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.  /2 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.

14 High Frequency Ultrasound Imaging Using Optical Arrays

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

16 High Frequency Ultrasonic Array Imaging (100MHz or greater) Complications: –Element spacing is 7.5  m at 100MHz. –Acoustic matching. –Electrical contact. –Acoustic and electrical isolation. –Interconnection.

17 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.

18 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  m’s.

19 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.

20 Detection System Set-up

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

22 Wire Images Using a 1D Array

23 Wire Images Using a 1D Array

24 Cyst Images Using a 2D Array

25 Cyst Images Using a 2D Array

26 Optical Biopsy Probe

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

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

29 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.

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

31 Thermo-elastic pressure waves Absorption -> Temperature rise -> Pressure rise. Under the condition of temporal stress confinement, i.e., insignificant stress relaxation during laser pulse.  d/c s. – Half-wavelength resonator.

32 Materials and Methods Fig. 2. Q-switched Nd:YAG laser: – =1064 nm. –1/e level 14 ns. –0.2 J/cm 2 (ANSI ). PVDF 5MHz bandwidth transducer, lithium-niobate 100MHz transducer (?).

33 Materials and Methods Breast phantom 1: –Normal tissue: gelatin+polystyrene spheres (900nm) or milk for scattering. –Tumors: bovine hemoglobin, 2-6mm. Breast phantom2: –Bovine liver (3mmX2mmX0.6mm). –Placed between chicken breast.

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

35 Complications Acoustic attenuation not present in gelatin phantoms: –Typically 0.5dB/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.

36 Depth Profiling of Absorbing Soft Materials Using Photoacoustic Methods

37 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).

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

39 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.

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

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

42 Materials and Methods Acrylamide gels were used to create layers of absorbers as thin as  m. 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.

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

44 Results Fig. 5 – Fig. 11.

45 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).