Normal Variants

The normal tracer uptake in lumbar vertebrae 4 and 5 is greater than in the other lumbar vertebrae because of a difference in size. The sacral promontory often shows prominent uptake on the coronal views (2) (Fig. 1).

STRESS FRACTURES, SPONDYLOLYSIS, AND SPONDYLOLISTHESIS

Spondylolysis is a defect in the pars interarticularis that may or may not be associated with spondylolysthesis. There is a familial predisposition for spondylolysis (3) and a strong association with repetitive mechanical load-

From: Interventional Radiology of the Spine

Edited by J. Kevin McGraw © Humana Press Inc., Totowa, NJ.

Spect Image Pars Stress
Fig. 1. Normal bone SPECT. The sacral promontory is located between the right (arrow) and left sacroiliac joints and is prominently seen on the coronal images but shows normal intensity on the sagittal views (middle row).

ing of the spine (3-6). Cadaveric studies show that fatigue fractures of the pars interarticularis occur with levels of repetitive force and deformation significantly below those required to produce failure in a single loading cycle (4,5,7). Furthermore, alternating flexion and extension movements cause large stress reversals on the pars that make it particularly vulnerable to spondylolysis (6).

Stress injuries of the pars interarticularis are a common cause of low back pain in athletes. Dancers, gymnasts, weight lifters, and basketball players are involved in vigorous and repetitious activities that concentrate stress over a small area of the pars interarticularis. The initial response to repeated stress injury is a stress injury that may progress to spondylolysis and spondylolytic spondylolisthesis (3).

Bone scintigraphy is valuable in the evaluation of stress injuries of the pars interarticularis. SPECT bone scintigraphy is more sensitive than either planar bone imaging or radiography in detecting pars injuries (8-10). Bellah et al. studied 162 young patients with low back pain referred for bone scintigraphy by orthopedic surgeons (8). SPECT imaging was abnormal in 71 patients. Planar scintigraphy was abnormal in only 32 of these patients. Furthermore, 16 of 56 patients with normal radiographs had abnormal SPECT studies. The authors postulated that this may be due to microfractures of the pars interarticularis that were too subtle to be detected radiographically. Bodner et al. compared SPECT imaging with planar scintigraphy and radiographic evaluation in 15 patients with low back pain (9). All patients were followed until resolution of symptoms. There were five patients in whom SPECT imaging was the only modality to detect a lesion. In all five, the posterior elements showed abnormally increased uptake which was thought to represent stress injuries of the pars interarticularis.

Quantitative techniques have also been used in assessing stress injuries of the pars interarticularis. Anderson et al. studied 34 patients with quantitative SPECT bone scanning before and after treatment with either bracing or activity restrictions (10). The SPECT studies were reported as the ratio of the activity in the affected vertebra over the activity in the superior, contiguous unaffected vertebra. The SPECT ratio before and after treatment was compared to the symptomatic response to treatment and proved to be a reliable indicator of the patient's progress. Patients with high initial SPECT ratios treated promptly with brace immobilization had complete symptom relief and greater improvement in the SPECT ratio than those treated with activity restrictions before bracing who were more likely to have persistent symptoms and more modest improvement on SPECT.

Spondylolysis is seen in 3-10% of adults and may or may not be associated with low back pain (11). Furthermore, adults with either spondylolysis or spondylolisthesis may have low back pain from unrelated causes. Collier et al. (11) studied the relationship between SPECT imaging and low back pain in 19 adults with radiographic evidence of either spondylolysis or spondylolisthesis.

SPECT imaging was positive in 1 of 6 asymptomatic patients and in 11 of 13 patients with low back pain. The authors hypothesized that the metabolic status of the radiographic abnormality may identify the lesion as the cause of the pain.

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