Select the vertebral level in the middle of the desired dermatomal spread. For example, if you wish to block levels T3-T7 then aim to place the injection at the level of the T5 transverse process (TP). Place the transducer in the midline using a saggital orientation. Identify the superficial hyperechoic shadows of the spinous processes in the midline. Scan laterally to visualise the square hyperechoic outline (‘tombstone’) of the transverse processes. Scanning further laterally this square outline of the TP will be replaced by a curved outline of the corresponding rib. Now move medially again to position over the lateral TP. Immediately superficial to the transverse processes you will visualise the erector spinae muscle seen it its long axis. Superficial to this the rhomboid major muscle is seen above T6 vertebral level (this level approximates to the inferior tips of the scapulae), and superficial to the rhomboid is the trapezius muscle. The erector spinae muscle fibres will be seen as horizontal striations running across the screen. Position the TP of the desired vertebral level in the middle of the screen.
Figure 2: a) saggital plane ultrasound image showing the erector spinae muscles sitting superficial to the Transverse processes of T5 and T6 vertebrae b) annotated ultrasound image (Trap=trapezius muscle, Rhomb=rhomboid muscle, ES=erector spinae muscle, IC=intercostal muscles, TP5=T5 transverse process, TP6=T6 transverse process)