P. Morán

Patient dose in digital mammography.

In the present investigation, we analyze the dose of 5034 patients (20,137 images) who underwent mammographic examinations with a full-field digital mammography system. Also, we evaluate the system calibration by analyzing the exposure factors as a function of breast thickness. The information relevant to this study has been extracted from the image DICOM header and stored in a database during a 3-year period (March 2001-October 2003). Patient data included age, breast thickness, kVp, mAs, target/filter combination, and nominal dose values. Entrance surface air kerma (ESAK) without backscatter was calculated from the tube output as measured for each voltage used under clinical conditions and from the tube loading (mAs) included in the DICOM header. Mean values for the patient age and compressed breast thickness were 56 years (SD: 11) and 52 mm (SD: 13), respectively. The majority of the images was acquired using the STD (for standard) automatic mode (98%). The most frequent target/filter combination automatically selected for breast smaller than 35 mm was Mo/Mo (75%); for intermediate thicknesses between 35 and 65 mm, the combinations were Mo/Rh (54%) and Rh/Rh (38.5%); Rh/Rh was the combination selected for 91% of the cases for breasts thicker than 65 mm. A wide kVp range was observed for each target/filter combination. The most frequent values were 28 kVp for Mo/Mo, 29 kVp for Mo/Rh, and 29 and 30 kV for Rh/Rh. Exposure times ranged from 0.2 to 4.2 s with a mean value of 1.1 s. Average glandular doses (AGD) per exposure were calculated by multiplying the ESAK values by the conversion factors tabulated by Dance for women in the age groups 50 to 64 and 40 to 49. This approach is based on the dependence of breast glandularity on breast thickness and age. The total mean average glandular dose (AGD(T)) was calculated by summing the values associated with the pre-exposure and with the main exposure. Mean AGD(T) per exposure was 1.88 mGy (CI 0.01) and the mean AGD(T) per examination was 3.8 mGy, with 4 images per examination on average. The mean dose for cranio-caudal view (CC) images was 1.8 mGy, which is lower than that for medio-lateral oblique (MLO) view because the thickness for CC images was on average 10% lower than that for MLO images. Mean AGD(T) for the oldest group of women (1.90) was 3% higher than the AGD(T) for the younger group (1.85) due to the larger compressed breast thickness of women in the older group (10% on average). Differences between the corresponding AGD(T) values of each age group were lowest for breast thicknesses in the range 40-60 mm, being slightly higher for the women in the older group.

Comparative study of dose values and image quality in mammography in the area of Madrid

Dose values and image quality in mammography resulting from a study carried out in the area of Madrid during 1990-1991 are presented in a two-part sequence. In this paper are reported the values of air kerma and mean glandular dose per film estimated at 14 centres by means of the Leeds TOR(MAX) mammographic phantom and from patient samples. The results obtained from patient measurements allow us to deduce that the TOR(MAX) phantom assembly used (4.5 cm thick) is equivalent to an average 5 cm thick compressed breast from the population in the area of Madrid. Approximately 75% of the air kerma values resulting from phantom estimations are below 6.6 and 12.2 mGy for X-ray units with and without grid, respectively. The mean glandular doses per film for 75% of the units with and without grid are below 1.5 and 1.7 mGy, respectively. The results showed that the higher values of both air kerma and mean glandular dose obtained for the units without grid result from the use of slow speed recording systems. On the other hand, the quality control programme carried out shows important faults in the performance of the X-ray units which have an important impact on both dose and image quality. Finally, the third quartile values of the air kerma and mean glandular dose per film (10.4 and 1.7 mGy, respectively), deduced by considering all the units (with and without grid), are proposed to be the reference values of the mammographic examinations during the aforementioned period. In a second paper, the image quality is analysed by using the same Leeds TOR(MAX) phantom. Results for each test object included in the phantom are discussed as a function of the X-ray unit characteristics. Subsequently, the phantom images obtained with each X-ray system are scored and the resulting values are compared with the corresponding dose values, so that the dose-image quality relationship for each unit is obtained.

Some indicative parameters on diagnostic radiology in Spain: first dose estimations

The Medical Physics Group at the Complutense University of Madrid has been co-ordinating, for approximately 1 year, a project on optimization of radiation protection in diagnostic radiology, in co-operation with the other states of the European Community. Exhaustive data on the subject, which offer accurate results on patient dosimetry for the different types of examination, are the final aim of the project. So far, it has been possible to analyse in detail the data from the National Institute of Health (NIH), which manages the care of about 96% of the Spanish population, plus the findings from several hospitals, outpatient centres and private clinics of the community of Madrid, which allowed us to perform the first dose estimations and to extrapolate them to the rest of Spain. The following estimations are presented: annual frequency of different examinations, their variation from 1985 to 1986, number of diagnostic rooms used for a given minimum of annual examinations, organ doses for different examinations, effective dose-equivalent, genetically significant dose for some examinations, as well as the collective dose.

Evolution of diagnostic radiology in a big hospital during a 5 year period, and the derived collective dose

An analysis is presented of the trends in the annual number of radiological examinations and in the average effective dose equivalent for each type of examination in a big Spanish hospital. Annual frequencies for each type of examination, annual average effective dose equivalent values for each study group, and the contribution of each examination group to the collective dose are presented. Also, sex and age distributions for several important examinations are given, and their impact on the collective dose is reviewed.

Measurement of the photon spectrum from an X-ray tube above 10 keV using the induced XRF technique

Abstract A method is described for determining the absolute spectrum of a photon beam coming from an X-ray tube above 10 keV. This is done by exciting the fluorescence of chemical elements whose K-shell binding energies are within the limits of the bremsstrahlung spectrum, using the photon beam coming from the tube as an excitation source. At the same time the efficiency curve of the detecting system is determined at the photoelectric peak. This method of measurement of the spectrum solves the problems associated with the detection of large numbers of photons as produced by an X-ray tube. In the numeric processing a parametric form is used for the photon spectrum which gives a physical description of the bremsstrahlung production on a thick target. The uncertainties in the calculation of the photon flux vary between 11% and 3% depending on the energy of the photons. The spectrum of the beam, when filtered with 1 mm of aluminium, has been measured, thus obtaining the spectral distribution compatible with the one obtained theoretically by filtering the original spectrum. The exposure values produced by the photon beam filtered with different thicknesses of A1 have been measured and the results are in agreement with those calculated from the initial spectrum.

Comparative study of dose values and image quality in mammography in the Madrid area

During 1990-1992 a study of dose and image quality in mammography was carried out in the Madrid area. Values of air kerma and mean glandular dose per film estimated at 14 public and private centres (17 X-ray units) by means of the Leeds TOR(MAX) mammographic phantom and from a sample of patients were reported in a previous paper. This second paper describes image quality results obtained by the same Leeds TOR(MAX) phantom. Inferior image quality was found for units without a grid (most of them older systems) and/or using aluminium filtration. In units with a grid, inferior image quality was due to the low optical density of the images, the use of inadequate image receptors and the use of an aluminium filter. Image quality results were compared with the standards of the UK Breast Screening Programme and proposed European Guidelines. It is concluded that mammography image quality might be considered as acceptable when TOR(MAX) phantom films achieved at least 10 lp mm-1 and 1.2% for high contrast resolution and threshold contrast visibility (6 mm details) respectively.

Spectrometry of the photon beam from an X-ray tube by X-ray fluorescence

Abstract A method to perform the measurement of photon beam spectra from X-ray tubes is described. It consists of exciting the fluorescence of elements with their K-binding energies lying within the energetic range of the bremsstrahlung spectrum, using the tube photon beam, and a 241 Am excitation source as a reference, to normalize individual results of each element. According to the numerical treatment described in this article, uncertainties in relative photon flux determinations range from 10% to 2% depending on the photon energies. This method overcomes the difficulties associated with the detection of high photon rates produced by an X-ray tube.

Experimental determination of the relative TL response of TLD-100 chips for 60 kVp X- and 60Co γ-rays

Abstract The relative response η of TLD-100 chips for low energy X-rays of well known spectral distribution has been determined, following two different experimental procedures. For the different X-ray spectral distributions used, values of η ⋍ 2 instead of the usually assumed value η ∼ 1 are obtained. Also, the values of η obtained for different filtrations show a decreasing trend as filtration increases. The use of experimental measurements for several aluminium filtration thicknesses and a numerical fit by a least-squares method lead to values of η in two adjacent energetic intervals.