Monte Carlo studies on the influence of focal spot size and intensity distribution on spatial resolution in magnification mammography.

Magnification is a special technique applied in mammography in cases where breast complaints have already been noticed, aiming to examine a specific area of the breast. Small-sized focal spots are essential in such techniques in order to reduce the resultant geometrical unsharpness. The x-ray intensity distribution of the focal spot is another crucial parameter for such a technique as it affects the mammographic resolution. In this study a Monte Carlo simulation model is utilized, in order to examine the effect of a wide range of focal spot sizes and three representative intensity distributions on spatial resolution under magnification. A thick sharp edge consisting of lead, non-transparent to x-rays was imaged under various conditions for this purpose, and the corresponding spatial resolution was calculated through the modulation transfer function (MTF). Results demonstrate that focal spots larger than 0.10 mm can mainly be used for low degrees of magnification, especially when combined with double peak Gaussian intensity distribution of the focal spot (sum of two single peak Gaussian distributions with different centers), as the resultant spatial resolution is not as high as the corresponding from smaller foci or uniform and single peak Gaussian distributions. Moreover, for the degrees of magnification usually utilized in clinical practice they do not reach the acceptable limit of 12 lp mm(-1). The replacement of the x-ray tube when the focal spot starts being destroyed is very crucial as the possible alteration of single peak Gaussian distribution to double peak Gaussian results in the degradation of spatial resolution. A focal spot of 0.10 mm or smaller, combined with single peak Gaussian intensity distribution, can be considered appropriate even for higher degrees of magnification and its use can contribute in the effort to optimize the magnification views in mammography.

Contrast-to-noise ratio in magnification mammography: a Monte Carlo study.

Magnification views are a common way to perform a secondary examination when suspicious abnormalities are found in a screening mammogram. The visibility of microcalcifications and breast lesions is restricted by the compromise between the image quality and the absorbed dose. In this study, image quality characteristics in magnification mammography were evaluated based on Monte Carlo techniques. A breast phantom was utilized, simulating a homogeneous mixture of adipose and glandular tissue in various percentages of glandularity, containing inhomogeneities of various sizes and compositions. The effect of the magnification degree, breast glandularity, tube voltage and anode/filter material combination on image quality characteristics was investigated in terms of a contrast-to-noise ratio (CNR). A performance index PI(nu) was introduced in order to study the overall performance of various anode/filter combinations under different exposure parameters. Results demonstrate that CNR is improved with the degree of magnification and degraded as the breast glandularity is increased. Degree of magnification 1.3 offers the best overall performance for most of the anode/filter combinations utilized. Under magnification conditions, the role of dose is demoted against the image quality, as magnification views are secondary, diagnostic examinations and not screening procedures oriented to non-symptomatic women. For decreased image quality weighting, some anode/filter combinations different from Mo/0.030 mmMo can be utilized as they offer a similar performance index. However, if the desired weighting for the image quality is high, the Mo/0.030 mmMo combination has the best overall performance.

Monte Carlo generated conversion factors for the estimation of average glandular dose in contact and magnification mammography.

Magnification mammography is a special technique used in the cases where breast complaints are noted by a woman or when an abnormality is found in a screening mammogram. The carcinogenic risk in mammography is related to the dose deposited in the glandular tissue of the breast rather than the adipose, and average glandular dose (AGD) is the quantity taken into consideration during a mammographic examination. Direct measurement of the AGD is not feasible during clinical practice and thus, the incident air KERMA on the breast surface is used to estimate the glandular dose, with the help of proper conversion factors. Additional conversion factors adapted for magnification and tube voltage are calculated, using Monte Carlo simulation. The effect of magnification degree, tube voltage, various anode/filter material combinations and glandularity on AGD is also studied, considering partial breast irradiation. Results demonstrate that the estimation of AGD utilizing conversion factors depends on these parameters, while the omission of correction factors for magnification and tube voltage can lead to significant underestimation or overestimation of AGD. AGD was found to increase with filter material's k-absorption edge, anode material's k-emission edge, tube voltage and magnification. Decrease of the glandularity of the breast leads to higher AGD due to the increased penetrating ability of the photon beam in thick breasts with low glandularity.

A Monte Carlo study of the influence of focal spot size, intensity distribution, breast thickness and magnification on spatial resolution of an a-Se digital mammography system using the generalized MTF.

In this study the generalized Modulation Transfer Function (GMTF) and the geometric sharpness (Sgeo) were used (i) to study the effects of various focal spot sizes (0.04 mm-0.3 mm), x-ray intensity distributions (Gaussian and double Gaussian), breast thicknesses (2-7 cm) and magnifications M (1.0-2.0) on the spatial resolution of an a-Se digital mammography system, (ii) to identify suitable focal spots for magnification mammography and (iii) derive optimum magnifications. For the calculation of GMTF the required components were: focal spot MTF, obtained from theory, detector MTF, scatter MTF and scatter fraction obtained from Monte Carlo simulations. The results showed that focal spots with sizes up to 0.18 mm are suitable for magnification mammography offering a GMTF which is >50% and >20% at the respective object frequencies of 6.5 mm(-1) and 9 mm(-1). Focal spots with sizes < 0.16 mm and Gaussian. intensity distribution, or sizes ≤ 0.1 mm and double Gaussian, offer a system resolution which improves or does not deteriorate with magnification for most object frequencies. For larger focal spots, i.e. 0.16-0.18 mm for a Gaussian and 0.12-0.18 mm for a double Gaussian. intensity distribution, optimum magnifications exist which depend on the object frequency and breast thickness. System resolution (in terms of Sgeo) is maximized at M = 1.8-2.0 (all breast thicknesses) for Gaussian intensity distribution, and at M = 1.4-1.6 (breast thicknesses ≤ 4 cm) and M = 1.6-1.8 (thicker breasts) for double Gaussian.

Justification in clinical radiological practice: a survey among staff of five London hospitals.

This study represents a survey performed among staff who, according to the Ionising Radiation (Medical Exposures) Regulations of 2000 (IRMER), are responsible for justifying radiological examinations in the UK. The aim of the survey is to map the current situation regarding knowledge of risks from X-ray exposures and the criteria used for their justification. An anonymous electronic questionnaire was emailed to 219 radiologists and radiographers of five National Health Service hospitals. The questions were designed to investigate the way the sample group defines/assesses risk and benefit when justifying medical exposures, and to test their knowledge on radiation doses, risk communication, and on relevant national legislation. The majority of the respondents are aware of the relevant legislation/guidelines. Patient's medical condition, age and sex, and alternative techniques using less or no ionising radiation are the main criteria used for justification. However, when estimating the effective dose of various examinations in chest radiograph equivalents, the majority of the responses were incorrect. Although there is good knowledge of legislation around justification of medical exposures, there seems to be a lack of knowledge on radiation doses and risks among IRMER practitioners.

Can electronic zoom replace magnification in mammography? A comparative Monte Carlo study.

Magnification, which is considered to be a relatively high "dose cost" mammographic technique, is a complementary examination performed on women exhibiting breast complaints or abnormalities. Particular attention is given to the imaging procedure as the primary aim is to confirm the existence of suspected abnormalities, despite the additional dose. The introduction of post-processing capabilities and the widespread use of digital mammography promoted some controversy in the last decades on whether electronic zoom performed on the derived initial screening mammogram can effectively replace this technique. This study used Monte Carlo simulation methods to derive simulated screening mammograms produced under several exposure conditions, aiming to electronically magnify and compare them to the corresponding magnification mammograms. Comparison was based on quantitative measurements of image quality, namely contrast to noise ratio (CNR) and spatial resolution. Results demonstrated that CNR was higher for geometric magnification compared to the case of electronic zooming. The percentage difference was higher for lesions of smaller radius and achieved 29% for 0.10 mm details. Although spatial resolution is maintained high in the zoomed images, when investigating microcalcifications of 0.05 mm radius or less, only with geometric magnification can they be visualised.