Transcript Opto-Acoustic Imaging

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:
- l/2 pitch between adjacent channels.
- l/2 thickness of the piezoelectrical material.
– Both are at the order of 10mm.
• 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 10mm.
• 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.
Output beam
Lasing
medium
Fully reflecting mirror
Partially transmitting mirror
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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.
E2
Lasing transition
E1
Pump
E0
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Characteristics of Laser
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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 l/2.
– Large numerical aperture.
– Wide bandwidth.
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High Frequency Ultrasonic Array
Imaging (100MHz or greater)
• Complications:
–
–
–
–
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Element spacing is 7.5mm 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 ↑
Doppler 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 mm’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.
- t<d/cs.
– Half-wavelength resonator.
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Materials and Methods
• Fig. 2.
• Q-switched Nd:YAG laser:
– l=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
•
•
•
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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 90mm.
• 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
ignored in practice.
• Potential application: laser-tissue welding
(measuring the chromophore deposition and
temperature profile).
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