OptoAcoustic Imaging 1 Conventional Ultrasonic Imaging Spatial resolution

Opto-Acoustic Imaging 台大電機系李百祺 1

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/2 thickness of the piezoelectrical material. – Both are at the order of 10 mm. • Other complications include bandwidth, matching, acoustic and electrical isolation, and electrical contact. 2

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. 3

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. 4

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. 5

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. 6

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 100 MHz is possible. • Remote detection. 7

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

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. 9

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 200 nm to 10 mm. • Visible light is from 400 to 700 nm. 10

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

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. E 2 Lasing transition E 1 Pump E 0 12

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

High Frequency Ultrasound Imaging Using Optical Arrays 14

Ultrasonic Array Imaging • Benefits: – Dynamic steering and focusing. – Adaptive image formation. • Requirements: – Element spacing at l/2. – Large numerical aperture. – Wide bandwidth. 15

High Frequency Ultrasonic Array Imaging (100 MHz or greater) • Complications: – Element spacing is 7. 5 mm at 100 MHz. – Acoustic matching. – Electrical contact. – Acoustic and electrical isolation. – Interconnection. 16

High Frequency Ultrasonic Imaging Using Optical Arrays • Generation: instantaneous absorption ↑ temperature change ↑ stress ↑ acoustic wave. • Detection: – Confocal Fabry-Perot interferometer. – Ultrasonic motion ↑ phase modulation ↑ Doppler shift. 17

High Frequency Ultrasonic Imaging Using Optical Arrays • Precise control of position and size. • Synthetic aperture with rapid scanning. • Element size and spacing at the order of a few mm’s. 18

High Frequency Ultrasonic Imaging Using Optical Arrays • Large bandwidth (both transmit and receive). • Transmission using fibers (low loss and high isolation). • Non-contact and remote inspection. 19

Detection System Set-up 20

Image Formation • Synthetic Aperture. • 1 D or 2 D aperture. • Image plane is defined by scanning of the laser beam. • Side-scattering vs. back-scattering. 21

Wire Images Using a 1 D Array 22

Wire Images Using a 1 D Array 23

Cyst Images Using a 2 D Array 24

Cyst Images Using a 2 D Array 25

Optical Biopsy Probe 26

Discussion • Optical generation of acoustic waves. • Improved receive sensitivity by active optic detection (displacement changes the laser cavity length). • Higher frequencies. 27

Sensitivity of Laser Opto-Acoustic Imaging in Detection of Small Deeply Embedded Tumors 28

Motivation • Develop an imaging technique for low contrast, small tumors. • Optical contrast mechanism (between normal tissue and tumor): – Absorption: blood content, porphyrins. – Scattering: micro-structures. 29

Advantages • High optical contrast in the NIR range. • Low acoustic scattering and attenuation. • Fig. 1. 30

Thermo-elastic pressure waves • Absorption -> Temperature rise -> Pressure rise. • Under the condition of temporal stress confinement, i. e. , insignificant stress relaxation during laser pulse. - t<d/cs. – Half-wavelength resonator. 31

Materials and Methods • Fig. 2. • Q-switched Nd: YAG laser: – l=1064 nm. – 1/e level 14 ns. – 0. 2 J/cm 2 (ANSI 0. 1 -0. 2). • PVDF 5 MHz bandwidth transducer, lithium -niobate 100 MHz transducer (? ). 32

Materials and Methods • Breast phantom 1: – Normal tissue: gelatin+polystyrene spheres (900 nm) or milk for scattering. – Tumors: bovine hemoglobin, 2 -6 mm. • Breast phantom 2: – Bovine liver (3 mm. X 2 mm. X 0. 6 mm). – Placed between chicken breast. 33

Results • Fig 4. To Fig. 6. • Fig. 7 to Fig. 8: Simulations based on existing measurements (2 mm sphere at 60 mm depth). • Wavelet transform for noise reduction. 34

Complications • Acoustic attenuation not present in gelatin phantoms: – Typically 0. 5 d. B/cm/MHz. – The smaller the tumor, the higher the attenuation. • Tissue inhomogeneities exist in breast tissue. • Receiver center frequency and bandwidth. • Lateral resolution vs. axial resolution. 35

Depth Profiling of Absorbing Soft Materials Using Photoacoustic Methods 36

Motivation • Characterize absorbing properties and detect boundaries of layered absorbing materials, such as skin. • Acoustic waves are generated by rapid deposition of laser energy into optically absorbing materials – thermoelastic effects. • Pressure(R) -> Absorption Coefficient(R). 37

Materials Under Investigation • India Ink (photo-stable absorber) in water solutions and acrylamide gels. • India-ink stained biomaterials. • Layered absorbing media using acrylamide gel. 38

Theory • Thermoelastic process: stress confinement. (eq. 1) • Highly attenuating materials: Beer’s law. Optical scattering, acoustic attenuation are ignored. (eq. 2) • Near field condition for plane wave assumption. (eq. 3) • Fig. 1 and Fig. 2. 39

Materials and Methods • • Fig. 3. Laser spot size: 3 -5 mm. Laser radiant exposure: 0. 2 -1. 2 J/cm 2. Lithium niobate transducer protected by a quartz window (800 ns delay). 40

Materials and Methods • Calibration using known concentration of India ink in solution (calibration factor m. V/bar). • India ink with absorption coefficient 2650 cm-1 was used to make absorbing solutions in the range from 15 to 188 cm-1. 41

Materials and Methods • Acrylamide gels were used to create layers of absorbers as thin as 90 mm. • Porcine aorta was processed such that only the elastin layer was used. • The intimal surface was stained by India ink. The opposite surface was in contact with the piezoelectric transducer. 42

Materials and Methods • Fig. 4. • Determination of absorption coefficient based on Beer’s law. Eqs. 7 -11. 43

Results • Fig. 5 – Fig. 11. 44

Discussion • Gel layer resolution is affected by acoustic attenuation and transducer bandwidth. • Stain diffusion of elastin biomaterial. Eq. 13. • The scattering coefficient may not be ignored in practice. • Potential application: laser-tissue welding (measuring the chromophore deposition and temperature profile). 45