PMC5394818_01
(a) Coronal short tau inversion recovery showing right piriformis involvement along with subtle right sacroiliac joint involvement appearing hyperintense as compared to left side. Hyperintense signal is also seen in the right sacroiliac joint space. (b) Axial T1-weight postcontrast image with fat suppression showing right-sided piriformis abscess. (c) Coronal short tau inversion recovery image showing increased hyperintensity of the right sacroiliac joint. Persistent right-sided piriformis involvement is seen. (d) Axial T1-weight postcontrast image with fat suppression showing persistent piriformis abscess with marrow enhancement of visualized right iliac bone
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F1
Pelvis magnetic resonance imaging (MRI) showed peripherally enhancing fluid collection within the right piriformis muscle measuring 3.2 cm x 3.0 cm size, representing an abscess with subtle bony involvement of the right sacroiliac joint [Figure 1a and b]. Repeat pelvis MRI after 2 weeks showed no interval change in size of abscess but marrow edema involving iliac and sacral portions of the right sacroiliac joint was more pronounced, with postcontrast enhancement [Figure 1c and d].
(a) Coronal short tau inversion recovery image showing a significant regression of altered signal intensity in right-sided piriformis muscle. Mild to moderate regression is seen in right sacral and iliac marrow involvement as compared to previous magnetic resonance imaging scan. (b) Axial T1-weight postcontrast image with fat suppression showing mild to moderate regression in marrow enhancement of right sacroiliac bone along with persistent fluid signal in the right sacroiliac joint space. (c) Axial T1-weight postcontrast image with fat suppression showing near complete regression in the right piriformis muscle involvement
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F2
Pelvis MRI after 4 weeks showed the signs of resolution with a significant reduction in abscess size and reduced marrow edema of the involved sacroiliac joint [Figure 2a-c].