Sukhoon Oh

Experimental and numerical assessment of MRI-induced temperature change and SAR distributions in phantoms and in vivo

It is important to accurately characterize the heating of tissues due to the radiofrequency energy applied during MRI. This has led to an increase in the use of numerical methods to predict specific energy absorption rate distributions for safety assurance in MRI. To ensure these methods are accurate for actual MRI coils, however, it is necessary to compare to experimental results. Here, we report results of some recent efforts to experimentally map temperature change and specific energy absorption rate in a phantom and in vivo where the only source of heat is the radiofrequency fields produced by the imaging coil. Results in a phantom match numerical simulation well, and preliminary results in vivo show measurable temperature increase. With further development, similar methods may be useful for verifying numerical methods for predicting specific energy absorption rate distributions and in some cases for directly measuring temperature changes and specific energy absorption rate induced by the radiofrequency fields in MRI experiments. Magn Reson Med, 2009.

Measurement of SAR-induced temperature increase in a phantom and in vivo with comparison to numerical simulation.

To compare numerically simulated and experimentally measured temperature increase due to specific energy absorption rate from radiofrequency fields.

An Approach to Rapid Calculation of Temperature Change in Tissue Using Spatial Filters to Approximate Effects of Thermal Conduction

We present an approach to performing rapid calculations of temperature within tissue by interleaving, at regular time intervals, 1) an analytical solution to the Pennes (or other desired) bioheat equation excluding the term for thermal conduction and 2) application of a spatial filter to approximate the effects of thermal conduction. Here, the basic approach is presented with attention to filter design. The method is applied to a few different cases relevant to magnetic resonance imaging, and results are compared to those from a full finite-difference (FD) implementation of the Pennes bioheat equation. It is seen that results of the proposed method are in reasonable agreement with those of the FD approach, with about 15% difference in the calculated maximum temperature increase, but are calculated in a fraction of the time, requiring less than 2% of the calculation time for the FD approach in the cases evaluated.

Permittivity and performance of dielectric pads with sintered ceramic beads in MRI: early experiments and simulations at 3 T.

Passive dielectric materials have been used to improve aspects of MRI by affecting the distribution of radiofrequency electromagnetic fields. Recently, interest in such materials has increased with the number of high‐field MRI sites. Here, we introduce a new material composed of sintered high‐permittivity ceramic beads in deuterated water. This arrangement maintains the ability to create flexible pads for conforming to individual subjects. The properties of the material are measured and the performance of the material is compared to previously used materials in both simulation and experiment at 3 T. Results show that both permittivity of the beads and effect on signal‐to‐noise ratio and required transmit power in MRI are greater than those of materials consisting of ceramic powder in water. Importantly, use of beads results in both higher permittivity and lower conductivity than use of powder. Magn Reson Med, 2013.

Complex-Difference Constrained Compressed Sensing Reconstruction for Accelerated PRF Thermometry with Application to MRI Induced RF Heating

Introduce a novel compressed sensing reconstruction method to accelerate proton resonance frequency shift temperature imaging for MRI‐induced radiofrequency heating evaluation.

Bloch‐based MRI system simulator considering realistic electromagnetic fields for calculation of signal, noise, and specific absorption rate

To describe and introduce new software capable of accurately simulating MR signal, noise, and specific absorption rate (SAR) given arbitrary sample, sequence, static magnetic field distribution, and radiofrequency magnetic and electric field distributions for each transmit and receive coil.

Improved Homogeneity of the Transmit Field by Simultaneous Transmission with Phased Array and Volume Coil

To improve the homogeneity of transmit volume coils at high magnetic fields (≥4 T). Due to radiofrequency (RF) field/tissue interactions at high fields, 4 T to 8 T, the transmit profile from head‐sized volume coils shows a distinctive pattern with relatively strong RF magnetic field B1 in the center of the brain.

MRI-based temperature and SAR mapping with a new dual-coil solenoid/birdcage heating/measurement system

We describe an MRI-based method for mapping temperature and specific absorption rate (SAR) using a solenoid coil and a birdcage coil for heating and imaging of a weakly conductive dielectric sample, respectively. The accuracy and the quality of SAR/temperature mapping are enhanced by separating the heating and imaging RF coils. 50 W of RF power is applied to the solenoid coil to heat the conductive agar-gel phantom for 120 sec. Maps of temperature increase were acquired with an MRI-based method. The MR-based measurements were in good agreement with fiber optic measurements. Finally, the dual-coil heating system was simulated using the finite difference time domain (FDTD) method. The distribution of numerically-calculated and experimentally-acquired SAR were also in good agreement.

Conventional volume coil and travelling-wave antenna for homogeneous excitation of the human head in MRI at 300 MHz

Two competing factors make for RF engineering challenges in MRI: 1) MRI at higher RF field frequencies provides higher signal-to-noise ratio and 2) MRI typically works best with a homogeneous (e.g., long wavelength) circularly-polarized RF magnetic field to excite nuclei in the region of interest. Conventional RF coils can be thought of as a series of antennas placed about the circumference of a cylinder and generating RF fields that travel into the body in the radial direction. Recently an alternative approach, with a circularly-polarized antenna sending fields along the axis of the cylinder has gained much interest. Here we use numerical methods to compare and combine the two approaches for homogeneous excitation of the human head at 300 MHz.

Design and implementation of a quadrature RF volume coil for in-vivo MR brain imaging of rhesus macaque monkey in a stereotaxic head frame

We describe the design and implementation of a unique coil for in-vivo rhesus macaque brain imaging in a stereotaxic device. The RF volume coil consists of a 2 turn solenoid and a saddle coil configured and fed in quadrature. Finite difference time domain method was used to design the coil. Images acquired show excellent homogeneity and SNR throughout the monkeys brain.