Purpose or Case Report: MRI is the gold standard for the diagnosis of acute septic sacroiliitis. However, appropriate timing of the procedure has not been addressed in the literature. MRI is highly sensitive for detecting long tubular bone osteomyelitis as early as 24-48 hours of symptom onset. The hypothesis of this study is that the onset of bone marrow edema of acute hematogenous osteomyelitis (AHO) of pelvis might be delayed as compared with long bone. The purpose of this study is to evaluate the optimum timing of MRI examination from the onset of disease.
Methods & Materials: Nine patients (8 – 16 years, median 14 years, four male and five female) with AHO of pelvis were selected from our radiology database between 2004 and 2016. We compared signal intensity of sacral/iliac bone marrow (BM) and gluteal muscle (M) (BM/M ratio) on the fat suppressed T2Weighted image (T2WI) and T1weighted image (T1WI) respectively. We also calculated the days elapsed from onset to MRI examination. Regression methods were used to evaluate the relationship between BM/M ratio and days elapsed. We also evaluated other five findings (subperiosteal infiltration, muscle infiltration, laba cleft phenomenon, erosive destruction, joint space enhance).
Results: BM/M ratio and elapsed days in patients with AHO of pelvis showed a statistically correlation (R²= 0.57). In five cases where MRI was performed twice, the BM/M ratio increased without exception. This tendency seemed to be more prominent in those who underwent initial MRI before day 7. The average BM/M ratio within the first 7 days was 3.6 ± 0.7 (mean ± SD), but increased markedly in those who underwent MRI more than one week after onset (6.5 ± 0.7 [mean ± SD]). There was a statistically significant difference in the BM/M ratio before and after day 7 (p ≤ 0.001). The BM/M ratio on the T1WI showed an inverse relationship to that on the T2WI with a statistically correlation (R²= 0.59) on the T2WI as well. All nine cases showed bone marrow edema, otherwise, un-even incidence of other five findings were identified.
Conclusions: The present research suggests that until day 7 there might be false negative or subtle finding of bone marrow edema in AHO. A follow-up MRI should be performed after seven days where the diagnosis is still in doubt.