PubMed Health⌕ Search

Biomedical subjects

L A DeWerd

Publications and source records attributed to L A DeWerd.

12 recordsLinked to original sources
PubMed1992

A new re-entrant ionization chamber for the calibration of iridium-192 high dose rate sources.

A re-entrant (well-type) ionization chamber has been designed and fabricated at the University of Wisconsin for use with iridium-192 high dose-rate (HDR) remote after-loading brachytherapy devices. The chamber was designed to provide an ionization current of about 10(-8) ampere with a nominal 10 curie iridium-192 source. A narrow opening is provided into the sensitive volume of the chamber to insert a Nucletron MicroSelectron catheter, or catheters with similar diameters from other HDR manufacturers. The chamber exhibits a flat response (+/- 0.1%) for any source position within +/-4 mm of the chamber center. A 300 volt chamber bias yields a 99.96% ion collection efficiency. The chamber is capable of being calibrated directly with an iridium-192 source which has in turn been calibrated with thimble-type ion chambers. Reproducibility for readings in the current mode for 10 consecutive insertions of the MicroSelectron iridium-192 HDR source is within 0.02% or less. Two thimble chambers calibrated by the U.S. National Institute of Standards and Technology provide calibration traceability of iridium-192 HDR sources and re-entrant chambers to a primary national standards laboratory. Results of activity measurements of 6 commercial iridium-192 HDR sources are reported.

Calibration↗
PubMed1990

Evaluation of a commercial diode monitor for mailed quality control of therapy units.

A commercial diode dosimeter was evaluated for use in a mailed radiotherapy quality control package. The quality control package contained the commercial diode dosimeter and four thermoluminescent dosimeters to measure the given dose, and it also contained a photographic film to measure the light/radiation field coincidence. The film also served as a backup dosimeter. Twenty small Midwestern hospitals participated in field measurements; approximately half of the hospitals had a consulting physicist and the others had one full-time physicist. Each hospital was asked to deliver 1.5 Gy (150 rads) to the device, which was returned to our laboratory for evaluation. Reliable results were obtained from three measurements at each hospital at about 3-month intervals. The diode dosimeter was more precise than thermoluminescent dosimeters or film.

Calibration↗
PubMed1978

Mailed thermoluminescent dosimeter determination of entrance skin exposure and half-value layer in mammography.

A method of providing in vivo measurement of entrance skin exposure for mammography using mailed thermoluminescent dosimeters has been developed. The dosimeters are calibrated to 137Cs (662 keV), and a correction is applied to account for energy response. A mailed dosimeter packet which can be used to measure the half-value layer at mammography energies is also described. The results for both systems agree well (to within 10%) with careful ion chamber determinations. These dosimeter measurements provide a simple. Inexpensive means to reliably determine exposure and to monitor machine parameters on a periodic basis.

Cesium Radioisotopes↗
PubMedDate not supplied

Direct-response ultraviolet thermoluminescent dosimeter.

An integrating solid-state dosimeter has been developed to detect ultraviolet radiation using the process of "direct" ultraviolet stimulation. This process yields a thermoluminescent signal after an exposure to ultraviolet radiation, without prior exposure to ionizing radiation. Magnesium oxide crystals were found to show a directly stimulated thermoluminescent glow peak at 145 degrees C which can be utilized as a linear, energy-independent ultraviolet dosimeter.

Magnesium Oxide↗
PubMedDate not supplied

Solid-state electrophotography with Al2O3.

Solid-state thermocurrent and radioconductivity (electrical conductivity during irradiation) experiments are described with attention to the feasibility of using ionic solids for radiographic imaging. The radioconductivity varies with temperature giving rise to temperature windows of potential usefulness. The electrophotographic image is formed at atmospheric pressure using Al2O3, collecting the charge on mylar film. Development is by the power cloud technique. A transparency, which can be viewed as a conventional radiographic image, is easily produced. A successful transfer image, free of electric discharge artifact, was produced.

Aluminum↗
PubMedDate not supplied

Calibration and use of the Wisconsin kVp test cassette.

The Ardran-Crooks kVp test cassette is widely used in diagnostic radiology to provide a rapid, simple, noninvasive measurement of x-ray tube potential. A modified version of this cassette called the Wisconsin kVp Test Cassette was introduced commercially in the U. S. in 1972. Since then, the method of calibration of these cassettes has changed significantly. Wisconsin kVp Test Cassettes calibrated by the manufacturer prior to August 1982 may yield underestimates of kVp measurements, particularly when using the 90-110 and 110-130 kVp regions with single-phase units. In August 1982 significant improvements in the calibration methods were implemented. The resultant change in calibration is demonstrated by data from the Centers for Radiological Physics. Present calibration methods are believed to be accurate within the greater of +/- 2 kVp or 2% of actual peak tube potential. Proper use of the cassette is necessary to achieve this level of accuracy.

Biophysical Phenomena↗
PubMedDate not supplied

Radiation therapy and clinical physics practices in the midwest (1982-1986).

As a part of an on-site physics review procedure, the Midwest Center for Radiological Physics (MWCRP) interviewed midwest clinical radiotherapy physicists and radiation therapists in institutions that participated in federally funded Cancer Control Programs. Information from 77 institutions was compiled regarding types of megavoltage units, daily patient load, radiotherapy personnel, treatment planning responsibilities, dosimetric practices, and treatment planning computers. Even though some practices, such as frequency of patient charge checks, were consistent throughout the midwest, other practices varied considerably such as patient load with the number of megavoltage units. This information may be useful either for planning a new facility, considering the needs for staff, or for comparing existing practices and responsibilities.

Cancer Care Facilities↗
PubMedDate not supplied

Thermal and scatter effects on the radiation sensitivity of well chambers used for high dose rate Ir-192 calibrations.

High dose rate (HDR) iridium sources must be calibrated regularly because of the short half-life of Ir-192. High dose rate sources can now be calibrated using a new well-type chamber that allows easy, reproducible source calibrations. The chamber includes a styrofoam insulator that surrounds the source in the well. A study of the radiation sensitivity of the well chamber exposed to an HDR Ir-192 source at two different activities (300 and 230 GBq) revealed that the sensitivity of the chamber varies by as much as 1.1% as the chamber is moved toward a scattering surface. Second, with the styrofoam insulator removed, the air temperature within the ion collecting volume increased during exposure, causing a gradual decrease in chamber sensitivity of 0.15% in 30 min. This temperature increase was caused by heat transfer from radiation emitted by the Ir-192 source, and diminished as the source decayed. However, with the styrofoam insulator around the central aluminum tube in the well, the source cannot heat the collecting volume and thus thermal equilibrium between the ion collecting volume and its environment is maintained throughout an exposure. The radiation sensitivity of the commercial well chamber was found to be constant for exposure times of 30 min.

Aluminum↗
PubMedDate not supplied

An electron density calibration phantom for CT-based treatment planning computers.

In order to correct for tissue heterogeneities on a voxel-by-voxel basis during CT-based treatment planning, the relationship between the correction factor (CF) and the CT number in Hounsfield units (HU) for the scanner in use must be established. Since the relationship between CF and electron density (rho e) of various materials is well documented, the rho e vs HU is required for direct computation of the correction factors by treatment planning computers. A CT phantom with 18 different tissue substitutes has been used to establish the rho e vs HU relationship. A description of the phantom and its contents is given and the calibration of the CT function of the planning computer is discussed.

Calibration↗