1 Opto-Acoustic Imaging 台大電機系李百祺

2 Conventional Ultrasonic Imaging Spatial resolution is mainly determined by frequency. Fabrication of high frequency array transducers is complicated:  /2 pitch between adjacent channels.  /4 thickness of the piezoelectrical material. –Both are at the order of 10  m. Other complications include bandwidth, matching, acoustic and electrical isolation, and electrical contact.

3 Conventional Ultrasonic Imaging Contrast resolution is inherently limited by differences in acoustic backscattered properties. Low contrast detectability is further limited by speckle noise. A new contrast mechanism is desired. One such example is the elastic property.

4 Opto-Acoustical Imaging Acoustic waves can be generated and detected using optical methods. Size limitations of conventional piezoelectrical materials can be overcome using laser techniques. Sensitivity and efficiency are critical issues.

5 Optical Generation of Acoustic Waves (I) Absorption of optical energy produces thermoelastic waves. A membrane with proper thermoelastic properties can be used to transmit acoustic waves.

6 Optical Generation of Acoustic Waves (II) Optical absorption can be viewed as a contrast mechanism (i.e., different tissues have different absorption coefficient, therefore produce acoustic waves of different amplitudes). Detection of such signals is still determined by inherent acoustic properties.

7 Optical Detection of Acoustic Waves Movement of a surface due to acoustic waves can be measured by using optical interference methods. Size of such detectors is determined by the laser spot size. Laser spot size can be a few microns, thus acoustic imaging up to 100MHz is possible. Remote detection.

8 High Frequency Opto-Acoustic Imaging Opto-acoustic phased array at very high frequency (>=100MHz). Resolution at a few microns. Rapid scanning. Synthetic aperture imaging. Compact.

9 Opto-Acoustical Imaging of Absorption Coefficient Rapid growing cancer cells often need extra blood supply. High blood content is related to high optical absorption coefficient. High optical contrast can be combined with low acoustic scattering and attenuation.

10 Basics of Laser Operations Light Amplification by Stimulated Emission of Radiation: a method to generate high power, (almost) single frequency radiation with wavelength ranging from 200nm to 10  m. Visible light is from 400 to 700 nm.

11 Basics of Laser Operations Two basic components: a resonator (cavity) and a gain medium (pump). Resonator: cavity length is half wavelength. Fully reflecting mirrorPartially transmitting mirror Output beam Lasing medium

12 Basics of Laser Operations Two basic components: a resonator (cavity) and a gain medium (pump). The gain medium can be gas, liquid or solid. It provides stimulated emission. E0 E1 E2 Pump Lasing transition

13 Characteristics of Laser Monochromaticity. Coherence. Directionality. High intensity.