Comparison of new digital mammography systems: physical characterisation and image quality evaluation

Transcription

1 Comparison of new digital mammography systems: physical characterisation and image quality evaluation Poster No.: C-2321 Congress: ECR 2011 Type: Scientific Exhibit Authors: N. Oberhofer; Bolzano/IT DOI: /ecr2011/C-2321 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 22

2 Purpose In recent years several new digital mammography equipments with different innovative features have been proposed on the market [1, 2]. The work compares 7 different so called direct radiography (DR) models with respect to physical characteristics, phantom image quality and dose. Fig.: Studied systems References: N. Oberhofer; Bolzano, ITALY Methods and Materials This work covers most of the innovations. It embraces 8 direct radiography (DR) systems of 7 different models and 5 different manufacturers; 2 computed radiography (CR) systems are reported for comparison (list in Fig. 2). System A1 and A2 are twin systems which underwent an x-ray tube replacement in 2010; Systems A3 and A4 are identical apart from the detector technology (Fig. 3 and 4). Data of System D1 are preliminary. The technological innovations introduced are related to 3 different aspects of a mammography unit, as explained in Fig. 1: Page 2 of 22

3 X-ray production [3, 4, 5, 6], X-ray acquisition technique, X-ray detection technology [7, 8, 9]. The work pursued two goals, the physical characterisation of the different systems and the comparison of their image quality with respect to dose. A. Physical Characterisation: Performed according to IEC with either beam quality RQA-M2 (Mo/30 mm Mo + 2 mm Al, 28 kv) or target/filter combination W/50 mm Rh in equal conditions and neither antiscatter grid nor compression paddle in place. Evaluation of modulation transfer function (MTF) with edge device placed directly on detector cover; entrance air kerma corresponding to levels in "normal" use (ca. 90 µgy); normalized noise power spectrum (NNPS) - mean value of 3 subsequent images; detective quantum efficiency (DQE). Evaluation has been carried out on linearized dicom images "for processing" with the open source software ImageJ and the plugin "qa-distri/dqe Panel v7" extended for DEQ calculation. Air Kerma values were determined with a regularly calibrated ion chamber (Radcal X6-6M, Radcal Corporation, USA). Uncertainty: <2% for MTF, <4% for NNPS, 8% for DQE (precision <5%). B. Phantom image quality evaluation: The process was carried out in two steps, applying two different methods for image quality evaluation [10, 11]: 1. optimization of exposure technique: Using contrast to noise ratio (CNR) as working parameter, for each phantom thicknesses (2 cm - 7 cm) those high voltage values and target/filter combinations were determined, which exhibited the best CNR at equal average glandular dose (AGD). CNR measurement followed the European Guidelines. The test object consisted of several polimethylmethacrylate (PMMA) layers covering the whole detector area with a small contrast object (foil of 0.2 mm Al) positioned above 2 cm PMMA, as indicated in Fig. 5. The adopted formula was Page 3 of 22

4 Fig.: Formula 1 References: N. Oberhofer; Bolzano, ITALY where MPV = mean pixel value SD = standard deviation. image quality assessment: The method of contrast detail (CD) evaluation was used with the phantom CDMAM ver. 3.4 and the commercial software CDMAM Analyser (both Artinis, The Netherlands) for automated image analysis (see Fig. 6 and Fig. 7). The CDMAM phantom consists of several PMMA layers and a detail tablet, which in this study was always positioned above 2 cm of PMMA. Additional PMMA layers needed to achieve the desired total phantom thickness were put on top of the detail tablet. In order to reduce uncertainty, for each exposure technique a set of 8 raw images was considered (detection rate of 75%) [12]. In DR systems the phantom block was slightly moved between successive images. Results are expressed in terms of a group contrast detail curve and, additionally, of an overall quality index, the Image Quality Figure Inverse (IQFinv), defined as Page 4 of 22

5 Fig.: Formual 2 References: N. Oberhofer; Bolzano, ITALY which increases when detail detection rises. This figure has prevously been shown to represent an objective and absolute measure of the image quality [10], suitable for comparison of different equipments. The linear relation between CNR and IQFinv was verified in all systems (see Fig. 8 for an example). Under the hypothesis that image noise is dominated by quantum noise, image 2 quality improves applying a higher dose according to CNR prop. dose. That means 2 CNR /dose is constant for each kv/phantom setting. In analogy we defined the dose 2 independent figure of merit IQFinv /AGD to compare systems performances in various conditions. Fig.: Formula 3 References: N. Oberhofer; Bolzano, ITALY Uncertainty of the CNR value was estimated as standard deviation of the evaluation of 10 repeated exposures. AGD was calculated according to Dance. Uncertainty: CNR < 3% in DR, < 5% in CR; IQFinv < 4%, AGD 10% (precision 3%), mean 2 IQFinv /AGD 5 % - 7%. Images for this section: Page 5 of 22

6 Fig. 1: Studied systems Page 6 of 22

7 Fig. 2: The technological innovations introduced in digital mammography in the last years are related to 3 different aspects of a mammography unit. Fig. 3: Scheme of flat panal imager with a-si TFT-matrix readout (left) and a-se photoconductive switching readout. Page 7 of 22

8 Fig. 4: Operational diagram of the photoconductive switching readout. Page 8 of 22

9 Fig. 5: Experimental setup for CNR measurement. A 0.2 mm Al foil is used as contrast object. Fig. 6: Phantom CDMAM 3.4: Radiography of the detail table for contrast detail evaluation. The contrast objects are accomplished as thin gold discs of exact diameter and thickness, arranged in squares. Within a column the disc-diameter is constant, with exponentially increasing thickness, and within a raw the disc thickness is constant, with exponentially increasing diameter. Each square contains two details, one in the center and one in a randomly chosen corner. The task consists in the correct detection of the corner detail position. Page 9 of 22

10 Fig. 7: Output example of the commercial software CDMAM Analyser 1.1 for automated CD evaluation. The darkness of the dots is related to the detection frequency of the corner details, being highest if one detail has been correctly detected in all images. Page 10 of 22

11 Fig. 8: Example of linear correlation between the parameters CNR and IQFinv related to System A3. Page 11 of 22

12 Results A. Physical characterisation: General comparison between CR and DR systems and between direct and indirect conversion technology in DR systems: Figures 1, 2 and 3 show for several systems the frequency dependence of the parameters MTF, NNPS and DQE, up to the Nyquist frequency. DR units clearly outperformed CR systems. Direct conversion flat panels (a-se, systems A1-A4) exhibited a higher MTF than detectors based on indirect conversion with needle-like CsI (systems B1, B2) (Fig. 1). Flat panels using direct conversion technology were generally affected by a higher noise than those using indirect conversion (Fig. 2). Considering DQE, units with direct conversion technology resulted superior (peak DQE > 65% versus ca. 50% for units with indirect conversion) (Fig. 3). Comparison between different a-se readout technologies: System A3 uses conventional a-si TFT-matrix readout, System A4 an innovative photoconducting switching readout obtained by a second a-se layer, separated by a thin electron trapping layer (ETL) [13]. Although the detector of System A4 has a smaller pixel pitch, its MTF value was lower compared to System A3 (Fig. 4). The detector with photoconductive switching readout exhibited low noise, in particular at higher frequencies, thus overcoming the high-noise-problem of a-se detectors with conventional TFT-matrix (Fig. 5). Peak DQE values were comparable in Systems A3 and A4, above 70%; System A4 with the photoconductive switching readout achieved a better overall DQE performance having a slower decrease (Fig. 6). B. Image quality assessment: Exposure technique optimization: In all systems target/filter combinations producing an x-ray spectrum of higher mean energy showed up to be more dose-efficient. For a phantom thickness # 3 cm PMMA the figure of merit 2 IQFinv /AGD was higher using Mo/Rh, Rh/Rh or W/Rh than with Mo/Mo. Fig. 7 reports an example. Systems A3 and A4 resulted already optimized with the factory AEC-settings. Comparison between systems with indirect and direct conversion AMFPI in optimized exposure technique: The studied direct conversion AMFPIs 2 scored a slightly higher IQFinv /AGD value than indirect conversion one; the difference is significant (Wilcoxon test, p < 5%). Page 12 of 22

13 Comparison between conventional direct AMFPI technology and innovative photoconducting switching readout: System A4 with the photoconductive 2 switching readout scored the highest IQFinv /AGD values among all direct conversion flat panel systems at every phantom thickness (Wilcoxon test, p < 1%). Fig. 8 shows the direct comparison between Systems A3 and A4, 2 which differ only by the detector readout. The higher IQFinv /AGD value of System A4 traduces into 8%-28% dose savings in terms of AGD compared to System A3. Comparison of image quality with target/filter of clinical use at equal AGD: Fig. 9 shows a comparison of the IQFinv value of all systems at 1.45 mgy, where the systems with a newer detector technology are superiore (Systems A3, A4 and D1). Fig. 10 compares the systems at a lower dose, ca. 0.8 mgy AGD, where the slit scanning System D1 excels. C. Evalution of different acquisition techniques: For the newer target/filter combinations not considered in the European Guidelines [14], the correction factors necessary for AGD calculation according to the method of Dance were kindly supplied by Dance. Comparison of AGD in 2D and 3D (Tomosynthesis) acquisition: The overall dose of a tomosinthesis acquisition turned out to be well below the dose of a standard screening exam (two projections), confirming the manufacturer's statement (-15% - -40%). Comparison of slit scanning/photon counting detection technology with conventional full field projection technology: At low AGD values System D1 with slit scanning acquisition and photon counting detection technology (intrinsic scatter radiation rejection) showed markedly higher IQFinv scores than the other systems with full field projection acquisition. On the other hand, increasing the exposure permitted only a reduced relative quality improvement. Accordingly System D1 operates in clinical use at particularly low exposure levels. At medium doses slit scanning and photoconductive switching readout technologies demonstrated comparable image quality. Images for this section: Page 13 of 22

14 Fig. 1: MTF of 7 digital mammography systems calculated parallel and perpendicularly to the chest wall side at ca. 90 µgy detector entrance air kerma. Uncertainty: MTF: < 2%. Fig. 2: NNPS of 7 digital mammography systems at ca. 90 µgy detector entrance air kerma. Uncertainty: NNPS: < 5%. Page 14 of 22

15 Fig. 3: DQE for 6 mammography systems (ca. 90 µgy). Uncertainty: ca. 8%. Fig. 4: Comparison of MTF between Systems A3 and A4 at similar exposure values. Both systems show a marked direction dependance. H = perpendicular to chest wall edge, V = parallel. Uncertainty: < 2%. Page 15 of 22

16 Fig. 5: Comparison of NNPS between Systems A and B at similar exposure values. H = perpendicular to chest wall edge, V = parallel, Rad = radial. Uncertainty: < 4%. Page 16 of 22

17 Fig. 6: Comparison of DQE between Systems A and B at similar exposure values. H = perpendicular to chest wall edge, V = parallel. Uncertainty: 8%. Fig. 7: Study of the figure of merit IQFinv2/AGD at different PMMA thickness and beam qualities. Example related to System A3. Reported data represent a mean value of several points corresponding to AGD at clinical values and above. Uncertainty:IQFinv2/ AGD 5-7%. Page 17 of 22

18 Fig. 8: Comparison of image quality performance of the systems A3 and A4 in terms of the figure of merit IQFinv2/AGD. Bars represent a mean value. Uncertainty: mean IQFinv/ AGD 5 % - 7%. Fig. 9: Comparison of image quality in different full field mammography systems by CD evaluation. Phantom composition: 2 cm PMM + CDMAM 3.4 detail plate + 2 cm PMMA. Results are given in terms of the overall image quality index IQFinv calculated Page 18 of 22

19 by the software CDMAM Analyser, ver. 1.1 (both Artinis, The Netherlands). Data refer to comparable AGD (ca mgy) after system "optimization". Fig. 10: Comparison of image quality in different full field mammography systems by CD evaluation for low dose exposures (ADG ca. 0.8 mgy). Phantom composition: 2 cm PMM + CDMAM 3.4 detail plate + 2 cm PMMA. Results are given in terms of the overall image quality index IQFinv calculated by the software CDMAM Analyser, ver. 1.1 (both Artinis, The Netherlands). Page 19 of 22

20 Conclusion DQE of all DR systems ranked between 55% and 80%, being higher in systems using a detector based on direct conversion and newer detector technology. Generally, a higher DQE traduced into a higher image quality expressed as IQFinv score. Target/filter combinations producing spectra of higher mean energy confirmed better performances than conventional Mo/Mo ones. The innovative photoconductive switching readout exhibited a potential average dose saving of 20% compared to state of the flat panels which use conventional a-si TFT-matrix readout. Also slit scanning acquisition showed improved image quality at equal dose compared to standard flat panel detectors. That technology revealed particularly suited for low dose applications. At medium doses slit scanning and photoconductive switching readout technologies demonstrated comparable good image quality. In summary, it could be confirmed that the technological innovations allow better image quality with lower AGD. References KCare, Report "Full Field Digital Mammography Systems. Sectra MicroDose. A technical report" (2006) Young, K.C., Oduko, J.M., Asad, M., "Technical Evaluation of Fuji Amulet Full Field Digital Mammography System", NHSBSP Equipment Report 0907 (2009), Bernhardt, P., Mertelmeier, T., Hoheisel, M., "X-ray spectrum optimization of full-field digital mammography: Simulation and phantom study", Med. Phys. 33 (11), (2006) Baldelli, P., Phelan, N., Egan, G., "Effect of Anode/Filter Combination on the Dose and Image Quality of a Digital Mammography System Based on Amorphous Selenium Detector", IWDM LNCS 5116, (2008) Varjonen, M., Strömmer, P.,"Optimizing the Target/Filter Combination in Digital Mammography in the Sense of Image Quality and Average Glandular Dose", IWDM LNCS 5116, (2008) Oduko, J.M., Young, K., Alsager, A., Gundogdu, O., "Effets of Using Tungsten-Anode X-Ray Tubes on Dose and Image Quality in Full-Field Digital Mammography", IWDM LNCS 5116, (2008) Irisawa, K., Yamane, K., Imai, S., Ogawa, M., Shouji, T., Agano, T., Hosoi, Y., Hayakawa, T., "Direct-Conversion 50 µm pixel-pitch Detector for Digital Mammography using Amorphous Selenium as a Photoconductive Switching Layer for Signal Charge Readout", In: Proc SPIE, vol. 7258, 72581I-1 (2009) Page 20 of 22

21 Kuwabara, T., Iwasaki, N., Sendai, T., Furue, R., Agano, T., "Image Quality Evaluation of Direct-Conversion Digital Mammography System with a New Dual a-se Layer Detector", In: Proc SPIE, vol. 7258, 72584P-1 (2009) Rivetti, S., Lanconelli, N., Bertolini, M., Borasi, G., Golinelli, P., Acchiappati, D., Gallo, E., "Physical and psychophysical characterisation of a novel clinical system for digital mammography", Med. Phys. 36 (11), pp (2009) Oberhofer, N., Parruccini, N. and Moroder, E., "Image Quality Assessment and Equipment Optimisation with Automated Phantom Evaluation in Full Field Digital Mammography (FFDM)", IWDM LNCS 5116, (2008) Oberhofer, N., Fracchetti, A., Springeth, M. and Moroder, E., "Digital Mammography: DQE versus Optimized Image Quality in Clinical Environment - an on Site Study", In: Samei, E., Pelc, N.J. (eds.) Proc SPIE Medical Imaging 2010, vol. 7622, 76220J-1 (2010) Pascoal, A., Lawinski, C.P., Honey, I., "Evaluation of a software package for automated quality assessment of contrast detail images - comparison with subjective visual assessment", Phys. Med. Biol. 50, (2005)" Oberhofer, N., Fracchetti, A., Nassivera, E., Valentini, A. and Moroder, E. "Comparison of two Novel FFDM Systems with Different a-se Detector Technology: Physical Characterization and Phantom Contrast Detail Evaluation in Clinical Conditions", IWDM LNCS 6136, (2010) Van Engen, R., Young, K., Bonmans, H. and Thijssen, M., "European Protocol for the Quality Control of the Physical and Technical Aspects of Mammography Screening. Part B: Digital Mammography" [European Guidelines for Quality Assurance in Breast Cancer Screening and Diagnosis], 4th Edition, European Communities, Luxembourg (2006) Personal Information Dr. Nadia Oberhofer Medical Physicist at Azienda Sanitaria dell'alto Adige Ospedale di Bolzano Via L. Böhler Bolzano (Italy) nadia.oberhofer@asbz.it Tel Page 21 of 22

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