Correlation Between Target / Filter Combinations

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Introduction

Currently, in the United Kingdom breast cancer is the most common type of cancer in women and early diagnosis is the key to reducing mortality rates (Skaane et al, 2007).

The National Health Service breast screening programme introduced digital mammography as the preferred option due to its increased sensitivity and specificity for denser breasts. Additionally, it has the ability to manipulate the images produced (NHSBSP, 2012). The aim of this study is to investigate the relationship between target/filter combinations, kilovoltage (kV), radiation dose and image quality using a mammographic phantom which is currently used for the quality assurance programme in the breast imaging department.

Digital x-ray technologies
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As the kV is reduced, scattered radiation is less penetrating. Therefore radiation dose to other radiosensitive organs can be reduced. However the skin radiation dose can increase as the kV is reduced (Allisy- Roberts and Williams, 2008). So by taking images at a higher kV should give a low contrast image which may not show the low density structures within the breast phantom.

If the kV is increased it is expected there are more photons reaching the detector resulting in an over exposed image which may possibly obscure small areas of micro calcifications and low density masses. However, digital mammography can compensate to some extent for over exposure. On the other hand over exposure should be avoided to comply with the ALARP (as low as reasonably practicable) principles (DoH, 2012).

Filter material

Filter materials in mammography are made of molybdenum or rhodium. The purpose of filters is to let through the useful photons and to absorb unwanted low and high energy photons so that they stop reaching the image detector thus contributing to the formulation of the radiographic image (Bushberg et al, 2002).

With the use of filters there is an increase in the quality of x-ray beam but a reduction in the intensity (Ball and Price, 1995). Therefore filtration is necessary but photons from the low end of spectrum if they are not
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