Large, complex cystic mass within the right psoas and iliacus muscles measuring 7.1 by 7.4 cm. The right sacro-iliac joint is abnormal, with subtle cortical erosion and fluid within the joint and marrow oedema evident in the adjacent sacrum and iliac bones. The marrow signal is of diffusely low to intermediate signal in both T1 and T2 weighted images. These findings are highly concerning for an infective sacroiliitis and right psoas abscess. The differential is psoas haematoma or less likely tumour. Diffuse low to intermediate signal marrow may represent red marrow hyperplasia or diffuse infiltration. There is also a focal central-right paracentral disc protrusion at L5-S1, contacting the right S1 nerve root as it buds off the thecal sac.