Acute hematogenous pelvic osteomyelitis: appropriate timing for MRI.

Background The precise time of appearance of bone marrow edema in acute hematogenous pelvic osteomyelitis (AHPO) is unknown. The purpose of present research is to clarify the time of appearance of bone marrow edema on MRI in AHPO. Our hypothesis was that onset is slower than in long-bone osteomyelitis. Methods We selected twelve 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 statistically evaluated and compared to lower extremity osteomyelitis. Results BM/M ratio of T2WI increased over time after 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 seven days or less had elapsed from the primary symptom and 6.4 when more than seven days had elapsed . BM/M ratios in the sacroiliac joint group were lower than in the non-sacroiliac joint group. Conclusion Unlike long-bone osteomyelitis, it took one 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 seven days or later.