Magnetic Resonance Thermometry at 7T for Real-Time Monitoring and Correction of Ultrasound Induced Mild Hyperthermia

While Magnetic Resonance Thermometry (MRT) has been extensively utilized for non-invasive temperature measurement, there is limited data on the use of high field (≥7T) scanners for this purpose. MR-guided Focused Ultrasound (MRgFUS) is a promising non-invasive method for localized hyperthermia and drug delivery. MRT based on the temperature sensitivity of the proton resonance frequency (PRF) has been implemented in both a tissue phantom and in vivo in a mouse Met-1 tumor model, using partial parallel imaging (PPI) to speed acquisition. An MRgFUS system capable of delivering a controlled 3D acoustic dose during real time MRT with proportional, integral, and derivative (PID) feedback control was developed and validated. Real-time MRT was validated in a tofu phantom with fluoroptic temperature measurements, and acoustic heating simulations were in good agreement with MR temperature maps. In an in vivo Met-1 mouse tumor, the real-time PID feedback control is capable of maintaining the desired temperature with high accuracy. We found that real time MR control of hyperthermia is feasible at high field, and k-space based PPI techniques may be implemented for increasing temporal resolution while maintaining temperature accuracy on the order of 1°C

Huygens-Fresnel wave-optics simulation of atmospheric optical turbulence and reflective speckle in CO2 differential absorption LIDAR (DIAL)

The measurement sensitivity of C02 differential absorption lidar (DIAL) can be affected by a number of different processes. We have previously developed a Huygens-Fresnel wave optics propagation code to simulate the effects of two of these processes: effects caused by beam propagation through atmospheric optical turbulence and effects caused by reflective speckle. Atmospheric optical turbulence affects the beam distribution of energy and phase on target. These effects include beam spreading, beam wander and scintillation which can result in increased shot-to-shot signal noise. In addition, reflective speckle alone has been shown to have a major impact on the sensitivity of C02 DIAL. However, in real DIAL systems it is a combination of these phenomena, the interaction of atmospheric optical turbulence and reflective speckle, that influences the results. In this work, we briefly review a description of our model including the limitations along with previous simulations of individual effects. The performance of our modified code with respect to experimental measurements affected by atmospheric optical turbulence and reflective speckle is examined. The results of computer simulations are directly compared with lidar measurements and show good agreement. In addition, advanced studies have been performed to demonstrate the utility of our model in assessing the effects for different lidar geometries on RMS noise and correlation "size" in the receiver plane.U.S. Department of EnergyW-7405-ENG-3

<i>In vitro</i> controlled heating results in a tofu phantom.

<p>(a) The temperature increase as measured by the MR (blue) versus the requested temperature increase (red) for a tofu phantom heated under PID control in a spiral pattern. (b) A single MR magnitude frame 15 minutes into the heating experiment illustrating the temperature increase (color overlay) on the magnitude image.</p

Comparison of measured and simulated pressure distributions of the annular array.

<p>Calculated pressure as function of distance derived from simulation of the 16 element annular array compared to the measured pressure in the lateral direction. The measured −6 dB beam width was 0.8 mm, which agreed well with simulated results.</p

Localization and heating of Met-1 tumor.

<p>Spin echo multi-slice MR magnitude images (acquired in the coronal plane) used to localize the Met-1 tumor prior to heating (top two rows). The animal is positioned on an acoustic gel pad. The gel pad has a central hole in which the tumor sits. Following localization of the tumor from the image series, the appropriate coronal plane is used for imaging and real-time temperature control. Temperature is overlaid in color on a magnitude MR image mid-way through the heating sequence (bottom).</p

Acoustic and thermal variables used during heating simulations for water and tofu.

<p>Acoustic and thermal variables used during heating simulations for water and tofu.</p

Experimental setup of the MRgFUS system.

<p>(a) A photograph of the MR compatible ultrasound insert (top) and a close up of the end of the insert in which the 16 element annular array is water coupled to an upper plastic membrane (white arrow) on which the sample or animal would rest. (b) Schematic of combined MRgFUS system for real-time feedback control of arbitrary heating trajectories. (c) Axial MR image illustrating the experimental setup utilized for <i>in vivo</i> experiments; the rat brain phased array was positioned on the mouse, but is not visible in the image due to decoupling and choice of TE (though the image was acquired with the volume resonator for both RF transmit and receive to facilitate better visualization of the entire experimental setup).</p