An inverse methodology for high frequency RF head coil design with pre-emphasized B1 field in MRI

An inverse methodology to assist in the design of radio-frequency (RF) head coils for high field MRI application is described in this work. Free space time-harmonic electromagnetic Green's functions and preemphasized B1 field are used to calculate the current density on the coil cylinder. With B1 field preemphasized and lowered in the middle of the RF transverse plane, the calculated current distribution can generate an internal magnetic field that can reduce the EM field/tissue interactions at high frequencies. The current distribution of a head coil operating at 4 T is calculated using inverse methodology with preemphasized B1 fields. FDTD is employed to calculate B1 field and signal intensity inside a homogenous cylindrical phantom and human head. A comparison with conventional RF birdcage coil is reported here and demonstrated that inverse-method designed coil with preemphasized B1 field can help in decreasing the notorious bright region caused by EM field/tissue interactions in the human head images at 4 T.

Numerical evaluation of e-fields induced by body motion near high-field MRI scanner

In modern magnetic resonance imaging (MRI), both patients and radiologists are exposed to strong, nonuniform static magnetic fields inside or outside of the scanner, in which the body movement may be able to induce electric currents in tissues which could be possibly harmful. This paper presents theoretical investigations into the spatial distribution of induced E-fields in the human model when moving at various positions around the magnet. The numerical calculations are based on an efficient, quasistatic, finite-difference scheme and an anatomically realistic, full-body, male model. 3D field profiles from an actively-shielded 4 T magnet system are used and the body model projected through the field profile with normalized velocity. The simulation shows that it is possible to induce E-fields/currents near the level of physiological significance under some circumstances and provides insight into the spatial characteristics of the induced fields. The results are easy to extrapolate to very high field strengths for the safety evaluation at a variety of field strengths and motion velocities.

An equivalent distributed magnetic current based FDTD method for the calculation of e-fields induced by gradient coils in MRI

This paper evaluates a low-frequency FDTD method applied to the problem of induced E-fields/eddy currents in the human body resulting from the pulsed magnetic field gradients in MRI. In this algorithm, a distributed equivalent magnetic current (DEMC) is proposed as the electromagnetic source and is obtained by quasistatic calculation of the empty coil's vector potential or measurements therein. This technique circumvents the discretizing of complicated gradient coil geometries into a mesh of Yee cells, and thereby enables any type of gradient coil modeling or other complex low frequency sources. The proposed method has been verified against an example with an analytical solution. Results are presented showing the spatial distribution of gradient-induced electric fields in a multilayered spherical phantom model and a complete body model.

Organochlorine and heavy metal concentrations in blubber and liver tissue collected from Queensland (Australia) dugong (Dugong dugon)

Tissue samples of liver and blubber were salvaged from fifty-three dugong (Dugong dugon) carcasses stranded along the Queensland coast between 1996 and 2000. Liver tissue was analysed for a range of heavy metals and blubber samples were analysed for organochlorine compounds. Metal concentrations were similar in male and female animals and were generally highest in mature animals. Liver concentrations of arsenic, chromium, iron, lead, manganese, mercury and nickel in a number of individual animals were elevated in comparison to concentrations previously reported in Australian dugong. Dieldrin, DDT (and its breakdown products) and/or heptachlor epoxide were detected in 59% of dugong blubber samples. In general, concentrations of organochlorines were similar to those reported in dugong 20 years earlier, and were low in comparison to concentrations recorded from marine mammal tissue collected elsewhere in the world. With the exception of lead, the extent of carcass decomposition, the presence of disease or evidence of animal starvation prior to death did not significantly affect dugong tissue concentrations of metals or organochlorines. The results of the study suggest that bioaccumulation of metals and organochlorine compounds (other than dioxins) does not represent a significant risk to Great Barrier Reef dugong populations, particularly in the context of other pressures associated with coastal development and other anthropogenic activities. (c) 2004 Elsevier Ltd. All rights reserved.