Estimation of signal transfer property for wireless digital detector in different measurement schemes

Transcription

1 Estimation of signal transfer property for wireless digital detector in different measurement schemes Anatoli Vladimirov, Kalle Kepler Training Centre of Medical Physics, University of Tartu, Estonia 11 TH International Conference & Workshop Medical Physics in the Baltic Satates, Kaunas, October, 2013

2 Introduction European criteria for acceptability of medical radiological equipment Radiation Protection 162 include and require measurement of signal transfer property (STP). STP refers to a test to be done during the acceptance testing of the computed radiography (CR) or digital radiography (DR) system in order to establish the relationship between detector air kerma (DAK) (or receptor dose) and pixel value (PV).

3 Radiation Protection 162

4 Simple STP ralationship Linear: PV lin = a+b DAK DAK = (PV-a)/b Logarithmic: PV log = a ln(dak)+b DAK = exp((pv-a)/b) Power: PV power = a DAK b +c DAK = ((PV-c)/a) 1/b

5 Linearization of standard deviation The measured standard deviation of the pixel values (σ) cannot simply be corrected to the true standard deviation (σ') using the inverse of the STP. For measurements of several objective parameters of image quality and imaging system: signal-to-noise ratio (SNR), modulation transfer function (MTF), noise power spectrum (NPS), detective quantum efficiency (DQE) the linearized standard deviation can then be estimated by subtracting the linearized mean pixel value: σ'= (PV + σ)' - PV' where ( ) is the linearization function using the inverse of the STP

6 Beam conditions used for DDI in CR Manufacturer Added filter kv Pre-processing Agfa 1.5 mm Cu 75 S=200 / System Diagnosis / Flat Field Fuji 0 mm Al 80 Semi-Auto / L=1 Kodak 0.5 mm Cu + 1 mm Al 80 Pattern mode Konica 0 mm Al 80 Test 1/G =2/F off

7 Beam conditions used for STP Spectrum Added filter kv Nominal HVL (mm Al) Measured HVL (mm Al) IPEM 32 1 mm Cu 70 ~ RQA 5 by IEC RQA 3 by IEC mm Al mm Al PMMA 0 mm Al

8 Materials and methods Scatter geometry (a) X-ray tube (b) Added filter X-ray tube Scatter-free geometry Wide X-ray beam Narrow X-ray beam beam Dose chamber Table PMMA Table Grid Image detector or dose chamber Grid removed Image detector or dose chamber IPEM. Measurement of the Performance Characteristics of Diagnostic X- Ray Systems: Digital Imaging Systems. IPEM Report Number 32, Part VII (2010)

9 Materials and methods Siemens Ysio and Pixium FE 3543pR (Trixell,France) with a CsI scintillator coupled to TFT matrix in a:si -technology, pixel size 155 µm, matrix 34.2 x 43.2 cm, saturation dose: 75 Gy. Barracuda MPD dosimeter (RTI, Sweden), accuracy of 5%. Siemens 2011

10 Setting the amplification factor Detector dose Indicative sensitivity Typical amplification factor 7.1 μgy μgy μgy μgy μgy μgy Siemens 2011

11 Setting the gradation curve (LUT) Siemens 2011

12 Pixel value Results and discussion PV = 54.5 DAK R 2 = PV = 54.0 DAK R 2 = PV = 27.2 DAK R 2 = ,00 10,00 20,00 30,00 40,00 50,00 60,00 Detector air kerm a (μgy) IPEM32 RQA5 RQA3 STP for RQA5, RQA3 and IPEM32 schemes, measured directly (without table, grid)

13 Pixel value Results and discussion PV = DAK R 2 = PV= 55.0 DAK R 2 = PV = 54.4 DAK R 2 = 0, ,00 10,00 20,00 30,00 40,00 50,00 60,00 Detector air kerm a (μgy) IPEM32, w ithout table and grid IPEM32, in Bucky w ithout grid IPEM32, in Bucky w ith grid STP by IPEM32 on table and inside table with and without grid.

14 Pixel value Results and discussion PV = 54.9 DAK R 2 = PV = 55.0 DAK R 2 = ,00 10,00 20,00 30,00 40,00 50,00 60,00 Detector air kerm a (μgy) IPEM32, scatter-free IPEM32, scatter (PMMA) STP in IPEM32 scatter (PMMA) and scatter-free geometry (with dose detector inside the Bucky).

15 Pixel value Results and discussion PV = DAK R 2 = PV = DAK R 2 = PV = 54.4 DAK R 2 = ,00 5,00 10,00 15,00 20,00 25,00 30,00 35,00 40,00 Detector air kerm a (μgy) IPEM32, Ampl.1 IPEM32, Ampl.2 IPEM32, Ampl.4 STP with the amplification factors of 1, 2 and 4.

16 Pixel value Results and discussion PV = Ln(DAK) R 2 = PV = Ln(DAK) R 2 = ,00 5,00 10,00 15,00 20,00 Detector air kerm a (μgy) RQA5 RQA3 STP for RQA5 and RQA3 with typical post-processing with DAK up to 20 µgy.

17 Pixel value Results and discussion RQA5: PV = Ln(DAK ) R 2 = RQA3: PV = Ln(DAK) R 2 = ,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 Detector air kerm a (μgy) RQA5 RQA3 STP for RQA5 and RQA3 with typical post-processing with DAK up to 70 µgy.

18 Linearized standard diviation Results and discussion 0,10 σ = DAK 0.47 σ = DAK 0.35 σ = DAK 0.48 RQA5 IPEM32 RQA3 0,01 0,10 1,00 10,00 100,00 Detector air kerm a (μgy) Linearized standard deviation of pixel value by RQA5, RQA3 and IPEM32 measured directly on the receptor.

19 Conclusions Several measurement schemes using beam qualities RQA3, RQA5 and IPEM32 were studied along with three different geometries: scatter (PMMA), scatter-free and direct measurement geometry for estimation of STP of a digital wireless detector. For the pre-processed images the more clinically applicable IPEM32 scheme is compared with the IEC RQA5 scheme. The results show that there is no significant difference in the coefficient of correlation R 2 for the STP curve. Also power coefficients for the measured linearized standard deviation dependence from DAK are comparable for these schemes. The different IPEM32 geometries for measurements without table, inside table with and without grid could be interchangeable in practice.

20 Conclusions The post-processed images could be linearizable up to 20 µgy if simple LUT was applied. One conclusion is that if we have to measure STP on non-wireless Bucky, where it is not possible to measure DAK directly on the entrance surface of the receptor, then the dose measurement could be done on the entrance of the table (or vertical stand), and recalculate it on the right distance. If we know transmission of grid and table we could calculate exact DAK.

21 References Bosmans H., Nens J., Delzenne L., et al. Exploration of exposure conditions with a novel wireless detector for bedside digital radiography. Proc. of SPIE, 2012, 8313, p K1-9. Samei E. DQE of wireless digital detectors: Comparative performance with differing filtration schemes. Med. Phys., 2013, 40, p European Commission. Criteria for acceptability of medical radiological equipment used in diagnostic radiology, nuclear medicine and radiotherapy. Rad. Prot Luxembourg: EC, 2012, 84 p. Institute of Physics and Engineering in Medicine. Measurement of the performance characteristics of diagnostic X-ray systems: digital imaging systems. IPEM Rep. 32, Part VII. IPEM, p. Institute of Physics and Engineering in Medicine. Recommended standards for the routine performance testing of diagnostic X-ray imaging systems. IPEM Report 91. IPEM, International Electrotechnical Commission. Medical diagnostic X-ray equipment Radiation conditions for use in the determination of characteristics Edition 2.0. IEC Geneva: IEC (2005). Kepler K., Vladimirov A. Survey of compliance with European acceptability criteria for HVL and AEC. Rad. Prot. Dos., 2013, 153(2), Siemens AG. FLUOROSPOT Compact. Operator manual. Imaging System for Ysio. Erlangen (2011). Mackenzie, A. Validation of correction methods for the non-linear response of digital radiography systems. The Brit. J. of Rad. 81, (2008). Marshall N. W., Mackenzie A., Honey, I. D. Quality control measurements for digital x-ray detectors. Phys. in Med. Biol. 56, (2011).

22 Thank you!