Localization of focused-ultrasound beams in a tissue phantom, using remote thermocouple arrays

Abstract

In focused-ultrasound procedures such as vessel cauterization or clot lysis, targeting accuracy is critical. To investigate the targeting accuracy of the focused-ultrasound systems, tissue phantoms embedded with thermocouples can be employed. This paper describes a method that utilizes an array of thermocouples to localize the focused ultrasound beam. All of the thermocouples are located away from the beam, so that thermocouple artifacts and sensor interference are minimized. Beam propagation and temperature rise in the phantom are simulated numerically, and an optimization routine calculates the beam location that produces the best agreement between the numerical temperature values and those measured with thermocouples. The accuracy of the method was examined as a function of the array characteristics, including the number of thermocouples in the array and their orientation. For exposures with a 3.3-MHz source, the remote-thermocouple technique was able to predict the focal position to within 0.06 mm. Once the focal location is determined using the localization method, temperatures at desired locations (including the focus) can be estimated from remote thermocouple measurements by curve fitting an analytical solution to the heat equation. Temperature increases in the focal plane were predicted to within 5% agreement with measured values using this method.

Enhanced thermal effect using magnetic nano-particles during high-intensity focused ultrasound

“…As a result, high resolution micro-CT (Inveon Multimodality System, Siemens Inc., Germany) was used to scan the tissue phantoms, in order precisely determine the location of the TC junctions, which are later used in the inverse algorithm [17]. The processing software used in conjunction with the CT scan images was Inveon Research Workplace 4.2.…”

“…As a first step in determining the focal temperature, the beam location within the TC array was ascertained, using an inverse algorithm published by Hariharan et al [17]. The inverse method was used because it significantly reduces TC artifacts.…”

“…Once the beam location was determined, it was used in a solution to the heat equation in the form of an exponential-integral [17, 19] to determine the temperature at locations and times of interest. Dillon et al[19] assumed a heat source that was constant in the axial direction and possessed a Gaussian profile in the radial direction.…”

“…In a previous study by the authors[17], it was determined that the uncertainty in temperature estimates performed using the exponential-integral technique was approximately 6% in the focal plane. The uncertainty arises due to inexact knowledge of the TC location, variation in thermal properties, and limitations on the assumption of source that is invariant in the axial direction and Gaussian in the radial direction.…”

“…An inverse algorithm [17] was used to estimate the temperature rise at the focus and other radial locations.…”