The ultrasound appearance of the normal psoas muscle.

One hundred and forty-four psoas muscles in 72 subjects were scanned to study the normal ultrasound appearance. For the purpose of description the psoas was divided into three sections. Ten of the 432 sections could not be adequately seen on ultrasound. The psoas demonstrated hyperechoic striations on a hypoechoic background typical of muscle. In addition the upper section, from the origin of the muscle to the lower pole of the kidney, contained echogenic planes in 15 (10%) and the mid section, from the lower pole of kidney to iliac crest, demonstrated prominent echogenic planes and focal areas of increased and decreased echogenicity in 65 (46%). The lower section, from the iliac crest to fusion with the iliacus, demonstrated a single echogenic plane in 96 (70.5%) which was best seen running obliquely in the transverse plane and in 40 (29.5%) there were more complex echogenic planes or focal areas of increased or decreased echogenicity. The cause of the prominent echogenic plane in the lower section was not apparent in the anatomical literature and therefore cadaveric dissection of nine psoas muscles was performed which demonstrated that the echogenic plane was caused by intramuscular tendon fibres formed from the more cranial origins of the psoas. The psoas minor was not identified as a separate structure.