Investigation of Effective DQE (edqe) parameters for a flat panel detector Poster No.: C-1892 Congress: ECR 2013 Type: Authors: Keywords: DOI: Scientific Exhibit D. Bor 1, S. Cubukcu 1, A. Yalcin 1, O. Birgul 1, N. Marshall 2 ; 1 Ankara/ TR, 2 Leuven/BE Quality assurance, Acceptance testing, Digital radiography, Radiation physics 10.1594/ecr2013/C-1892 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. www.myesr.org Page 1 of 10
Purpose The effective detective quantum efficiency (edqe) was recently introduced to assess the performance of the digital radiographic systems in clinical examinations since it includes the effects of scatter and focal spot unsharpness [1,2,3]. The aim of this work is to investigate the influence of these factors on edqe. An alternative calculation of edqe is also suggested in this work that includes the influence of scatter in the MTF measurement. Methods and Materials A wireless flat panel imaging detector (Carestream DRX1) based on indirect conversion with a CsI phosphor with pixel size of 0.139 mm was used with an analog X-ray system (GE Silhouuette VR) including a wall-stand bucky. Focus to detector distance was 180 cm and a stationary grid with 10:1 grid ratio and grid density of 60 lines per cm was employed. In order to see the effects of scatter and focus blurring, edge phantom and uniformity images were collected at three different acquisition geometries for the assessment of modulation transfer function (MTF) and noise power spectrum (NPS); 1) detector only: PMMA slabs (5, 10, 20, and 25cm) were placed in close proximity to the X-ray tube and the edge phantom (8x10 cm in size, 1mm in width and W in material) was put on the detector surface to determine the detector sharpness and noise performance alone. 2) focal spot blurring: Slabs were placed similar to above geometry, but images of the edge phantom were acquired at different distances from the detector face to see the additional effect of focus blurring. The distances in air were adjusted equal to the thicknesses of the slabs. 3) focal spot blurring and scatter: The PMMA slabs were put in front of the detector and edge phantom was positioned at the entrance surface of these slabs to assess the combined effect of scatter and focal spot blurring. Images were collected using tube voltages of 70, 90 and 120 kvp with phototimed exposure to establish a fixed detector exposure around 3 ugy for PMMA thicknesses of 5, 10, 20 and 25 cm. Effective detective quantum efficiency (edqe) values for second and third geometries were determined using [1]; Page 2 of 10
edqe(f) = MTF 2 (f) (1-SF) 2 /(NNPS(f).q.E.TF) E is the measured prephantom free-in-air exposure which is inverse- squared corrected to detector plane and q is the ideal squared signal- to-noise ratio per unit exposure at that plane. The narrow-beam transmission fraction (TF) and scatter fraction (SF) were measured as described Samei et.al. [1,2] for each beam quality. In their approach, MTF was obtained by smoothing and truncating the line spread function (LSF). An alternative calculation of edqe is suggested in this study; MTF assessments were carried out by obtaining LSF without any smoothing or truncation, and edge profiles were obtained using a large region of interest (50x50mm 2 ) in the edge images, so that the long tails of scatter were included in the LSF. This eliminates the factor from the above equation. Results Fig. 1 on page 3, Fig. 2 on page 4 and Fig. 3 on page 4 give the MTF, NNPS and edqe results for three different acquisition geometries. MTF is degraded with focus blur and scatter. The use of grid suppresses the effect of scatter, and therefore, focus blurring becomes dominant (Fig. 1 on page 3). Lower influence of The same uniformity images were used for the first and second geometries, so NNPS is the same for these geometries (Fig. 2 on page 4). Although the detector exposures are the same (3 ugy), scatter image has a lower NNPS. As expected, DQE of the detector alone is higher and reduces progressively when the focus blurring and scatter were introduced subsequently (Fig. 3 on page 4). Fig. 4 on page 5, Fig. 5 on page 6 and Fig. 6 on page 6 compare the MTF, NNPS and edqe results obtained for different PMMA thicknesses. edqe degrades with increasing thickness of PMMA slabs especially at low frequency range. As it is shown in Fig. 7 on page 7, no noticeable kvp dependency was observed for edqe results. Fig. 8 on page 8 and Fig. 9 on page 8 compare the edqe calculated from two MTFs, one obtained with truncated and smoothed LSF and the other including the scatter and no processing. Images for this section: Page 3 of 10
Fig. 1: MTF results for three acquisition geometries obtained at 90 kvp. Fig. 2: NNPS results for three acquisition geometries obtained at 90 kvp. Page 4 of 10
Fig. 3: DQE and edqe results for three acquisition geometries obtained at 90 kvp. DQE is used for detector only geometry and edqe is used for the others. Page 5 of 10
Fig. 4: MTF results for different thicknesses of PMMA slabs at 90 kvp using the third geometry. Fig. 5: NNPS results for different thicknesses of PMMA slabs at 90 kvp using the third geometry. Page 6 of 10
Fig. 6: edqe results for different thicknesses of PMMA slabs at 90 kvp using the third geometry. Fig. 7: edqe results obtained at different tube voltages using the third geometry. Page 7 of 10
Fig. 8: MTFs used for the calculation of edqe using two approaches. Red graph is obtained with truncated LSF and blue graph is obtained with the proposed approach that does not apply any processing to be able to see blurring effects. Third geometry and a slab of 20cm thickness were used. Page 8 of 10
Fig. 9: Comparison of two edqe approach using MTFs presented in (a). Page 9 of 10
Conclusion The edqe has been demonstrated as a useful technique in evaluating the performance of digital radiographic imaging for clinical radiographic examinations. It is shown that edqe can demonstrate the effect of different blurring factors. Although the use of an LSF without any processing can introduce noise into the MTF and subsequently into the edqe, inclusion of scatter in the LSF for MTF calculation simplifies the edqe estimation since the measurements for scatter fraction can be omitted. References [1] Samei, E., Ranger, N. T., MacKenzie, A., Honey, I. D., Dobbins, J. T., & Ravin, C. E. (2009). Effective DQE (edqe) and speed of digital radiographic systems: an experimental methodology. Medical Physics, 36(8), 3806-3817. [2] Samei, E., Ranger, N. T., MacKenzie, A., Honey, I. D., Dobbins, J. T., & Ravin, C. E. (2009). Effective DQE (edqe) and speed of digital radiographic systems: an experimental methodology. Medical Physics, 36(8), 3806-3817. [3] Salvagnini E., Bosmans H., Struelen L., Marshalls N. W. (2012). Effective detective quamtum efficiency (edqe) and effective noise equivalent quanta (eneq) for system optimization purposes in digital mammography. Proc. of SPIE vol. 8313. Personal Information Page 10 of 10