Gradient Modification Research Articles

Eddy current compensation by direct field detection and digital gradient modification

We demonstrate an empirical magnetic field gradient waveform optimization technique which compensates for eddy current distortions and nonideal gradient amplifier performance. The desired gradient response is used as a trial function, and a pickup coil in the magnet bore detects the actual pulsed gradient. The temporal behavior of the gradient is digitized and recorded with the existing NMR spectrometer hardware and is used for the calculation of a new trial function table. The method and some typical results as well as some practical considerations are described.

Intracranial circulation: pulse-sequence considerations in three-dimensional (volume) MR angiography.

The technique and feasibility of magnetic resonance (MR) angiography of intracranial vessels were studied in 35 healthy volunteers. Variations in image orientation, repetition time (TR), and flip angle were evaluated to determine their effects on flow-related enhancement. Gradient modifications--including echo time (TE), motion compensation, bandwidth, and field of view--were also studied in an effort to reduce motion-induced phase shifts. Results indicated that a FISP (fast imaging with steady precession) sequence with a TR of 50 msec, TE of 15 msec, velocity compensation in the read and section-select directions, acceleration compensation in the read direction, anisotropic volume, and a 1.25-mm partition thickness produced three-dimensional angiographic MR images that were accurate and reproducible in the depiction of the major intracranial vessels. Difficulties with field of view, persistent signal void secondary to higher-order motion, and spatial resolution remain major problems requiring additional study.

Improved SSD growth of InP single crystals

Single crystals of InP were grown by the synthesis solute diffusion (SSD) method improved by using a thermal screen for temperature gradient modification. Improved temperature conditions near the growth-front permit the growth of InP single crystals apparently free from inclusions. The crystals were used as substrates for the preparation of GaInAsP/InP double heterojunctions.

Electrofocusing on flat pH gradients: Systematic pH gradient modification leading to improved protein separation

AbstractA pH gradient providing maximal separation between pH 4.7 and 4.9 was generated by systematic variation of carrier constituent mixtures, and/or selection of anolyte/catholyte. In the optimized pH gradient, separation on gel electrofocusing between proteins with pI's between 4.7 and 4.9 was shown to be superior to that achieved on relatively steep pH gradients. This was the case in the separation of either human growth hormone isohormones D and E, or of bovine serum albumin and ovalbumin.

Electrofocusing in natural pH gradients formed by buffers: gradient modification.

Abstract Natural pH gradients formed in buffers (1) can be shifted in slope or in parallel along the pH scale by addition or substitution of buffers to obtain, for each particular fractionation problem, a gradient in which the species of interest occupies the center position. Similar to the natural pH gradients formed in Ampholine (2), the gradients formed in buffer decay concomitantly with a cathodic drift and progressive acidification of the pH gradient from the anodic end (3). Reproducibility of the pH gradient in buffer electrofocusing was found to be adequate. The stability of pH gradients with time is not affected by addition of 10 amine (basic) buffers to the standard mixture of 10 amphoteric buffers.

Plasma heating and gradient modifications by resonant absorption

Results of two-dimensional electromagnetic simulations of linear mode conversion in an inhomogeneous, freely expanding plasma are reported. Important modifications of the plasma profile are shown to be intrinsic to the process.