Biomedical subjects

M Grecescu

Publications and source records attributed to M Grecescu.

8 recordsLinked to original sources

PubMed2001

Calibration of ionization chambers in air kerma length.

Pencil ionization chambers are employed for the direct determination of the computed tomography dose index (CTDI), which is used for patient dosimetry in CT examinations. The chambers are calibrated in air kerma length in a reference x-ray field. The precision of calibration is influenced by several factors which are investigated in the present work. It is shown that the best irradiation length is 50% of the chamber rated length. The reading must be corrected for a residual signal due to the irradiation of the sensitive chamber volume by scattered radiation. The origin of this radiation is identified mainly at the edges of the diaphragm limiting the useful field. The residual signal is determined by a linear fit of measurements performed at minimum of three diaphragm apertures. The optimum geometric conditions are also discussed.

Air↗

PubMed1997

Use of passive detectors to characterize neutron field hardness.

Most personnel neutron dosimeters and field monitors suffer from an energy dependence. The knowledge of the energy distribution of the measured neutron field is necessary to correct the response of the detectors. However, the response of the detectors can be significantly improved when only a simple idea of the spectrum hardness is available. This paper describes a way of characterizing the neutron spectrum hardness in a large variety of neutron fields (with energies extending from thermal to 100 MeV) by using the various indications of different types of passive neutron detectors. These indications allow the choice of the appropriate factors established by calibration measurements or taken from the literature to correct the energy dependent response of personnel neutron dosimeters and field monitors.

Air Pollution, Indoor↗

PubMed1996

The fission track detector revisited: application to individual neutron dosimetry.

A system based on fission fragment tracks had previously been developed for individual neutron dosimetry. The dosimeter detects both fast neutrons by means of the 232Th(n,f) reaction, and thermal and albedo neutrons by means of the 235U(n,f) reaction. The fission tracks produced in a plastic foil are chemically etched and counted by spark discharges. The response of the dosimeter has recently been re-investigated in 36 different neutron fields: monoenergetic beams, reference fields near isotopic sources, and radiation fields encountered in a variety of situations inside nuclear power plants. The results obtained have been compared to those computed by convolution of the neutron spectra with the energy response functions of the dosimeters. In practical situations, it is essential to know the shape of the neutron spectrum, approximately at least, in order to perform an acceptably accurate dose evaluation. For that purpose, the neutron fields encountered inside nuclear power plants have been grouped into four categories, for which algorithms for dose evaluation have been developed. Concerning the neutron equivalent dose, the error associated with this approach does not exceed a factor of 2, a performance which is comparable to other detection systems used in the field of individual neutron dosimetry.

Neutrons↗

PubMed1991

Image quality index (IQI) for screen-film mammography.

A suitable quantity for evaluating the image quality in mammography is the smallest visible size of an object. This quantity, called the image quality index (IQI), can be derived from the basic image parameters: contrast, MTF, Wiener spectrum. Several evaluation methods of the IQI, all based on statistical decision theory, have been considered. An experimental visibility test using simulated microcalcifications has been performed in order to compare the results obtained with different IQI models. A previous approach, based on simplifying assumptions, yields a good correlation with the visibility test but fails to predict the actual size of the visible objects. Improved models have been derived for an ideal observer and for a 'quasi-ideal' one with perfect or with realistic visual characteristics. The experimental visual results are well modelled by the IQI method, provided that a suitable threshold signal-to-noise ratio is used for each of these models.

Animals↗

PubMed1991

Influence of anode and filter material on image quality and glandular dose for screen-film mammography.

The influence of anode and filter materials on the performance (image quality and dose) of a mammography system is investigated. The image quality is evaluated with the image quality index method. A computer simulation has been developed to calculate the physical parameters of the image quality index (contrast, resolution and noise) as well as the mean glandular dose. The calculations take into account the successive steps of the process: x-ray production, filtration, interaction with the test object, anti-scatter grid, interaction with the image detector (screen-film system). An excellent correlation is obtained between the results predicted by the model and those of experimental measurements, suggesting that the model may be used for the prediction of the performance of mammographic equipment. The experimental conclusions are confirmed: the use of a tungsten anode with a rhodium filter allows a dose reduction without a significant degradation of image quality. The computer program can also be used to simulate the influence of factors which are difficult to combine in practice, e.g., various anode and filter materials, monoenergetic x-rays, etc.

Electrodes↗

PubMed1986

[Large breasts, small breasts; which mammographic technic?].

The variation of dose and image quality parameters in the voltage range 25...33 kV have been studied on 3 mammographic phantoms simulating thin, average and thick breasts respectively. The global image quality has been objectively evaluated by an image quality index. The average glandular dose has been chosen as relevant indicator of risk. The study shows that the surface dose and the midplane dose are not suitable indicators of risk if the breast thickness is variable. For all phantoms, the deterioration of image quality with voltage increase was found to be relatively less important than the decrease of the average glandular dose. Consequently, the following optimum voltages are proposed: 28-29 kV for thin breasts, 29-30 kV for average breasts and 33-35 kV for thick ones. The study showed that breast thickness variation is not suitably compensated by the automatic exposure devices currently available.

Breast↗

PubMed1985

Objective assessment of mammography systems. Part I: Method.

The authors have developed an experimental method for simultaneous determination of dose and image parameters in mammography. A global and objective quality concept, the image quality index, is proposed and its reliability demonstrated by tests of reproducibility. Objective quality tests and subjective evaluation by radiologists showed good correlation.

Evaluation Studies as Topic↗

PubMed1985

Objective assessment of mammography systems. Part II: Implementation.

A quality control program for mammography units was carried out, based on objective tests of image quality and dose. Results are reported for 31 units, including correlation between various parameters. Satisfactory results were obtained in the case of 21 installations; for the other 10, comparison between measured quality parameters and those of the reference system suggested ways in which quality might be improved.

Female↗