Fig. 4. Paramedian sagittal transverse process view of the lumbar spine and corresponding magnetic resonance imaging (MRI) scan (T1-weighted). The probe is placed over the tips of the transverse processes (TP), which appear as hyperechoic curvilinear structures with “finger-like” acoustic shadowing beneath. This appearance is also called the “trident sign.” The erector spinae muscle and the psoas muscle lie superficial and deep to the transverse processes, respectively. The peritoneum often us visible if the depth setting is increased appropriately. (Image used with permission from www.usra.ca.)