Thermoacoustic and photoacoustic sensing oftemperatureManojit PramanikLihong V. WangWashington University in St. LouisDepartment of Biomedical EngineeringOptical Imaging LaboratoryCampus Box 1097One Brookings DriveSt. Louis, Missouri 63130Abstract. We present a novel temperature-sensing technique usingthermoacoustic and photoacoustic measurements. This noninvasivemethod has been demonstrated using a tissue phantom to have hightemporal resolution and temperature sensitivity. Because both photoa-coustic and thermoacoustic signal amplitudes depend on the tempera-ture of the source object, the signal amplitudes can be used to monitorthe temperature. A temperature sensitivity of0.15°C was obtained ata temporal resolution as short as2s, taking the average of 20 signals.The deep-tissue imaging capability of this technique can potentiallylead us to in vivo temperature monitoring in thermal or cryogenicapplications.© 2009 Society of Photo-Optical Instrumentation Engineers.关DOI: 10.1117/1.3247155兴Keywords: thermoacoustics; photoacoustics; temperature sensing; tissue phantom;tissue temperature.Paper 09158R received Apr. 23, 2009; revised manuscript received Jul. 7, 2009;accepted for publication Aug. 6, 2009; published online Oct. 12, 2009.1 IntroductionDuring thermotherapy or cryotherapy, it is necessary to moni-tor the temperature distribution in the tissues for the safedeposition of heat energy in the surrounding healthy tissueand efficient destruction of tumor and abnormal cells. To thisend, real-time temperature monitoring with high spatial reso-lution共⬃1mm兲 and high temperature sensitivity 共1°C orbetter兲 is needed.1The most accurate temperature monitoringis by directly measuring the temperature with a thermocoupleor thermistor. However, it is invasive, hence, generally notpreferred and often not feasible. Several noninvasive tempera-ture monitoring methods have been developed. Infrared ther-mography is a real-time method with0.1°C accuracy but islimited only to superficial temperature.2Ultrasound can beapplied for real-time temperature measurements with goodspatial resolution and high penetration depth, but the tempera-ture sensitivity is low.3–5Magnetic resonance imaging has theadvantages of high resolution and sensitivity, but it is expen-sive, bulky, and slow.6,7Therefore, an accurate, noninvasive,real-time temperature measurement method needs to be devel-oped.The thermoacoustic 共TA兲 and photoacoustic 共PA兲 effectsare based on the generation of pressure waves on absorptionof microwave and light energy, respectively. A short micro-wave and laser pulse is usually used to irradiate the tissue. Ifthermal confinement and stress confinement conditions aremet, then pressure waves are generated efficiently. The pres-sure rise of the generated acoustic wave is proportional to adimensionless parameter called the Grueneisen parameter, andto the local fluence. The local fluence depends on the tissueparameters, such as the absorption coefficient, scattering co-efficient, and anisotropy factor, and does not change signifi-cantly with temperature. However, the Grueneisen parameter,which depends on the isothermal compressibility, the thermalcoefficient of volume expansion, the mass density, and thespecific heat capacity at constant volume of the tissue,changes significantly with temperature. Thus, the generatedTA/PA signal amplitude changes with temperature. Here, weshow that by monitoring the change in the TA/PA signal am-plitude, we were able to monitor the change in temperature ofthe object.The TA/PA technique has been widely applied in biomedi-cal imaging applications, such as breast cancer imaging, brainstructural and functional imaging, blood-oxygenation and he-moglobin monitoring, tumor angiogenesis, and, recently, formolecular imaging.8–22Lately, PA sensing has also been usedto monitor tissue temperature.1,23–27However, TA sensing oftemperature has never been studied. These two techniques donot interact and can be used independently. Depending on theneed, one has to choose which technique to use. The maindifference between these two techniques is the contrastmechanism. For example, water and ion concentrations arethe main sources of contrast in TA measurements, whereasblood and melanin are the main sources of contrast in PAmeasurements. Therefore, if we need to monitor the tempera-ture of a blood vessel, then the PA technique will be moreuseful; whereas if we need to monitor the temperature ofmuscles, then the TA technique will be preferred. TA/PA tem-perature sensing is a noninvasive, real-time method. TheTA/PA technique has the ability to image deeply 共up to5cm兲with high spatial resolution 共scalable: millimeters to microns兲.We can monitor temperature with high temporal resolutionand high temperature sensitivity 共scalable with temporal res-olution:⫾0.015 and ⫾0.15° C at 200 s 共2000 measurementsaveraged兲 and2s 共20 measurements averaged兲 resolutions,1083-3668/2009/14共5兲/054024/7/$25.00 © 2009 SPIEAddress all correspondence to: Lihong Wang, Washington University in St.Louis, Department of Biomedical Engineering, Optical Imaging Laboratory,Campus Box 1097, One Brookings Drive, St. Louis, MO 63130. Tel: 共314兲 935-6152; Fax: 共314兲 935-7448; E-mail: [email protected] of Biomedical Optics 14共5兲, 054024 共September/October 2009兲Journal of Biomedical Optics September/October 2009쎲Vol. 14共5兲054024-1Downloaded from SPIE Digital Library on 13 Oct 2009 to 128.252.20.193. Terms of Use: http://spiedl.org/termsrespectively兲. Because microwaves penetrate more deeplyinto tissue than light, we can potentially monitor temperaturein vivo for locations deep inside the body.2 Theoretical BackgroundIf the microwave/laser excitation is much shorter than boththe thermal diffusion 共i.e., the excitation is in thermal confine-ment兲 and the pressure propagation 共i.e., the excitation is instress confinement兲 in a heated region, the fractional volumeexpansiondV/ V can be expressed asdVV=−␬p +␤T,where␬is the isothermal compressibility,␤is the thermalcoefficient of volume expansion, andp and T denote changesin pressure 共measured in Pascal兲, and temperature 共in Kelvin兲,respectively.When the fractional change in volume is negligible underrapid heating, the local pressure rise immediately after themicrowave/laser excitation pulse can be derived asp0=␤T␬=␤␬␳Cv␩thAe= ⌫␩thAe,where␳denotes