Acute hematogenous pelvic osteomyelitis: appropriate timing for magnetic resonance imaging.

BACKGROUND:The precise time of appearance of bone marrow edema in acute hematogenous pelvic osteomyelitis (AHPO) is unknown. The purpose of the present research is to clarify the time of appearance of bone marrow edema on magnetic resonance imaging (MRI) in AHPO. Our hypothesis was that onset is slower than in long-bone osteomyelitis. METHODS:We selected 12 patients (mean, 11.8 years) with MRI findings and clinical diagnosis of AHPO. The signal ratios of bone marrow (BM) and gluteus maximus muscle (M, BM/M ratio) in fat-suppressed T2- and T1-weighted images (T2WI, T1WI) were calculated to evaluate changes in bone-marrow signals. The correlation between BM/M ratios and days from onset was evaluated statistically and compared with lower extremity osteomyelitis. RESULTS:Bone marrow/M ratio of T2WI increased over time after the onset of the primary symptom in all patients and showed a statistically positive correlation (r = 0.36). In seven patients in whom an MRI scan was conducted twice, all showed higher values for the second MRI, and changes were more pronounced over time. The mean BM/M ratio of T2WI was 4.1 when 7 days or less had elapsed from the primary symptom, and 6.4 when more than 7 days had elapsed. The BM/M ratios in the sacroiliac joint group were lower than in the non-sacroiliac joint group. CONCLUSIONS:Unlike long-bone osteomyelitis, it took 1 week before findings for AHPO became fully evident. A definitive diagnosis can be made in patients with suspected sacroiliitis by performing a further MRI scan at 7 days or later.