A large loculated fluid collection lies within the distal right psoas muscle, and measures 4.2 x 7.7 x 11.5 cm in dimension. The proximal aspect of this fluid collection lies at the L5 vertebral level, and its distal most aspect lies approximately at the level of the superior pubic rami and is above the psoas' distal insertion. The affected portion of the right psoas muscle is expanded. Erosion of the right sacroiliac joint consistent with a septic arthritis. Subtle fat stranding is seen around the posterior aspect of the caecum, which lies directly anterior to the fluid collection. Large right distal psoas fluid collection is consistent with a right psoas abscess secondary to an infected right SI joint.