Background and objectives Ultrasound visibility of regional block needles is a critical component for safety and success of regional anesthetic procedures. The aim of the study was to formally assess factors that influence ultrasound visibility of needles used in regional anesthesia.

Methods Regional block needles between 17- and 22-G diameter were inserted in a tissue equivalent phantom at angles from 0° to 65° relative to the phantom surface. For visibility enhancement, the needles were primed with air or water in combination with stylets and different size guide wires. Ultrasound measurements of needle tips and shafts were performed using transversal and longitudinal imaging with a linear 15-MHz transducer. Univariate and multivariate statistical analyses were performed on 719 visibility measurements.

Results Hustead tip needles exhibited best ultrasound visibility. Ultrasound visibility of the needle tip was increased by insertion of a medium size guide wire. Water or air priming of the needle, insulation, and the insertion of a stylet did not influence needle visibility. Long axis imaging of the needle for shallow insertion angles (<30° in relation to the phantom surface) and short-axis imaging for steep angles (>60°) provided the best ultrasound visibility of the needle tips. Needle visibility decreased linearly with steeper insertion angles (P < .001) and smaller needle diameters (P < .001).

Conclusions The results of our in-vitro study suggest a number of factors enhancing ultrasound visibility of regional block needles. The use of needles in the largest possible size inserted with a medium-size guide wire provides the best ultrasound visibility. Analysis of the approach angle favors needle insertion parallel to the transducer. The consideration of these factors may improve safety and success of ultrasound-guided regional blocks.