In summary, FLUKA can be used as a tool for simulating scatter. Although the program itself produces an ideal situation, the ability to add multiple geometries and materials into the setup means that a more realistic situation can be modelled.
Producing a spectrum via SpekCalc that is similar to the experimental beam is difficult to achieve. However, with some manipulation of the intensity a spectrum with sufficient simi- larities can be formed.
Using the scatter spectra attained using FLUKA and simply subtracting them from experi- mental spectra has shown a 15% increase in the contrast. This was not achievable without the spectroscopic capabilities of the HEXITEC detector and shows an alternative method of scatter removal by utilising this ability.
This improvement is less than was found when imaging using monochromatic X-rays and windowing the scatter from the spectrum. It is possible that this smaller improvement in contrast is due to the underestimation of the scatter when modelling it in FLUKA. This is the limitation of such a method as scatter is produced from surrounding objects in the room and as this is not constant for all mammographic facilities it is difficult to model an accurate scatter spectrum for all cases.
However although precise spectra of the scatter would be difficult to model, estimates can be made and have shown to have a good effect on the contrast once removed from the ex- perimental spectrum. In practise it would be possible to provide estimates of the scatter for a series of breast thicknesses which is a very simple method to introduce into the screening process without the need for expensive equipment as many methods for the improvement of image quality require.
5
Conclusions
The purpose of this study was to investigate alternative methods to the anti-scatter grid for the removal of scatter from X-ray images in mammography. Alternatives are needed because of the increase in dose that is required to compensate for the loss of statistics experienced when using an anti-scatter grid. Other modalities of breast imaging, such as breast CT and MRI, have been developed to improve on image quality but these are often more time consuming or expensive and therefore not efficient enough for the screening of women across the UK and only used for women at high risk of developing cancer or women already diagnosed. Two methods of removing scatter using the spectroscopic capabilities of a pixellated spec- troscopic detector were investigated. These methods have the advantage that they are modi- fications to conventional mammography X-ray, resulting in a practical solution to the image quality in mammography without needing very large and expensive equipment such as MRI and CT scanners and would therefore be more feasible for the use in breast screening. Both of the methods rely on the use of the pixellated spectroscopic detector HEXITEC. The detector used has a couple of limitations at this stage in its development. Firstly the detector size as the detector used in the study is only 2 x 2 cm and therefore to produce an image the size of a conventional mammogram a detector with a much larger area is required. There have been investigations made into a larger area HEXITEC detector which is 10 x 10 cm in size [99]. Although this is still not large enough for a conventional mammogram, it does show the possible development of this type of detector to a much larger size that could be used for such a purpose.
There is also a limitation in the rates that it operates at as HEXITEC is designed for low rates and imaging for a medical purpose involves high rates. The Medipix detector described in in the introduction chapter is an example of a spectroscopic detector designed for high rates but it has the limitation of only being able to access the spectroscopic information in bands instead of the full spectroscopic information acquired with HEXITEC. Therefore a possible compromise between the two detectors that dealt with higher rates for imaging with but compromised with a lesser energy resolution. The methods described both require the use of the detected energy spectra, however both methods could work equally as well with a
reduced resolution. The monochromatic method needs an energy resolution around the size of the monochromatic peak so that it can be distinguished from the scatter and the FLUKA simulations only require the full energy spectrum that scatter can be subtracted from so a lesser energy resolution could still be used.
The first method investigated was based on monochromatic X-rays produced with a con- ventional X-ray tube and a mosaic crystal monochromator. Compton-scattered X-rays from the monochromatic beam would appear at a lower energy than the primary beam in the detected spectrum and were therefore simply windowed out from the spectrum. Imaging with synchrotron monochromatic beams has been previously investigated and has seen some promising results. The downside with this method is that synchrotrons are large and expen- sive which is why a conventional X-ray tube was chosen for this study. The method used in this study also has the additional advantage of combining it with a spectroscopic detector so that scattered X-rays can be removed from the image which was not achievable before without the use of a spectroscopic detector.
Results showed an increase in contrast of 40% in comparison to imaging with a full energy beam. The main problem with this method is the limited statistics and hence the long acqui- sition time. This time was increased further as the size of the mononchromatic beam incident on the detector was very small and so multiple slices of the image were acquired and then recombined. This problem has been investigated previously by using multiple crystals to produce a much wider beam so less slices need to be acquired at one time and this is some- thing that could be investigated in order to see how much of a decrease in acquisition time it has.
The second method that was investigated was the removal of scattered X-rays by first mod- elling them using a Monte Carlo modelling package. A scatter-only spectrum was generated from an input beam with a peak energy the same as the beam used to acquire the experimen- tal image. Then using the spectroscopic capabilities of the detector, the scatter spectrum was subtracted from the experimental one; leaving a scatter-free image.
Results showed that, by subtracting the scatter from background and detail regions, the con- trast improved by about 15% when compared to the full spectrum image. The limitations
of this approach is the accuracy in modelling the scattered spectrum for every experimental situation. Apart from that the method is reasonably simple as there is limited change to a conventional mammographic set up and simulations of varying breast thicknesses could be carried out off line and the resulting scatter spectra stored. Then the scatter removal could be part of the post-processing of the image.
Further work on this method is to find a way to measure how accurate this method is from removing scatter. Then a more clinically realistic phantom could be imaged with the spec- troscopic detector and the scatter removed from the whole image.
In conclusion, this project aimed to investigate methods of removing scatter in order to im- prove image quality using a pixellated spectroscopic detector. Two methods were researched and both showed positive results by improving on the contrast of low contrast details when compared to conventional imaging.
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