Laser photothermoacoustic heterodyned lock-in depth profilometry in turbid tissue phantoms Ying Fan, 1 Andreas Mandelis, 1 Gloria Spirou, 2 I. Alex Vitkin, 3 and William M. Whelan 4 1 Center for Advanced Diffusion-Wave Technologies, Department of Mechanical and Industrial Engineering, 5 King’s College Road, University of Toronto, Toronto, Canada M5S 3G8 2 Department of Medical Biophysics, University of Toronto and Ontario Cancer Institute/Princess Margaret Hospital, 610 University Ave, Toronto, Canada M5G 2M9 3 Department of Radiation Oncology, University of Toronto and Ontario Cancer Institute/Princess Margaret Hospital/University Health Network, 610 University Ave, Toronto, Canada M5G 2M9 4 Ryerson University, Dept. of Mathematics, Physics and Computer Science, 350 Victoria Street, Toronto, ON, Canada M5B 2K3 Received 26 January 2005; revised manuscript received 5 July 2005; published 4 November 2005 Frequency-domain correlation and spectral analysis photothermoacoustic FD-PTA imaging is a promising new technique, which is being developed to detect tumor masses in turbid biological tissue. Unlike conven- tional biomedical photoacoustics which uses time-of-flight acoustic information induced by a pulsed laser to indicate the tumor size and location, in this research, a new FD-PTA instrument featuring frequency sweep chirp and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wave- length is constructed and tested for its depth profilometric capabilities with regard to turbid media imaging. 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-fluence modulation frequencies with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. Combining with the depth information carried by the back-propagated acoustic chirp signal at each scanning position, one could rapidly generate subsurface three-dimensional images of the scanning area at optimal signal-to-noise ratios and low laser fluences, a combination of tasks that is difficult or impossible by use of pulsed photoa- coustic detection. In this paper, results of PTA scans performed on tissue mimicking control phantoms with various optical, acoustical, and geometrical properties are presented. A mathematical model is developed to study the laser-induced photothermoacoustic waves in turbid media. The model includes both the scattering and absorption properties of the turbid medium. A good agreement is obtained between the experimental and numerical results. It is concluded that frequency domain photothermoacoustics using a linear frequency sweep method and heterodyne lock-in detection has the potential to be a reliable tool for biomedical depth- profilometric imaging. DOI: 10.1103/PhysRevE.72.051908 PACS numbers: 87.57.Nk, 87.80.Tq, 87.57.Gg, 87.57.Ce I. INTRODUCTION In recent years, the field of photoacoustic PA applica- tions to soft tissue imaging, cancerous lesion detection, and subdermal depth profilometry has enjoyed very rapid devel- opment 1–3. This is so because PA detection has shown concrete promise of depth profilometric imaging in turbid media at depths significantly larger than accessible by purely optical methodologies 4. In state-of-the-art photoacoustic imaging systems, pulsed lasers have always been the choice of signal source 4–6. Some continuous-wave experimental system configurations, however, have been reported with the reverse effect, acousto- optic imaging 7. The advantages of using pulsed laser sources include the following. a Considerable signal strength, which yields acceptable signal-to-noise ratios SNR, can be obtained right after the short pulse. b Fol- lowing optical absorption of a short laser pulse by turbid tissue, optical-to-thermal energy conversion and localized photothermoacoustic PTA volume expansion, the acoustic signals received within approximately 1 s after the end of the laser pulse are essentially thermally adiabatic. Therefore, the signal carries information about the thermal shape of the absorber, which substantially coincides with its geometric shape before any significant heat conduction can deform the image 8. Pulsed PTA detection, however, presents disad- vantages in terms of laser jitter noise and thermal noise within the wide bandwidth of the transducer and hard-to- control depth localization of the contrast-generating subsur- face features. Furthermore, in order to avoid any detrimental effects to human tissue, the pulse energies must be limited to below 5 mJ/pulse at the expense of SNR. Frequency-domain PTA methodologies can offer alterna- tive detection and imaging schemes with concrete advan- tages over pulsed laser photoacoustics. This paper reports the theoretical and experimental development of a new frequency-domain PTA analytical technique and instrumen- tation featuring frequency sweep chirp and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wavelength. The advantages of the system include the following. a Low fluence of harmonically- modulated cw laser, with the concomitant advantage that a much higher optical energy density can be deposited during a given length of time without damaging the tissue, the ther- mal effects being further mitigated by thermal diffusion. b The superior SNR of the lock-in amplifier compared to pulsed laser averaged transients can offset much of the SNR deterioration at high frequency MHz range. Frequency PHYSICAL REVIEW E 72, 051908 2005 1539-3755/2005/725/05190811/$23.00 ©2005 The American Physical Society 051908-1