Thermal effects associated with acoustic radiation force impulse imaging
Acoustic Radiation Force Impulse (ARFI) imaging uses brief, high-intensity, focused ultrasound pulses to generate radiation force in tissue. The absorption of acoustic energy also generates tissue heating. The purpose of this work is to study the maximum acoustic intensities that can safely be used in ARFI imaging, and to characterize the effects of tissue heating on ARFI displacement tracking. FEM models are implemented to determine the temperature increases associated with ARFI imaging for different transducer configurations and tissue properties. Model validation is accomplished with experimental measurements of heating using thermocouples in gelatin-based tissue-mimicking phantoms and porcine muscle. ARFI imaging typically employs in situ intensities of up to 1000 W/cm2that are applied over a period of less than 1 ms. FEM temperature increases of 0.2°C are demonstrated for a single ARFI interrogation for an absorption of 0.5 dB/cm/MHz, with cooling time constants on the order of several seconds. Tissue heating associated with two-dimensional ARFI imaging is a function of the spatial overlap of adjacent heated tissue volumes, with FEM temperature increases not exceeding 0.4°C in current ARFI configurations. Real-time two-dimensional ARFI imaging temperature increases reach steady-state values of less than 2.0°C after 1 minute at 1 frame per second. Tissue displacement due to thermal expansion is negligible for ARFI imaging, however changes in sound speed due to temperature changes may be appreciable. Experimental studies demonstrate that temperature increases of 0.15°C are associated with a single ARFI interrogation for an absorption of 0.5 dB/cm/MHz, with cooling time constants on the order of seconds. Tissue temperature increases do not exceed 1.5°C for two-dimensional ARFI imaging. ARFI-induced thermal diffusivity patterns are shown to be functions of the transducer f-number, the tissue absorption, and the temporal and spatial spacing of adjacent ARFI imaging locations. Good agreement is observed between FEM and experimental data. We conclude that ARFI imaging is safe in soft tissue, although thermal response must be monitored when developing ARFI beam sequences.
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