Frederic Vimeux

Local hyperthermia with MR-guided focused ultrasound: Spiral trajectory of the focal point optimized for temperature uniformity in the target region

The objective of hyperthermia treatment is to deliver a similar therapeutic thermal dose throughout the target volume within a minimum amount of time. We describe a noninvasive approach to this goal based on magnetic resonance imaging (MRI)‐guided focused ultrasound (FUS) with a spherical transducer that can be moved along two directions inside the bed of a clinical MR imager and that has an adjustable focal length in the third dimension. Absorption of FUS gives rise to a highly localized thermal buildup, which then spreads by heat diffusion and blood perfusion. A uniform temperature within a large target volume can be obtained using a double spiral trajectory of the transducer focal point together with constant and maximum FUS power. Differences between the real and target temperatures during the first spiral are evaluated in real time with temperature MRI and corrected for during the second spiral trajectory employing FUS focal point velocity modulation. Once a uniform temperature distribution is reached within the entire volume, FUS heating is applied only at the regions boundaries to maintain the raised temperature levels. Heat conduction, together with the design and timing of the trajectories, therefore ensures a similar thermal dose for the entire target region. Good agreement is obtained between theory and experimental results in vitro on gel phantoms, ex vivo on meat samples, and in vivo on rabbit thigh muscle. Edema in muscle was visible 1 hour after hyperthermia as a spatially uniform rise of the signal intensity in T2‐weighted images. J. Magn. Reson. Imaging 2000;12:571–583.

Hyperthermia by MR-guided focused ultrasound: Accurate temperature control based on fast MRI and a physical model of local energy deposition and heat conduction

Temperature regulation in MR‐guided focused ultrasound requires rapid MR temperature mapping and automatic feedback control of the ultrasound output. Here, a regulation method is proposed based on a physical model of local energy deposition and heat conduction. The real‐time evaluation of local temperature gradients from temperature maps is an essential element of the control system. Each time a new image is available, ultrasound power is adjusted on‐the‐fly in order to obtain the desired evolution of the focal point temperature. In vitro and in vivo performance indicated fast and accurate control of temperature and a large tolerance of errors in initial estimates of ultrasound absorption and heat conduction. When using correct estimates for the physical parameters of the model, focal point temperature was controlled within the measurement noise limit. Initial errors in absorption and diffusion parameters are compensated for exponentially with a user‐defined response time, which is suggested to be on the order of 10 sec. Magn Reson Med 43:342–347, 2000.

Fast lipid-suppressed MR temperature mapping with echo-shifted gradient-echo imaging and spectral-spatial excitation

The water proton resonance frequency (PRF) is temperature dependent and can thus be used for magnetic resonance (MR) thermometry. Since lipid proton resonance frequencies do not depend on temperature, fat suppression is essential for PRF‐based temperature mapping. The efficacy of echo‐shifted (TE > TR) gradient‐echo imaging with spectral‐spatial excitation is demonstrated, resulting in accurate and rapid, lipid‐suppressed, MR thermometry. The method was validated on phantoms, fatty duck liver, and rat thigh, demonstrating improvements in both the speed and precision of temperature mapping. Heating of a rat thigh with focused ultrasound was monitored in vivo with an accuracy of 0.37°C and a time resolution of 438 msec. Magn Reson Med 42:53–59, 1999.

On-line correction and visualization of motion during MRI-controlled hyperthermia.

Displacement of tissue during MRI‐controlled hyperthermia therapy can cause significant problems. Errors in calculated temperature may result from motion‐related image artifacts and inter‐image object displacement, leading to incorrect spatial temperature reference. Here, cyclic navigator echoes were incorporated in rapid gradient‐echo MRI sequences, used for temperature mapping based on the proton resonance frequency. On‐line evaluation of navigator information was used in three ways. First, motion artifacts were minimized in echo‐shifted (TE > TR) gradient‐echo images using the phase information of the navigator echo. Second, navigator profiles were matched for a quantitative evaluation of displacement. Together with a novel processing method, this information was employed to correct the reference temperature maps, thereby avoiding persistence of motion‐related temperature errors throughout the hyperthermic period. Third, on‐line visualization of displacement, together with temperature maps and thermal dose images, was developed, allowing physician intervention at all times. Examples are given of on‐line corrections during hyperthermia procedures with focused ultrasound and radiofrequency heat sources. Magn Reson Med 45:128–137, 2001.

Automatic control of hyperthermic therapy based on real-time Fourier analysis of MR temperature maps.

Local hyperthermia is increasingly being used for therapeutic purposes, such as tumor ablation. Heat conduction and energy absorption in vivo during the hyperthermic procedure are largely unknown, thus making feedback temperature control highly desirable. Here, a general method for temperature control based on Fourier transformation (FT) of the bio‐heat equation is presented, taking into account heat diffusion (D) and energy absorption (α) together with temperature distribution derived from rapid, continuous MR temperature mapping. The main advantages of the new method are: 1) the spatial distribution of heat deposition and conduction over the full region of interest (ROI) is taken into account, and 2) the high speed resulting from the use of fast FT (FFT) of temperature maps allows rapid feedback coupling. Initial tests based on MRI‐guided focused ultrasound (FUS) demonstrated that high‐quality temperature regulation can be obtained even for erroneous values of D and α, so long as their relative error remained in the same range. Performance of the automated control procedure was validated ex vivo and in vivo on rabbit thigh using moderate FUS heating. During the procedure, the standard deviation (SD) of the temperature remained in the range of temperature noise obtained by MRI, indicative of the performance of the regulation algorithm. Magn Reson Med 47:1065–1072, 2002.