Estimation of thermal dose from MR thermometry during application of nonablative pulsed high intensity focused ultrasound

Abstract

Purpose To evaluate whether MR thermometry is sufficiently fast, accurate, and spatially resolved for monitoring the thermal safety of non-ablative pulsed HIFU (pHIFU) treatments. Materials and Methods A combination of real MR thermometry data and modeling was used to analyze the effects of temporal and spatial averaging as well as noise on the peak temperatures and thermal doses that would be measured by MR thermometry. Results MR thermometry systematically underestimates the temperature and thermal doses during pHIFU treatment. Small underestimates of peak temperature can lead to large underestimates of thermal dose. Spatial averaging errors are small for ratios of pixel dimension to heating zone radius less than 0.25, which may be achieved by reducing the voxel size or steering the acoustic beam. Thermal dose might also be underestimated for very short, high power pulses due to temporal averaging. A simple correction factor based on the applied power and duty cycle may be applied to determine the upper bound of this effect. Conclusion The temperature and thermal dose measured using MR thermometry during pulsed HIFU treatment is probably sufficinet in most instances. Simple corrections may be used to calculate an upper bound where this is a critical factor.

Effects of MRTI sampling characteristics on estimation of HIFU SAR and tissue thermal diffusivity

“…Finally, the spatially distributed temperatures are averaged to create discrete temperature measurements for each voxel. This spatial averaging may cause misrepresentation of temperatures and has already been assessed for its effect on temperature and thermal dose calculations (Todd et al 2011, O’Neill et al 2012). …”

Modulation of the interstitial fluid pressure by high intensity focused ultrasound as a way to alter local fluid and solute movement: insights from a mathematical model

“…This may, in principle, be obtained directly from MRI thermometry data of the treatments, however, for simulation purposes in Comsol Multiphysics, it is easier to model the temperature rise due to a theoretical acoustic beam than work with noisy data. This may be done using the bioheat equation, that is, we can solve (see O’Neill et al 2012, Wissler 1998): $$\rho C\frac{\partial T}{\partial t}-k{\nabla }^{2}T=-\rho C{\omega }_{b}false(T-T_{b}false)+Q_{US},$$ with boundaries in the +R direction fixed at 37 °C, in the ±z direction set to 21 °C, and an insulation condition at the R=0 axis. For simplicity, we modeled the ultrasound as a Gaussian beam (Wu and Du 1990, O’Neill et al 2012) in cylindrical coordinates, that is: $$Q_{US}=\frac{2P_{\mathit{tap}}{\alpha }_z}{2\pi {\sigma }_R^{2}false(1-z^2/{\sigma }_z^{2}false)}exp\left(\frac{R^2}{2{\sigma }_R^{2}false(1-z^2/{\sigma }_z^{2}false)}-\frac{z^2}{2{\sigma }_z^2}\right).$$…”

“…The constants used in the simulation come from literature and earlier studies that fit our experimental thermometry data to a similar expression (see O’Neill et al 2012). The total acoustic power, P tap , was set so that the peak temperature reached 52 °C at the end of 60 s of sonication, matching several of our experimental data sets.…”

“… a Data taken from O’Neill et al (2012) b Data taken from O’Neill et al (2013) c Data taken from Netti et al (1997) d Data taken from Jain et al (2007) e Data taken Jain (1987) f Data taken from Baxter and Jain RK (1989) …”

MRI-guided focused ultrasound surgery in musculoskeletal diseases: the hot topics

“…The second is low-temperature ablation, with temperatures in the range of 43-45°C applied for a longer time interval (tens of minutes) to directly induce cancer cells' death or to make them more sensitive to cytotoxic agents and/or radiation. [144][145][146][147][148][149][150][151][152][153] Staruch et al 154 studied control strategies and drug delivery with temperature-sensitive liposome hyperthermia in bone, generated with MRgFUS. Significant increases in doxorubicin concentration occurred in heated vs unheated marrow (8.2-fold) and muscle (16.8-fold).…”