MRI in a healthy young person, semi-coronal T1, T1FS and STIR image in addition to two semi-axial STIR images at the upper and lower part of the SIJ, respectively. The coronal slices located in the middle of the cartilaginous joint compartment visualize a normal joint space and subchondral bone without subchondral edema or fat deposition. The axial slices display both joint compartments with the cartilaginous joint portion located anteriorly (marked with # at the sacral side) and the ligamentous portion posteriorly (marked with asterisks at the iliac side of the joints). On these standard sequences the normal capsular structures may not be seen. Only the upper capsule on the right side is visible (arrow). This is partly due to the sequences used, but the capsule may also be invisible on dedicated high resolution images as shown beneath as they can be slim.

Semi-axial T1FS MR image not visualizing the ventral sacroiliac ligament on the left side, but it is clearly demonstrated on the right side (arrow). A microscopic image showing the slim capsule anteriorly is shown for comparison. 

STIR image demonstrating fluid filled pouches anteriorly as well as posteriorly at both SIJs (arrow). A microscopic image of the region anteriorly with an arrow on the synovial pouch is shown for comparison.