Improvement in quantifying optical absorption coefficients based on continuous wavelet-transform by correcting distortions in temporal photoacoustic waveforms

Abstract

Quantification of the optical absorption coefficients of optical absorbers using photoacoustic (PA) pressure waves with broadband frequency was reported. We proposed to use continuous wavelet-transform (CWT) to obtain time-resolved frequency spectra of PA signals and demonstrated the relationship between optical absorption coefficients of optical absorbers and CWT of PA signals. However, the optical absorption coefficients of the optical absorbers were not quantified. Thus, in this research, we quantified optical absorption coefficients of optical absorbers by using the calibration curve which relates the optical absorption coefficients of optical absorbers and CWTs of PA signals. The calibration curve is derived from the simulation. However, due to the frequency response of the acoustic sensor, the simulated PA pressure waves differed from the measured PA signals. Thus, we measured the frequency response of the acoustic sensor. By convolving the frequency response of the acoustic sensor to the simulated pressure waves, we simulated the PA signals which were obtained by measuring the PA pressure wave using the acoustic sensor. The calibration curve derived from the simulated PA signal enabled to quantify optical absorption coefficients of optical absorbers. We verified the method by quantifying optical absorption coefficients of blood vessel phantoms which is tubes filled with diluted inks with optical absorption coefficients from 10 to 40 cm-1. As results, the simulated PA signals demonstrated close similarity with the measured PA signals, and the optical absorption coefficients of the blood vessel phantoms were quantified with root mean square error of 2.42 cm-1.