653 ノート 2013 9 17 2014 5 29 Code No. 522 DR DQE 1 國友博史 4 服部真澄 2 小山修司 5 岡田陽子 1 2 3 東出林 1 了 6 則夫 3 市川勝弘 7 澤田道人 4 5 6 7 有限会社 緒言 digital radiography: DR detective quantum efficiency: DQE DQE DR q X signal-to-noise ratio: SNR 1, 2 SNR DQE DR X Investigation of Measurement Accuracy of Factors Used for Detective Quantum Efficiency Measurement in Digital Radiography Hiroshi Kunitomo, 1 Shuji Koyama, 2 Ryo Higashide, 1 Katsuhiro Ichikawa, 3 Masumi Hattori, 4 Yoko Okada, 5 Norio Hayashi, 6 and Michito Sawada 7 1 Department of Central Radiological Technology, Nagoya City University Hospital 2 Radiological Sciences, Department of Radiological and Medical Laboratory Sciences, Nagoya University Graduate School of Medicine 3 Institute of Medical, Pharmaceutical and Health Sciences, Kanazawa University 4 Department of Radiology, Tokai Memorial Hospital 5 Department of Radiological Technology, Anjo Kosei Hospital 6 School of Radiological Technology, Gunma Prefectural College of Health Sciences 7 Mutsuda Shoukai Corporation, Ltd. Received September 17, 2013; Revision accepted May 29, 2014 Code No. 522 Summary In the detective quantum efficiency (DQE) evaluation of detectors for digital radiography (DR) systems, physical image quality indices such as modulation transfer function (MTF) and normalized noise power spectrum (NNPS) need to be accurately measured to obtain highly accurate DQE evaluations. However, there is a risk of errors in these measurements. In this study, we focused on error factors that should be considered in measurements using clinical DR systems. We compared the incident photon numbers indicated in IEC 62220-1 with those estimated using a Monte Carlo simulation based on X-ray energy spectra measured employing four DR systems. For NNPS, influences of X-ray intensity non-uniformity, tube voltage and aluminum purity were investigated. The effects of geometric magnifications on MTF accuracy were also examined using a tungsten edge plate at distances of 50, 100 and 150 mm from the detector surface at a source-image receptor distance of 2000 mm. The photon numbers in IEC 62220-1 coincided with our estimates of values, with error rates below 2.5%. Tube voltage errors of approximately ±5 kv caused NNPS errors of within 1.0%. The X-ray intensity non-uniformity caused NNPS errors of up to 2.0% at the anode side. Aluminum purity did not affect the measurement accuracy. The maximum MTF reductions caused by geometric magnifications were 3.67% for 1.0-mm X-ray focus and 1.83% for 0.6-mm X-ray focus. Key words: detective quantum efficiency (DQE), measurement accuracy, incident photon numbers, normalized noise power spectrum (NNPS), modulation transfer function (MTF) *Proceeding author
654 3 5 International Electrotechnical Commission IEC 2003 IEC62220-1IEC DQE 3 5 IEC 6 DQE presampled modulation transfer function MTF normalized noise power spectrum NNPS DQE(u) Dobbins Siewerdsen X 7, 8 NNPS DR 9, 10 11 12 IEC presampled MTF 13, 14 15, 16 presampled MTF X DQE(u) 1. 方法と使用機器 X IEC 1 μgy mm -2 μgy -1 X NNPS NNPS presampled MTF 1-1 1-1-1 IEC IEC RQA5 4 X X X Philips OPTIMS50 SRO2550 Siemens AXION Luminos drf Luminos UD150B DK-85 UD150L DK-85 Radcal 9015 10X5-6 Fig. 1 a X 2000 mm 99.5 IEC 7.1 mmal Fig. 1 a 73 kv X Fig. 1 b RAMTEC413 CdTe 0.1 mm 0.05 mm X CdTe 2000 mm 40 ma 20 μgy/s X 1.0 10 4 X X electron gamma shower Ver.5 17 EGS5 Fig. 1 c X 2000 mm 1.0 cm 2 1.0 10 9 X 18 0.2 kev 0.03
655 Fig. 1 Setup for measurement and calculation of photon number. (a) Geometry for defined X-ray beam quality (RQA5; International Electrotechnical Commission), (b) Geometry for measurement of X-ray spectrum, (c) Geometry for Monte Carlo simulation K air=ψ (μ tr/ρ) air 1 K air Ψ(µ tr/ρ) air (µ tr/ρ) air (µ en/ρ) air 2 K air Ψ( E ) ( µ / ρ) en 2 air 3 K Ψ( E ) ( μ / ρ) air en air 3 3 1 μgy 3, 5 IEC 30174 mm -2 μgy -1 1-1-2 RQA5 X AXION Luminos drf RQA5 73 kv 68 70 71.5 73 75 79 kv RAMTEC413 X 1-1-1 0.4 mm 0.2 mm 1-1-3 RQA5 IEC61267 99.9 H4000 19 99.5 A 1050 RAMTEC413 1-1-1 1-2 NNPS 1-2-1 X NNPS X IEC
656 source-image receptor distance SID 1500 mm NNPS 125 125 mm 3, 5 X SNR SNR 2 NNPS NNPS X X X UD150B DK-85 Konica Minolta flat panel detector FPD AeroDR0.175 mm SID 1500 mm RQA5 73 kv 40 μc/kg 400 400 mm 64 N=64 X ±179 mm 64 region of interest ROI 256 256 420 NNPS 22 mm 128 NNPS NNPS 1-2-2 RQA5 RQA5 73 kv 68 70 71 73 75 77 79 kv 2.58 10-7 C/kg 1-2-1 NNPS 768 768 7 X 20 NNPS exposure NNPS exposure NNPS 12, 21 NNPS RQA5 73 kv exposure NNPS 1-2-3 NNPS 1-1-3 RQA5 99.9 99.5 21 mm exposure NNPS 1-2-2 1-3 MTF 1-3-1 X computed radiography CR X 22, 23 X Philips SRO 2550 1.0 mm 0.6 mm CR Konica Minolta Regius170 87.5 μm 1-3-2 IEC 1 mm SID 2000 mm d 0 50 100 150 mm 1.000 1.026 1.053 1.081 15 line spread function LSFpresampled MTF 1-3-1 X CR IEC RQA5 MTF α 4 d α = fs l d 4 fs l α d 0 mm f MTF p(f) 5 sin( αfπ) p( f ) = 5 αfπ 2. 結果 2-1 2-1-1 IEC 4 X Table 1 IEC
657 Table 1 Calculation data of photon number using EGS5 and error rate with photon numbers of 30174 mm -2 µgy -1 at RQA5 X-ray equipment Tube voltage (kv) Photon number (mm -2 μgy -1 ) Difference from IEC (%) Philips 73 30889 2.370 OPTIMS50/SRO2550 Siemens 73 30929 2.502 AXION Luminos drf Shimadzu 72 30393 0.726 UD150B/DK-85 Shimadzu 75 30213 0.129 UD150L/DK-85 Table 2 Calculation data of photon number: variation in tube voltage/purity of aluminum filter X-ray equipment Tube voltage Purity of Aluminum filter Photon number Difference from IEC (kv) (%) (mm -2 μgy -1 ) (%) Siemens 68 99.5 30459 0.945 AXION Luminos drf 70 30896 2.393 71.5 31247 3.556 73 31526 4.481 75 31873 5.631 79 32402 7.384 73 99.9 31498 4.388 +2.50 +0.13 2-1-2 RQA5 73 kv Table 2 IEC 73 kv +4.48 +7.38 2-1-3 RQA5 73 kv Table 2 99.9 +4.39 99.5 +4.48 2-2 NNPS 2-2-1 X NNPS Fig. 2 NNPS 1.47 cycles/mm X Fig. 2 1.47 cycles/mm IEC SID 1500 mm 125 125 mm 2.0 1.3 Fig. 2 The error ratio of NNPS above 1.47 cycles/mm by the X-ray quantum number distribution to NNPS value at the center of the irradiated field. 2-2-2 exposure NNPS Fig. 3
658 Fig. 3 Error in NNPS from RQA5 due to variation in tube voltage. Fig. 4 Exposure NNPS due to difference in purity of aluminum filter. exposure NNPS -5 kv +6 kv ±1 2-2-3 exposure NNPS Fig. 4 99.5 NNPS 5 2-3 MTF CR X 1.0 mm 1.49 mm 1.15 mm 0.6 mm 0.999 mm 0.768 mm MTF Fig. 5 MTF Fig. 6 MTF MTF MTF 1.000 1.026 1.053 1.081 1.0 cycle/mm MTF 0.32 1.76 3.67 0.230 1.03 1.83 Fig. 5 Result of presampled MTF from two focal spot sizes. 2.0 cycles/mm MTF 3.09 6.69 11.7 0.243 2.81 5.33 Table 3 1 100 mm 3 cycles/mm 3.6 3. 考察 3-1 IEC RQA5
659 a b Fig. 6 Influence of presampled MTF on geometric magnification. (a) Large focal spot, (b) Small focal spot Table 3 Comparison of theoretical values and measured values in MTF reduction rate due to geometric magnification and blurring with effective focal spot size (a) Large focal spot Object-to-detector distance (mm) 50 100 150 Spatial frequency Theoretical Measured Theoretical Measured Theoretical Measured (mm -1 ) value value value value value value 0.5 0.999 0.999 0.997 0.995 0.994 0.989 1.0 0.998 0.997 0.990 0.982 0.976 0.963 2.0 0.990 0.969 0.960 0.933 0.906 0.883 3.0 0.978 0.958 0.910 0.879 0.796 0.785 (b) Small focal spot Object-to-detector distance (mm) 50 100 150 Spatial frequency Theoretical Measured Theoretical Measured Theoretical Measured (cycles/mm) value value value value value value 0.5 1.000 0.999 0.999 0.998 0.997 0.994 1.0 0.999 0.998 0.995 0.990 0.989 0.982 2.0 0.996 0.997 0.982 0.972 0.957 0.947 3.0 0.990 0.990 0.959 0.950 0.905 0.910 4 X IEC +2.5 Fig. 7 7 IEC RQA5 Table 2 IEC IEC 99.9 99.5
660 Fig. 7 X-ray energy spectra for two different beam collimation sizes. 3-2 NNPS IEC SID 1500 mm NNPS 125 125 mm NNPS 2 IEC 125 125 mm RQA5 NNPS ±1.0 exposure NNPS IEC 99.9 99.5 DQE 3-3 MTF DR 5 10 cm SID 1500 mm 2000 mm SID 5 10 15 cm 3.75 cm 7.50 cm 11.25 cm 7.3 cm 1.0 cycle/mm 1.76 2.0 cycles/mm 6.69 MTF 1.0 cycle/mm 1.03 2.0 cycles/mm 2.81 MTF MTF X 2000 mm DQE MTF MTF 5 DQE 10 DQE DQE MTF 4. 結論 X DQE 4 X IEC FPD RQA5 ±5 kv NNPS 1 NNPS IEC X DQE MTF 23 DQE
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