Ultimate sensitivity of time-resolved opto-acoustic detection

The major limitation in sensitivity of the optical tomography is associated with strong optical attenuation in human tissues. Opto-Acoustic Tomography overcomes this limitation utilizing detection of acoustic waves instead of detection of transmitted photons. Exceptional sensitivity of the opto-acoustic tomography allows early detection of small tumors located deep in human tissues, such as breast. This paper demonstrates that an optimally designed opto-acoustic imaging system can detect early l-mm tumors with minimal blood content of only 7% at the depth of up to 7-cm within the breast attenuating laser irradiation 3.3 times per each 1-cm of its depth. A theoretical consideration of the ultimate sensitivity of piezo-detection in a wide ultrasonic frequency band is developed. The detection sensitivity is presented as a function of the ultrasonic frequency, tumor dimensions and optical absorption coefficient. Comparative analysis of piezo and optical interferometric detection of opto-acoustic transients is presented. The theoretical models of piezo-detection were developed for the open-circuit and short-circuit schemes of operation. The ultimate sensitivity limited by thermal noise of electric capacitor of the piezo-element was estimated. It was shown that the limit of detection depends on the frequency band, the electric capacity of the transducer and the speed of sound in the piezo-element. Comparative analysis of various piezo-materials was made from the point of view of their utility for sensitive opto-acoustic detection.