R. Mark Henkelman

Heat-source orientation and geometry dependence in proton-resonance frequency shift magnetic resonance thermometry

The proton‐resonance frequency (PRF) shift method of thermometry has become a promising tool for magnetic resonance image‐guided thermal therapies. Although the PRF thermal coefficient has recently been shown to be independent of tissue type when measured ex vivo, significant discrepancy remains on its value for tissues measured in vivo under a variety of experimental conditions. The authors identify a potential source of variation in the PRF thermal coefficient that arises from temperature‐induced changes in the volume magnetic susceptibility of tissue and is dependent on the orientation and geometry of the heat‐delivery device and its associated heat pattern. This study demonstrates that spatial variations in the apparent PRF thermal coefficient could lead to errors of up to ±30% in the magnetic resonance estimated temperature change if this effect is ignored. Magn Reson Med 41:909–918, 1999.

Does IVIM measure classical perfusion

MR measurements based on motion encoding gradients provide interesting information about diffusion in tissues and have also been advanced as a way to measure tissue perfusion. This communication shows why IVIM cannot measure perfusion in the classical sense. Attempts to do so result from an unclear understanding of classical perfusion measurements and from confusion between terminal deposition (or uptake) and blood volume flow.

180° refocusing pulses which are insensitive to static and radiofrequency field inhomogeneity

Abstract Using simulated annealing, 180° composite pulses which are insensitive to a wide range of B 0 and B 1 variation have been designed. The pulses give low phase dispersion and are suitable for spin refocusing. Constraint on integrated power has been incorporated in the optimization, leading to more practical pulse designs. Extensive evaluation, experimental confirmation, and comparison with previous pulse designs demonstrate the superior performance of the pulses presented in this work.

A new way of averaging with applications to MRI.

Averaging is often used to increase the quality of an image degraded by noise or artifacts. A method is developed in which several degrees of freedom are introduced in the averaging process, this freedom making possible the choice of different weighting factors for different portions of the Fourier space. If a weighting factor is associated with each line of a magnetic resonance acquisition, we show that we obtain some freedom to eliminate motion artifacts. The process minimizes a quantity called the gradient energy over a region of interest in the image plane. A processed image is obtained from a mosaic of such regions of interest scanned over the whole image plane. The method is shown to yield greater motion artifact suppression in magnetic resonance images than that achieved with regular averaging. The main strength of the method is probably its ability to diminish the intensity of unstructured artifacts which are usually poorly managed by other postprocessing methods of artifacts suppression.