Tissue microscopy using optical generation and detection of ultrasound

Optical detection of ultrasound provides a unique and appealing way of forming detector arrays (1D or 2D). Etalon based optical techniques are of particular interest, due to their relatively high sensitivity resulting from multiple optical reflections within the resonance structure. Detector arrays formed by etalon based techniques are characterized by high element density and small element active area, which enables high resolution imaging at high ultrasonic frequencies (typically 10-50 MHz). Laser generated ultrasound using the photoacoustic effect has been demonstrated in recent years as a powerful imaging modality for medical and biological applications. A short laser pulse illuminates a tissue creating rapid thermal expansion and acoustic emission. Detection of the resulting acoustic field by a detector array enables the imaging of the tissue optical absorption using ultrasonic imaging methods. One of the most appealing features of photoacoustic imaging is that it provides access to tissue composition at the molecular level using multiple wavelength illumination. elements are easily accomplished by optical focusing. These techniques are typically characterized by a flat bandwidth. Laser generation of ultrasound was studied and demonstrated for several decades but gained renewed interest since the late nineties for applications in medical imaging (7-10). The method is based on illuminating tissue with a short laser pulse. The optical pulse is absorbed by the tissue causing it to heat rapidly. The temperature rise is followed by thermal expansion producing acoustic emission. Detection of the resulting acoustic field allows reconstructing the initial distribution of heat deposition and therefore the distribution of optical absorption in the tissue. The most appealing feature of this method is that image contrast is based on the optical properties of tissue. This makes it possible for ultrasonic imaging to probe tissue composition at the molecular level and facilitates the interaction with optical based contrast agents for functional imaging (11, 12).