The muscles of the abdominal wall (external and internal obliques and transversus abdominis) are visualized
The probe is moved dorsal until the quadratus lumborum is seen medial to the aponeurosis of the tranversus abdominis
With a slight caudad tilt of the probe the body and transverse process (TP) of vertebral body L4 can be seen surrounded by the quadratus lumborum (QL) at the tip of the TP, erector spinae posterior to the TP and psoas major anterior to the TP
This is the “shamrock sign” – the treefoil of the stem of the TP surrounded by the three muscle “leaves”
TP of L4 (gray) surrounded by the QL (white asterix), psoas major (cyan asterix), and erector spinae (green asterix). The lumbar plexus (yellow shadow) is just anterior to the TP. The external and internal obliques and the transversus abdominis are blue, green and purple. A = anterior, P = posterior, M = medial and L = lateral
