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Opto-Acoustic Imaging

Opto-Acoustic Imaging. 台大電機系李百祺. Conventional Ultrasonic Imaging. Spatial resolution is mainly determined by frequency. Fabrication of high frequency array transducers is complicated: - l /2 pitch between adjacent channels. - l /4 thickness of the piezoelectrical material.

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Opto-Acoustic Imaging

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  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: - l/2 pitch between adjacent channels. - l/4 thickness of the piezoelectrical material. • Both are at the order of 10mm. • 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 10mm. • Visible light is from 400 to 700 nm.

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

  12. E2 Lasing transition Pump E1 E0 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.

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

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