Effects of ultrasound coupling gel on photoacoustic signal attenuation

Photoacoustic imaging systems are now commercially available and built into traditional ultrasound hardware. However, unlike ultrasound imaging, the choice of coupling gel may potentially alter attenuation and fluency of the laser transmission. Hence, the objective of this study was to determine the attenuating effects of various ultrasound coupling gels on photoacoustic signals. Signals from 3 tissue phantoms were acquired using the Vevo LAZR system with a LZ250 probe. All experiments were performed in triplicate. Two agar phantoms were dyed with methylene blue or IR820 dye. A third phantom was made with no dye. Phantoms were scanned from 680–970 nm at a gain of 45 dB and a depth of 8 mm with four different types of acoustic coupling gel: a clear medium viscosity gel, a clear high viscosity gel, a blue medium viscosity gel, and a white, medium viscosity opaque ultrasound lotion. After scanning, Vevo software was used to graph the absorption spectra of each phantom. The clear phantom's spectra were subtracted from the spectra of the methylene blue and IR820 phantoms in order to eliminate the photoacoustic signal from agar. The average maximum intensities of the photoacoustic signal in the methylene blue phantom using the white, blue, clear high viscosity, and clear medium viscosity gels were 0.28±0.01, 0.42±0.17, 0.56±0.16, and 0.76±0.20 linear arbitrary units (au), respectively (p=0.03). While variations in intensities were observed between clear and blue gels, these differences were not statistically significant (p=0.12). For the IR820 phantom, the average maximum signal intensities, due to the white, blue, clear high viscosity, and clear medium viscosity gels were 0.38±0.04, 2.31±0.22, 2.25±0.43, and 2.30±0.23 au, respectively (p<0.001). However, unlike the methylene blue phantom, no differences were observed between clear and blue coupling gels (p=0.9). In both phantoms, no significant differences were observed in peak absorbance wavelengths between the blue and either clear coupling gel groups (p>0.49). In clinical ultrasound, little consideration is given to the coupling gel used during scanning. However, for photoacoustic imaging, the selection of acoustic coupling gels can lead to signal attenuation and should be kept constant during serial imaging studies.