One hundred and forty-four psoas muscles in 72 subjects were scanned t
o study the normal ultrasound appearance. For the purpose of descripti
on the psoas was divided into three sections. Ten of the 432 sections
could not be adequately seen on ultrasound. The psoas demonstrated hyp
erechoic striations on a hypoechoic background typical of muscle. In a
ddition 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 p
rominent echogenic planes and focal areas of increased and decreased e
chogenicity in 65 (46%). The lower section, from the iliac crest to fu
sion 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 ec
hogenic plane in the lower section was not apparent in the anatomical
literature and therefore cadaveric dissection of nine psoas muscles wa
s performed which demonstrated that the echogenic plane was caused by
intramuscular tendon fibres formed from the more cranial origins of th
e psoas. The psoas minor was not identified as a separate structure.