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Proceedings Paper

Determination of mass attenuation coefficients for threshold contrast evaluation in digital mammography
Author(s): Johann Hummel; Friedrich Semturs; Susanne Menhart; Michael Figl
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Paper Abstract

According to the 'European protocol for the quality control of the physical and technical aspects of mammography screening' (EPQC) image quality digital mammography units has to be evaluated at different breast thicknesses. At the standard thickness of 50 mm polymethyl methacrylate (PMMA) image quality is determined by the analysis of CDMAM contrast detail phantom images where threshold contrasts are calculated for different gold disc diameters. To extend these results to other breast thicknesses contrast-to-noise ratios (CNR) and threshold contrast (TC) visibilities have to be calculated for all required thicknesses. To calculate the latter the mass attenuation coefficient (MAC) of gold has to be known for all possible beam qualities in the tube voltage range between 26 and 32 kV. In this paper we first determined the threshold contrast visibility using the CDMAM phantom with the same beam quality at different current-time products (mAs). We can derive from Rose theory that CNR • CT • α = const, where α is the diameter of the gold cylinder. From this the corresponding attenuation coefficients can be calculated. This procedure was repeated for four different beam qualities (Mo/Mo 27kV, Rh/Rh 29kV, Rh/Rh 31 kV, and W/Rh 29 kV)). Next, we measured the aluminium half value layer (HVL) of all x-ray spectra relevant for mammography. Using a first order Taylor expansion of MAC as a function of HVL, all other desired MAC can be calculated. The MAC as a function of the HVL was derived to MAChvl = -286.97 * hvl+186.03 with R2 = 0.997, where MAChvl indicates the MAC for all specific x-ray spectrum defined by its aluminium half value layer. Based on this function all necessary MACs needed for quality assurance (QA) were calculated. The results were in good agreement with the data found in the protocol.

Paper Details

Date Published: 23 March 2010
PDF: 6 pages
Proc. SPIE 7622, Medical Imaging 2010: Physics of Medical Imaging, 762242 (23 March 2010); doi: 10.1117/12.844483
Show Author Affiliations
Johann Hummel, Medizinische Univ. Wien (Austria)
Wilhelminenspital (Austria)
Friedrich Semturs, Medizinische Univ. Wien (Austria)
Susanne Menhart, Medizinische Univ. Wien (Austria)
Michael Figl, Medizinische Univ. Wien (Austria)


Published in SPIE Proceedings Vol. 7622:
Medical Imaging 2010: Physics of Medical Imaging
Ehsan Samei; Norbert J. Pelc, Editor(s)

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