Light Conversion, S/N Characteristics of X-Ray Phosphor Screens
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Light conversion, S/N characteristics of x-ray phosphor screens Item Type text; Thesis-Reproduction (electronic) Authors Lum, Byron Kwai Chinn Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 28/09/2021 05:29:31 Link to Item http://hdl.handle.net/10150/557456 LIGHT CONVERSION, S/N CHARACTERISTICS OF X-RAY PHOSPHOR SCREENS by Byron Kwai Chinn Lum A Thesis Submitted To the Committee on COMMITTEE ON OPTICAL SCIENCES (GRADUATE) In Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 19 8 0 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of re quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg ment the proposed use of the material is in the interests of scholar ship. In all other instances, however, permission must be obtained from the author. , SIGNED: 'K.C. / - APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: OltiUV 4- HANS ROEHRIG I Date Adjunct Associate Professor of Radiology ACKNOWLEDGMENTS I would like to express my deep appreciation to my advisor. Dr. Hans Roehrig, whose guidance and patience made this thesis possible. I am also very grateful for the efforts of Ms. Betty Porter and Ms. Delia Bryant in the preparation of the final copy of this thesis. This work was sponsored under the project "Evaluation of PEID Systems for Radiology", awarded through the Bureau of Radiological Health, Food, and Drug Administration under Grant No. 5R01FD00804-04RAD. TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS...................................... v LIST OF TABLES .......... viii ■ A B S T R A C T ....................... ix 1. INTRODUCTION ...................................... 1 Justification .......................................... 2 Physical Processes of X-ray Induced Emission ......... 9 Characteristic X-ray Reabsorption................ 14 Properties of New and Traditional X-ray Phosphors . 20 2 NOISE ...................... 26 Introduction .................................... 26 Statistics of Screen Amplification ......... 34 Scintillation D Q E .............. 38 Simulation of Screen Statistical Processes . 40 Some Conclusions . .......... 44 3. EXPERIMENTAL PROCEDURE AND SE T U P ........................... 45 Introduction ................... 45 The Photomultiplier ........ .................... 47 The X-ray Source ..................... 35 Electronics............. ... 36. Computer Analysis .............. 38 4. RESULTS AND CONCLUSIONS......................... 39 R e s u l t s ................. ... ........................... 39 Analysis .......... 72 Conclusions .................. 79 REFERENCES............ ........ .................... .. 81 iv LIST OF ILLUSTRATIONS Figure Page 1. Screen-film combination (Messier 1973) 3 2. Theoretical and experimental values for radiographic noise; Experimental-theoretical (Rossmann, 1962) . ............. 6 3. PEID, using fluorescent screen optically coupled to intensified video camera tube ................................ 7 4. Schematic of x-ray intensifier video camera system..... 7 5. Cross section for interaction in a calcium tungstate (CaWO^) screen (Vybomy, 1978) . .......... 11 6 . X-ray absorption processes in a Csl phosphor and the resulting absorbed energy spectrum for monochromatic x-rays (Swank, 1973) . .................................. 12 7. Inner-electron transitions for characteristic x-ray emission (Weidner and Sells, 1973) . ........ 15 8. Characteristic x-ray spectrum for a thulium (Tm) secondary t a r g e t ........................................ 15 9. X-ray attenuation as a function of energy for a Ba phosphor (Vybomy, 1 9 7 8 ) ........................... ........ 16 10. Characteristic x-ray re absorption diagram (Vybomy, 1978) . 16 11. CsBr:(Tl)1, CsI:Na(2), ZnCdS:Ag(3), and CsI:Tl(4) ......... 22 12. BaS04 :Eu2+ (5), BaFCl:Eu2+ (6), and CaW04(7) . ............. 22 13. Gd„(LS:Tb(8) , La„CLS:Tb(9), and Y 0„S :Tb(10) . (Stevens, 1975) 7 7 .............. 23 14. Radiographic mottle (Shaw, 1976) 27 v vi LIST OF ILLUSTRATIONS--Continued Figure Page 15. Components of radiographic density fluctuations (Rossmann, 1962) . 27. 16. Spatial Frequency content of noise in radiographic images (Rossmann, 1962). .... 29 17. Random fluctuations in density (SPSE Handbook of Photographic Science and Engineering, 1973)......... 30 18. Processes in the variation of x-ray screen scintillations. 32 19. Schematic for the serial combination of two statistical devices (RCA Photomultiplier Handbook, 1970) ........ 34 20. Simulation screen probability distributions . 41 21. Simulation results for Fig. 20(a), P^ - P^ = 0.5 ...... 42 22. Simulation results for Fig. 20(a), P^ - 1/4, P^ = 3/4 . 42 23. Simulation results for Fig. 20 . = . 43 24. Block Diagram of U of A evaluation facility . ....... 46 25. Schematic of PMT pulse counting method . .......... 46 26. RCA 8850 photomultiplier pulse height spectrum . ..... 48 27. Photomultiplier output corresponding to a Poisson distribution . ........... .... 49 28. Phosphor screen output decay characteristics ........ 50 29. Decay time constant for a CaWO^ screen ...... 50 30. Calibration of photomultiplier counting efficiency ..... .51 31. Counting efficiency of the system ..... 53 32. Variable energy x-ray source . ......... 55 33. System electronics .................... 57 vii LIST OF ILLUSTRATIONS--Continued Figure Page 34. Measured probability distributions (P ) for a ZnCdS screens: Cd K-edge: 26.7 keV . ....... f ........... 60 35. Measured probability distributions (P ) for a CaWO. screen; W k-edge = 69.5 k e V ............... f .............. 61 36. Measured probability distributions (P ) for a BaSO. screen; Ba K-edge: 37.5 k e V ...............7 .............. 62 37. Measured probability distributions (P ) for a La.O S-Gd^O S screen; La k-edge: 38.9 keV Gd k-edge: 50.2 k e V ............. 63 38. Measured probability distributions for a Csl x-ray image intensifier; 1 k-edge: 33.2 keV, Cs k-edge: 35.9 keV .... 64 39. Bremstrahlung spectrums for different amounts of filtrations .......... 65 ■40. Measured output emissions for a CaWO^ screen ................. 66 41. Measured output emissions for a BaSO^ screen ................. 67 42. Measured output emissions for a series of ZnCdS screens . 6 8 43. Measured output parameters for a La^OgS-GdgO^S screen........... 69 44. Measured output parameters for a Csl x-ray image intensifier .......... 70 45. Measured signal to noise ratios from the output of Csl x-ray image intensifier experimental values below 10 absorbed x-ray photons in error due to the fact that the threshold counter did not sample continuously, but was triggered by signal pulses ............................. 73 46. Measured signal to noise ratios from the output of a CaWO^ screen; 44 keV incident x-ray photons ............... .... 74 47. Csl image intensifier output distributions . ............... 77 48. CaWO^ screen output distributions .....'.....................78 LIST OF TABLES Table Page 1. Probability of barium k reabsorption in a pair of barium strontium sulfate screens versus incident x-ray energy .... 19 2. Reabsorption probabilities forthe K x-rays emitted by the principla phosphor elements in the screens studied .......... 19 3. Basic properties of phosphors ................................ 21 4. X-ray source characteristics .................................. 53 5. Measured output efficiencies of some phosphor screens .... 71 viii ABSTRACT The variations in gain or amplification are measured for a variety of x-ray phosphor screens and for a Csl image intensifier as a function of incident x-ray energy. These variations result in a reduction of the output SNR (signal to noise ratio) by a factor of /DQEsc^nt• The scintillation detective quantum efficiency, DQ^scint' is evaluated theoretically and experimental results are presented. The findings show that the newer rare earth phosphor screens possess a higher gain than do the traditional calcium tungstate (CaWO^) screens and that the values for DQEsc^n t , do not vary considerably for a different phosphor materials. CHAPTER 1 INTRODUCTION Since the incident x-ray photons follow Poisson statistics, the input SNR (signal to noise ratio) of an x-ray imaging system is readily known. However, for systems in which x-ray intensifying phosphor screens are utilized, the output SNR following screen amplification will be degraded due to variations in the amplification or gain of the phosphor screen. In this thesis, the average gain and the associated variations for various phosphor materials are measured and its effect on the SNR is investigated theoretically and experimentally. The experimental measurements are done with a photon counting system with which individual x-ray absorption events may be detected and