The psoas sign in normal subjects without bowel preparation: the influence of scoliosis on visualisation

The lumbar spine films of 1494 subjects who were examined for low back pain or prior to job placement were studied to assess psoas muscle visualisation. The study group were almost all aged 18-39 years. None of the subjects had prior bowel preparation for a contrast examination. In the subjects without scoliosis only 36% had well seen psoas shadows on both sides. Taking the psoas shadows separately, 72% of left psoas shadows were well seen, while only 40% were seen on the right side. In the scoliotic group, the psoas shadow was seen bilaterally in 30% of subjects while in another 50% the muscle was seen only on the convex side of the curve. In no case was it seen only on the concave side. The direction of the curve to left or right did not influence visualisation. Thus in 80% of subjects with scoliosis of 5 degrees or more, the psoas was seen on the convex side, while in only 30% was it seen on the concave side.