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Ivanović, Sonja, 1972-
Improving QA/QC in mammography screening and breast diagnosis in Montenegro
2017

https://phaidra.ucg.ac.me/detail_object/o:1581?tab=0#mda

Autorstvo 3.0 Srbija (CC BY 3.0)
Academic metadata
Phd. theses

Prirodno-matematičke nauke

doktor nauka - fizičke nauke
Univerzitet Crne Gore
Prirodno-matematički fakultet
Studijski program Fizika

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The leading idea for this PhD project was to contribute, in a scientifically driven way, to Quality Control (QC) of mammography systems. During the project, we obtained however both scientific and practical results. Seen the very poor level in Quality Control of diagnostic radiology in Montenegro at the start of the project, the practical contribution obtained through this PhD on National level is huge. This is of course also thanks to the Technical cooperation project with the International Atomic Energy Agency (IAEA) that provided the Clinical center of Montenegro with a complete kit for mammography QC. A first task was to perform a QC test on every mammography system in Montenegro, to compare data with European standards and to consequently find out about the situation. Concerning patient doses we were in line with the EU protocol. Concerning image quality we should be on a higher level and aim for the European acceptable level. This situation is mostly due to inadequate image processing devices and absence of any regular maintenance, but there is a factor of lack of staff and proper training, also. The next phase of the PhD project was performed in the University Hospital of the Catholic University of Leuven, Belgium, with my remote mentor. I learned about the details of QC tests, both for film-screen and full field digital mammography. This center has also 2 breast tomosynthesis systems for which a QC protocol was being developed. More importantly, we made plans for a first scientific paper in which we would test a new feature of the MagicMax multimeter (IBA), the dosimeter that had been provided by the IAEA. This new dosimeter provides an automated estimate of the half value layer (HVL) from just a single exposure. This characteristic was tested for 5 different anode/filter combinations and results were compared to measurements with an ionization chamber, considered gold standard. First, our scientific contributions will be presented. In this work a critical analysis of the methodology for evaluating the x-ray half value layer was conducted. Our analysis was performed on mammography units with different anode-filter combinations. Solid state dosimeters can be very energy sensitive and may not be reliable for an automated HVL determination, while ionization chambers are known to be largely energy independent and served as gold standard. A two-phase project was started: to investigate different methods and their associated accuracies to manually calculate the HVL and to subsequently verify the accuracy of the new solid state dosimeter. First, we showed that all manually acquired, experimental data fitted well to an exponential function (correlation coefficients were in most cases more than 99%). Next, the MATLAB curve fitting toolbox was used to assess the errors on the curve fits. The uncertainty of manual HVL determination was less than 10% (usually less than 6%). These results define the errors that can be tolerated with automated equipment. From a series of measurements for analyzing accuracies, repeatability and energy dependencies of the detectors [23], and other factors [24], we realized that they are not the critical factors in HVL determination. We decided that uncertainties of HVL should not be more than 10%. A critical factor for the HVL is the number of experimental data. The HVL, determined from the first or last four data points, obtained with successive sheets of Aluminum, was significantly different from HVL values obtained from all eight points. In every experiment three significantly different values of the HVL were obtained: the lowest, obtained from the first four data; an intermediate value, obtained from all experimental data, and the highest value, obtained from the last four points. In some cases these differences were about 25%. We concluded that the X-ray beam is hardening by passing through aluminum sheets. The beam that is being characterized is the beam as measured with the first measurement of the series that is considered in the calculation, and that it defined as the initial measurement. Next, because new (solid state) dosimeters allow all-in-one shot data acquisition, we used the opportunities of different mammography units in Leuven and Gent to verify whether we can trust kV measurements at all anode/filters and over the complete kV range. In other words, we studied the question: are solid state dosimeters sufficiently energy corrected for (all) beam qualities? Can we trust actual dosimeters which provide ‘readymade’ data on HVL from a single exposure? Is Robson’s approach, a method used for extrapolation of tube output and HVL from a measurement at a single tube voltage, also valid on newly introduced beam qualities? We showed that differences between the automated, direct HVL measurements of Magic Max and manual measurements did not exceed 0.02 mm Al. Standard deviation was approximately 0.1. We concluded that the direct HVL measurements with the Magic Max are within the confidence interval of manual HVL determinations. Likewise, we used a linear model and more MATLAB software to show that Robson’s model shows a satisfactory agreement between measured and predicted values of air kerma and HVL for the conditions that we have tested. To derive useful information from a mammogram, image processing plays important role. The x-ray beam that exits the patient contains the ‘available information’. However, the imaging system, with its specific image formation part, deteriorates this information to some extent. Part of the information content could be restored or enhanced. The presence of non-avoidable (quantum) noise makes this procedure more complex from a mathematical point of view. We have explored the use of the Wiener filter to restore the information content via ‘restoration by deconvolution’ [50]. MATLAB software was used to restore image by deconvolution procedures, using the Wiener filter in two ways: 1. with a scalar estimate of the noise/signal power ratio (NSR), 2. with a frequency dependent estimate of the noise/power ratio. In order to proof the concept, the bar-pattern was used, instead of a real breast. It is found that with both approaches, the spatial resolution is improved, although the best result is depicted in methodology 2. where a frequency dependent estimate of the noise/signal power ratio is done via respective autocorrelation functions. The conclusion was that a careful estimation of the quality of the imaging system through PSF (Point Spread Function) or MTF (Modulation Transfer Function), together with an estimation of the sort and magnitude of noise in the image, could be used to improve the spatial resolution. A first practical contribution in the frame of radiation safety is that for the first time, Average Glandular Doses (AGD) have been measured in Montenegro. Dose reference levels were established and it is in accordance with EU protocols. We performed sensitometry and densitometry measurements at all screen-film mammography systems and they were shown to have low image quality in general. The optimization process is demanding. Optimization of image quality is already done at Podgorica’s Health care center. During the QC procedures on screen-film mammographs, we found discrepancy between the indicated and measured tube voltages, with deviations that are larger than what is allowed following the European protocol (namely ±2 kV) on the Planmed Sophie Classic. Paper 4., 5. and 6. from the list of published papers reflect that work performed in the frame of present PhD project. List of published papers: 1. "Dose estimations for persons occupationally exposed to ionizing radiation in Montenegro", published in Arch. of oncology 2008, 16(1-2) 5-6, A.Milatovic, S.Ivanovic, S.Jovanovic 2. Radiation protection of patients in diagnostic radiology: Status of practice in five Eastern-European countries, based on IAEA project; O. Ciraj-Bjelac, A. Beganovic, D. Faj, V. Gershand, S. Ivanovice, 1. R. Videnovicf, M. M. Rehani; European Journal of Radiology 79 (20 II) e70-e73, 3. Status of radiation protection in interventional cardiology in five Eastern -Europian Countries, O. Ciraj-Bjelac, A. Beganovicb, D. Faj , S. Ivanovice,I. R. Videnovicf, M. M. Rehani; Book of abstracts of International Conference on Radiation Protection in Medicine, Varna, Bulgaria, 1-3 September, 2010, Radiologija, Suppl 10, P 22. 4. "Getting started with protocol for Quality Assurance of Digital Mammography in the Clinical centre of Montenegro", published in Radiation Protection Dosimetry 2015, S.Ivanovic, H.Bosmans, S.Mijovic 5. Uncertainties in half value layer determination in mammography, S. Mijovic, S.Ivanovic, F. Bemelmans, H.Bosmans, AIP Conference Proceedings 1722, 300004 (2016); doi: 10.1063/1.4944308 View online: http://dx.doi.org/10.1063/1.4944308 6. Mammograms restoration by using Wiener filter, M. Dakovic, S. Ivanović, and S. Mijovic, AIP Conference Proceedings 1722, 300005 (2016); doi: 10.1063/1.4944309 View online: http://dx.doi.org/10.1063/1.4944309