MR imaging of septic sacroiliitis.

Septic sacroiliitis is difficult to diagnose, causing delayed treatment and increased morbidity. The traditional imaging techniques for diagnosis have been CT and nuclear medicine. Our purpose was to determine the ability of MR imaging to detect septic sacroiliitis, to evaluate the features of septic sacroiliitis with MR, and to compare the relative detection rate of MR, CT, and nuclear medicine. All patients with a discharge diagnosis of septic sacroiliitis who were evaluated by MR imaging of the pelvis were retrospectively evaluated. Five patients were collected with six septic sacroiliac joints, which were also evaluated with CT, 99mTc-methylene diphosphonate bone scans, and 67Ga-citrate scans. Abnormalities consistent with sacroiliitis were seen in all sacroiliac joints both prospectively (impression from the initial report) and retrospectively on MR. In addition to the nonspecific MR findings of inflammation and/or fluid in the sacroiliac joint space, bone marrow of the sacrum and/or ilium, and iliopsoas muscle, fluid/inflammation was uniquely identified tracking posterior to the iliopsoas muscle in each of these patients with septic sacroiliitis. Even in retrospect, a definite diagnosis of sacroiliitis could be made in only five of six joints by 67Ga-citrate scans, three of six joints by CT scans, and one of six joints by 99mTc-methylene diphosphonate bone scans. These results suggest MR imaging may be a sensitive modality in the early diagnosis of septic sacroiliitis.