Jennifer Diffey

Practitioner compression force variability in mammography: a preliminary study

OBJECTIVE This preliminary study determines whether the absolute amount of breast compression in mammography varies between and within practitioners. METHODS Ethics approval was granted. 488 clients met the inclusion criteria. Clients were imaged by 14 practitioners. Collated data included Breast Imaging Reporting and Data System (BI-RADS) density, breast volume, compression and practitioner code. RESULTS A highly significant difference in mean compression used by different practitioners (p<0.0001 for each BI-RADS density) was demonstrated. Practitioners applied compression in one of three ways using either low, intermediate or high compression force, with no significant difference in mean compression within each group (p=0.99, p=0.70, p=0.54, respectively). Six practitioners showed a significant correlation (p<0.05) between compression and BI-RADS grade, with a tendency to apply less compression with increasing BI-RADS density. When compression was analysed by breast volume there was a wide variation in compression for a given volume. The general trend was the application of higher compression to larger breast volumes by all three practitioner groups. CONCLUSION This study presents an insight into practitioner variation of compression application in mammography. Three groups of practitioners were identified: those who used low, intermediate and high compression across the BI-RADS density grades. There was wide variation in compression for any given breast volume, with trends of higher compression demonstrated for increasing breast volumes. Collation of further studies will facilitate a new perspective on the analysis of practitioner, client and equipment variables in mammography imaging. ADVANCES IN KNOWLEDGE For the first time, it has been practically demonstrated that practitioners vary in the amount of compression applied to breast tissue during routine mammography.

A new step-wedge for the volumetric measurement of mammographic density

The volume of dense breast tissue can be calculated from an x-ray mammogram by imaging a calibrated step-wedge alongside the breast and determining the compressed breast thickness. Previously published work used a step-wedge made of PTFE with a maximum height of 35mm, length 175mm and width 15mm. Although fulfilling all theoretical requirements, it can be difficult to find space on the film for a large step-wedge when examining bigger breasts. Furthermore, the step-wedge is lead-lined, making it heavy and difficult to attach to the bucky. A more compact aluminium step-wedge has been designed to overcome these limitations, and experiments have been carried out on a prototype to evaluate its performance. Initial results show that the maximum and minimum heights of the prototype step-wedge are inadequate to sufficiently cover the range of optical densities within a breast image at the higher and lower exposures required for 6cm and 2cm Perspex (>200mAs and <40mAs respectively). However, the step increment appears to be satisfactory. Analysis of the mean pixel value and standard deviation within Regions of Interest of varying size and position indicates an optimum step length of 3mm. A new step-wedge has been constructed with an improved specification informed by the evaluation of the prototype.

Quantifying Breast Thickness for Density Measurement

The largest source of error in the estimation of volumetric breast density typically arises from inaccuracies in the measurement of breast thickness. We present a method which accounts for paddle tilt and show that the variation within the compressed breast region may be as much as 21.2mm in the chest wall to nipple direction. The system-indicated value of breast thickness results in an average underestimation of compressed breast volume of 10.5% which has implications for density measurement. Paddle deformation in the lateral direction is shown to be insignificant in comparison. A method of modelling the thickness at the breast periphery is presented with examples of the thickness maps generated.

Feasibility and acceptability of stepwedge-based density measurement

A link between increased breast density, as visualised in mammograms, and increased risk of developing breast cancer has been established. Recently, a number of objective, quantitative methods for measuring breast density have been described. One such method requires a calibration object to be imaged alongside the breast. However, it is important that this should not interfere with the routine imaging process. In this paper, we investigate the amount of space in mammographic images which is not currently occupied by the breast or existing patient labels and markers, and which would therefore be available for imaging an additional calibration device. We do this with a view to estimating the likelihood of failure of the method, and also to determining whether, without detriment to the imaging process, a device could be permanently fixed to the breast support platform. We also examine the impact of markers attached to the compression plate on the visibility of breast tissue. The results show that our existing calibration device may be used successfully without interfering with the routine imaging process, although permanently fixing such a device may present problems in a small minority of cases, and we demonstrate that the number of cases which would fail can be reduced by using a smaller stepwedge.

Investigating the replacement of the physical anti-scatter grid with digital image processing

Scattered photons degrade mammographic image quality, so, almost universally, a physical anti-scatter grid is used to limit their effect Physical grids are not completely effective in rejecting only scattered photons, so patient dose must be increased in order to maintain low levels of quantum noise The standard attenuation rate (SAR), a quantitative normalised representation of breast tissue for image analysis applications, incorporates a model of scatter, and a software correction of the image blurring arising from scatter within the image signal A tissue equivalent phantom is used to investigate the possibility, in terms of both image sharpness and noise, of replacing the physical grid with the software correction in the SAR Encouraging results are reported, software correction almost matching the performance of the grid, whilst maintaining a superior signal-to-noise ratio.

Estimating Individual Cancer Risks in the UK National Breast Screening Programme: A Feasibility Study

Conventional risk models for the development of breast cancer use inputs such as age, weight, hormonal factors and family history to compute individual breast cancer risk. These are employed in the management of women at high risk. The addition of breast density as an input has been shown to improve the accuracy of such models. An improved risk model could facilitate risk-based population screening. However, in order to use breast density in risk models there is a need to employ objective methods for measuring the density. A feasibility study has been carried out to assess the practicality of using a stepwedge-based technique for measuring breast density from mammograms in the UK National Health Service Breast Screening Programme and to determine whether additional information, relevant to risk, can be collected by questionnaire. Preliminary results suggest that it is practical to use such a technique in the screening environment. In a sample of 100 women, the mean density was 27% (range 2 - 81%). A negative trend in breast density was observed with Body Mass Index.

Optimising beam quality selection in mammographic acquisition using the standard attenuation rate

A common metric used to optimise digital mammography image acquisition is contrast-to-noise ratio Using the standard attenuation rate (SAR), a quantitative normalised representation of breast tissue for image analysis applications, we demonstrate that the image contrast may be completely separated from the acquisition parameters, in particular the beam quality, used for acquisition Optimising the contrast-to-noise ratio at acquisition is therefore suboptimal, since the contrast may be manipulated by post processing A tissue equivalent phantom is used to investigate the variation in both signal-to-noise ratio, and image sharpness within the SAR images The results show that the primary effect of varying the acquisition parameters through the various automated optimisation of parameter modes, and hence the mean glandular dose, is to vary the global contrast of the acquired image, an effect successfully mapped to a common normalised basis using the SAR The signal-to-noise ratio and image sharpness are second order effects, and are therefore dominated by the global image contrast when image acquisition is optimised using the contrast-to-noise ratio.

Effect of Image Quality on Film Reading

Good image quality is essential for breast screening to ensure that low contrast and very small features are not misinterpreted or missed. Furthermore, results and performance standards using older screening films are often cited in more recent studies for comparative purposes. In this study we examine whether older films are of comparable standard to more recent ones in a film reading test, investigating density and difference of opinion between readers. We also compare the most recent results for screen-film screening with current digital reading performance.

Inter and intra observer variability in a semi-automatic method for measuring volumetric breast density

If breast density is to be incorporated into breast cancer risk prediction models, the technique used for measurement must be quantitative, accurate, objective and reproducible We present a semi-automated method that has been used by three independent operators to measure glandular volume from the digitised mammograms of 29 women (116 images) Additionally, one operator used the method on 10 separate occasions on a sample of 24 images Intra-observer variability was found to be acceptably low, with coefficients of variation ranging from 3.5 – 5.7% depending on mammographic view (intra-class correlation coefficient close to 1 in all cases) However, inter-observer variability was greater with significant differences in glandular volume recorded between observers This was attributed to the method of breast edge detection The development of a new automatic breast edge detection algorithm has resolved the issue The average difference in glandular volume measurement between two independent operators in the cranio-caudal view is -0.89cm3 (95% confidence interval -2.77 – 0.99 cm3) using the new method, compared to 5.99cm3 (95% confidence interval 2.72 – 9.76 cm3) using the old method.

Volumetric breast density and breast cancer risk factors in a screening population

Breast density is positively linked to the risk of developing breast cancer Furthermore, the addition of breast density as an input to breast cancer risk prediction models has been shown to improve their predictive power Such models are used in the management of women at high risk but could potentially be used to determine screening strategy A stepwedge-based technique has been used to measure volumetric density from the mammograms of 1,289 women in the UK screening programme who additionally completed a questionnaire on risk-related factors The sample had a mean age of 60.1 (range 48.0 – 78.0), a mean breast thickness of 59mm (range 21 – 102mm) and a mean volumetric breast density of 11% (range 0.5 – 58%) Using Pearsons correlation coefficient, breast density was found to be significantly correlated with weight (r = -0.45), body mass index (r = -0.48), age (r = -0.13) and breast thickness (r =-0.65) at the p = 0.01 level Absolute glandular volume was also found to be significantly correlated with these parameters although the extent of correlation was weaker.