Optimization of anode-filter combinations in digital mammography: An experimental study

Transcription

1 Optimization of anode-filter combinations in digital mammography: An experimental study Poster No.: C-2915 Congress: ECR 2010 Type: Scientific Exhibit Topic: Physics in Radiology Authors: A. Peruzzo Cornetto 1, A. Taibi 2, V. Rossetti 1, P. Vignoli 3, R. Keywords: DOI: Ropolo 1 ; 1 Torino/IT, 2 Ferrara/IT, 3 Bologna/IT Digital mammography, Optimization, Image quality /ecr2010/C-2915 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 15

2 Purpose Most of digital mammography systems are now developed with a tungsten anode because of the good matching of its physical characteristics (high atomic number, high x-ray output fluence) with the digital detectors features. Tungsten spectrum shows considerable useless L-characteristic x-rays (Figure1 on page 3) at 8-10 kev that must be removed, by the use of suitable k-edge filters, in order to achieve clinical optimization. The use of a 50 µm-thick rhodium filter allows the elimination of the tungsten L- characteristic x-rays (Figure2 on page 4). Experimental studies pointed out that the use of a tungsten/rhodium (W/Rh) anode/ filter combination with a digital amorphous selenium detector is always able to achieve equivalent contrast to noise ratio (CNR) values with a net Mean Glandular Dose (MGD) reduction, if compared to classic molybdenum/molybdenum (Mo/Mo) and molybdenum/ rhodium (Mo/Rh) combinations [1, 2]. The choice of rhodium filter is used to optimize the so-called "standard breast" (5 cm, 50% fat - 50% fibro-glandular). The purpose of this study was to investigate the feasibility of using alternatives to rhodium filter for a digital mammography system based on a tungsten-anode x-ray tube, so as to optimize the x-ray spectrum used to image breasts of various thicknesses. Page 2 of 15

3 Images for this section: Page 3 of 15

4 Fig. 1: Unfiltered W anode Bremsstrahlung simulated spectrum at 28 kvp. Figure shows marked presence of L-characteristic x-rays. Page 4 of 15

5 Fig. 2: Filtered W anode simulated spectrum at 28 kvp, with mm Rh filter, shows the elimination of the L-characteristic x-rays. Rhodium k-edge@23.22 kev. Page 5 of 15

6 Methods and Materials First, the choice of alternative filters was evaluated by simulating the X-ray spectra with the Spectrum Processor program produced by the IPEM (Institute of Physics and Engineering in Medicine, United Kingdom). This program yields as output parameters the first HVL (Half Value Layer), the mean photon energy and the Entrance Surface Air Kerma (ESAK), calculated on the basis of a Catalogue of Diagnostic X-Ray spectra referred as IPEM Report No 78 [3]. Spectral data for the tungsten target in the field of mammographic energy, not included in the software, was obtained through the TASMIP (Tungsten Anode Spectral Model using Interpolating Polynomials) spectral model described by Boone [4], which enables us to estimate the values of the photon fluence #(E,V) (photons/mm 2 ) as a function of the energy E and the applied tube voltage V. Data was added in the Spectrum Processor database as well as linear attenuation coefficients for lexan (compression paddle) and silver (Ag) filtrations, which were calculated by the use of XCOM software [5]. All spectra were simulated setting target angle to 16 and using lexan 2 mm-thick and beryllium (Be) 0.8 mm-thick fixed attenuators. Spectral simulation was also used for the evaluation of the HVLs necessary to estimate the mean glandular dose (MGD) according to the method described by Dance et al [6]. Experimental measurements were performed on a Giotto Image 3DL mammography system (IMS S.r.l., Bologna, Italy), which integrates a tungsten/rhodium (W/Rh) combination and an amorphous selenium digital detector with a field of view of 24 x 30 cm 2 (Anrad, Canada). Contrast to noise ratio (CNR) was measured by using a sheet of aluminium 0.2 mm-thick and increasing the thickness of the polymethyl methacrylate (PMMA) slab according to the EC protocol [7]. CNR analysis was performed using a dedicated software that allows to make measurements within a region of interest in a reproducible way. Mean glandular dose (MGD) was kept constant for all thicknesses cm PMMA thicknesses were investigated with mGy MGDs respectively; these values are lower or comparable with the achievable levels set per PMMA thickness by EC protocol. Because of its physical characteristics, Rh filter (Z=45, k-edge@23.22 kev) was compared with two alternatives elements whose k-edges are lower and higher respectively: Mo (Z=42, k-edge@20.00 kev) and Ag (Z=47, k-edge@25.51 kev). W/Mo and W/Ag combinations were compared to the standard W/Rh for small-medium (2-3 cm) and medium-high (3, 5, and 7 cm) PMMA thicknesses, respectively. In the range from 22 kv p to 32 kv p, at least four kv p /mas configurations providing the desired MGD Page 6 of 15

7 values were studied for each alternative combination and thickness. W/Rh was studied setting IMS clinical standard kv p values. Each combination was studied after a complete specific calibration of the detector and air kerma measurements, necessary to estimate MGD, were performed with a dedicated ionization chamber (Radcal 20X6-6M, Monrovia, CA, USA) positioned in the reference area (6 cm far from the chest wall). Figure of merit FOM=CNR 2 /MGD was used for comparative evaluation. Results Spectral analysis led to the choice of 90-µm thick molybdenum (Mo) and 50-µm thick silver (Ag) filtrations. K-edge energy of the elements chosen and thickness-modulation modify the W x-ray spectrum and the mean photon energy, acting as "bandpass filters". Simulation pointed out that these thicknesses were able to remove the L-characteristic x-rays in the spectrum of the tungsten, as shown in Figure1 on page 7 and Figure2 on page 8. Experimental FOM evaluations, which maximum values are collected in Figure3 on page 9, pointed out a substantial equivalence (i.e. the difference was inside the error bars), in terms of FOM values between the standard W/Rh and the combinations W/Mo and W/ Ag for a PMMA thickness of 2 and 3 cm, respectively (Figure4 on page 9, Figure5 on page 10). W/Ag combination achieved higher FOM values than W/Rh for a thickness of 5 and 7 cm, showing improvements of almost 20% (Figure6 on page 11) and 40% (Figure7 on page 12), respectively. Images for this section: Page 7 of 15

8 Fig. 1: Filtered W anode simulated spectrum at 28 kvp with mm-thick Mo filter showing elimination of the L-characteristic x-rays. Molybdenum k-edge@20.00 kev. Page 8 of 15

9 Fig. 2: Filtered W anode simulated spectrum at 28 kvp with mm-thick Ag filter showing elimination of the L-characteristic x-rays. Silver k-edge@25.51 kev. Fig. 3: Maximum values and per cent differences between the Figure of Merit (FOM), experimentally evaluated for W/Mo-W/Rh-W/Ag anode/filter combinations, various PMMA thicknesses, keeping MGD fixed for each thickness. FOM uncertainties are of about 10%. Page 9 of 15

10 Fig. 4: Figure of Merit (FOM) as a function of nominal tube voltage (kvp) for W/Mo-W/Rh anode/filter combinations, PMMA 2 cm-thick and MGD fixed at 0.6 mgy. Uncertainties of about 10% for FOM are shown with error bar. Page 10 of 15

11 Fig. 5: Figure of Merit (FOM) as a function of nominal tube voltage (kvp) for W/Ag-W/Rh anode/filter combinations, PMMA 3 cm-thick and MGD fixed at 0.9 mgy. Uncertainties of about 10% for FOM are shown with error bar. Page 11 of 15

12 Fig. 6: Figure of Merit (FOM) as a function of nominal tube voltage (kvp) for W/Ag-W/Rh anode/filter combinations, PMMA 5 cm-thick and MGD fixed at 1.5 mgy. Uncertainties of about 10% for FOM are shown with error bar. Page 12 of 15

13 Fig. 7: Figure of Merit (FOM) as a function of nominal tube voltage (kvp) for W/Ag-W/Rh anode/filter combinations, PMMA 7 cm-thick and MGD fixed at 3.1 mgy. Uncertainties of about 10% for FOM are shown with error bar. Page 13 of 15

14 Conclusion This study demonstrated experimentally the possibility of replacing standard Rh filtration, currently used by the commercial mammography in study, with the double filtration of 90 µm-mo for small breast thicknesses (PMMA 2 cm-thick equivalent) and 50 µm-ag for higher thicknesses. By replacing Rh filter, it is possible to optimize the x-ray beam energy used to image breast of various thicknesses by exploiting the different physical characteristics of the alternatives filtrations studied. Indeed, the results showed that an improvement of physical image quality, expressed in terms of FOM, can be achieved by replacing the classic filter. This improvement grows with thickness, being already significant for a medium-high-thick breast. References [1] P. Toroi, F. Zanca, K.C.Young, C.Van Ongeval, G. Marchal, H. Bosmans, Experimental investigation on the choice of tungsten/rhodium anode/filter combination for an amorphous selenium-based digital mammography system, Eur Radiol, 17, 2007, pp [2] P. Bernhardt, T. Mertelmeier, M. Hoheisel, X-ray spectrum optimization of full-field digital mammography: Simulation and phantom study, Med. Phys., Vol 33 No 11, 2006, pp [3] K. Cranley, B.J. Gilmore, G.V.A. Fogarty, L. Desponds, Catalogue of Diagnostic X-ray Spectra and other data, Report No 78. Institute of Physics and Engineering in Medicine, York, UK, [4] J.M Boone, Molybdenum, rhodium, and tungsten anode spectral models using interpolating polynomials with application to mammography, Med. Phys., Vol 24 No 12, 1997, pp [5] Berger MJ, Hubbel JH, XCOM: photon cross sections on a personal computer, National Bureau of Standards Report NBSIR , 1987 [6] D.R. Dance, C.L. Skinner, K.C. Young, J.R. Beckett, C.J. Kotre, Additional factors for the estimation of mean glandular breast dose using the UK mammography dosimetry protocol, Phys. Med. Biol., Vol. 45, 2000, pp [7] R. van Engen, K.C. Young, H. Bosmans, M. Thijssen. European protocol for the quality control of the physical aspects of mammography screening: Digital mammography. In: Page 14 of 15

15 N. Perry, M.Broeders, C. de Wolf, S. Tornberg, R. Holland, L. von Karsa. European guidelines for quality assurance in breast cancer screening and diagnosis, 4 th ed. pp European Commission Personal Information Andrea Peruzzo Cornetto Ph.D. Medical Physics Department, S. Giovanni Battista University Hospital Torino, ITALY Page 15 of 15