Search PubMed⌕ Search

Biomedical subjects

K J Weeks

Publications and source records attributed to K J Weeks.

23 records · Page 2Linked to original sources
PubMedDate not supplied

Selenium-75: a potential source for use in high-activity brachytherapy irradiators.

A study was made to find a high-specific-activity radioisotope having a longer half-life and lower gamma-ray energies than 192Ir in order to replace 192Ir in high-activity afterloading brachytherapy devices. Based upon a number of considerations, 75Se was selected. The present paper describes the physical properties of 75Se, its method of production, and the properties of an encapsulated source that was fabricated to prove the accuracy of our initial calculations.

Biophysical Phenomena↗
PubMedDate not supplied

Improvements in selective nuclear magnetic resonance excitation using nonlinear gradients.

The potential role of nonlinearities in the magnetic field gradient in magnetic resonance imaging for producing sharper boundaries for the excited spin slice region is investigated. A theoretical analysis based on solving the Bloch equations numerically is presented. Simple criteria for measuring the performance of selective excitation are defined and then used to evaluate the results for selective 90 degrees and 180 degrees pulses.

Humans↗
PubMedDate not supplied

Gypsum mixtures for compensator construction.

The characteristics and properties of a new material used for the fabrication of compensators are presented. This material is a special, refined gypsum. It requires a factor of 3 less water to prepare than ordinary gypsums and as a result the attenuation properties are stable over time. The material may be used by itself or mixed with fine metal particles to increase the attenuation per unit thickness. Gypsum, gypsum + steel, and gypsum + iron were investigated. The results of attenuation measurements in narrow- and broad-beam geometries appropriate to design of clinical dose modifying compensators are presented. Practical and technical details associated with the use of these materials are given. These compounds are found to be easy to use, versatile, reliable, environmentally safe, and inexpensive. In addition, an example of their use for dose compensation is given.

Calcium Sulfate↗
PubMedDate not supplied

3-D dose-volume compensation using nonlinear least-squares regression technique.

A method for external beam dose-volume optimization is presented. The Gauss-Marquardt nonlinear least-squares regression technique is applied to compensator design and determination. The dose distribution (uniform or otherwise) desired throughout a volume is specified. Compensators optimized to produce the necessary variation of beam intensity across the surface of each beam are simultaneously determined for all the beams. Solutions for homogeneous dose, homogeneous target dose, and restricted dose to exterior target volume structures, and inhomogeneous target dose cases are presented. Dependence of the results on the number of parameters as well as the role of degree of desirability weighting is explained and illustrated via examples. Discussion of the significance and limitations of this optimization method is also presented.

Humans↗