Computed tomography (CT) and conventional radiography of the sacroiliac joint were compared in 43 patients. CT appeared to be far more sensitive and equally specific in the recognition of sacroiliitis. In a number of patients with sacroiliitis diagnosed by both techniques, CT demonstrated abnormalities that were not demonstrated by conventional radiographs. Of those patients with clinical evidence of sacroiliitis and HLA-B27 positivity, 50% had negative or equivocal radiographs compared to 19% who had negative computed tomographic images for sacroiliitis.Radiographic demonstration of sacroiliitis is essential for the early diagnosis of ankylosing spondylitis (1,2) and related spondylarthropathies. Conventional radiographic methods are relatively insensitive because of the curved, oblique orientation of the sacroiliac joint and because of overlying bony and soft tissue structures. Recently, scintigraphic methods were introduced for early demonstration of sacroiliitis, .and the results were encouraging (3,4). Unfortunately, subsequent studies have shown that this technique is not specific for sacroiliitis (5-9).We have investigated the value of high-resolution computed tomography (CT) for the detection of sacroiliitis. This technique should have the same or better specificity as conventional radiography because identical morphologic criteria are used for analysis and images are more distinct. Moreover, CT should be more sensitive than conventional radiography because the confusing effects of overlying soft tissue or bony structures may be minimized. A computed radiograph (Scout view), performed at the time of CT examination, demonstrates the lumbosacral junction in lateral projection. The correct gantry level (black line) and gantry angle (white line) necessary to produce transaxial images of the sacroiliac joint are obtained from this preliminary study, using a computer program. Consecutive CT images are taken in the plane Of the sacrum.
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