Acousto-optic imaging of tissue
Steve Morgan
Electrical Systems and Optics Research Division,
University of Nottingham, UK
[email protected]
Optical imaging is useful
• Functional imaging of tissue
• Main chromophores; oxy- and de-oxyhaemoglobin
(oxygen saturation imaging)
• Fluorescence
Oxygen saturation imaging
• Skin imaging, inflammatory responses
• 2D imaging
• 3D functional imaging?
Oxygen saturation imaging
Ulcer right plantar view
2D imaging
O2 saturation map of the same
(Dr D Clark, Nottingham University Hospitals Trust)
Light scattering
Light scattered along random paths, degradation of imaging resolution
Ultrasound modulated optical
tomography
(acousto-optic imaging)
• Potential for high resolution 3D imaging of tissue
• Optical illumination and optical detection
• Light that passes through the ultrasound column is phase modulated
(‘tagged’ light)
• Scanning the ultrasound builds up an image
Murray and Roy, Acoustics Today 3:17-24 (2007)
Ultrasound modulated optical
tomography
(acousto-optic imaging)
•
•
•
•
Modulation mechanisms (x3)
Detection
Learning from conventional ultrasound
Applications
Modulation mechanisms
(i) Change in the refractive index of the background
medium causes a change in light paths & modulation of a
detected speckle pattern
(ii) Motion of scatterers causes a change in light paths &
modulation of a detected speckle pattern
(iii) Change in the scattering coefficent in a region
(i) and (ii) can only be observed in coherent light e.g. laser light
(iii) Can be observed in incoherent light but is a weak effect
LV Wang, Phys Rev Lett 87: 043903 (2001)
What is laser speckle?
When coherent light (e.g. from a laser) is split and then
recombined interference fringes can be observed
e.g. Young’s slits experiment
http://en.wikipedia.org/wiki/File:Doubleslit.svg
What is laser speckle?
Extending this to multiple light paths causes a granular
interference pattern to be observed – ‘speckle’
Mechanisms of modulation
(i) change in refractive index
3
-3
1cm
2cm
3cm
10cm
• Compression and rarefaction of material properties changes
refractive index
• Sets up a diffraction grating in the background medium
• Phase modulates the speckle pattern
• conventional acousto-optic effect observable in transparent medium
Mechanisms of modulation
(ii) motion of scatterers
Motion of light scatterers causes a change in the paths taken by
the light – change in speckle pattern
Mechanisms of modulation
Incoherent light
source
US off
Contrast: 0.52
US on
Contrast: 0.49
• Both mechanisms cause a change in speckle contrast
• Imaged on a camera, speckle is averaged over the exposure time
More motion = lower contrast
Mechanisms of modulation
(iii) change of scattering coefficient
3
-3
1cm
2cm
3cm
10cm
• different light paths not only affects speckle pattern
• overall intensity distribution changes
Mechanisms of modulation
(iii) change of scattering coefficient
12000
rarified
edge
compressed
10000
8000
6000
4000
2000
0
-20
-15
-10
-5
0
5
10
15
20
• Monte Carlo, spatial distribution at detector plane
• different grating positions
Mechanisms of modulation
(iii) change of scattering coefficient
4
x 10
ran3(new) inside -- ran at a point and then move
3.76
3.74
3.72
3.7
Intensity
3.68
3.66
3.64
3.62
3.6
3.58
-0.1
-0.05
0
0.05
0.1
0.15
Grating
• Motion of the grating produces a modulated signal
• Weakest of the 3 effects
Summary - Modulation mechanisms
(i) Change in the refractive index
(ii) Motion of scatterers
(iii) Change in the scattering coefficent
(i) and (ii) can only be observed in coherent light
(iii) Can be observed in incoherent light but is a weak effect
Detection
Mix down by
interference with a
reference
DC or intermediate freq
1015Hz
• acousto-optic modulation causes change in optical frequency (a few
MHz on a carrier of ~1015 Hz)
• This is observed at the ultrasound frequency because the unshifted
reference interferes with the modulated term to produce sum and
difference frequencies.
• can be detected directly (small colour shift)
Detection methods
(i) Single detector
(ii) speckle difference imaging
(iii) parallel lock-in detection
Sensitive to
mixed down
signals
(speckle)
(iv) photorefractive crystals
(v) Fabry Perot
(vi) Spectral hole burning
Sensitive to
optical
wavelength
(colour)
changes
Detection methods (i) Single detector
Simplest but
• large detector for scattered light detection
• averages out speckle
• use a small detector
Detection methods (i) Single detector
• to collect scattered light a large detector is ideal
• however this averages out speckle
• use a small detector
Detection methods (ii) Speckle
difference imaging
Function
Generator
Transducer
Amplifier
Camera
Computer
Laser
Aperture
Sample
• Pixelated detectors allow collection of scattered light while
maintaining speckle information
• simple speckle contrast difference measurement can indicate
modulated signal
Detection methods (ii) Speckle
difference imaging
US off
Contrast: 0.52
US on
Contrast: 0.49
• simple speckle contrast difference measurement
M Hisaka, Appl. Phys. Lett. 88, 033901 (2006).
Detection methods (iii) parallel lockin detection
•
Each speckle is the result of many
summed E-field components
•
Can use a ‘lock-in’ ccd to extract amplitude
and phase of speckle at each pixel
•
Summing the amplitudes across the array
provides sqrt N improvement in SNR
Detection methods (iii) parallel lockin detection
UT
PA
FG
AP
LD
y
x
PC
CA
z
ST
• how does one detect a signal modulated at ~MHz range using a
camera that operates at ~30Hz?
Detection methods (iii) parallel lockin detection
• Strobe the laser to sample 1 part of modulated signal many
times over the exposure time of the camera
S Leveque-Fort, Appl Opt 40:1029-36 (2000)
Detection methods (iii) parallel lockin detection
Detection methods (iv) photorefractive
a) Scattered light and reference beam write a hologram to PRC
b) Diffracted reference and transmitted signal have max. interference as
wavefronts are matched
c) US distorts wavefront and reduces detected signal
d) Detected signal
Lai et al, Ultr. in Med & Biol 37:239-52 (2011)
PRC- adapting conventional US scanner
• PRC detection built around a conventional US scanner
Bossy et al, Opt Lett 30:744-746 (2005)
Detection methods (v) Fabry Perot
Constructive and destructive
interference provides
narrowband optical filters
• Fabry Perot interferometer can be used to detect slight wavelength
changes at the optical wavelength (colour)
• tune length of cavity to slight shift in optical frequency
• tricky alignment, not widely used
Detection methods (v) Fabry Perot
•
•
•
15MHz US
Optically absorbing rod in chicken breast
Sakadzic & Wang, Opt Lett 23:2770-2 (2004)
Detection methods (v) spectral hole burning
Li et al Opt Expr
16:14862-74 (2008)
• Crystal is highly absorptive across a wideband
• Pump at a particular frequency (colour), electrons are excited to higher
energy state
• Once all electrons excited, incoming photons cannot be absorbed
• provides a narrow band filter capable of detecting optical sidebands
Detection methods (v) spectral hole burning
• Spectral hole encoded at 70MHz
above optical frequency
• 1MHz US applied, reduction in peak,
appearance of sidebands
Summary - Detection methods
(i) Single detector
(ii) speckle difference imaging
(iii) parallel lock-in detection
Sensitive to
mixed down
signals
(speckle)
(iv) photorefractive crystals
(v) Fabry Perot
(vi) Spectral hole burning
Sensitive to
optical
wavelength
(colour)
changes
Learning from conventional ultrasound
•
Pulsed ultrasound
•
Harmonic imaging/pulse inversion
•
Contrast agents
•
Time reversal
Pulsed US
-3
x 10
0.01
Cut 2
1
cut 1
(a)
2
Black
absorber
(b)
Lateral (m)
3
Cut 1
4
Transparent
Amplitude
0.008
0.006
0.004
5
0.002
6
0
7
8
0.002
-3
0.004
0.006
Lateral (m)
0.008
0.01
x 10
9
3.5
cut 2
3
10
1
2
-3
U/S (m)
x 10
Estimated resolution:
• Lateral: 250μm
• Axial: 90μm
2.5
Amplitude
(a) Optical absorbing object
and (b) object in scattering
medium
0
2
1.5
1
0.5
Axial resolution by time gating optical pulse
(Maximum likelihood algorithm)
0.5
1
1.5
U/S scan (m)
2
-3
x 10
Harmonic imaging
•
•
•
Harmonics caused by different US velocity at different pressure
At higher pressures harmonics generated
Used to obtain smaller zone, reduced sidelobes, higher resolution
Harmonic imaging
•
Measured US linewidths from hydrophone (2.25MHz
focused US transducer)
Pulse Inversion Harmonic Imaging
Short Pulse
High resolution
Overlapping bands
Long Burst
Easy to filter out the
fundamental frequency
Low resolution
Ref.: W. R. HEDRICK, JOURNAL OF DIAGNOSTIC
MEDICAL SONOGRAPHY 2005 21, NO. 3
Pulse Inversion Harmonic Imaging
linear
non-linear
• Summing inverted pulses cancels fundamental and retains
second harmonic
Harmonic imaging
Function
Generator
Transducer
Amplifier
Camera
Computer
Laser
Aperture
Sample
•
Same set up as previously described can perform SHG
pulse inversion
Harmonic imaging
• Optical ultrasound modulated images
• objects cannot be observed with naked eye
Harmonic imaging
• Optical line spread function calculated from an edge response
Acousto-optic sensing with Microbubbles
in blood vessel
PhD student: Jack Honeysett
Light
Ultrasound
1.5c
m
StO2
NIR
1cm
SvO2
•NIR measurement more sensitive to StO2
•Acousto-optic measurement more sensitive to
SvO2
J.E. Honeysett, E. Stride, and T.S. Leung, Advances in Experimental
Medicine and Biology (2011).
Acousto-optics
Time Reversal
• Difficult to focus scattered USMOT waves
• Sense the aberrated wavefront
• Propagate conjugated wavefront back to focus
http://www.ndt.net/article/0498/fink/fink.htm
Time Reversal
•
Recent images from time reversal system
Xu et al, Nat. Photonics 5:154-7 (2011)
Applications
• 3D oxygen saturation imaging
• Imaging optical scattering changes
• Regenerative medicine, imaging fluorescence
3D Oxygen saturation imaging
• Lots of work on phantoms
(Bratchenia et al JBO 14:034031 (2009))
• Ex-vivo tissue sample work
(Kothapalli and Wang JBO 14:014015 (2009))
3D Oxygen saturation imaging
• More quantitative algorithms needed
• Imaging acquisition slow (minutes)
Region of interest imaging?
Only take optical measurements in suspicious region
‘optical biopsy’
http://nexradiology.blogspot.com/2009/04/breast-cancer-on-ultrasound.html
Region of interest monitoring
AO signals used to monitor volume of tissue necrosis in high
intensity focused ultrasound
Lai et al, Ultr. in Med & Biol 37:239-52 (2011)
Imaging in tissue engineering
•
•
•
•
Growth of tissue in 3D in bioreactors
Tissue grown in a scaffold e.g. gel, polymer
Monitor growth (necrotic core)
relatively static samples
NT Huynh et al Proc SPIE 7897, 789719 (2011)
Imaging in tissue engineering
1
Fluorescence
1MHz
1.5MHz
2MHz
0.9
0.8
Normalized signal
Excitation light
0.7
0.6
0.5
0.4
0.3
U/S focus
Target
Emission Filter
0.2
0.1
0
2
• Imaging fluorescence
• Incoherent light, weak modulation
• Observable but very challenging!
4
6
x (mm)
8
10
Summary
• Combine ultrasound and optics to reduce the effects of
light scattering
•3 mechanisms of modulation
• Much effort put into detection
• Applications, medical, fluorescence imaging challenging
Challenges
• Reconstruction algorithms/quantitative imaging
• Can anything else be adapted from ultrasonics?
• Imaging speed (related to low SNR)
• Increase SNR (bubbles, radiation force ….)
• ROI imaging or ex vivo samples
Acknowledgments
Funding BBSRC
Nottingham; NT Huynh, H Ruan, D He, M Mather, BR Hayes-Gill, JA
Crowe, FRAJ Rose, D Clark.
Leeds; M Povey, N Parker, N Watson
UCL; T Leung