This study was reviewed and approved by the local ethics committee (Kyorin University Ethical Review Board, Reception No. 1408, approved date 2019.12.6 ) and registered in the University Hospital Medical Information Network Center Clinical Trials Registration System (UMIN000036974, 01/07/2019). The study was conducted in accordance with Consolidated Standards of Reporting Trials (CONSORT) guidelines (Supplemental information; CONSORT Diagram).
The cadaver used in this study was donated to Kyorin University School of Medicine for anatomical education, research, and clinical skills training. The research protocol was prepared in strict accordance with the "Guidelines for the research involving cadavers" of the Japanese Association of Anatomists, and was approved by the Ethics Committee of Kyorin University Faculty of Medicine (Permit No. 986). A comprehensive consent form was obtained from the donor prior to donation and from the family prior to and at the time of donation. The research outline was published on the Kyorin University website to ensure that the families had the opportunity to refuse (opt-out).
Ultrasound-emitting from needle tip using photoacoustic effect
The principle of an ultrasound-emitting system using a photoacoustic effect is described (Fig. 1)6). An optical fiber is inserted into the lumen of a needle, and fixed in place maintaining the lumen open for administration of local anesthetic agents. The fiber tip is covered with black resin containing carbon-black pigment, and fixed to the inside wall at the needle bevel. Pulsed laser from an external laser light source is transmitted through the optical fiber. The black resin at the needle tip absorbs the pulsed laser light, which causes adiabatic thermal expansion, and is translated to high frequency vibrations. As a result, ultrasound waves are generated by the photoacoustic effect. The ultrasound wave is received by the ultrasound transducer in the ultrasound probe, which is converted into electrical signals and transferred to the ultrasound unit for imaging. Typical ultrasound imaging for scanning tissue involves a “round trip” for the ultrasound waves, emitted from the transducer in the probe and reflected by the needle back to the probe. However, the ultrasound wave generated by the photoacoustic effect is a one-way trip, emitted at the needle tip to the probe. The ultrasound frame to create a view for the ultrasound wave generated by the photoacoustic effect needs 11% of all frames. This reduction in the frame rate did not inhibit smooth dynamic ultrasound views in the preliminary study. Safety evaluation of the system was proven in a previous study according to the “Safety of laser products – Part 1: Equipment classification and requirements: IEC60825-1:2014” 6). The ultrasound waves generated by the photoacoustic effect is colored green or white.
In the study, we used an ultrasound machine FC1 ( FUJIFILM Medical Co., Ltd., Tokyo, Japan) and ultrasound probe L38xp/13 − 6 and C35xp/8 − 3 (FUJIFILM SonoSite, Inc., Bothell, WA, USA). The nerve block needle was a Stimuplex® Ultra 360® (insulated echogenic needle; size 22 G, length 80 mm, bevel angle 30°, B. Braun Medical Inc., Melsungen, Germany), in which an optical fiber was incorporated as described above). Figure 2 shows ultrasound views at several trajectory needle angles of the echogenic needle or ultrasound-emitting needle using the simulator.
Ultrasound-guided deep nerve block
An expert (K.W.) in ultrasound-guided nerve blocks performed these nerve blocks on a cadaver under ultrasound guidance using the novel ultrasound-emitting needle. The cadaver (an 85-year-old man fixed with N-vinyl-2-pyrrolidone) had soft and pliable tissue7). The deep nerve blocks performed included a paravertebral block (transversal technique with lateral to medial direction, in-plane approach), lumbar plexus block (transverse technique with lateral to medial direction, in-plane approach) and sciatic nerve block (parasacral approach).
On the ultrasound display, ultrasound views with and without the ultrasound generated by the photoacoustic effect were displayed during the procedure. In the study, the ultrasound generated by the photoacoustic effect was shown in white to prevent information bias.
The side showing ultrasound generated by the photoacoustic effect was blinded to the expert with an opacity board. Then, the expert performed the nerve block watching with a typical ultrasound view, then both of the movies were recorded. To confirm whether the nerve blocks were successful, 10 ml of acrylic paint solution was injected instead of a local anesthetic agent, and dissection performed after the experiment.
Survey to evaluate needle tip visibility
Needle tip visibility was evaluated by a survey of participating anesthesiologists acting as volunteers. Recruitment of participants was performed through the local community of anesthesiologists, including department colleagues and anesthesiologists in associated hospitals. Exclusion criteria was rejection to participate in the survey. The authors and collaborators of the study were excluded from participating in the survey.
Ultrasound movies were randomly selected using a random number table. Participating anesthesiologists watched the ultrasound images and evaluated needle tip visibility in each movie using a Likert scale (score 1: very poor, 2: poor, 3: fair, 4: good, 5: very good). Demographic data of participants were also collected, including overall clinical experience and experience with ultrasound-guided nerve block. The survey was conducted on the Internet using questions posted with an invitation limit. Informed consent was obtained from all participants prior to the Internet survey. The primary outcome of the study was evaluation of needle tip visibility with or without the photoacoustic ultrasound. The secondary outcome was a relationship between experience with ultrasound-guided nerve block and needle tip visibility with or without the photoacoustic ultrasound.
Statistical Analysis
Likert scale scores from participant responses are expressed as median [first quartile, third quartile]. Clinical experience of participants is expressed in four rank groups (1–5, 6–10, 11–15, > 15 years) and the number of ultrasound-guided nerve block procedures expressed in four rank groups (0–50, 51–100, 101–200, > 200). Wilcoxon matched-pairs signed rank test was used to evaluate the scores. Spearman’s correlation coefficients (rS) were used to evaluate strength of associations among the variables. A preliminary study for estimating power analysis was performed using the authors and collaborators. The median score and standard deviation of photoacoustic ultrasound were expected to be superior in one score than without photoacoustic ultrasound. The sample size required for 80% power at ɑ = 0.05 was estimated to be sixteen participants. A p-value less than 0.05 was considered statistically significant. Statistical analyses were performed with GraphPad Prism, version 7.02 (GraphPad Software Inc., San Diego, USA) .