Beam localization in HIFU temperature measurements using thermocouples, with application to cooling by large blood vessels

• Published in: Ultrasonics Vol. 51; no. 2; pp. 171 - 180
• Main Authors: Dasgupta, Subhashish, Banerjee, Rupak K., Hariharan, Prasanna, Myers, Matthew R.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.02.2011; Elsevier
• Subjects: acoustics; Algorithms; Beam positioning error; Beams (structural); Biological and medical sciences; Blood flow; Blood flow cooling; blood vessels; cooling; Equipment Design; Errors; Exact sciences and technology; exposure duration; Gels; heat; HIFU; Humans; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Localization; Medical sciences; Models, Theoretical; Phantoms, Imaging; Physics; Position (location); Radiotherapy. Instrumental treatment. Physiotherapy. Reeducation. Rehabilitation, orthophony, crenotherapy. Diet therapy and various other treatments (general aspects); Regional Blood Flow; Reproducibility of Results; shrinkage; Surgery (general aspects). Transplantations, organ and tissue grafts. Graft diseases; Technology. Biomaterials. Equipments. Material. Instrumentation; Temperature; temperature profiles; Thermal dose; Thermal instruments, apparatus and techniques; Thermocouple artifacts; Thermocouples; Thermometry; Transducers; Ultrasonic Therapy - adverse effects; Ultrasonic Therapy - methods; ultrasonics; Uncertainty; HIFU; Thermal dose; Thermocouple artifacts; Blood flow cooling; Beam positioning error; Uncertainty; Pipe flow; Temperature effect; Artefact; Acoustic beam; Wire; Convection; Transients; Uncertain system; Focusing; Blood vessel; Model test; Exposure time; Localization; Target detection; Transient response; Cooling; Ultrasonic treatment; Inference; Experimental study; Blood flow; Temperature measurement; Heat source; Power device; Ultrasonic transducer; Induced hyperthermia; Thermocouple
• DOI: 10.1016/j.ultras.2010.07.007
• ISSN: 0041-624X; 1874-9968

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 inferences 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 narrow 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 maximum artifact was found to be double the measured temperature rise, during initial few seconds of sonication.