Beam localization in HIFU temperature measurements using thermocouples, with application to cooling by large blood vessels Subhashish Dasgupta a , Rupak K. Banerjee a,b,⇑ , Prasanna Hariharan c , Matthew R. Myers c a Mechanical Engineering Department, University of Cincinnati, Cincinnati, OH 45220, USA b Biomedical Engineering Department, University of Cincinnati, Cincinnati, OH 45220, USA c Division of Solid and Fluid Mechanics, Center for Devices and Radiological Health, Food and Drugs Administration, Silver Spring, MD 20993, USA article info Article history: Received 9 November 2009 Received in revised form 16 July 2010 Accepted 22 July 2010 Available online xxxx Keywords: HIFU Beam positioning error Thermocouple artifacts Blood flow cooling Thermal dose abstract Experimental studies of thermal effects in high-intensity focused ultrasound (HIFU) procedures are often performed with the aid of fine wire thermocouples positioned within tissue phantoms. Thermocouple measurements are subject to several types of error which must be accounted for before reliable infer- ences can be made on the basis of the measurements. Thermocouple artifact due to viscous heating is one source of error. A second is the uncertainty regarding the position of the beam relative to the target location or the thermocouple junction, due to the error in positioning the beam at the junction. This paper presents a method for determining the location of the beam relative to a fixed pair of thermocouples. The localization technique reduces the uncertainty introduced by positioning errors associated with very nar- row HIFU beams. The technique is presented in the context of an investigation into the effect of blood flow through large vessels on the efficacy of HIFU procedures targeted near the vessel. Application of the beam localization method allowed conclusions regarding the effects of blood flow to be drawn from previously inconclusive (because of localization uncertainties) data. Comparison of the position-adjusted transient temperature profiles for flow rates of 0 and 400 ml/min showed that blood flow can reduce temperature elevations by more than 10%, when the HIFU focus is within a 2 mm distance from the vessel wall. At acoustic power levels of 17.3 and 24.8 W there is a 20- to 70-fold decrease in thermal dose due to the convective cooling effect of blood flow, implying a shrinkage in lesion size. The beam-localization technique also revealed the level of thermocouple artifact as a function of sonication time, providing investigators with an indication of the quality of thermocouple data for a given exposure time. The max- imum artifact was found to be double the measured temperature rise, during initial few seconds of sonication. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction In the pre-clinical investigations of high-intensity focused ultra- sound (HIFU) devices and procedures, tissue phantoms and animal tissues are frequently used to study thermal effects in HIFU proce- dures. The phantoms or animal tissues can be instrumented with thin-wire thermocouples to provide temperature information at relatively low cost. Thermocouples embedded in animal tissue or tissue-mimicking materials are subject to two types of errors that must be quantified in order to obtain reliable measures of temperature rise arising from ultrasound absorption. The first type is thermocouple artifact [1–3]. Thermocouple artifacts can arise due to conduction of heat along the thermocouple wire, or from differences in heat capacity or acoustic absorption between the thermocouple and the sur- rounding medium. These artifacts can be minimized with the use of sufficiently small-diameter thermocouples [2]. However, an- other type of artifact arising from viscous heating at the surface of the thermocouple occurs even with thin thermocouples. Fry and Fry [4], Parker [5], Dickinson [3], Huang et al. [1] and Morris et al. [2] have developed methods for quantifying and treating vis- cous-heating artifacts. The second type of error associated with thermocouple measurements is the uncertainty in the position of the ultrasound beam relative to the thermocouple junction. A com- mon method for positioning the beam on the junction is to move the beam until the position of maximum temperature rise during a brief sonication is located. As noted by O’Neill et al. [6], this imprecise beam-positioning technique can lead to significant underestimates of temperature in HIFU applications, due to the small widths of the HIFU beams. O’Neill et al. [6] performed soni- cations remote from the junction and used the measured transient 0041-624X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2010.07.007 ⇑ Corresponding author. Address: 593 Rhodes Hall, ML 0072, Department of Mechanical, Industrial and Nuclear Engineering, University of Cincinnati, Cincinnati, OH 45221, USA. Tel.: +1 513 556 2124; fax: +1 513 556 3390. E-mail address: rupak.banerjee@uc.edu (R.K. Banerjee). Ultrasonics xxx (2010) xxx–xxx Contents lists available at ScienceDirect Ultrasonics journal homepage: www.elsevier.com/locate/ultras Please cite this article in press as: S. Dasgupta et al., Beam localization in HIFU temperature measurements using thermocouples, with application to cool- ing by large blood vessels, Ultrasonics (2010), doi:10.1016/j.ultras.2010.07.007