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327 MR Imaging of the Pars Interarticularis David W. Johnson1 Gregory N. Farnum2 Richard E. Latchaw S. Michael Erba3 This article appears in the November/Decem- ber 1988 issue of AJNR and the February 1989 issue of AJR. Aeceived October 21, 1987; accepted after re- vision April 5, 1988. Presented at the annual of the American of Neuroradiology, New York, May 1987. 1 All authors: Department of Radiology, Univer- sity Health Center of Pittsburgh, One Children’s Place, Pittsburgh, PA 15213. Address re- quests to D. W. Johnson. 2Prosent address: Department of Radiology, Fountain Valley Aegional Medical Center, 17100 Euclid at Warner, Fountain Valley, CA 92708. 3Present address: Department of Radiology, Salem Hospital, Salem, OR 97309. AJR 152:327-332, February 1989 0361 -803x/89/1522-0327 © American Roentgen Aay MR imaging of the lumbar spine has become a useful for the noninvasive evaluation of low back pain. However, bone abnormalities are more difficult to than soft-tissue lesions, such as herniated disk. We reviewed MR images of the lumbar spine in adults with spondylolisthesis. These were correlated with CT scans and plain films in all cases. From the CT scans and plain films we found that seven patients had spondylolysis and that seven had other causes for their spondylolisthesis. It was our opinion that the MR images suggested an abnormality of the pars interarticularis in all seven of the cases confirmed to have spondylolysis and in six of the seven patients that did not spondylolysis. We also studied four cadaver lumbar spines, obtained as blocks of tissue, and scanned in the coronal, sagittal, and axial planes with MR and in the sagfttal and axial planes with CT. The tissue blocks were then sectioned in the sagittal plane. Spondylolysis is suggested on sagittal MR images when there is an inabilfty to resolve the marrow signal in the pars as uninterrupted from the superior to the inferior This is caused by a dark signal on all sequences in the pars resulting from marginal sclerosis at the sIte of the break. If there is also a gap at the site of the break then there will also be an increased signal in the gap resulting from the presence of soft tissue. We found four sftuations in which the pars can simulate spondylolysis on sagittal MR images: (1) sclerosis of the neck of the pars; (2) partial volume imaging of the degen- erative spur of the superior slightiy lateral the pars; (3) partial and (4) osteoblastic replacement of the marrow of the pars. MR imaging is well-suited to the evaluation of disk disease [1 -5]. As a single technique, however, it may not be well-suited to the evaluation of all causes of low back pain, because bone abnormalities are more difficult to The study of bone architecture and abnormalities has remained in the realm of high- resolution CT. In our institution most lumbar spine MR studies requested for patients with low back pain to rule out herniated disk. Rarely do CT and plain films accompany the patient; for some, MR is the initial study. Therefore, are frequently made without correlative studies. Despite the fact that bone abnor- malities cannot be evaluated as well by MR as by CT or plain films, we believe there is information that can be gleaned from the routine MR that can lead to the correct impression or at least raise the level suspicion sufficiently to suggest the need for CT or plain-film corroboration. This report describes the MR, CT, and plain-film findings of the pars interarticularis in 14 patients with spondylolisthesis with MR, CT, and anatomic studies of the pars in four randomly selected cadaver specimens. Materials and Cadaver Study We studied four randomly selected fixed cadaver lumbar spines. Each spine was obtained as a block of tissue and scanned with MR and CT. The tissue specimens were then sectioned