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G Drexler

Publications and source records attributed to G Drexler.

At least 19 recordsLinked to original sources
PubMed2001

Backscatter factors for mammography calculated with Monte Carlo methods.

The objective of this study is to establish a comprehensive set of backscatter factors for mammography based on the exposure model proposed by the European Protocol on Dosimetry in Mammography. The Monte Carlo calculated backscatter factors (BSFs) presented in this study are for various exposure conditions encountered in mammographic practice as well as in calibration procedures. The data demonstrate the variation of the BSF as a function of the exposure parameters, hence enabling a better match with calibration conditions and, at the same time, reviewing the BSF data already recommended by the European Protocol. Furthermore, earlier data for BSF for general diagnostic radiology are validated.

Calibration↗
PubMed1998

Calculation of backscatter factors for diagnostic radiology using Monte Carlo methods.

Backscatter factors were determined for x-ray beams relevant to diagnostic radiology using Monte Carlo methods. The phantom size considered most suitable for calibration of dosimeters is a cuboid of 30 x 30 cm2 front surface and 15 cm depth. This phantom size also provides a good approximation to adult patients. Three different media were studied: water, PMMA and ICRU tissue; the source geometry was a point source with varying field size and source-to-phantom distance. The variations of the backscatter factor with phantom medium and field geometry were examined. From the obtained data, a set of backscatter factors was selected and proposed for adoption as a standard set for the calibration of dosimeters to be used to measure diagnostic reference doses.

Adult↗
PubMed1995

An analysis of the equivalent dose calculation for the remainder tissues.

In the 1990 Recommendations of the International Commission on Radiological Protection, the risk-weighted quantity "effective dose equivalent" was replaced by a similar quantity, "effective dose." Among other alterations, the selection of the organs and tissues contributing to the risk-weighted quantity and their respective weighting factors were changed, including a modified definition of the so-called "remainder." Close consideration of this latter definition shows that it causes certain ambiguities and unexpected effects which are dealt with in the following. For several geometries of external photon irradiation, the numerical differences of two possible methods of evaluating the remainder dose from the doses to ten single organs, namely as arithmetic mean or as mass weighted average, are assessed. It is shown that deviation from these averaging procedures, as prescribed for those cases where a remainder organ receives a higher dose than an organ with a specified weighting factor, causes discontinuities in the energy dependence of the remainder dose and, consequently, also non-additivity of this quantity. These problems are discussed, and it is shown that, although the numerical consequences for the calculation of the effective dose are small, this unsatisfactory situation needs clarification. One approach might be to abolish some of the ICRP guidance relating to the appropriate tissue weighting factors for the remainder tissues and organs and to make other guidance more precise.

Adult↗
PubMed1994

Realistic computerized human phantoms.

To estimate the risk resulting from exposures to ionizing radiation, the organ and tissue doses should be assessed. A convenient method is the calculation of these doses using representations of the human body, called models or phantoms, together with computer codes simulating the transport of radiation in the body. Most commonly used are mathematical phantoms whose external and internal volumes are defined by simple geometric bodies. More recently, phantoms constructed from computed tomographic data of real persons were introduced as an improvement. These phantoms present advantages concerning the location and shape of the organs, in particular the hard bone and bone marrow, whose distribution can be assessed with high resolution. So far, three of these phantoms were constructed at the GSF, a fourth is under process. The construction technique is described, and some calculational results of organ doses due to external photon irradiation are presented.

Adult↗
PubMed1993

Effective dose--how effective for patients?

The question discussed in this paper is whether effective dose can reflect the risk to patients from radiological procedures and can be used, for example, to optimise procedures and compare risks of various methods, to define dose constraints, and to estimate the risks to individuals or populations attributed to medical exposures. This report demonstrates that the use of effective dose for patients could be misleading or even wrong due to inappropriate simplifications of the underlying biological mechanisms and inappropriateness of the weighting factors connected with the definition of effective dose for a given patient population. We show that the choice of the most meaningful quantities to express patient exposure depends strongly on the respective situation.

Bone Marrow↗
PubMed1992

Effective dose and effective dose equivalent--the impact of the new ICRP definition for external photon irradiation.

In a recent recommendation, the International Commission on Radiological Protection substituted the effective dose equivalent, HE, with a similar quantity--the "effective dose," E--changing both the set of organs considered and the respective weighting factors. To quantify the impact of these changes, calculations of E and HE were performed for various photon energies and external irradiation geometries using a Monte Carlo code and mathematical anthropomorphic phantoms to which an esophagus was introduced for this purpose. For energies greater than 15 keV, E less than HE, the difference depending on photon energy and exposure geometry.

Female↗
PubMed1988

The construction of computer tomographic phantoms and their application in radiology and radiation protection.

In order to assess human organ doses for risk estimates under natural and man made radiation exposure conditions, human phantoms have to be used. As an improvement to the mathematical anthropomorphic phantoms, a new family of phantoms is proposed, constructed from computer tomographic (CT) data. A technique is developed which allows any physical phantom to be converted into computer files to be used for several applications. The new human phantoms present advantages towards the location and shape of the organs, in particular the hard bone and bone marrow. The CT phantoms were used to construct three dimensional images of high resolution; some examples are given and their potential is discussed. The use of CT phantoms is also demonstrated to assess accurately the proportion of bone marrow in the skeleton. Finally, the use of CT phantoms for Monte Carlo (MC) calculations of doses resulting from various photon exposures in radiology and radiation protection is discussed.

Bone Marrow↗
PubMed1988

[A field study to determine dosages in computed tomography].

122 CT scanners were studied in order to determine close values free in air on the axis of rotation during the most common types of examination. In addition, other exposure parameters were collected, which may be important in relation to dose to patients. The results showed a surprising variation in the dose values of CT examinations. It is concluded that it is not possible to estimate patient exposure reliably by using the information on exposure parameters supplied by the operator.

Humans↗
PubMed1987

Calibration of a NaI spectrometer in dose equivalent quantities.

A sodium iodide spectrometer was calibrated in terms of dose equivalent quantities. The detector was irradiated with a known dose (dose equivalent) at 12 ISO x-ray qualities between 33 keV and 248 keV. The method of calibration is based on the relationship between the considered dose equivalent delivered by each x-ray quality and the linear combination of the number of counts in a set of six fixed energy ranges. This procedure allows for the evaluation of the effective dose equivalent, the whole-body dose equivalent and the testes dose equivalent for parallel irradiation with a maximum and minimum relative deviation from the reference values of +11% and -21%, respectively.

Calibration↗
PubMed1987

Improved radioimmunoassay of melatonin in serum.

Melatonin was extracted from serum by using Baker reversed-phase C-18 columns. More than 99% of the applied melatonin was retained by the columns, and more than 97% was eluted from the columns in 300 microL of methanol. We then determined melatonin in the serum extract by a modification of a standard radioimmunoassay, using filtration instead of centrifugation to collect the [3H]melatonin-antibody complex precipitated by saturated ammonium sulfate. These modifications allow more rapid, accurate, and reproducible determination of melatonin than do previously published procedures.

Circadian Rhythm↗
PubMed1986

A rapid and simple method for efficient coating of microtiter plates using low amounts of antigen in the presence of detergent.

Bio-Beads SM-2 have previously been used for the removal of non-ionic detergents from protein solutions. Addition of Bio-Beads SM-2 to detergent solubilized antigen significantly enhanced the immobilization of antigen to microtiter wells. Depending on the incubation time used 35-45% of the applied antigen could be immobilized to the microtiter wells. Using this method and a subsequent ELISA procedure it was possible to detect monoclonal antibodies in hybridoma supernatants after coating microtiter wells with 100 microliters of a solution containing 16 ng antigen/ml in the presence of 0.01% Triton X-100.

Allergy and Immunology↗
PubMed1985

Experimental determination of conversion factors between organ doses and measured quantities for external photon irradiation.

For external photon radiation, conversion factors between organ doses and the quantities normally measured in radiation protection were determined experimentally using an anthropomorphic phantom and thermoluminescent dosimeters. The measurements were made for different energies and irradiation geometries. As normalization quantities, the exposure measured in free air and the dose equivalent in soft tissue measured at a personal dosimeter site were used. The results are compared with conversion factors calculated by other authors.

Female↗
PubMed1984

[35S]tert.-butylbicyclophosphorothionate and avermectin bind to different sites associated with the gamma- aminobutyric acid-benzodiazepine receptor complex.

Low concentrations of avermectin B1a (AVM) stimulated the specific high affinity binding of [35S]tert.-butylbicyclophosphorothionate [( 35S]TBPT) to membranes from rat cerebral cortex in the absence or presence of chloride or bromide ions. In contrast, TBPT either weakly stimulaes or does not significantly influence the specific high affinity binding of [3H]AVM to the same membranes in the absence or presence of chloride ions, respectively. These results indicate that [3H]AVM and [35S]TBPT bind to different but closely associated binding sites.

Animals↗