Detection of Optoacoustic Transients with a Rectangular Transducer of Finite Dimension Valeriy G. Andreev1, Alexander A. Karabutov1, Anatoliy E. Ponomaryov1, Alexander A. Oraevsky*2 1M.V.Lomonosov Moscow State University, Moscow 119899, RUSSIA, 2University of Texas Medical Branch at Galveston, Texas, 77555 USA ABSTRACT Laser optoacoustic tomography system (LOIS) for breast cancer detection and diagnostics utilizes optical generation of acoustic pressure profiles in tumors and piezoelectric detection of these pressure transients within an ultra- wideband of ultrasonic frequencies. Temporal profile of optoacoustic pulses provides information on tumor dimensions and its location inside the breast. A finite spatial dimension of receiving transducers results in distortions of the optoacoustic profile and corresponding reduction of the resolution in LOIS. The impulse response approach for calculation of the pulse profiles originated from a uniformly absorbing sphere and detected with a rectangular transducer was employed. The pressure profiles were expressed as convolution integrals of velocity-potential distribution over the transducer surface with the corresponding impulse response function. The impulse response function was evaluated for different locations of optoacoustic sources with respect to the receiving transducer. Numerical simulations were performed for acoustic transducers with dimensions of lxi 0 mm and located on the cylindrical surface with radius of 60-mm. Results demonstrated that detected N-shaped pressure profiles become smoother and their duration increases with increased linear dimensions of the transducer. This effect depends on relative position of the spherical acoustic source and the detector. Keywords: optoacoustic transients, spatial impulse response, ultrason ic detector 1. INTRODUCTION The waveform distortion effect related to the finite aperture of ultrasonic transducer is of great importance in pulse-echo measurements [1-2]. In particular, the knowledge of the transient sensitivity of transducer allows to correct the temporal profile of a detected ultrasonic transient and reduce corresponding errors in the medical ultrasonic image. Undistorted detection of laser-induced acoustic pulses in optoacoustic imaging system is of principal importance because temporal pressure waveform provides information on the tumor size and its location [3-4]. A PVDF wide-band transducers employed in LOIS demonstrated feasibility of the small (5- 1 0)-mm tumors detection [4]. However it was noticed that the accuracy of the 2-mm tumor detection or smaller ones is reduced. One of the possible reason of this inaccuracy can be distortion of a pressure pulse waveform resulted from finite aperture of transducer. Unfortunately the dimensions of the transducer could not be reduced significantly because it leads to corresponding capacity reduction and electric noise growth [5]. The performance of LOIS will be improved when distortion effects related to finite transducer aperture will be taken into account. We focused our study on the transducer with rectangular shape since most of transducer elements employed in optoacoustic imaging represent this geometry. Radiation of transient pressure fields from planar transducers of various shapes has been widely investigated. Excellent review of theoretical approaches and mathematical methods was presented by Harris [6]. We employed impulse response approach proposed by Stepanishen [7-8]. Originally the author employed the impulse response method for the evaluation of radiation from piston in a rigid infinite baffle. The approach is based on Green's function solution to the time dependent boundary problem. It utilizes a transformation of coordinate to simplify the calculation of resultant surface integral. The approach can be applied for the calculation of pressure profile both in the near- and farfield of the radiator. According to reciprocity principle the same method can be employed for evaluation of the pressure waveform, which is detected by planar transducer of finite dimensions. * <aaraevs@utmb.edu> Biomedical Optoacoustics III, Alexander A. Oraevsky, Editor, Proceedings of SPIE Vol. 4618 (2002) © 2002 SPIE · 1605-7422/02/$15.00 153 Downloaded from SPIE Digital Library on 08 Aug 2011 to 69.15.164.234. Terms of Use: http://spiedl.org/terms