1 opto-acoustic imaging 台大電機系李百祺. 2 conventional ultrasonic imaging spatial...
TRANSCRIPT
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Opto-Acoustic Imaging
台大電機系李百祺
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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 10m.
• Other complications include bandwidth, matching, acoustic and electrical isolation, and electrical contact.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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 10m.
• Visible light is from 400 to 700 nm.
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Basics of Laser Operations
• Two basic components: a resonator (cavity) and a gain medium (pump).
• Resonator: cavity length is half wavelength.
Fully reflecting mirror Partially transmitting mirror
Output beamLasing medium
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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
PumpLasing transition
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Characteristics of Laser
• Monochromaticity.
• Coherence.
• Directionality.
• High intensity.
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High Frequency Ultrasound Imaging Using Optical Arrays
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Ultrasonic Array Imaging
• Benefits:– Dynamic steering and focusing.– Adaptive image formation.
• Requirements:– Element spacing at /2.– Large numerical aperture.– Wide bandwidth.
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High Frequency Ultrasonic Array Imaging (100MHz or greater)
• Complications:– Element spacing is 7.5m at 100MHz.– Acoustic matching.– Electrical contact.– Acoustic and electrical isolation.– Interconnection.
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High Frequency Ultrasonic Imaging Using Optical Arrays
• Generation: instantaneous absorption ↑ temperature change ↑ stress ↑ acoustic wave.
• Detection:– Confocal Fabry-Perot interferometer.– Ultrasonic motion ↑ phase modulation ↑ Doppl
er shift.
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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.
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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.
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Detection System Set-up
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Image Formation
• Synthetic Aperture.
• 1D or 2D aperture.
• Image plane is defined by scanning of the laser beam.
• Side-scattering vs. back-scattering.
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Wire Images Using a 1D Array
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Wire Images Using a 1D Array
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Cyst Images Using a 2D Array
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Cyst Images Using a 2D Array
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Optical Biopsy Probe
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Discussion
• Optical generation of acoustic waves.
• Improved receive sensitivity by active optic detection (displacement changes the laser cavity length).
• Higher frequencies.
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Sensitivity of Laser Opto-Acoustic Imaging in Detection of Small Deeply Embedded Tumors
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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.
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Advantages
• High optical contrast in the NIR range.
• Low acoustic scattering and attenuation.
• Fig. 1.
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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/cs.
– Half-wavelength resonator.
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Materials and Methods
• Fig. 2.
• Q-switched Nd:YAG laser:– =1064 nm.– 1/e level 14 ns.– 0.2 J/cm2 (ANSI 0.1-0.2).
• PVDF 5MHz bandwidth transducer, lithium-niobate 100MHz transducer (?).
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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.
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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.
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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.
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Depth Profiling of Absorbing Soft Materials Using Photoacoustic Methods
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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).
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Materials Under Investigation
• India Ink (photo-stable absorber) in water solutions and acrylamide gels.
• India-ink stained biomaterials.
• Layered absorbing media using acrylamide gel.
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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.
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Materials and Methods
• Fig. 3.
• Laser spot size: 3-5mm.
• Laser radiant exposure: 0.2-1.2 J/cm2.
• Lithium niobate transducer protected by a quartz window (800ns delay).
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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.
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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.
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Materials and Methods
• Fig. 4.
• Determination of absorption coefficient based on Beer’s law. Eqs. 7-11.
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Results
• Fig. 5 – Fig. 11.
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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 ignore
d in practice.• Potential application: laser-tissue welding
(measuring the chromophore deposition and temperature profile).