Calorimetric determination of the absorbed dose to water for medium-energy x-rays with generating voltages from 70 to 280 kV.

For medium energy x-rays produced with tube voltages from 70 to 280 kV, the absorbed dose to water, D(w), has been determined by means of water calorimetry with relative standard uncertainties ranging from 0.45% to 0.98% at 280 and 70 kV. The results were confirmed by Monte Carlo calculations, in which the ratios of D(w) at 5 cm depth in a reference water phantom to the air kerma free in air, K(a), at the same point in space were compared to the corresponding ratios determined experimentally. The general agreement between measurement and calculation was better than 1%. These results confirm earlier investigations in which the absorbed dose to graphite was determined by means of a graphite extrapolation chamber. For the Monte Carlo calculations, an attempt was made to present a complete uncertainty budget, taking into account type B contributions also.

Extension of the range of definition of the practical peak voltage up to 300 kV.

The practical peak voltage (PPV) was introduced with special emphasis being placed on the X-ray tube voltage range pertinent to diagnostic radiology, i.e. from 20 kV to 150 kV. In this paper, an extension of the range of definition of the PPV up to X-ray tube voltages of 300 kV is presented. This is realized by combining the weighting function derived for the field of diagnostic radiology with a newly derived one that covers the voltage range from 150 kV to 300 kV.

Measurement of the x-ray mass energy-absorption coefficient of air using 3 keV to 10 keV synchrotron radiation.

For the first time absolute photon mass energy-absorption coefficients of air in the energy range 3 keV to 10 keV have been measured with relative standard uncertainties less than 1%, significantly smaller than those of up to 5% assumed hitherto for calculated data. Monochromatized synchrotron radiation was used to measure both the total radiant energy by means of silicon photodiodes calibrated against a cryogenic radiometer and the fraction of radiant energy that is deposited in dry air by means of a free air ionization chamber. The measured ionization charge was converted into energy absorbed in air by calculated effective W values of photons as a function of their energy based on new measurements of the W values in dry air for electron kinetic energies between 1 keV and 7 keV, also presented in this work. The measured absorption coefficients were compared with state-of-the art calculations and found to agree within 0.7% with data calculated earlier by Hubbell at energies above 4 keV but were found to differ by values up to 2.1% at 10 keV from more recent calculations of Seltzer.

A new method for an improved determination of continuous photon fluence spectra for X-ray tube voltages up to 150 kV.

The experimental determination of bremsstrahlung spectra used in radiation protection dosimetry as well as in medical physics is generally composed of the following steps: acquisition of pulse-height spectra, determination of the detector's inherent response function and unfolding of the measured pulse-height spectra with this function. On the whole, this procedure leads to photon fluence spectra of good quality. It is, however, generally not completely free of artefacts. These occur at both the low- and high-energy sides of the spectrum, with a predominance at the low-energy side. In this work, a new method to overcome this problem is presented. It consists of reconstructing the distorted part of a spectrum on the basis of the total filtration pertinent to the radiation quality considered. The formalism is derived and the ability of the method to provide spectra free of artefacts is demonstrated.

Calibration of dosemeters used in mammography with different X ray qualities: EUROMET project no. 526.

The effect of different X ray radiation qualities on the calibration of mammographic dosemeters was investigated within the framework of a EUROMET (European Collaboration in Measurement Standards) project. The calibration coefficients for two ionization chambers and two semiconductor detectors were established in 13 dosimetry calibration laboratories for radiation qualities used in mammography. They were compared with coefficients for other radiation qualities, including those defined in ISO 4037-1, with first half value layers in the mammographic range. The results indicate that the choice of the radiation quality is not crucial for instruments with a small energy dependence of the response. However, the radiation quality has to be chosen carefully if instruments with a marked dependence of their response to the radiation energy are calibrated.

Results supporting calculated wall correction factors for cavity chambers.

Thick walled cavity ionization chambers are used by primary standard laboratories as primary air kerma standards in 137Cs and 60Co gamma-rays. Application of the cavity theory requires correction for the effects of photon attenuation and scattering in the chamber walls. For more than a decade there have been intensive discussions about the validity of wall correction factors determined by more traditional extrapolation methods versus those calculated by Monte Carlo methods. For existing primary standards the alternative methods lead to results that differ by up to 50% of the correction itself. This report presents both experimental and theoretical results which strongly support the validity of calculated wall correction factors. Moreover, it is demonstrated that, in selected cases, the application of a linear extrapolation method leads to errors in the determination of the air kerma reaching up to 13%.

The effect of 29 kV X rays on the dose response of chromosome aberrations in human lymphocytes.

The induction of chromosome aberrations in human lymphocytes irradiated in vitro with X rays generated at a tube voltage of 29 kV was examined to assess the maximum low-dose RBE (RBE(M)) relative to higher-energy X rays or 60Co gamma rays. Since blood was taken from the same male donor whose blood had been used for previous irradiation experiments using widely varying photon energies, the greatest possible accuracy was available for such an estimation of the RBE(M), avoiding the interindividual variations in sensitivity or differences in methodology usually associated with interlaboratory comparisons. The magnitude of the linear coefficient alpha of the linear-quadratic dose-effect relationship obtained for the production of dicentric chromosomes by 29 kV X rays (alpha = 0.0655 +/- 0.0097 Gy(-1)) confirms earlier observations of a strong increase in alpha with decreasing photon energy. Relating this value to previously published values of alpha for the dose-effect curves for dicentrics obtained in our own laboratory, RBE(M) values of 1.6 +/- 0.3 in comparison with weakly filtered 220 kV X rays, 3.0 +/- 0.7 compared to heavily filtered 220 kV X rays, and 6.1 +/- 2.5 compared to 60Co gamma rays have been obtained. These data emphasize that the choice of the reference radiation is of fundamental importance for the RBE(M) obtained. A special survey of the RBE(M) values obtained by different investigators in the narrow quality range from about 30 to 350 kV X rays indicates that the present RBE is in fairly good agreement with previously published findings for the induction of chromosome aberrations or micronuclei in human lymphocytes but differs from recently published findings for neoplastic transformation in a human hybrid cell line.

The role of de-excitation electrons in measurements with graphite extrapolation chambers.

A method is described for determining the absorbed dose to graphite formedium energy x-rays (50-300 kV). The experimental arrangement consists of an extrapolation chamber which is part of a cylindrical graphite phantom of 30 cm diameter and 13 cm depth. The method presented is an extension of the so-called two-component model. In this model the absorbed dose to graphite is derived from the absorbed dose to the air of the cavity formed by the measuring volume. Considering separately the contributions of the absorbed dose to air in the cavity from electrons produced in Compton and photoelectric interactions this dose can be converted to the absorbed dose to graphite in the limit of zero plate separation. The extension of the two-component model proposed in this paper consists of taking into account the energy transferred to de-excitation electrons, i.e. Auger electrons, which are produced as a consequence of a photoelectric interaction or a Compton scattering process. For the system considered, these electrons have energies in the range between about 200 eV and 3 keV and hence a range in air at atmospheric pressure of 0.2 mm or less. As the amount of energy transferred to the de-excitation electrons is different per unit mass in air and in graphite, there is a region, about 0.2 mm thick, of disturbed electronic equilibrium at the graphite-to-air interface. By means of the extension proposed, the x-ray tube voltage range over which a graphite extrapolation chamber can be used is lowered from 100 kV in the case of the two-component model down to at least 50 kV.

Mixed high energy photon and electron radiation fields for calibrating radiation protection dosemeters.

According to ISO 4037-3, calibrations of radiation protection dosemeters with photon radiation of energies above 3 MeV are performed under conditions of charged particle equilibrium. No information is provided concerning how to determine the response of dosemeters to radiation fields in the more general case when these conditions are not fulfilled. This paper deals with the production of mixed high energy photon and electron fields characterised by a lack or an excess of charged particles relative to conditions of equilibrium and describes a new procedure for the dosimetry in such fields. Through variation of the charged particle fluence fraction with respect to a nearly constant photon fluence, Hp(10) and H'(10) values varied by up to a factor of 1.74. The above mentioned basic study was utilised in the recent IAEA intercomparison (Co-ordinated Research Project 1996-1998) and EURADOS 'trial performance test' (1996-1998) for individual monitoring of photon radiation in testing response characteristics of individual dosemeters in non-charged particle equilibrium conditions.

Response of radiation protection dosemeters in mixed high-energy photon and electron radiation fields.

The response of radiation protection dosemeters in terms of the phantom-related operational quantities Hp(10) and H'(10.0 degrees) was measured for personal and area monitoring systems in mixed high-energy electron and photon radiation fields with energies up to 7 MeV. Using mixed radiation fields composed of different fractions of charged particle and photon fluence, three conditions were produced at the point of measurement: charged particle equilibrium (CPE) (a), a lack (b) and an excess (c) of charged particles relative to the conditions of CPE. Personal and area dosemeters of different types were investigated under conditions (a)-(c). A large variability of the response of the different dosemeter types was observed. The results are presented and discussed.

Experimental determination of practical peak voltage.

In diagnostic radiology the practical peak voltage was initially derived by postulating that, for a given combination of X-ray tube and contrast geometry, a constant X-ray tube voltage should produce the same low level contrast as an arbitrarily pulsating X-ray tube voltage. It has been shown previously that the practical peak voltage can be properly defined as a weighted average of the X-ray tube voltage. Up to now the concept of practical peak voltage was based entirely on the results of calculations. The present paper describes the experimental investigations for measuring and comparing the contrast-equivalent X-ray tube voltage and practical peak voltage derived from an invasive measurement of the time-dependent X-ray tube voltage. Within the experimental uncertainties, the results demonstrate the mutual equivalence of the practical peak voltage and the contrast-equivalent X-ray tube voltage.

The practical peak voltage of diagnostic X-ray generators.

A new quantity termed the "practical peak voltage" is proposed. This quantity is derived by equating the low level contrast in an exposure made with an X-ray tube connected to a generator delivering any arbitrary wave form, to the contrast produced by the same X-ray tube connected to a constant potential generator. Out of the great number of possible contrast configurations one is selected as being suitable for diagnostic radiology. By means of an eigenvalue problem a direct link is established between the electrical quantity X-ray tube voltage and the practical peak voltage which was initially defined through the properties of the X-ray field. It is shown that the spread in total X-ray tube filtration as encountered in medical diagnostic radiology can influence the result of a measurement of the practical peak voltage only marginally.

Arrhythmias in mitral valve prolapse. Effect of selection bias.

To assess the contribution of bias in subject selection to the prevalence of arrhythmias in cases of mitral valve prolapse (MVP), we compared ambulatory arrhythmias in 63 patients with MVP and 28 symptom-matched control subjects. All subjects were in sinus rhythm. Mean 24-hour heart rate of the prolapse population was lower than that of the control group (76 v 82 beats per minute). Mean atrial premature complex (APC) density per 1,000 beats (0.9 v 0.7 for patients with MVP and control subjects, respectively) and mean ventricular premature complex (VPC) density per 1,000 total beats (1.2 v 1.5) did not differ between groups. Small differences between groups in APC and VPC complexity did not reach statistical significance. Our findings suggest that, compared with similarly symptomatic controls, patients with MVP do not have as high an excess prevalence of arrhythmias as previously believed.