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Acousto-optic imaging in liquids: a step towards in-vivo measurements
Pedro Santos, Michael Atlan, Benoit Forget, Emmanuel Bossy, Claude Boccara, Michel Gross
To cite this version:
Pedro Santos, Michael Atlan, Benoit Forget, Emmanuel Bossy, Claude Boccara, et al.. Acousto- optic imaging in liquids: a step towards in-vivo measurements. The Seventh Conference on Biomed- ical Thermoacoustics, Optoacoustics, and Acousto-optics, Mar 2006, United States. pp.608613,
�10.1117/12.641676�. �hal-00261656�
Acousto-optic imaging in liquids : a step towards in-vivo measurements
Pedro Santos a
,Mihael Atlan a
, BenoîtC. Forget a
,Emmanuel Bossy a
, A.Claude Boara a
and
Mihel Gross b
a
Laboratoired'Optique, ESPCI, CNRS UPRA0005,
Université Pierre etMarie Curie, 10rue Vauquelin 75231 Parisedex 05. Frane
b
LaboratoireKastler-Brossel, Département de Physique de l'Eole Normale Supérieure,
UMR 8552 (ENS, CNRS, UMPC), 10rue Lhomond 75231Parisedex 05. Frane
ABSTRACT
The aim of this paper is to show that we an perform aousto-optial signal aquisition of one datapoint (or
voxel of a 3D image) in a very short time (2 - 4 ms), in order to overome the spekle deorrelation eet.
Todemonstratethis, wehaveperformed experimentsin in dynami sattering media suh as liquids. We will
show that wean work with pulsed wave ultrasound, to redue the sound irradiation duration in order to be
ompatiblewithsafetylimits. Thesearesigniantstepstowardsin-vivoexperiments.
Keywords: Medial and biologial imaging, Spekle interferometry, Turbid media, Ultrasound, Holographi
interferometry
1.INTRODUCTION
Aousto-optiimaging
1–4
aimsatobtainingimagesofoptialontrastinentimeterthikbiologialtissueswith
themillimeterspatialresolutionofultrasoundimaging. Itisbasedonthemodulationofthephotontravelpaths
within the areaof interation withthe foused ultrasoundbeam. This proessis often referredto as "photon
tagging". Thedetetion of tagged photons is diult : thesignal is weak, spatially inoherent and sinethe
detetion shemes are usually based on a modulation of the spekle intensity they require that this pattern
remain orrelatedthroughout the whole measurement time. In the ase of in-vivomeasurements, blood ow
limitsthisorrelationtimetoroughly1ms.
5, 6
Lastbutnotleast,thelevelofappliedlaserlightandultrasound
mustomplywithsafetyregulations.
The heterodyne parallel spekledetetion shemewehave developed allowsto detet tagged photonswith
optimal (shotnoise limited) sensitivity.
7, 8
Using either apulse or apseudo randomphase modulation ofthe
ultrasoundandthereferenebeamitispossibleto obtainaousto-optiimagesindynami satteringmediaat
speeds(afewms pervoxel)ompatiblewithfuture appliationtoin-vivoimaging.
2.HETERODYNE PARALLEL SPECKLE DETECTION
Theexperimental setupis representedin gure1. Thesetupitself as wellas theexperimental methodologyof
digitalholographyand heterodynedetetion applied to AO imaginghavebeen desribedpreviously
7, 9
and we
will only briey reall them here. As seenin gure1, the setup is basially a Mah-Zender interferometer in
whihthethereferenebeamis frequenyshiftedbyaousto-optishifters(modulators). Thisisdonein order
toensurethatthestatiinterferenepatternreordedbytheCCDorCMOSameraresultsfromtheinteration
of the referene eld and the so-alled tagged-photons
2
oming from the diusing sample whih have been
Furtherauthorinformation: (Sendorrespondene toBenoîtC.Forget)
BenoîtC.Forget: E-mail: forgetoptique.espi.fr ,Telephone: +33(0)1 40794590
Mihael Atlan: E-mail: atlanoptique.espi.fr
FrançoisRamaz: E-mail: ramazoptique.espi.fr
A.ClaudeBoara: E-mail: boaraoptique.espi.fr
refereneisfrequenyshiftedbyanextraamountin ordertoperformparallellok-indetetion
10
withtheCCD
or CMOSamera. Tofurtherimprovethesignalto noise,aspatial lter(slit) isintroduedat theoutputside
ofthediusingsampleandthereferenebeamisshiftedto performo-axisholography.
11
Figure 1. Setup. Near-infrared light (wavelength λL = 780 nm, optial frequeny fL) is provided by a CW, single
axialmode400mWoutputpowertitanium:sapphirelaser(Coherent,MBR110). Thelight pathissplitintoareferene
(loalosillator, LO)andanobjetarm,byabeamsplitter. Intheobjetarm,asetoflenses(notshown)expandsthe
beamilluminating the sample. The sampleis immersedina transparent watertank, set ona 1D displaement stage.
Transmittedlightpassesthroughaspatiallter(SF)madeofa3×100mmretangulardiaphragm. Twoaousto-opti modulators(AOM1,2: CrystalTehnologie s,dirationeieny: 50%)are plaedinthereferenearm,toshift theLO
optial frequenyfLO=fL+fAOM1−fAOM2,wherefAOM1,2 arethefrequeniesofsignalssentto AOM1,2. A10mm
foallengthlensisplaedinthereferenearminordertoreateaslightlyo-axis(θ≈1◦tiltangle)virtualsourepoint intheSFplane. A 10bit,1Megapixel(square pixels,pixelsize: d=17.5 µm)CMOSamera (HSS4,LaVision)isset
at a distane L =40 mfrom SF,faing the aperture, reordingthe interferene patternof light from both arms, at frameratefC. An aoustitransduer (Panametris AusanA395S-SU, foallength: 68mm)providesthe ultrasoni pressurewaveatthefrequenyfA=2.25MHz,0.25MPaatfoalpoint.
3.PULSED AND PSEUDO RANDOM MODULATION TECHNIQUES
Toaddresstheproblemofspatialresolutionalongtheultrasoniaxisz(axialresolution),Wangetal. developed frequeny-sweptAOimaging withamonodetetor.
12, 13
Forget et al.
14
have extended thishirptehniqueto
PSD byreording asequene ofCCD images while sweepingthe frequenyof the US.These hirptehniques
requirethatthespekleremainsorrelatedthroughoutthedurationofthefrequenyhirp(severalseonds),and
are thus inompatible within vivotomography, beausethe spekle loosesits memoryin atime (1ms)muh
shorterthatthehirpduration(∼10s).
sample reference light
(LO beam)
ultrasound light pulse (LP)
vsü c ü
acoustic pulse (AP)
detection
(i) (ii) (iii)
x y
z
Figure 2. Strobosopidetetionoftheaousti pulseAP(pulselengthvsτ wherevs≃1500m/sissound veloity)by
theLObeamlightpulseLP(pulselengthcτ wherecislightveloity). (i)theAPpropagatesinthesamplewhiletheLO
beamisturnedo. (ii)APandLPoverlapwithintheAOvolumeofinterest. Heterodyneampliationanddetetionof
ultrasound-taggedphotonsours. (iii)theLObeamisturnedountilthenextAPreahestheAOvolume.
Togethighaxialresolutioninvivo,LevandSfez
5, 15
useontinuous-wave(CW)illuminationandpulsed-wave
(PW)ultrasound. However,theSNRisintrinsiallylowbeausetheyworkwithasingledetetorofalowoptial
throughput. Wehaveadapted ourparallel detetiontehniquetoaommodatepulsed-waveultrasound.
9
The
prinipleisdesribedingure2.
The ideais to gateone of the optial beamin order to ash the propagatingaousti pulse at a spei
loation along axisy (dened in gure 2). Tobetter illustrate the tehnique, in gure 2 the objet beam is
gated, butfor experimentsitis morepratialtogatethe referenebeam. The interferenepatternis present
(andreordedbytheCCDorCMOSamera)onlyforashorttimeorrespondingtothepositionoftheaousti
pulseat thistimeinthemedia. Byintroduingaontrollabledelaybetweenthelaunhoftheaoustiandthe
referenelightbeampulsesitispossibletosanthepositionsalongthepropagationaxis.
The axialresolutionis learlydeterminedbytheonvolutionprodutofthetwotimegates (aoustialand
optial). Asin anbeseenin ref.,
9
inreasingthepulsedurationdegradesthespatial resolutionresolution(as
well astheontrastwhih isintrinsiallylinkedtotheresolution). This hasled ustoproposeanewshemeto
obtainaxialresolutionbasedonpseudorandomphasemodulation.
16
Consideragaintheinterferenepatternreordedbytheamera,expressedastheprodutoftheobjeteld
and the onjugatereferene eld, but this time wereplae thegated sine phasemodulation induedby the a
pseudorandomphasemodulationfrnd(t):
I=O×R∗=I0ejfrnd(t−z/v)e−jfrnd(t−τ) (1)
Wemustnowtakeinto aountthattheamerawill integrate,during oneframetimetcam,thissignal:
Icam= Z tcam
0
I0ejfrnd(t−z/v)e−jfrnd(t−τ)dt (2)
As mentioned frnd is a pseudo random funtion, therefore so are exp(jfrnd(t−z/v)) and exp(jfrnd(t−τ)).
Theresultsoftheintegrationineq. 2anbeinterpretedastheautoorrelationfuntion ofthepseudorandom
funtion exp(jfrnd(t−z/v)).
If exp(jfrnd(t−z/v)) is onsidered a purely random funtion, then this autoorrelation is delta funtion meaningthat thesignalreordedbytheamera,Icam iszeroexept whenτ=z/v. Ourpseudorandomsignal
from0toπ. Inthisasetheautoorrelationisnotadeltafuntionbutstillaverynarrowone. Asseeningure 3,evenforadurationmorethantwoordersofmagnitudegreater(1msomparedto 3µs),theautoorrelation funtionisnarrowerandthereforethespatialresolutionis3timesbetterwithapseudorandommodulationthan
withapulsedsinemodulation.
Figure 3.Numerialalulationoftheautoorrelationof(left)a1mspseudorandomsignal,(enter)a3µssinepulse
and(right)a5µs sinepulse
One again, by adjusting the delay τ between the aousti and referene beam signals, we an selet a
speipositionalongtheUSpropagationaxis.
4.EXPERIMENT
A50mmthikaquariumislledwithsatteringliquid: adilutedsolutionofintralipid10%. Thissolutionisthen
further dilutedto vary theoptial stteringproperties ofthe liquid. Measurementsof thespetralbroadening
of the diuse light using the methodology disribed in ref.
6
as shown are in the range of the kHz, whih is
omparable to suh mesurementsmade in vivo.
6, 15
The inlusion is asmall latex pouh a few milimiters in
diameterlled withthesamesolutiondyedwithblakink.
Experimental sans alongtheUS propagationaxisusing thepulsedaoustiand referenebeamtehnique
are shown in gures 4 and 5. The signalis plotted in normalized units. As desribed in ref
7
our tehnique
allowsusthmeasure thesignalandtheshotnoisesimultaneously. Wedeneournormalizedsignalas (signal-
shotnoise)/shot noise. Figure 4omparesresultsontwodilutions(3%and 5%)and showsin eahasde two
sansalongtheUSpropagationaxis: oneovertheinlusion(irles)andoneawayfromtheinlusion(squares).
Theexperimentisrepeated(8 timesforeahsan)toimprovethesignaltonoiseratio. Theinlusionislearly
visible evenforasmallnumbeofaumulations. Figure5showresultsonfurther dilutedliquids. As expeted
theinlusionismoreeasilyseenin themostdilutedsolution.
Experimental resultsusingthepseudorandom modulationtehniqueareshown in gure6. Thedilutionis
5%andtheinlusionislearlyseeninthesanontheleftandthe2Dimageontheright. Thenormalizedsignal
levelisroughly5timeshigherthantheoneobtainedwiththepulsetehnique,theresolutionomparbleandthe
signalto noiseratioismuh better,withasinglesan.
5.CONCLUSION
Theseresultsareanimportantsteptowardsinvivo measurementinwhiheetsofdeorrelationofthespekle
bymovementof, orin,thebiologialtissuesisimportant. Theomparisonof thetwoexperimental tehniques
mustbepushedfurther determinewhih oersthebeast ompromisebetweensignalleveland resolutionwhile
remaininginthesafetylimitsfortheamplitudeanddurationoftheUSpulses. Thesetupanandmustalsobe
adapted forthe onurrentaquisition ofthe signalat multiple wavelengths, addingspetrosopiinformation
totheimages.
ACKNOWLEDGMENTS
ThisworkhasbeensupportedbytheRégion Ile-de-Frane ,aspartoftheCanerpled'Ile deFrane . Wewould
and away fromthe inlusion for 2dilutions of the sattering liquid. Averagingthe signal improvesthe signal to noise
ration
0 50 0.08
0.1 0.12
transverse axis (mm) C = 5%, 2 accumulations
0 50
0.08 0.1 0.12
transverse axis (mm) C = 5%, 4 accumulations
0 50
0.08 0.1 0.12
transverse axis (mm) C = 5%, 8 accumulations
inclusion no inclusion
0 50
0.05 0.06 0.07 0.08
transverse axis (mm) C = 3%
0 50
0.05 0.1
transverse axis (mm) C = 1.5%
0 50
0 1 2 3
transverse axis (mm) C = 0.75%
Figure 5. Experimentalsans along theUS propagationaxis using thepulsetehnique. Theinlusion islearly more
easilydistinguishedasthesolutionismorediluted,thuslesssattering.
0 10 20 30 40 50 60 70 80 0.25
0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65
Cut along x
Normalized signal
Ultrasound propagation axis(mm)
Random phase modulation
x axis (mm)
Ultrasound propagation axis
10 20 30
0
10
20
30
40
50
60
70
Figure6. Experimentalresultsusingthepseudorandommodulationtehnique.
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