ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 51, no. 5, may 2004 551On the Thermal Effects Associated withRadiation Force Imaging of Soft TissueMark L. Palmeri, Senior Member, IEEE,andKathrynR.Nightingale,Member, IEEEAbstract—Several laboratories are investigating the useof acoustic radiation force to image the mechanical prop-erties of tissue. Acoustic Radiation Force Impulse (ARFI)imaging is one approach that uses brief, high-intensity, fo-cused ultrasound pulses to generate radiation force in tis-sue. This radiation force generates tissue displacements thatare tracked using conventional correlation-based ultrasoundmethods. The tissue response provides a mechanism to dis-cern mechanical properties of the tissue.The acoustic energy that is absorbed by tissue generatesradiation force and tissue heating. A finite element methodsmodel of acoustic heating has been developed that modelsthe thermal response of different tissues during short du-ration radiation force application. The beam sequences andfocal configurations used during ARFI imaging are modeledherein; the results of these thermal models can be extendedto the heating due to absorption associated with other radi-ation force-based imaging modalities. ARFI-induced ther-mal diffusivity patterns are functions of the transducer f-number, the tissue absorption, and the temporal and spatialspacing of adjacent ARFI interrogations. Cooling time con-stants are on the order of several seconds. Tissue displace-ment due to thermal expansion is negligible for ARFI imag-ing. Changes in sound speed due to temperature changescan be appreciable. These thermal models demonstrate thatARFI imaging of soft tissue is safe, although thermal re-sponse must be monitored when ARFI beam sequences arebeing developed.I. IntroductionThe use of acoustic radiation force to interrogate themechanical properties of soft tissues is becoming awidely investigated research area. In general, acoustic radi-ation force is used to excite tissue, and the tissue responseis monitored using either ultrasonic or magnetic resonancemethods. For an analytic description of the mechanical re-sponse of soft tissue to focused acoustic radiation force,the reader is referred to Sarvazyan et al. [2].There are many methods using acoustic radiation forcecurrently under investigation. Vibroacoustography usesfrequency-shifted, confocal beams to generate an oscillat-ing radiation force within tissues, and the tissue responseis monitored either with a hydrophone [3], [4], or by ultra-sonic methods [5]. The KAVE method uses radiation forceto generate a steady-state stress within soft gels and thevitreous humor of the eye, and ultrasonic displacementsManuscript received May 2, 2003; accepted November 25, 2003.This work was supported by NIH grant 8 R01 EB002132, theWhitaker Foundation, and the Medical Scientist Training Programgrant T 32 GM-07171.The authors are with Duke University, Department of BiomedicalEngineering, Durham, NC (e-mail: [email protected]).TABLE INomenclature and Physical ConstantsT Temp erature◦CK Thermal Conductivity 6.0 mW/cm/◦Cκ Thermal Diffusivity (K = κcv) 0.00143 cm2/sτ Perfusion Time Constant sqvHeat Production/Volume J/cm3cvHeat Capacity/Volume 4200 mW·s/cm3/◦Cα Absorption Coefficient dB/cm/MHzpoAcoustic Pressure Paρ Tissue Density 1.0 g/cm3I In situ Acoustic Beam Intensity W/cm2c Speed of Sound 1540 m/sβ Thermal Expansion Coefficient310000◦C−1fcCenter Frequency 7.2 MHzThermal constants correspond to values appropriate for soft tis-sue [1].are monitored to characterize the steady-state response ofthese materials [6]. Supersonic imaging [7], and shear waveelasticity imaging (SWEI) [2], [8], monitor the shear wavesgenerated from short-duration acoustic radiation force tocharacterize the shear modulus of the medium. AcousticRadiation Force Impulse (ARFI) imaging, which is the fo-cus of this paper, uses a commercial diagnostic scannerto generate multiple, short-duration radiation forces to in-terrogate a two-dimensional region of interest (ROI), andmonitors the tissue response using ultrasonic, correlation-based methods [9]–[12]. Similar work is also under inves-tigation using a high-intensity focused ultrasound (HIFU)transducer to generate radiation force in order to monitortreatment [13]. In addition to soft tissue imaging applica-tions, radiation force is being used to manipulate ultra-sonic contrast agents in vitro and in vivo [14], [15].The purpose of the work herein is to determine the in-crease in tissue temperature that is associated with thepulse sequences used in single-location, two-dimensional,shear wave and ARFI M-mode imaging in soft tissue.In addition, the impact of the heating on displacementtracking is evaluated with respect to thermal expansionand sound speed changes. Nomenclature and physical con-stants used in this paper are given in Table I.II. BackgroundA. ARFI ImagingARFI imaging is a radiation force-based imagingmethod that studies the local mechanical properties of0885–3010/$20.00c 2004 IEEE552 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 51, no. 5, may 2004tissue [16]. ARFI imaging uses short-duration (<1ms),high-intensity acoustic pulses to generate localized dis-placements in tissue, and the tissue recovery response ismonitored using ultrasonic correlation-based methods [17].Images of two-dimensional ROIs are generated by sequen-tially interrogating multiple lateral locations, as is donein color Doppler imaging. During the application of high-intensity pulses, energy is absorbed by the tissue that re-sults in the generation of acoustic radiation force, tissuedisplacement, and tissue heating.Herman et al. [18] recently published a study inves-tigating ultrasonic imaging modalities that use short-duration, high-intensity pulses, such as streaming detec-tion [19] and shear wave ARFI imaging [11]. This studydemonstrates, for moderately absorbing soft tissues (0.3–0.5 dB/cm/MHz), that repeated interrogations in a givenlocation result in peak heating at the focus versus the tis-sue surface. Short duration, in situ intensities higher thanthe currently accepted steady-state limit of 0.72 W/cm2are shown to be reasonable in soft tissue, but the presenceof bone interfaces increases the potential for thermal tissuedamage.B. Bio-Heat Transfer EquationAs an acoustic wave propagates through a dissipativemedium, an energy gradient is established in