Ultrasound characteristics of needles for regional anesthesia

Needle visibility is a crucial requirement for successful and safe ultrasound-guided peripheral nerve blocks. We performed a prospective study on the ultrasonic imaging quality of various commercially available needles. We tested the hypothesis that different nerve block needles would have different ultrasonic appearances. Furthermore, we examined the influence of needle angle with regard to the ultrasound plane, 2 types of media surrounding the needle, and the impact of 3 different ultrasound machines.

Twelve needles were prospectively tested for 3 ultrasound planes (longitudinal, axial tip, and axial shaft) at 2 different angles (0 degrees and 45 degrees). Quality of needle visibility and ultrasound scans were described by using 6 criteria (visibility score range 0-10): (1) visibility of needle; (2) visibility of surrounding media; (3) consistency of needle surface; (4) formation of artifacts; (5) degree of shadowing; and (6) detection and distinction of the needle from the surrounding media. Additionally, every ultrasound scan was performed in 2 media (water bath and animal model) with 3 ultrasound devices and evaluated by 2 investigators. Evaluation of the ultrasound scans was blinded with regard to needle but not to the ultrasound machine and media.

In the animal model, visibility was good at 0 degrees (visibility score greater than 6) but was decreased for all needles at a 45 degree angle (criterion 6). In this setting, 2 needles were difficult to identify (score less than 3; criterion 6) and only 3 of 12 reached a score of 7 or more (criterion 6). Depiction quality for all 3 planes was significantly lower in the animal model when compared with the water bath (P < .001) and at an angle of 45 degrees when compared with 0 degrees (P < .001). There was no significant impact of the ultrasound machine on image quality.

In a tissue-equivalent model we found significant differences among different types of needles at a 45 degree angle. In clinical use, angles between 30 degrees and 60 degrees are required. Because visibility of the needle is a keystone of ultrasound-guided peripheral nerve blocks, our results suggest the need to optimize the echogenicity of needles used for ultrasound-guided nerve blocks.