Abnormal uptake in the pars interarticularis is usually due to a stress injury or spondylolysis. Facet lesions are usually arthritic although fractures and osteoid osteomas may be present. Lesions in the vertebral arch must be accurately localized to differentiate between these disorders. SPECT is particularly useful in lesion location because multiple projections can be reconstructed (1). It is difficult to distinguish a pars injury from facet arthritis in the coronal and transverse reconstructions. However, these lesions can be distinguished in sagittal projection constructed slightly off midline toward the affected side (Fig. 2). Sagittal imaging shows the relationship between the lesion to the disc space and the posterior column of the vertebral body (1). The apophyseal joints are on the same horizontal plane as the disc space whereas the pars
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