Experimental Studies of the Thermal Effects Associated with Radiation Force Imaging of Soft Tissue MARK L. PALMERI, KRISTIN D. FRINKLEY AND KATHRYN R. NIGHTINGALE Department of Biomedical Engineering Duke University Durham, NC 27708 mark.palmeri@duke.edu Many groups are studying acoustic radiation force-based imaging modalities to determine the me- chanical properties of tissue. Acoustic Radiation Force Impulse (ARFI) imaging is one of these modali- ties that uses standard diagnostic ultrasound scanners to generate localized, impulsive, acoustic radiation force in tissue. This radiation force generates tissue displacements that are tracked using con- ventional correlation-based ultrasound methods. The dynamic response of tissue to this impulsive radia- tion force provides information about the mechanical properties of the tissue. The generation of micron-scale displacements using acoustic radiation force in tissue requires the use of high-intensity acoustic beams, and the soft tissue heating associated with these high-intensity beams must be evaluated to ensure safety when performing ARFI imaging in vivo. Experimental studies using thermocouples have validated Finite Element Method (FEM) models that simulate the heating of soft tis- sue during ARFI imaging. Spatial maps of heating measured with the thermocouples are in good agree- ment with FEM model predictions, with cooling time constants measured and modeled to be on the order of several seconds. Transducer heating during ARFI imaging has been measured to be less than 1 °C for current clinical implementations. These validated FEM models can now be used to simulate soft tissue heating associated with different transducers, beam spacing, focal configurations and thermal material properties. These experiments confirm that ARFI imaging of soft tissue is safe, although thermal response must be monitored when developing ARFI beam sequences for specific tissue types and organ systems. KEY WORDS: Acoustic radiaton force; ARFI;FEM; heating; thermal; thermocouple. 1. INTRODUCTION The use of acoustic radiation force to interrogate the mechanical properties of soft tissues is becoming a widely investigated research area. In general, acoustic radiation force is used to excite tissue, and the tissue response is monitored using either ultrasonic or magnetic reso- nance methods. Vibro-acoustography uses frequency-shifted, confocal acoustic beams to generate oscillatory radiation force within tissues, and the tissue response is monitored with either a hydrophone, 1,2 or ultrasonic methods. 3 The Kinetic Acoustic Vitreoretinal Examina- tion (KAVE) method uses radiation force to generate steady-state stresses within soft gels and the vitreous of the eye and ultrasonic displacements are monitored to characterize the steady-state response of these materials. 4 Supersonic imaging 5 and Shear Wave Elasticity Imaging (SWEI) 6, 7 monitor the shear waves generated from short duration, acoustic radia- tion force to characterize the shear modulus of the medium. Acoustic Radiation Force Im- pulse (ARFI) imaging, which is the focus of this manuscript, uses a commercial diagnostic scanner to generate multiple, short duration radiation forces to interrogate a two-dimensional region of interest (ROI), and monitors the tissue response using ultrasonic correlation-based methods. 8-11 Similar work is also under investigation using a High Intensity Focused Ultra- sound (HIFU) transducer to generate radiation force in conjunction with a piston to track dis- placement in order to monitor ablation treatment. 12 In addition to soft tissue imaging, radiation force is being used to manipulate ultrasonic contrast agents in vitro and in vivo. 13, 14 ULTRASONIC IMAGING 26, 100- 114 (2004) 100 0161-7346/04 $18.00 Copyright 2004 by Dynamedia, Inc. All rights of reproduction in any form reserved.