplate, the result is a Schmorl node. (b) Sagittal CT image obtained in a patient with Schmorl nodes shows depressions in the superior endplates of three contiguous vertebral bodies (white arrowheads) with surrounding sclerosis. A similar depression is seen in the inferior endplate of the middle vertebral body (black arrowhead). These depressions are characteristic of Schmorl nodes. (c) Sagittal fused SPECT/CT image shows focal radiotracer uptake corresponding to the superior endplate depressions at multiple vertebral levels (arrows). (d) Sagittal SPECT image obtained in a patient with a limbus vertebra shows focal abnormal radiotracer uptake at the anterior aspect of a lumbar vertebra (arrow). (e) Coregistered CT image shows that the uptake corresponds to a depression in the anterior superior vertebral endplate with surrounding sclerosis (black arrow) and isolation of the ossified ring apophysis (white arrow).

Once established, Schmorl nodes and limbus vertebrae will persist into adulthood. Radiotracer uptake will generally not be present, particularly in chronic lesions, and the clinical significance of uptake within these lesions is not known. However, both Schmorl nodes and limbus vertebrae can be a source of back pain due to associated inflammation (32).

At planar bone scintigraphy, Schmorl nodes and limbus vertebrae can be difficult to distinguish, appearing as linear or, rarely, focal uptake at the vertebral endplate. At SPECT/CT, Schmorl nodes appear as foci of uptake that are relatively centrally located within the vertebral endplate (Fig 8) (33). This uptake is associated with a focal depression with well-defined margins and often peripheral sclerosis (33). Limbus vertebrae have a distinctive appearance at SPECT/CT, with radiotracer uptake located at the anterior corner of the vertebral body. This uptake is associated with a triangular bone fragment (the ring apophysis) separated from the remainder of the vertebral body by an intervening lucency (Fig 8) (34). The isolated bone fragment is variably corticated, and sclerosis may be present within the affected vertebral body depending on the chronicity of injury.

Degenerative Disk Disease.Adolescents and young adults may develop degenerative spinal changes similar to those encountered in adults. Degenerative endplate changes are caused by segmental spinal dysfunction, which results in increased motion at the discovertebral and facet joints (28) and ultimately leads to disk degeneration and reactive bone changes in the adjacent vertebral endplates. There is some indication that impactful activities may accelerate the degenerative process (28).

Findings at planar bone scintigraphy can be subtle, with focal uptake at the level of an intervertebral disk, often anteriorly or laterally. SPECT/CT allows better localization of the uptake to facing vertebral endplates, with a narrowed intervertebral disk space and formation of osteophytes (Fig 9). Depending on the chronicity of the degenerative process, sclerosis may be present along the involved endplate.

Figure 9a

Figure 9a Degenerative disk disease. (a) Coronal SPECT image shows lateral radiotracer uptake at the level of the lumbosacral junction (arrow). (b) Coronal fused SPECT/CT image reveals that the region of uptake (arrow) corresponds to narrowing of the lateral aspect of the L5-S1 intervertebral disk space with associated endplate sclerosis and peripheral osteophytes.

Figure 9b

Figure 9b Degenerative disk disease. (a) Coronal SPECT image shows lateral radiotracer uptake at the level of the lumbosacral junction (arrow). (b) Coronal fused SPECT/CT image reveals that the region of uptake (arrow) corresponds to narrowing of the lateral aspect of the L5-S1 intervertebral disk space with associated endplate sclerosis and peripheral osteophytes.

Endplate Compression Fractures.—Vertebral endplate compression fractures are typically the result of axial loading. Fractures can occur in healthy children in the setting of trauma or can occur with relatively minor trauma in the setting of osteoporosis. Osteoporotic compression fractures disproportionately involve the superior endplate (35).

The pattern of radiotracer uptake associated with vertebral compression fractures depends on the chronicity of the injury (36). Within the first 4 weeks, radiotracer uptake will appear somewhat diffuse and surround the fracture. Uptake condenses and becomes more linear over the next 2 months, gradually fading over approximately 6 months (36).

In general, an endplate fracture seen at planar bone scintigraphy manifests with a “wafer-like” distribution of activity along the vertebral endplate (37). If the fracture is also seen on correlative images (eg, radiographs), SPECT/CT may not be needed. In the absence of correlative images, SPECT/CT can provide a definitive diagnosis and exclude other causes of endplate uptake. Findings of vertebral endplate fractures at SPECT/CT include linear radiotracer uptake along the endplate with corresponding height loss at the involved vertebral level (Fig 10). Sclerosis may be apparent along the involved endplate, a finding that indicates compacted bone or healing response.

Figure 10a

Figure 10a Vertebral endplate compression fractures. (a) Sagittal CT image shows diffuse osteoporosis with depression of the superior endplates of a lower thoracic vertebra and an upper lumbar vertebra (arrows). (b) Sagittal fused SPECT/CT image shows abnormal radiotracer uptake (arrows) corresponding to the depressed superior endplates, findings compatible with vertebral compression fractures.

Figure 10b

Figure 10b Vertebral endplate compression fractures. (a) Sagittal CT image shows diffuse osteoporosis with depression of the superior endplates of a lower thoracic vertebra and an upper lumbar vertebra (arrows). (b) Sagittal fused SPECT/CT image shows abnormal radiotracer uptake (arrows) corresponding to the depressed superior endplates, findings compatible with vertebral compression fractures.

Transverse Process Abnormalities

The vertebral transverse processes typically demonstrate symmetric lower-intensity radiotracer uptake than do the vertebral endplates. Uptake in the transverse processes should be similar at all vertebral levels in the lumbar spine. Entities that cause asymmetrically increased transverse process uptake at SPECT include a persistent transverse process ossification center, transverse process fractures, and transverse process lesions (eg, osteoid osteoma).

Persistent Transverse Process Ossification Center.—There is a normal physis within the transverse process that, like other growth centers in the skeleton, can demonstrate increased uptake at skeletal scintigraphy. The secondary ossification center related to this physis usually starts to ossify at puberty, with fusion of the physis by 25 years of age (37). Delayed or failed fusion of the physis can appear as asymmetrically increased radiotracer uptake. At correlative imaging, the unfused secondary ossification center is located at the tip of the transverse process and appears smooth and curved, paralleling the margin of the adjacent transverse process. No associated callus formation or periosteal reaction should be present (37).

Transverse Process Fractures.—Transverse process fractures are considered minor injuries that do not result in spinal instability (38). However, major trauma is generally required to produce a transverse process fracture; thus, other fractures and abdominal or retroperitoneal injuries are commonly present and must be excluded in the acute setting (39).

Transverse process fractures can occur at any level of the lumbar spine but are most commonly seen at the L3 level (39). These fractures can be subtle on conventional radiographs and may not be identified (39). SPECT demonstrates focal increased uptake at the fracture site in association with new bone related to fracture healing (Fig 11). At CT, fracture lines are usually straight and tend to occur near the base of the transverse process. In contrast, the persistent transverse process epiphysis has a curved appearance and is located at the tip of the transverse process. In the subacute or chronic setting, callus formation and periosteal reaction may be seen in association with the fracture.

Figure 11a

Figure 11a Transverse process fracture. (a) Axial fused SPECT/CT image shows focal uptake (arrow) at the level of the transverse process, which is irregular in appearance. (b) Axial CT image shows irregularity of the transverse process with a subtle fracture lucency (white arrow). Callus formation associated with the fracture (black arrow) helps distinguish the fracture from a persistent ossification center.

Figure 11b

Figure 11b Transverse process fracture. (a) Axial fused SPECT/CT image shows focal uptake (arrow) at the level of the transverse process, which is irregular in appearance. (b) Axial CT image shows irregularity of the transverse process with a subtle fracture lucency (white arrow). Callus formation associated with the fracture (black arrow) helps distinguish the fracture from a persistent ossification center.

Sacroiliac Abnormalities

Normal radiotracer uptake related to the sacroiliac joints is generally symmetric on planar and SPECT bone scintigraphic images and often has an intensity similar to that of unfused physes in skeletally immature patients. In skeletally mature patients, normal radiotracer uptake related to the sacroiliac joints is often greater than at any other site in the skeleton. Minor obliquity in positioning on planar images can create apparent asymmetry in radiotracer uptake and thus should be confirmed on both anterior and posterior projections. SPECT is less susceptible to this position-related issue.

Radiotracer uptake within the body of the sacrum should be at a level similar to that of other large bones (eg, iliac wings, femurs). When sacroiliac disease is suspected as a cause of the patient’s symptoms, an empty bladder is of paramount importance in preventing excreted activity from obscuring sacroiliac abnormalities. Common entities that result in increased or asymmetric sacroiliac uptake at SPECT include transitional vertebra (Bertolotti syndrome), sacral stress fractures, and sacroiliac joint syndrome.

Transitional Vertebrae (Bertolotti Syndrome).—Transitional lumbosacral vertebrae (including partially sacralized L5 vertebrae and partially lumbarized S1 vertebrae) are present in 3%–21% of the population (40). Transitional vertebrae are characterized by an enlarged, usually unilateral transverse process that articulates or fuses with the sacrum with a vestigial intervening disk space. Abnormal biomechanics at the transitional level can result in stress at the vertebrosacral articulation and the intervertebral disk immediately above it. Low back pain associated with a transitional vertebra is referred to as Bertolotti syndrome.

Transitional vertebrae are visible on radiographs, but changes from associated stress are not usually apparent. At SPECT, abnormal radiotracer uptake is present at the articulation between the enlarged transverse process and the sacrum (Fig 12). CT shows the transitional anatomy and can demonstrate findings of stress identifiable as sclerosis, joint surface irregularity, or subchondral cysts at the articulation (40).

Figure 12a

Figure 12a Transitional vertebra (Bertolotti syndrome). (a) Coronal SPECT image shows asymmetrically increased radiotracer uptake at the level of the right cranial sacroiliac joint (arrow). (b) Coronal fused SPECT/CT image reveals that the uptake corresponds to an abnormal articulation (arrow) between an enlarged right L5 transverse process (T) and the right aspect of the sacrum. The CT component of the image shows the articulation to be irregular and sclerotic.

Figure 12b

Figure 12b Transitional vertebra (Bertolotti syndrome). (a) Coronal SPECT image shows asymmetrically increased radiotracer uptake at the level of the right cranial sacroiliac joint (arrow). (b) Coronal fused SPECT/CT image reveals that the uptake corresponds to an abnormal articulation (arrow) between an enlarged right L5 transverse process (T) and the right aspect of the sacrum. The CT component of the image shows the articulation to be irregular and sclerotic.

Sacral Stress Fractures.—Stress fractures can be either fatigue fractures (resulting from abnormal or repetitive stress on normal bone) or insufficiency fractures (resulting from normal stress on abnormal bone). Sacral stress fractures are uncommon in young healthy patients. When these fractures are present, they are generally fatigue fractures, often occurring in athletes. Insufficiency fractures can be seen in chronically ill patients.

In general, sacral stress fractures are radiographically occult. SPECT demonstrates asymmetric, often linear increased uptake at the fracture site (Fig 13). Stress fractures may also be occult at CT but may appear as linear sclerosis along the fracture line (41,42).

Figure 13a

Figure 13a Sacral stress fracture. (a) Coronal fused SPECT/CT image shows linear abnormal radiotracer uptake at the level of the left sacral ala (arrow). (b) Coronal CT image shows a linear band of sclerosis (arrow) at the location of the abnormal radiotracer uptake.

Figure 13b

Figure 13b Sacral stress fracture. (a) Coronal fused SPECT/CT image shows linear abnormal radiotracer uptake at the level of the left sacral ala (arrow). (b) Coronal CT image shows a linear band of sclerosis (arrow) at the location of the abnormal radiotracer uptake.

Sacroiliac Joint Syndrome.—The sacroiliac joint is a large joint with relatively limited mobility. Athletic activities, overload injuries, and direct trauma can produce abnormal motion or stress at the sacroiliac joint, resulting in pain (43). Skeletal scintigraphy has a high specificity but low sensitivity for sacroiliac joint syndrome; therefore, normal findings at bone scintigraphy do not exclude the diagnosis (43,44). Scintigraphic findings that are suggestive of sacroiliac joint syndrome include focal abnormally increased radiotracer uptake in the sacroiliac joint (Fig 14). Radiographs and coregistered CT images may show sacroiliac joint irregularity, sclerosis, and subchondral cysts, but are usually normal until degenerative changes become more advanced (43,44).

Figure 14a

Figure 14a Sacroiliac joint syndrome. (a) Coronal SPECT image shows asymmetric radiotracer uptake in the right sacroiliac joint (arrow). Unlike planar images, SPECT images are not subject to positional asymmetry. (b) Coronal fused SPECT/CT image helps confirm that the abnormal radiotracer uptake is related to the right sacroiliac joint (arrow). The margins of the joint are irregular, and there are adjacent subchondral cystic changes.

Figure 14b

Figure 14b Sacroiliac joint syndrome. (a) Coronal SPECT image shows asymmetric radiotracer uptake in the right sacroiliac joint (arrow). Unlike planar images, SPECT images are not subject to positional asymmetry. (b) Coronal fused SPECT/CT image helps confirm that the abnormal radiotracer uptake is related to the right sacroiliac joint (arrow). The margins of the joint are irregular, and there are adjacent subchondral cystic changes.

Other Abnormalities

In addition to the abnormalities described earlier, which are the most commonly encountered causes of pediatric and adolescent low back pain, other less common entities can be a source of pain and are depicted at SPECT/CT. Patients with these lesions may have unique clinical presentations. Specifically,

focal short-segment scoliosis or painful scoliosis should raise suspicion for an underlying lesion.

Diskitis and Osteomyelitis.—In the pediatric population, diskitis may be a primary infectious or inflammatory process or may be related to extension of infection from vertebral endplate osteomyelitis. In either case, scintigraphy reveals radiotracer uptake along facing endplates (45). SPECT/CT can add value in such cases by (a) providing improved localization of the abnormal uptake to the vertebral endplates and (b) demonstrating associated CT changes of disk height loss, endplate irregularity, and subchondral sclerosis (45).

Chronic Recurrent Multifocal Osteomyelitis.—Chronic recurrent multifocal osteomyelitis is a poorly understood process that primarily occurs in children and adolescents and has imaging features similar to those of osteomyelitis (46). Because it is a multicentric process, chronic recurrent multifocal osteomyelitis can involve the spine. Scintigraphy typically shows radiotracer uptake at sites of involvement on the blood pool and delayed phase images (Fig 15). Whole-body imaging helps confirm multifocality and identify asymptomatic disease sites (46). SPECT/CT allows better definition of the extent of local involvement and the associated osseous changes. At CT, lesions typically appear initially as areas of permeation or lysis with development of sclerosis (occasionally laminated) over time (46).

Figure 15a

Figure 15a Chronic recurrent multifocal osteomyelitis. (a) Axial SPECT image shows ill-defined radiotracer uptake at the level of the right transverse process and vertebral pedicle (arrow). (b) Coregistered axial CT image shows the pedicle with a mixed lytic and sclerotic appearance (arrow), with surrounding sclerosis reflecting reactive bone formation.

Figure 15b

Figure 15b Chronic recurrent multifocal osteomyelitis. (a) Axial SPECT image shows ill-defined radiotracer uptake at the level of the right transverse process and vertebral pedicle (arrow). (b) Coregistered axial CT image shows the pedicle with a mixed lytic and sclerotic appearance (arrow), with surrounding sclerosis reflecting reactive bone formation.

Benign Tumors.—Osteoid osteomas and osteoblastomas are benign bone tumors that are related to abnormal formation of osteoid or woven bone (47). Osteoid osteomas are more common than osteoblastomas but rarely occur in the spine, whereas up to one-third of osteoblastomas occur in the spine (47). Both lesions have a predilection for the posterior elements and produce back pain, albeit with different symptoms (47,48). Osteoblastomas have the potential for either slow progressive or rapid growth, resulting in progressive symptoms or, potentially, neural impingement (47).

The imaging appearances of osteoid osteoma and osteoblastoma differ, which reflects differences in structure. Osteoid osteomas are typically small (<1.5 cm) (47). SPECT of osteoid osteomas reveals focal radiotracer uptake in the nidus with a surrounding halo of less intense uptake (47). If multiphase imaging is performed, increased activity will be present on both the blood flow and blood pool images. At CT, osteoid osteomas typically appear radiolucent with surrounding sclerosis (48). The nidus may calcify centrally to varying degrees. Osteoblastomas are larger (average, >4 cm) and less well organized. SPECT of osteoblastomas demonstrates a larger area of radiotracer uptake within the lesion (49). At CT, osteoblastomas are expansile with central mixed sclerosis and lysis and only a thin rim of reactive sclerosis (47,49).

Aneurysmal bone cysts are benign but locally aggressive lesions (50) that occur in the spine both primarily or as secondary lesions associated with other bone tumors. Scintigraphic findings have been described as uniformly positive in aneurysmal bone cysts, demonstrating either peripheral uptake with central photopenia or diffuse uptake throughout the lesion (51). SPECT/CT provides improved characterization of the lesion, its location, and its local effects. At CT, aneurysmal bone cysts appear as expansile lytic lesions. Internal blood products may be hyperattenuating, and septations may be calcified.

Malignant Tumors.—Malignant tumors in the spine are more commonly metastatic than primary in origin. Metastatic lesions are typically multifocal and, at scintigraphy, may be either lytic with relative photopenia or blastic with associated radiotracer uptake. SPECT/CT generally provides a better sense of the extent of disease involvement and improves specificity and positive predictive value (52).

Primary malignant tumors of bone (osteosarcoma, Ewing sarcoma) do not commonly occur in the spine. By virtue of matrix production and reactive bone changes, these lesions generally show radiotracer uptake, although the uptake is known to extend beyond the margins of the tumor (53,54). In these cases, SPECT/CT allows improved definition of tumor extent, primarily on the basis of CT findings. The CT appearance of these tumors is variable and depends on tumor type, aggressiveness, and chronicity.

Conclusion

SPECT/CT plays an important role in assessment of low back pain in children and young adults. Spondylolysis is optimally identified and characterized with SPECT/CT, but other entities with characteristic imaging features are also apparent. Knowledge of these entities and their imaging features can maximize the diagnostic utility of SPECT/CT in children and young adults with low back pain.

Recipient of a Certificate of Merit award for an education exhibit at the 2013 RSNA Annual Meeting.

For this journal-based SA-CME activity, the author C.T.M. has provided disclosures (see “Disclosures of Conflicts of Interest”); all other authors, the editor, and the reviewers have disclosed no relevant relationships.

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Article History

Received: Mar 20 2014
Revision requested: July 24 2014
Revision received: Aug 13 2014
Accepted: Aug 14 2014
Published online: May 13 2015
Published in print: May 2015