Acousto-optic imaging of tissue Steve Morgan Electrical Systems and Optics Research Division, University of Nottingham, UK Steve.morgan@nottingham.ac.uk
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 O2 saturation map of the same 2D imaging (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 2cm 1cm 10cm 3cm different light paths not only affects speckle pattern overall intensity distribution changes
Mechanisms of modulation (iii) change of scattering coefficient 12000 10000 rarified edge compressed 8000 6000 4000 2000 0-20 -15-10 -5 0 5 10 15 20 Monte Carlo, spatial distribution at detector plane different grating positions
Intensity Mechanisms of modulation (iii) change of scattering coefficient x 10 4 3.76 ran3(new) inside -- ran at a point and then move 3.74 3.72 3.7 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 10 15 Hz acousto-optic modulation causes change in optical frequency (a few MHz on a carrier of ~10 15 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 (iv) photorefractive crystals (v) Fabry Perot (vi) Spectral hole burning Sensitive to mixed down signals (speckle) 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 Amplifier Transducer 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 FG PA PC x y z LD ST AP CA 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 (iv) photorefractive crystals (v) Fabry Perot (vi) Spectral hole burning Sensitive to mixed down signals (speckle) Sensitive to optical wavelength (colour) changes
Learning from conventional ultrasound Pulsed ultrasound Harmonic imaging/pulse inversion Contrast agents Time reversal
Amplitude Lateral (m) Amplitude Pulsed US (a) (b) x 10-3 1 2 3 4 5 6 Cut 2 Cut 1 Black absorber Transparent 0.01 0.008 0.006 0.004 0.002 cut 1 (a) Optical absorbing object and (b) object in scattering medium Estimated resolution: Lateral: 250μm Axial: 90μm Axial resolution by time gating optical pulse (Maximum likelihood algorithm) 7 8 9 10 1 2 U/S (m) x 10-3 3.5 2.5 1.5 0.5 0 0 0.002 0.004 0.006 0.008 0.01 Lateral (m) x 10-3 3 2 1 cut 2 0.5 1 1.5 2 U/S scan (m) x 10-3
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 Amplifier Transducer 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 PhD blood vessel student: Jack Honeysett Light Ultrasound NIR 1.5c m 1cm S t O 2 S v O 2 Acousto-optics NIR measurement more sensitive to S t O 2 Acousto-optic measurement more sensitive to S v O 2 J.E. Honeysett, E. Stride, and T.S. Leung, Advances in Experimental Medicine and Biology (2011).
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)
Normalized signal Imaging in tissue engineering Excitation light Fluorescence 1 0.9 0.8 0.7 1MHz 1.5MHz 2MHz 0.6 0.5 0.4 U/S focus Target Emission Filter 0.3 0.2 0.1 0 2 4 6 8 10 x (mm) Imaging fluorescence Incoherent light, weak modulation Observable but very challenging!
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