Temperature monitoring of interstitial thermal tissue coagulation using MR phase images.

The temperature-dependent water proton frequency shift was investigated for temperature monitoring of interstitial thermal coagulation. A procedure for on-line temperature calculation was developed, and errors due to temperature-dependent susceptibility were investigated by finite element analysis and reference measurements. The temperature coefficient of magnetic susceptibility and proton chemical shift were determined for brain tissue and other substances. With the proposed procedure, the location of isotherms could be well visualized during laser-induced interstitial coagulation in vitro and in vivo. Systematic errors caused by magnetic susceptibility changes with temperature depend strongly on the characteristics of the heat source and can exceed susceptibility effects caused by physiologic tissue changes. For the laser applicators discussed here, however, a first order compensation for this effect was found to be satisfactory, because it reduces the absolute error to the range of +/- 1 degrees C. The proposed method represents a very promising approach for monitoring of the interstitial thermal coagulation.

Analysis of eddy currents originating from switched magnetic gradient fields in magnetic resonance imaging.

The fact that time-varying magnetic fields cause eddy currents in conductive objects is very well known. Switched magnetic gradient fields, used in echo planar imaging, were shown in the past to be able to elicit a stimulation process in peripheral nerves. We report about the distribution and values of induced current densities in the human torso, modeled by the Finite Element Method using isoparametric formulation. Since the applied numerical method is a frequency-domain one, the trapezoidal waveform is decomposed into a Fourier series. The simulation was made for four different exposures to switched magnetic gradient fields: transverse (x- and y-gradients) coil systems, longitudinal (z-gradients) coil system and for all the three coil systems working simultaneously. Special attention was paid to the region of the heart, since stimulation of the heart muscle could be extremely dangerous for human health. Therefore, the three components of the current density in the region of the heart muscle were spatially analyzed in all directions (x, y, and z), trying to find out the 'worst case' position of the heart muscle relative to the gradient coil system at which the highest current density is induced. Finally, the calculated values were compared to existing recommendations, showing that the simulated 'worst case' amplitude is relatively close to the limiting value for sinusoidal currents.

Spatial distribution of high-frequency electromagnetic energy in human head during MRI: numerical results and measurements.

Finite Element Method (FEM) using 26-node isoparametric finite elements was applied for modeling saddle-shaped head coils used in Magnetic Resonance Imaging (MRI) generating linearly polarized radiofrequency (RF) pulses at 64 MHz. The human head was modeled from MR scans of a volunteer and additional information were taken from Atlas of Sectional Human Anatomy. The physical dimensions of the head coil and the head permit a calculation of the outside magnetic field by a quasistatic approach. Of course, a full-wave approach was applied within the head. Values of specific energy--specific absorption (SA)--as well as of specific power--specific absorption rate (SAR)--were calculated by the method, simulating the real exposure conditions during MRI. Although the results of the used numerical method were compared previously to the results of the analytical solution with homogeneous sphere and to the results of RF measurements on heterogeneous phantom, a comparison between the numerical results of the modeled human head and in vivo measurements performed on the human head of the volunteer was made once more. Since the results are in excellent agreement, they argue for the correctness of the numerical method. The "worst-case" temperature elevations delta theta of the "hot-spots" were calculated, as well. Finally, the results of SA, SAR, and delta theta are compared to the existing recommendations.