Quantitative comparison of PZT and CMUT probes for photoacoustic imaging: Experimental validation

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Quantitative comparison of PZT and CMUT probes for

photoacoustic imaging: Experimental validation

Maëva Vallet, Francois Varray, Jérôme Boutet, Jean-Marc Dinten, Giosue

Caliano, Alessandro Savoia, Didier Vray

To cite this version:

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Research

article

Quantitative

comparison

of

PZT

and

CMUT

probes

for

photoacoustic

imaging:

Experimental

validation

Maëva

Vallet

a

,

François

Varray

a,

*

,

Jérôme

Boutet

b

,

Jean-Marc

Dinten

b

,

Giosuè

Caliano

c

,

Alessandro

Stuart

Savoia

c

,

Didier

Vray

a

UnivLyon,INSA-Lyon,UniversitéLyon1,UJM-SaintEtienne,CNRS,Inserm,CREATISUMR5220,U1206,F-69621Lyon,France

b

CEA-LETI,MINATEC,F-38054Grenoble,France

c

DipartimentodiIngegneria,UniversitàdegliStudiRomaTre,Rome,Italy

ARTICLE INFO Articlehistory:

Received14September2016 Receivedinrevisedform27July2017 Accepted8September2017 Availableonline22September2017 Keywords: Photoacoustic Ultrasoundimaging CMUT PZT ABSTRACT

Photoacoustic(PA)signalsareshortultrasound(US)pulsestypicallycharacterizedbyasingle-cycle shape,oftenreferredtoasN-shape.Thespectralcontentofsuchwidebandsignalsrangesfromafew hundredkilohertz toseveral tensofmegahertz.Typical receptionfrequencyresponses ofclassical piezoelectricUSimagingtransducers,basedonPZTtechnology,arenotsufficientlybroadbandtofully preservetheentireinformationcontainedinPAsignals,whicharethenfiltered,thuslimitingPAimaging performance.Capacitivemicromachinedultrasonictransducers(CMUT)arerapidlyemergingasavalid alternativetoconventionalPZTtransducers inseveral medicalultrasoundimagingapplications.As comparedtoPZTtransducers,CMUTsexhibitbothhighersensitivityandsignificantlybroaderfrequency responseinreception,makingtheiruseattractiveinPAimagingapplications.Thispaperexploresthe advantagesoftheCMUTlargerbandwidthinPAimagingbycarryingoutanexperimentalcomparative studyusingvariousCMUTandPZTprobesfromdifferentresearchlaboratoriesandmanufacturers.PA acquisitionsareperformedonasuturewireandonseveralhome-madebimodalphantomswithbothPZT andCMUTprobes.Threecriteria,basedontheevaluationofpurereceiveimpulseresponse, signal-to-noise ratio(SNR) andcontrast-to-noiseratio (CNR)respectively,have beenusedforaquantitative comparisonofimagingresults.Themeasuredfractionalbandwidths oftheCMUTarraysarelarger comparedtoPZTprobes.Moreover,bothSNR andCNRare enhancedbyatleast 6dB withCMUT technology.ThisworkhighlightsthepotentialofCMUTtechnologyforPAimagingthroughqualitative andquantitativeparameters.

1.Introduction

Photoacousticimaging(PAI)hasbeenproventobeapromising imaging techniquedue to itsability toprovide highresolution imagesatenhancedcontrastrelatedtotheopticalabsorption[1,2]. In addition, PAI does not cause harmful effects tothe patient. Photoacoustic(PA)wavesaregeneratedfromatissuewhenitis subjectedtoapulsedlaserirradiation.Theenergycarriedbyeach laserpulsecausesa localincreaseof temperatureofthetissue, relatedtoitsopticalabsorption,leadingtoathermalexpansion, which generate an acoustic perturbation in the ultrasound

frequency range [3,4]. The resulting ultrasound (US) waves propagate through the tissue to the body surface where they canbedetected[4].

The advantages of PAI rely on its hybrid nature and the combination of thetwo imaging methods:opticalimaging and ultrasoundimaging.Bycouplingthem,thismodalityovercomes someoftheirlimitations.Moreprecisely,PAIfeaturesresolutionof ultrasound imaging while its contrast derives from the optical absorption.Thisimagingmodality is particularlyinterestingfor vascularimagingasbloodisastrongopticalabsorberinthenear infrared and presents a good contrast with the surrounding medium[5].

The theoretical PAsignal generated by a spherical absorber surroundedbyalosslessmediumisashortpulse,characterizedby asingle-cycleshape,whichisoftenreferredtoasN-shaped.This *Correspondingauthor.

E-mailaddress:[email protected](F.Varray).

http://dx.doi.org/10.1016/j.pacs.2017.09.001

ContentslistsavailableatScienceDirect

Photoacoustics

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shape results from the summation of a diverging compressive

wave coming from the absorber and a converging

compressivewave comingfromthecenter oftheabsorber and reaching the detector with a delay, as a rarefaction wave [6]. ThefrequencybandwidthofaPAsignalincreasesastheabsorber gets smaller. Consequently, the spectral content of a PA signal generated by biological tissues may range from 1MHz up to 100MHz[4,6].

PAsignalscan berecordedusing anUS probe coupledwith an US scanner. However, classical piezoelectric US probes, using PZT technology, have a limited bandwidth in both transmissionandreception.Anemergingalternativetechnology, CMUT(capacitivemicromachinedultrasoundtransducer,[7]),can overcome this limitation. As compared to conventional PZT transducers, CMUTs may offer higher sensitivity and wider bandwidth[8].As speci_fically regardsreception (RX) operation inidealelectricalloadingconditions,whilethevoltagefrequency responseofPZTtransducershasaband-passcharacteristic,CMUTs exhibit low-pass voltage [9] and charge [10] RX frequency responsestheoreticallyreaching a200%fractionalbandwidthin reception.Suchpeculiarbroadbandcharacteristic,together with thehigherRXsensitivity,motivatedthefirstinvestigationsonthe use of CMUTs in PAI [11,12]. Several research groups have researched on the potentialof CMUTs for PAI. In [13], in vitro three-dimensional PAI results using 2D CMUT arrays were successfullyobtained.In[14],aparticularCMUTtechnologywas establishedtofabricateanoptical-acousticintegratedPAimager, consistingofaninfrared-transparentUSarraybackedbyanoptical source.Furthermore,thetechnologicaladvantagesofCMUTswere exploitedintherealizationofminiaturizedarraysforboth two-dimensional [15] and three-dimensional [16] endoscopic PAI. However,thecurrentliteraturelacksinformationonthepotential performance increase achievable byusing CMUT technologyin place of classical PZT technology. Only recently, a RX-mode operationperformance comparison betweenaPZT linearprobe andanequivalent CMUTprobewas carriedout[17,18],and the benefits achievable by the higher sensitivity and the wider acceptance angle of the CMUT probe were discussed in a PA imagingapplicationcontext.

This paper explores the potential advantages achievable by usingCMUTtechnologyforPAapplicationsthroughaqualitative and quantitative imaging assessment. A preliminarystudy was conductedin[19]andisextendedhereafter.Acomparativestudyis carriedoutbyconductinginvitroimagingexperimentsonasuture wire and onbimodal phantoms using differentPZTand CMUT probes.TwoCMUTlinearprobes,developedandmanufacturedby different research laboratories, were used in conjunction with openUSscannerstoacquirePAsignalsandgeneratePAimagesof thephantoms.Thesameexperimentswerecarriedoutusingtwo linear PZT commercial probes from different manufacturers. Qualitativeand quantitative criteria, basedon thecomputation ofpurereceiveimpulseresponse,signal-to-noiseratio(SNR)and contrast-to-noise ratio (CNR), were used to assess PA image quality.

2.Materials

2.1.Photoacousticexperimentalset-up

ThePAimagingset-upemployedforthisstudyconsistsofa Q-switched Nd:YAG laser at 1064nm (Quanta-ray INDI, Spectra-Physics)delivering5-nslaserpulsesatarepetitionrateof10Hz. The beamdiameteris 8mm. Thespatialimpulse responsewas measuredusinga100

m

mblackabsorbingsuturewire(Ethilon5-0, Polyamid6,Ethicon)placedinawatertank.Thesuturewirewas illuminated through the Nd:Yag sourcecoupledwith a 10-mm fiberbundle(CeramOptecGmbH,Bonn,Germany)composed of 431individualopticalfibers,eachwith0.3mmsilicacorediameter and0.22numericalaperture.Inallmeasurements,the illumina-tionpositionwasfixedwithrespecttothesuturewireposition.The photoacoustic signal detection was made _first with a needle hydrophone (Preamplifier W235052, PrecisionAcoustics), verti-callyplaced25mmabovethesuturewire,usinganoscilloscope. Thefrequencybandofthehydrophoneiswideenoughinorderto acquirethecompletespectralcontentofthePAsignalgeneratedby the suture wire. The measured signal is averaged over 100 acquisitions.Then,thedetectionofthesamePAsignalisconducted usingthedifferentUSprobesusedinthisstudy.Inall measure-ments, the probes werepositioned in a way that the distance betweenthesuturewireand theclosestprobeelement,i.e.the borderelementoftheactiveaperture,issettothesamedistanceof 25mm.ThePAsignalwasthenacquiredusingalltheelementsof theactiveapertureinordertoevaluatetheangularacceptanceof theprobes,i.e.theevolutionofthereceptionfrequencyresponse asafunctionofthearrivalangleofthePAsignalontheprobe.The experimentalsetupishighlightedinFig.1.

For the phantom acquisitions, the laser beam was directed towards the phantoms while the US probe was positioned perpendicularly tothelaser excitation,as shownin Fig. 2. The signal reception was optimized by improving the US coupling betweentheprobeandthephantomusingUSgel.Thedescribed set-upallowsrecordingsimultaneouslybothPAandUSimages. Fig.1.Experimentalset-upforthepurereceiveimpulseresponseoftheUSprobesofthestudy.Thecomputationismadeintwosteps:(a)receivingthesignalonthe hydrophone,(b)receivingthesamesignalontheUSprobes.The’angleallowsevaluatingtheacceptanceangleofthevariousprobes.

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2.2.Ultrasoundimagingexperimentalset-up

In ordertocompareCMUTand PZTtransducertechnologies, fourdifferentlinearprobeswithrathersimilarcharacteristicswere employedintheexperiments.ThetwoCMUTprobesarea 128-element,8.9MHzlinearprobefromVermon(VermonSA,Tours, France),and a 192-element,10MHz linear probefrom ACULAB (DepartmentofEngineering,RomaTreUniversity,Rome,Italy).The two commercial PZT probes are the L14-5W/60 128-element, 7.5MHzlinearprobe(ProsonicCo.,Gyeongsangbuk-do,Korea)and the LA523E 192-element, 8.5MHz linear probe (Esaote S.p.A., Florence,Italy).

Being provided with different system connectors, the four probescouldnotbeusedonthesameUSscanner.Therefore,two differentUSscannershavebeenusedinthisstudy.Theﬁrstoneis the clinical US scanner SonixMDP (Ultrasonix, Analogic Corp, Peabody, MA, USA), coupled with the additional SonixDAQ acquisitionmodule. This module allowsthe acquisition of raw radiofrequency(RF)prebeamformingdataon128independentRX channels,andthesynchronisationoftheUSscannerandthelaser system.ThesecondUSscanneristheUltrasoundAdvancedOpen Platform(ULA-OP,MSDLab,UniversityofFlorence,Florence,Italy), whichcanbefullyprogrammedandallowscontrollingupto64 independentRXchannels[20].

Thebasiccharacteristicsoftheprobesandthecorresponding USsystememployedaresummarizedinTable1.

2.3.Phantoms

Bimodalphantoms,constitutedofopticalabsorbersembedded in tissue mimicking materials, havebeen specifically manufac-tured for this experimental study. Two components havebeen selectedforthebulkmaterial:PVAcryogel(PolyVinylAlcohol,10%) andagar(Agaragar4%).Thesetwomaterialsexhibitadifferent reducedscatteringcoefficient.A highreduced scatteringcoef fi-cientimpliesthatonlyasmallfractionoftheincominglaserlight willreachtheabsorber,leadingtoalowPAsignaltobedetectedby theUSprobe. After5 freezecycles, thePVAcryogel presentsa reduced scatteringcoefficient of about

m

0s¼4cm1 [21], along withanegligibleabsorptioncoefﬁcient(

m

a1cm1)andaspeed

ofsoundof1520ms1.Thereducedscatteringcoefﬁcientinagaris

about

m

0

s¼1cm1,theabsorptioncoefﬁcientisnegligibleandthe speedofsoundisabout1475ms1.Foreachmaterial,aspherical inclusion(10mmindiameter)ofthebulkmaterialisincluded.The inclusionwaspreviouslycolouredbyIndiaink(concentrationof 0.03%). They are approximatively 1cm deep from the external surface.Picturesofthephantomsusedforthiscomparativestudy arepresentedFig.3.InFig.3(c),apictureofthetaintedinclusion beforebeingembeddedinthephantomisalsoprovided. 3.Methods

3.1.CalibrationoftheSonixMDPandtheULA-OP

ToobtaincomparablevaluesandsignalsontheSonixMDPand theULA-OPsystems,acalibrationprocedurehasbeenestablished. Acustom-builtadaptorwasdesignedand fabricatedinorderto connectthePZTL14-5W/60andtheCMUTVermonprobes,which areboth providedwiththeSonixMDPsystem connector,tothe ULA-OP,allowingtherawRFsignalsacquisition.However,because theULA-OPhardwareallowssimultaneousacquisitionofonly64 signals,theacquisitionswereconductedintwostepsinorderto collectthesignalsof theentirearrays.Anagarphantomwitha strongtaintedinclusionwasilluminatedwiththelaser.Foreach probeandforagivendrivinglaseramplitude,thereceivedechoes werestoredonboththeSonixMDPandtheULA-OPsystems.For each reconstructed image, the maximum amplitude inside the inclusionwasextracted.Theratiobetweenthetwovaluesgavethe conversionratiobetweentheSonixMDPandtheULA-OPforthe same laser excitation,correspondingtothesame initial photo-acousticsignal.Bychangingtheexcitationamplitudeoflaser,the conversionratioevolutionbetweentheSonixMDPandtheULA-OP wascomputed.

3.2.Evaluationofthereceiveimpulseresponse

UsingtheproposedexperimentalsetupproposedinFig.1,the purereceiveimpulseresponseofthetransducercanbecomputed from the signals detected by the hydrophone and the probe element.Indeed,thesignalreceivedbytheprobeelement,s(t),can beexpressedas

sðtÞ¼IRRXðtÞsPAðtÞ ð1Þ

Table1

CharacteristicsoftheUSprobeusedintheexperimentsinconjunctionwiththeSonixMDPandtheULA-OPsystem.Thevaluesaregivenbythemanufacturersoftheprobes.

SonixMDP ULA-OP

PZTL14-5W/60 CMUTVermon PZTLA523E CMUTACULAB

Centralfrequency(MHz) 7.5 8.9 8.5 10

6dBfractionalbandwidth(pulse-echomeasurement) 80% 70% 92% 100%

Numberofelements 128 128 192 192

Pitch(mm) 472 150 245 200

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whereIRRX(t)isthepurereceiveimpulseresponseoftheprobe

elementandsPAisthePAsignalgeneratedbythesuturewireand

detected by the hydrophone. The IRRX can be conveniently

computedinthefrequencydomainas[10]: log 10ðIRRXðfÞÞ¼ log 10ðsðfÞÞ

log 10ðsPAðfÞÞ ð2Þ

The 6dB center frequency in reception and bandwidth are computedfromtheresultingspectrumbyﬁndingthetwo6dB cutofffrequenciesfhighandflow.Then,thefractionalbandwidthis

computedaspercentageratiobetweenthe6dBbandwidthand centerfrequencyas

BW_6dB¼2fhighflow fhighþflow

100% ð3Þ

Toevaluatetheacceptanceangleofeachprobe,thespectraofthe signalsreceivedbyalltheprobeapertureelementsareevaluated asafunctionoftheangle

’

,followingthemeasurementprocedure proposedin[18].Thesubtractionofthehydrophonesignalisalso conductedtoevaluateonlythereceiveIRofeachprobeelement. Foreachprobe,thespectralsensitivityisevaluatedinthe[0;35] range.

3.3.Phantomacquisitions

Threecomparativestudieshavebeencarriedout.Theﬁrsttwo studiescomparetwopairsofprobes(PZTandCMUT)connectedto twodifferentUSscannersusingthesametargetedagarmedium withthecolouredinclusion.ThethirdstudycomparestheULA-OP PZTand CMUT probes onthe PVA tissue mimicking materials, which present a higher optical scattering coefﬁcient. The acquisitionsstrategyissummarisedinTable2.

The three studiesare made withboth US probesat several energy levels up to 195mJ/pulse. For each conﬁguration, a backgroundimagehasbeentaken,withoutanylaserexcitation, toevaluatetheglobalnoiseofthesystem.Foreachenergylevel, thirtyframeswiththefullarrayonafewcentimetresdepthare saved before proceeding to theimage reconstruction. For each phantom, an ultrasonic image (B mode image) has also been recordedinthesamecon_ﬁguration.HencethePAsignalscanbe overlaidonthecorrespondingB-modeimage.

3.4.Imagereconstruction

Theacquireddataarereconstructedusingk-Wavepackage[22]. Theechoesinitiallyreceivedbythelinearprobesareexpressedin theFourierdomainandthenresampledtobeamformtheimage. TheusedsamplingintheFourierdomainisdeﬁnedby:

kt2½0;

p

=dt kx2½0;c

p

=dx k¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffik2tþk 2 x q 8 > < > : ð4Þ

where dt being thesampling time, dx being thelateral spatial sampling,cisthespeedofsound,ktandkxarethetemporaland

spatial wave numbers. The focalisation of received echoes is

processedbyre-samplingthespectra.Hence,foreachspatialline ofthespectra,aninterpolationismadefromthektaxisonthe

modiﬁedk axis.This way,the echoesare focusedand a better reconstructionfromPAsignalsascomparedtothedelay-and-sum algorithm is obtained[22]. Anotheradvantageis itscalculation speedanditsreal-timeimplementation.

3.5.Quantiﬁcation

Twocriteriahavebeenusedtoevaluatetheimagesqualityand comparetheacquisitions madewithPZTand CMUTprobes:the signal-to-noiseratio(SNR)andthecontrast-to-noiseratio(CNR). To calculate such indexes, two regions of interest (ROI) were deﬁnedontheimagesofthesphericalinclusions:oneforthesignal (S)andoneforthenoise(N).TheseROIarecirclesof10mmin diameter.Fig.4showsanexampleoftwoROI.

TheSNRandCNRarecalculatedfromthefollowingexpressions

[23]: SNR¼20 log 10 S N ! ð5Þ CNR¼20 log 10 SN

s

0 0 @ 1 A _ð6Þ

ð:Þbeingthemeaninthede_ﬁnedROIand

s

0beingthestandard

deviationofpurenoiseintheROI. 4.Results

4.1.SonixMDPandULA-OPcalibration

Forthecalibration,5differentenergylevelswereusedonthe laser.Theusedenergylevelsare125,160,175,190and195mJ/ impulsion.ForeachacquisitionontheULA-OPortheSonixMDP, themeanandstandarddeviationofthereceivedamplitudewere computed. The resulting curve is presented in Fig. 5. A linear regressionwascomputedwithanR2_value_of_98.5%._For_each_new

SonixMDP acquisition, this calibration allowed converting the acquiredPAsignalsintotheULA-OPrange,makingitpossibleto compare acquisitions carried out with different scanners. Table2

Acquisitionstrategy.Instudy1and2,thesameAgarphantomisscannedusingthe twoUSscannerswiththefourprobes.Instudy3,thePVAphantomisscannedusing theULA-OPandthecorrespondingprobes.

Study Probes USscanner Phantom

1 PZTL14-5W/60 – CMUTVermon SonixMDP Agar

2 PZTLA523E – CMUTACULAB ULA-OP Agar

3 PZTLA523E – CMUTACULAB ULA-OP PVA

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However,giventhelimitednumbersofactiveelementsofthe ULA-OP, the acquisitions of study 1 are still conducted with the SonixMDPinordertodirectlyacquirethePAsignalonthewhole arrayforitsnativeprobes(PZTL14-5W/60andCMUTVermon). 4.2.Receiveimpulseresponsemeasurement

ForeachacquisitionontheULA-OPsystem,anaccumulationof 100acquisitionshasbeenconducted.Fig.6depictstheobtained frequencyresponsesin purereception, and Table 3 reportsthe correspondingcenter frequencies,the 6dBcutoff frequencies, thefractional bandwidths and thepeak sensitivities. It can be noticed that the reception frequency response of both CMUT probesiscenteredatalowerfrequency,andischaracterizedbya signiﬁcantlylargerfractionalbandwidthascomparedtothePZT probes.Intermsofpeaksensitivity,thevaluesreportedinTable3

arereferredtothehighestvaluethatwasachievedwiththeCMUT ACULABprobe.

InFig.7,thespectralsensitivityofthefourprobesasafunction of the incident angle is proposed. A quantitative comparison betweentheresultsisnotstraightforwardsincethefourprobes have different element widths resulting in different element directivity,beingtheacceptanceanglepartiallyrelatedtothese parameters.Qualitatively,thespectralsensitivityofthetwoCMUT probes shows a lower variation as a function of the angle as compared tothe PZTones.In theparticular case of the CMUT ACULABandtheLA523EPZTprobes,theresultscanbemorefairly compared.Indeed,theCMUTACULAB arrayhasa200

m

mpitch with a practically zero kerf, and the LA523E PZT probe has a 245

m

mpitchwitha 30

m

mkerf,leadingtoa 215

m

melement width.Thesimilaritybetweenthetwoelementwidthsallowsafair comparisonbetweenthespectralsensitivities.Ascanbeobserved, thesensitivityreductionasafunctionoftheincidentangle,inthe frequencyrange3.2–7.1MHz,wherethereception6dBbandsof thetwoprobesoverlap,islowerfortheCMUTprobe,evidencingits broaderacceptanceangle.

4.3.ComparisonofthePZTandCMUTprobesontheagarphantom Thestudies1and2comparethetwoprobetechnologiesonthe same phantom.This comparison isconducted ontwo different scanners:onepairof PZT/CMUTprobesisusedoneachsystem (Table2).

Fig. 8 shows PA images obtained on the agar phantoms containing the coloured spherical inclusions withthe different tested probes. On this phantom, the SNR and CNR have been calculatedforthedifferentvaluesoftheexcitationenergyandfor bothprobesofeachpair.ResultsareshowninFig.9.

Thiscomparativestudyshowsasigniﬁcantimprovementofthe SNRandCNRwhenusingtheCMUTtechnologyforeachprobes pair.TheSNRisincreasedbyupto14dBandtheCNRupto20dB. Nonetheless,itcanbeobservedthattheresultsobtainedbythe Fig.5.CalibrationcurveoftheSonixMDPandULA-OPsystemusingthesame

probes.

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LA523EPZTprobeandVermonCMUTprobearesimilarandthat theLA523EhasevenabetterCNRthattheVermonCMUTprobe. 4.4.EnhancementofthePAimagequalityonahighlyscattering phantom

Thepreviouscomparison hashighlightedtheinterestof the CMUTtechnology forPAimaging.Itenhancestheimagequality evenatarelativelylowinputenergy,whichalsomeansalowPA signal reachingthe probe. This is crucial in clinical conditions when the medium complexity and high scattering coefﬁcient preventsmostoftheinputphotonstoreachtheabsorber.

Withthisinmind,theCMUTprobegivingthebestresults,i.e. CMUTACULAB, hasbeentested ona more scatteringphantom madeofPVA.Theresultsarethencomparedwiththeonesfromthe PZTprobeofthesamepair,onbothagarandPVAphantoms.The correspondingreconstructedimagesaredisplayedinFig.10.Fig.11

presentsthecalculatedSNRandCNRforbothprobesandonboth phantomsfordifferentvaluesoftheexcitationenergy.

FromFig.10,itcanbeobservedthat,wherethePZTprobefails todetectenoughsignaltoreconstruct theinclusion image,the CMUTprobedetectsmoresignalcomingfromtheinclusion,giving moreinformation abouttheimagedmedium.TheSNRvaluesin thePVAphantomarehigherwiththeCMUTACULAB,fromafewdB upto20dB(Fig.11).

5.Discussion

The imaging performance of CMUT and PZT probes was

quantitativelyassessed and compared ina PAexperimental set up.Invitroimagesof customdesignedbimodalphantomswere achievedusingfourdifferentCMUTandPZTprobesandtwoUS scanners.By usingtheproposed calibrationprocedure,thedata acquiredwiththetwodifferentscannersweremadecomparable. The characterizationin purereception showed that the CMUT probes, which are both provided with high-input-impedance voltagebuffersintegratedintheprobehandle,exhibitalow-pass frequencyresponse,resultinginanincreasedsensitivityatlower frequenciesandagreaterfractionalbandwidthascomparedtothe

PZT probes. Moreover, the CMUT ACULAB probe showed an

increased peak sensitivity as compared to the CMUT Vermon probe.Thespectralsensitivitymeasurementsasafunctionofthe incidentanglegenerallyshowedthat,withtheCMUTprobes,the frequencycontentisbetterpreservedasafunctionoftheangle betweenthePAsignalsourceandtheprobeelementascompared tothePZTprobes.TheperformanceoftheCMUTVermonisslightly better than the CMUT ACULAB due to the smaller pitch and element width(150

m

mvs.200

m

m). For thephantoms acquis-itions,inordertoensurerepeatabilityofmeasurements,particular attentionwasdevoted,duringphantomsdesignandfabrication,on the dimensioning and positioning of the inclusions in the Table3

Receptionfrequencyresponseperformancecomparison.

PZTL14-5W/60 CMUTVermon PZTLA523E CMUTACULAB

Centralfrequency(MHz) 7.2 2.6 7.3 3.8

6dBfrequencyinterval(MHz) [3.6;10.8] [0.5;4.8] [3.2;11.3] [0.5;7.1]

Bandwidth 99.3% 161.3% 111.3% 175.6%

Peaksensitivity(dB) 1.0 4.2 11.6 0

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surroundingmedium.Theexperimentalimagingresultsobtained in the experiments showed that, in general, a qualitative and quantitativeimprovementwasachievedusingtheCMUTprobes.In fact,oneachscanner,theSNRandCNRcomputedontheimages reconstructedfromthedataacquiredwiththeCMUTprobewere higher. In the PVA phantom experiments, in particular, the detection and correct reconstruction of the coloured inclusion waspossibleonlyusingtheCMUTprobe.

The proposed study suffers from some limitations. First, to comparethesignalsreceivedonthefourprobes,acalibrationstep hasbeen conductedina photoacoustic experiment.Based ona custom-builtadaptor,itallowstoacquiredonasingleUSscanner thePAsignalsreceivedonallprobes.Forthecomparisonbetween twoprobes,theacquisitions havebeenconductedonthesame phantom.Thepositionwasnotchangedduringtheexperimentand thelaserexcitationwassetatthesamelocation.However,note thatfor thecomparisonstudywe chosetokeep connectedthe probestotheirnativescannersinordertogetthebeneﬁtsofthe settings parameters adapted to each scanner. Of course, the positionoftheprobeonthephantommayhaveslightlychanged

between two acquisitions but this change is limited and the alignment of the probe has been carefully optimized for each situation.However,theSNR andCNRcomparison didnotallow directlyconcludingthatCMUTprobesarebetterthanPZT.Indeed, basedonthesetwocriteria,theperformanceoftheLA523Eand CMUT Vermonprobes areclose.Last,the fourusedprobes are different in terms of pitch and or number of active elements. However,theobserved maximal PAamplitudesdetected in the wholeimagesareinaccordancewiththeobtainedSNRandCNR trends. The study of PZT and CMUT probes having the same geometricaldesignmaybetheoptimalsolutiontoevaluateboth probe performances. However, such hardware solution is not currentlyavailable.

In the proposed comparison, the noise equivalent pressure (NEP) of our arrays has not been conducted. Indeed, we the variabilityoftheusedprobesandsystems,suchmeasurementwill surely by very sensitive to other parameters that cannot be practicallycontrolledduringtheacquisition.Similarly,valuesof theNEPcanbefoundintheliteratureforCMUTarrays,2mPA/

ffiffiffiffiffiffi Hz p

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resolution may appear as a good criterion to evaluate the performance of the various probes. Indeed, in our study, the numberofelementandtheirpitcharedifferentoneachprobeand

such parameters are highly related to the lateral resolution. Moreover,theaxialresolutionisrelatedtotheabsolutefrequency bands, which are also quite different among the probes. The Fig.9.EvolutionsoftheSNRandCNRasafunctionoftheexcitationenergyforbothprobestypesofeachpairintheagarphantom(study1and2,Table2).

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obtainedresolution will then be difﬁcult tocompare between them.

TheapplicationofCMUTtechnologytoPAIisadvantageousfor many reasons. As it was proven in this paper, the larger RX bandwidthallowsdramaticallyimprovingthedetection sensitivi-ty, leading to an increased quality of PA images. As concerns medicalimagingapplications,ahighersensitivitymaybeahighly appreciatedpropertyofatransducerusedinPAclinicalconditions sinceit allows both reducing theoptical excitationenergy and improvingthepenetrationdepth.Moreover,otherCMUT charac-teristics, mainly related to the fabrication technology, may be leveraged in PA applications. The possibility of fabricating transducerarrayswithparticularshapes,suchassparseorring arrays, together with the opportunity of making the device transparenttolightpropagation,opensa bigopportunityinthe fabrication of high-performance miniaturized PA integrated imagers.Besidestheﬁrstinvestigationsreportedin[14],where infrared transparency was achieved by thinning the silicon microfabrication substrate down to 100

m

m, the successful microfabricationofCMUTsonanopticallytransparentsubstrate (i.e. glass), was shown in [26,27]. Furthermore, CMUT with through-glass-viainterconnectsfabricationwasreportedin[28], possibly enabling the access to 3D-integration technologies. Finally,ReverseFabricationProcess[29],i.e.theCMUTtechnology usedforthefabricationoftheCMUTACULABprobetestedinthis paper,allowstheapplicationofacustombackingdirectlytothe CMUTarray[30],afterthe microfabricationonstandard silicon substrates.CurrentresearchisdevotedtothestudyofnovelCMUT probesforthree-dimensionalPAIapplications,basedontheuseof particular 2D array conﬁgurations, and both acoustically and opticallyoptimizedpackagingmaterials.

6.Conclusion

Theproposedstudyconﬁrmedthatthehigheracceptanceangle obtainablewithCMUTsrepresentsoneoftheadvantagesofusing such transducer technology in PAI applications. In addition to previousstudies,itdemonstratedthattheCMUThighersensitivity and wider reception fractional bandwidth represents another importantcharacteristicfordetectingsignalswherethespectral contentisdistributedoveraverywidefrequencyrange,suchasin PAI.Eventhoughthefourprobesusedhavedifferent character-istics in terms of arraygeometry and frequency response, the experimentalresultssupporttheconclusions,suggestingthatthe potential of CMUT technologycan be further leveraged in PAI applicationsbyoptimizingthedesignforhighreceptionsensitivity andwidebandoperation.

Conﬂictofinterest

Theauthorsdeclarenoconﬂictsofinterest. Acknowledgments

ThisworkwassupportedbytheLABEXCELYA (ANR-10-LABX-0060)and PRIMES(ANR-11-LABX-0063) ofUniversitéé deLyon, within the program “Investissements d’Avenir” (ANR-11-IDEX-0007)operatedbytheFrenchNationalResearchAgency(ANR).The CMUTprobefromVermonwaspartoftheprogramANRBBMUTof UniversitédeLyon.TheCMUTprobefromACULABwaspartofthe collaborative research agreement between the Department of Engineeringof Roma Tre Universityand CREATIS, Universityof Lyon.ThisworkwasalsopartiallysupportedbyITN-FP7OILTEBIA. References

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MaëvaValletwasborninNîmes,France,in1988.She graduatedfromtheengineeringschoolPhelma(Grenoble INP)inGrenoble,France,in2012withafocusinphysics andnanosciencesandobtainedherPh.D.in2015.HerPh. D.researchonphotoacousticimagingwasrealizedin co-agreementbetweentheCentredeRechercheen Acquisi-tionetTraitementdel’ImagepourlaSanté(CREATIS), Lyon,FranceandtheLaboratoired’ImagerieetSystèmes d’Acquisition(LISA)ofCEA-LETI,Grenoble,France.Sheis currentlyworkingasanR&Dengineerinabiotech start-upinCambridge,UnitedKingdom.

FrançoisVarraywasborninMontpellier,France,in1985. HereceivedtheEngineeringDiplomaandthemaster's degreeinimageandsignalprocessingfromtheEcoledes MinesdeSaint-Etienne,Saint-Etienne,France,in2008, andthePh.D.degreewithafocusonnonlinearultrasound simulationin2011.HisPh.D.researchwasrealizedin co-agreementbetweentheCentredeRechercheen Acquisi-tionetTraitementdel’ImagepourlaSanté(CREATIS), Lyon,France andtheMSDLaboratory, Florence, Italy. Since 2013, he hasbeenan Associate Professor with CREATIS. Hisresearchinterests includethe nonlinear ultrasound propagation simulation, nonlinear image simulation,multi-resolutionmotionestimation,cardiac imaging,andphotoacousticimaging.

Dr.JérômeBoutet,graduatedasanengineerinPhysics fromInstitutNationalPolytechniquedeGrenoble(INPG). Then,heobtainedaPh.D.inPhysicsforLifeSciencesfrom GrenobleUniversity.AfterjoiningCEA-LETI,hebecame project managerand coordinator of theANR-TECSAN PROSTAFLUOprojectwhichaimedtodevelopanoptical probetoimproveprostatecancerdiagnostics.Heisnow project manager of project BITUM (Investissements d’avenir,callNanobiotechnologies)onthesamesubject. Thesedevelopmentshaveledtotheﬁleof17patentsand the publication of 18 papers, 1book chapter and 8 reviewedproceedings.

Jean-MarcDINTENisSeniorScientistattheBiologyand HealthDivisioninCEA-LETI.Formorethan20years,he hasbeenwidelydevelopingmedicalimageprocessing andreconstructionalgorithmsassociatedtothe develop-mentofinnovativeX-RaysandOpticalimagingsystems. HenowheadstheImagingReadoutSystemsLaboratory whichdevelopsnewopticalimagingsystemsforhealth andbiologyapplications.

Giosuè Caliano received a M.S. degree in electronic engineeringfromtheUniversityofSalernoin1993.After receivingthedegree,heservedasapostgraduatefellow intheDepartment ofElectronicsof theUniversityof Salerno,bothindidacticandresearchfields.Hisinterests wereindevelopingpiezoelectricpressuresensorsandin measurementtechniquesforceramics’characteristics.In 1995,hejoinedPirelli-FOSasanIndustrialAutomation Engineer.Inthisposition,heworkedasdesignengineer foropticalfiberproduction.Since1997,hehasworkedat theACULAB,DepartmentofEngineering,UniversityRoma Tre,asheadofthelaboratory,andsince2012,hehasbeen AdjunctProfessorof“SensorsandTransducers”.In2014, he obtained the“National AcademicQualification”as AssociateProfessorinElectronics.

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AlessandroStuartSavoiawasborninEdinburgh, Scot-land(UK),in1978.HereceivedtheLaureaandthePh.D.in ElectronicEngineeringfromUniversitàdegliStudiRoma Tre,Rome,in2003and2007,respectively.Hehashelda postdoctoral research position at the Department of Electronics Engineering of the same University since 2007.Intheyears2008–2010heparticipated,asa co-founder and R&D Manager, in an academic spin-off companyofRomaTreUniversityincollaborationwiththe medicaldevicecompanyEsaoteS.p.A.,grantedbythe ItalianMinistryofEducation(MIUR),fortheindustrial exploitation of the scientific results on MEMS-based ultrasonictransducers(CMUTs),mostofthemachieved duringhisPh.D.andPost-Doctoralresearch.In2014,hebecameAssistantProfessor inElectronicsattheDepartmentofEngineeringofRomaTreUniversity.In2017he obtainedtheNationalScientificQualification(ASN)forAssociateProfessorinthe scientificfield“Electronics”.

HehasconductedresearchactivityintheAcoustoelectronicsLaboratory(ACULAB) mainlyintheﬁeldofultrasonictransducersandtheirapplications.Duringhis scientiﬁc career, he has focused on analytical and FEM modeling, design, microfabricationandpackaging,characterization,electronicsandsystem integra-tionofMEMS-basedCapacitiveMicromachinedUltrasonicTransducers(CMUTs). His research interests also include piezoelectric ultrasonic transducers, and

ultrasound beamforming and imaging techniques for medical and biometric applications.

Dr.Savoiahasauthoredandco-authoredabout60papersininternationaljournals andconferences,andthreebookchapters.Heholds4internationalpatents.Hehas carriedoutconsultancyactivitiesasascientiﬁcadvisorintheﬁeldof acousto-electronicsforseveralsemiconductorandmedicaldevicecompanies.