PubMed Health⌕ Search

Biomedical subjects

W Grant

Publications and source records attributed to W Grant.

62 records · Page 4Linked to original sources
PubMedDate not supplied

Evaluation of the Therac 6 linear accelerator for radiation therapy.

The Therac 6 is new generation of low-energy linear accelerator. It incorporates a PDP-11/05 computer for beam control, treatment-factor input, and beam shutdown in the event of failure of the system. The performance of the unit has not been hindered by computer or software malfunction, and the computer has provided an excellent means for preventive maintenance and repair. Dosimetry parameters are similar to other 6 MV x-ray beams, and comparison to 60Co therapy beams shows that this unit may be more like 60Co units in penumbra and absence of off-axis peaking than other low-energy accelerators.

Cobalt Radioisotopes↗
PubMedDate not supplied

A review of the reliability of chamber factors used clinically in the United States (1968--1976).

One of the principal concerns of a physicist responsible for calibrating megavoltage radiotherapy equipment is the validity and stability of the 60Co exposure correction factor assigned to his ionization-chamber and electrometer system. It is the practice of the AAPM Radiological Physics Center (RPC) to perform an intercomparison between the RPC chamber and electrometer system and the chamber and electrometer in use at each of the various institutions visited by the RPC. The results of 202 such intercomparisons are reviewed to determine (1) the consistency in the assignment of exposure correction factors by a calibrating agency with itself and with other calibrating agencies, and (2) the dependence of the reliability of the exposure correction factors upon the type of field instrument and the time since calibration.

Cobalt Radioisotopes↗
PubMedDate not supplied

Effects of filters and wedges on skin sparing and gamma/neutron dose ratios in neutron teletherapy.

The effects of skin sparing and the gamma/neutron dose ratios in the clinical situations presently in use at the TAMVEC neutron teletherapy facility are not appreciably affected by the presence of filters and/or wedges. It is also shown that if skin sparing is lost due to close proximity of a hydrogenous scattering source, it can be restored by the use of thin lead filters.

Gamma Rays↗
PubMedDate not supplied

Evaluation of patients with local reactions to insulin with skin tests and in vitro techniques.

To better understand the part played by IgE and IgG antibody in the production of dermal reactions to insulin and the usefulness of skin tests in the evaluation of these reactions, we studied 21 diabetic patients referred for evaluation of large local insulin reactions, 46 diabetic patients without local insulin reactions, and 22 healthy nondiabetic controls. Study subjects were skin tested with 15 different insulins, and the results were evaluated over 48 h. All control subjects and 41 of 46 diabetic patients without local reactions were skin-test negative to insulin. The 11% of diabetic patients who reacted had positive wheal-and-flare reactions at 20 min to animal-species insulin but negative skin tests to human insulin. Study revealed two subgroups of patients with histories of local reactions. Ten (48%) of these patients had negative skin tests to insulin. Five of this subgroup remained skin-test negative to quantities of less than or equal to 8 U insulin/skin test. Eleven (52%) of the patients formed a subgroup with positive insulin skin tests; most of these patients were skin-test positive to human insulin and to beef, pork, or both insulins as well. Although the group mean insulin-specific IgE values of this latter subgroup were significantly higher than those of any other study group, overlap of these individual IgE values did not allow separation of specific individuals with positive skin tests from those of patients on insulin without dermal reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMedDate not supplied

Comparison of two-dimensional numerical approximation and measurement of SAR in a muscle equivalent phantom exposed to a 915 MHz slab-loaded waveguide.

Computer predictions of the specific absorption rate (SAR) distribution in a uniform muscle-equivalent phantom with an overlying bolus have been compared to those measured experimentally. The microwave source was a 10 cm x 10 cm slab-loaded waveguide applicator operating at 915 MHz. The modelling technique (theory) combines the equivalence principle and a two-dimensional finite element technique to determine the incident and the scattered electric fields separately. The E-field was measured using a small dipole device oriented parallel to the polarized field of the waveguide source. Comparisons of the predicted and measured SAR were made for various bolus properties, and reasonable agreement with theory was found in each case. The results demonstrate the usefulness of numerical modelling in characterizing the fields from microwave applicators used in clinical hyperthermia.

Computer Simulation↗
PubMedDate not supplied

Non-invasive thermometry using magnetic resonance diffusion imaging: potential for application in hyperthermic oncology.

The proposition to use non-invasive thermometry based on magnetic resonance diffusion imaging for applications in therapeutic hyperthermia is examined. The measurement of proton motion predominantly associated with the self-diffusion of water can be characterized by a Boltzmann temperature dependence (i.e. e-Ea/kT). The activation energy (Ea) is on the order of 0.2 eV and, for a restricted range (approximately 30 degrees) at a base temperature of approximately 300 K, the relationship between the effective diffusion coefficient and temperature is approximately linear. This response has been empirically demonstrated in water-based gel phantoms using magnetic resonance imaging (MRI). Additionally, it is feasible to have compatibility between radiofrequency (RF) heating devices and MRI equipment. An MRI-compatible heating applicator that includes a hexagonal array of coherently phased dipoles was assembled. This heating array easily fits into a standard 1.5 T head imaging coil (diameter 28 cm). The RF fields associated with heating (130 MHz) and imaging (64 MHz) were decoupled using bandpass filters providing isolation in excess of 100 dB. This isolation was sufficient to allow simultaneous imaging and RF heating without deterioration of the image signal-to-noise ratio. In this report temperature, spatial and time resolution achieved in phantom are examined in order to assess the potential for using this non-invasive temperature measurement in applications of hyperthermic oncology. Using this system and conventional multi-slice imaging techniques, 0.5 degrees C resolution in a voxel size of less than 1 cm3 has been achieved using an acquisition time of 4.15 min.

Diffusion↗