Fig. 8.  Surface marking to guide needle insertion. In the paramedian sagittal (PS) oblique view, each interspace (L3–L4 in this case) is centered in turn on the ultrasound screen (A  ). A corresponding skin mark is made at the midpoint of the probe's long edge (B  ). The probe is then turned 90 degrees to obtain the transverse interlaminar view (C  ). The midline is centered on the ultrasound screen, and skin marks are made at the midpoint of the probe's long and short edges (D  ). The intersection of these two marks provides an appropriate needle insertion point for a midline approach to the epidural or intrathecal space at that level. 


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