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Thermoacoustic and photoacoustic sensing of temperature



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Journal of Biomedical Optics 14 5 054024 September October 2009 Thermoacoustic and photoacoustic sensing of temperature Manojit Pramanik Lihong V Wang Washington University in St Louis Department of Biomedical Engineering Optical Imaging Laboratory Campus Box 1097 One Brookings Drive St Louis Missouri 63130 Abstract We present a novel temperature sensing technique using thermoacoustic and photoacoustic measurements This noninvasive method has been demonstrated using a tissue phantom to have high temporal resolution and temperature sensitivity Because both photoacoustic and thermoacoustic signal amplitudes depend on the temperature of the source object the signal amplitudes can be used to monitor the temperature A temperature sensitivity of 0 15 C was obtained at a temporal resolution as short as 2 s taking the average of 20 signals The deep tissue imaging capability of this technique can potentially lead us to in vivo temperature monitoring in thermal or cryogenic applications 2009 Society of Photo Optical Instrumentation Engineers DOI 10 1117 1 3247155 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 Introduction During thermotherapy or cryotherapy it is necessary to monitor the temperature distribution in the tissues for the safe deposition of heat energy in the surrounding healthy tissue and efficient destruction of tumor and abnormal cells To this end real time temperature monitoring with high spatial resolution 1 mm and high temperature sensitivity 1 C or better is needed 1 The most accurate temperature monitoring is by directly measuring the temperature with a thermocouple or thermistor However it is invasive hence generally not preferred and often not feasible Several noninvasive temperature monitoring methods have been developed Infrared thermography is a real time method with 0 1 C accuracy but is limited only to superficial temperature 2 Ultrasound can be applied for real time temperature measurements with good spatial resolution and high penetration depth but the temperature sensitivity is low 3 5 Magnetic resonance imaging has the advantages of high resolution and sensitivity but it is expensive bulky and slow 6 7 Therefore an accurate noninvasive real time temperature measurement method needs to be developed The thermoacoustic TA and photoacoustic PA effects are based on the generation of pressure waves on absorption of microwave and light energy respectively A short microwave and laser pulse is usually used to irradiate the tissue If thermal confinement and stress confinement conditions are met then pressure waves are generated efficiently The pressure rise of the generated acoustic wave is proportional to a dimensionless parameter called the Grueneisen parameter and to the local fluence The local fluence depends on the tissue parameters such as the absorption coefficient scattering coAddress 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 314 9356152 Fax 314 935 7448 E mail lhwang biomed wustl edu Journal of Biomedical Optics efficient and anisotropy factor and does not change significantly with temperature However the Grueneisen parameter which depends on the isothermal compressibility the thermal coefficient of volume expansion the mass density and the specific heat capacity at constant volume of the tissue changes significantly with temperature Thus the generated TA PA signal amplitude changes with temperature Here we show that by monitoring the change in the TA PA signal amplitude we were able to monitor the change in temperature of the object The TA PA technique has been widely applied in biomedical imaging applications such as breast cancer imaging brain structural and functional imaging blood oxygenation and hemoglobin monitoring tumor angiogenesis and recently for molecular imaging 8 22 Lately PA sensing has also been used to monitor tissue temperature 1 23 27 However TA sensing of temperature has never been studied These two techniques do not interact and can be used independently Depending on the need one has to choose which technique to use The main difference between these two techniques is the contrast mechanism For example water and ion concentrations are the main sources of contrast in TA measurements whereas blood and melanin are the main sources of contrast in PA measurements Therefore if we need to monitor the temperature of a blood vessel then the PA technique will be more useful whereas if we need to monitor the temperature of muscles then the TA technique will be preferred TA PA temperature sensing is a noninvasive real time method The TA PA technique has the ability to image deeply up to 5 cm with high spatial resolution scalable millimeters to microns We can monitor temperature with high temporal resolution and high temperature sensitivity scalable with temporal resolution 0 015 and 0 15 C at 200 s 2000 measurements averaged and 2 s 20 measurements averaged resolutions 1083 3668 2009 14 5 054024 7 25 00 2009 SPIE 054024 1 September October 2009 Downloaded from SPIE Digital Library on 13 Oct 2009 to 128 252 20 193 Terms of Use http spiedl org terms Vol 14 5 Pramanik and Wang Thermoacoustic and photoacoustic sensing of temperature respectively Because microwaves penetrate more deeply into tissue than light we can potentially monitor temperature in vivo for locations deep inside the body 2 Theoretical Background If the microwave laser excitation is much shorter than both the thermal diffusion i e the excitation is in thermal confinement and the pressure propagation i e the excitation is in stress confinement in a heated region the fractional volume expansion dV V can be expressed as dV p T V where is the isothermal compressibility is the thermal coefficient of volume expansion and p and T denote changes in pressure measured in Pascal and temperature in Kelvin respectively When the fractional change in volume is negligible under rapid heating the local pressure rise immediately after the microwave laser excitation pulse can be derived as p0 T A thAe Cv th e where denotes mass density Cv denotes specific heat capacity at constant volume Ae is the specific optical microwave absorption and th is the percentage of absorbed


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