F R Zypman | PubMed Health

PubMed1996

MRI electromagnetic field penetration in cylindrical objects.

Magnetic Resonance Imaging information depends on the knowledge of the behavior of the transverse, radio frequency (RF) electromagnetic fields inside biological objects. Here we present exact results for the RF vector potential field inside and outside a cylindrical object that mimics the human body. These results are obtained by solving Maxwell Equations with appropriate boundary conditions, and take into consideration the object conductivity and dielectric constant and the presence of a cylindrical RF uniform-current-carrying coil. A computer code that evaluates RF magnetic field phase and magnitude is presented for two cases of interest: for a longitudinal and for a phi-directed current. Each of those cases is studied in free space and in the presence of a metallic shielding. The symbolic code will be of use for order-of-magnitude calculations in the clinical world and, for pedagogical uses in medical imaging courses.

Electric Conductivity↗

PubMed1990

Optimizing blood vessel contrast in fast three-dimensional MRI.

Magnetic resonance angiography has matured to the point where clinically useful images can be acquired in half an hour or less. In this paper, the role of 3D imaging techniques is primarily considered. Specifically, the optimal imaging parameters, sequences, and reconstruction techniques are evaluated for moving spins. A variant of FISP known as ROAST with low flip angles, short repeat times, and a thick slab has been found to yield the best 3D survey scan of the cranial vessels with roughly 1 X 1 X 1-mm3 resolution in each of the processed images (slices). For the faster flowing carotids, a sagittal scout with as short a TE as possible is required to avoid spin dephasing. Localization is accomplished in both cases by acquiring thin slab 3D, thin partition, larger flip angle, longer repeat time FLASH sequences. Different choices of dephase/rephase sequences and directions are also reviewed. These choices are discussed from a practical and theoretical perspective. In particular, improvements in contrast and resolution are evaluated using half-Fourier, 512 acquisition, small fields of view and constrained reconstruction for both rephased gradient echo sequences and dephased thin slice long TR spin-echo sequences. A resolution of 0.5-0.75 mm is recommended to obtain sufficient image quality for consistent clinical interpretation of stenoses and vessel abnormalities.

Blood Flow Velocity↗