Development of a laser photothermoacoustic frequency-swept system for subsurface imaging: Theory and experiment

Abstract

In conventional biomedical photoacoustic imaging systems, a pulsed laser is used to generate time-of-flight acoustic information of the subsurface features. This paper reports the theoretical and experimental development of a new frequency-domain (FD) photo-thermo-acoustic (PTA) principle featuring frequency sweep (chirp) and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wavelength. PTA imaging is a promising new technique which is being developed to detect tumor masses in turbid biological tissue. Owing to the linear relationship between the depth of acoustic signal generation and the delay time of signal arrival to the transducer, information specific to a particular depth can be associated with a particular frequency in the chirp signal. Scanning laser modulation with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. Preliminary results performed on rubber samples and solid tissue phantoms indicate that the FD-PTA technique has the potential to be a reliable tool for biomedical depth-profilometric imaging.

Thermally enhanced signal strength and SNR improvement of photoacoustic radar module

“…The response amplitude spectrum $|\tilde{p}(\omega )|$ of the acoustic pressure is generated by the spatially distributed energy source spectrum$\tilde{E}(\omega )$ [J/m 2 ] in a chromophore and can be described in one dimension by [16–18]:$\left|\tilde{p}(\omega )\right|\approx \frac{{\mu }_a\beta c_a^2{e}^{-{\mu }_{ef}z}}{C_p\sqrt{{\mu }_a^2{c}_a^2+{\omega }^2}}{E}_0=\frac{{\mu }_a\Gamma e^{-{\mu }_{ef}z}}{\sqrt{{\mu }_a^2{c}_a^2+{\omega }^2}}{E}_0$where $\omega$[rad/s] represents the angular modulation frequency, ${\mu }_a$ [m −1 ] is the absorption coefficient, $\beta$ [°C −1 ] is the volume thermal expansion coefficient, $c_a$[m/s] is the speed of sound, ${\mu }_{ef}$[m −1 ] is the effective attenuation coefficient, z [m] is the distance, $E_0$[J·Hz/m 2 ] is the frequency modulated light intensity at the surface of the absorber,$C_p$[J/kg°C] is the specific heat capacity at constant pressure, and $\Gamma =\frac{\beta c_a{}^2}{C_p}$is the Gruneisen parameter. From Eq.…”

Photoacoustic imaging of biological tissue with intensity-modulated continuous-wave laser

“…A cw PA detection with a narrow-band ultrasonic transducer and a narrow-band or lock-in amplifier provides high sensitivity and strong noise rejection. A cw PAM based on lock-in detection has been demonstrated to measure surface 15 and subsurface 16 properties of solids. Although single-frequency detection does not enable unequivocal phase-based axial resolution owing to the 2 phase ambiguity, chirping ͑frequency sweeping͒ over a bandwidth enables full axial resolution, which is comparable to the time-domain resolution acquired with the same bandwidth.…”

A multi-dimensional transfer function approach to photo-acoustic signal analysis

“…Consider the Fourier transform of the detected wave measured in the z ¼ z d plane. Since detection in one of the z planes is being considered, we proceed by taking the spatial Fourier transform of both sides of the governing equation (3), however, only with respect to the x and y variables leaving the z variable untransformed. This gives…”

“…Several groups of researchers have made significant progress towards achieving these imaging goals [2][3][4][5][6]. However, a number of different approaches have been developed by each group.…”