Gamma Ray Electronic Collimation Using a Large Volume CZT Crystal
Estanislao Aguayo Navarrete
Gamma Ray Electronic Collimation Using a Large Volume CZT Crystal
Gamma Ray Electronic Collimation Using a Large Volume CZT Crystal
Estanislao Aguayo Navarrete
May 4, 2007
GAMMA RAY ELECTRONIC COLLIMATION USING A LARGE VOLUME CZT CRYSTAL
by
Estanislao Aguayo Navarrete
Submitted to the Department of Computer Science in partial fulfilment of the requirements for the Degree of
Telecommunications Engineering Doctorate at the ESCUELA POLIT´ECNICA SUPERIOR
Universidad Aut´onoma de Madrid May 4, 2007
Directed by;
Jos´e Manuel P´erez Morales
Electronic and Sensors division at the Centro de investigaciones energ´eticas, medio ambientales y tecnol´ogicas (CIEMAT)
Eduardo Boemo Scalvinoni
Professor at the Computer Science Department EPS-UAM
COLIMACI ´ ON ELECTR ´ ONICA DE RAYOS GAMMA BASADA EN UN CRISTAL DE CZT
DE GRAN VOLUMEN
Memoria presentada para la obtenci´on del t´ıtulo de Doctor en Ingenier´ıa de Telecomunicaciones por
Estanislao Aguayo Navarrete
ESCUELA POLIT´ECNICA SUPERIOR Universidad Aut´onoma de Madrid
May 4, 2007
Dirigida por;
Jos´e Manuel P´erez Morales
Divisi´on de electr´onica y sensores en el Centro de investigaciones energ´eticas, medio ambientales y tecnol´ogicas (CIEMAT)
Eduardo Boemo Scalvinoni
Profesor del departamento de Ingenier´ıa Informatica en la Escuela Polit´ecnica Superior de la Universidad Aut´onoma de Madrid EPS-UAM.
“I am among those who think that science has great beauty. A scientist in his laboratory is not only a technician: he is also a child placed before natural phenomena which impress him like a fairy tale”
Marie Curie (1867 - 1934)
Dedicated to Eduardo de Burgos.
Large portions of chapter 4 and chapter 6 have appeared in the following works:
“Viability study of soft-processor usage for electronic collimation con- trol in medical applications”, E. Aguayo, R. Mart´ın, J. M. P´erez and E. Boemo, IEEE press, presented at the Southern Programmable Logic Conference, SPL 2007. Mar del Plata, Argentina.
“Position Sensitive Compton Detector Readout Electronic System: An Application Study”, E. Aguayo, J. Carrascal, G. Martinez and J. M. P´erez, Congreso de Computaci´on, Inform´atica, Biom´edica y Electr´onica, CONCIBE 2006. Guadalajara, Mexico.
“First Characterization Tests of CIEMAT Pixel CdZnTe Detectors”, Jos´e M. P´erez, Estanislao Aguayo, Raquel Gonz´alez, Oscar Vela, Presentation at the IEEE conference on nuclear Science, Nov.
2006. San Diego, California.
“Una aproximaci´on a la Validaci´on de Circuitos Integrados: Casos Pr´acticos de Testbenches en VHDL”, P. Aguayo and E. Aguayo, CIEMAT tech. rep. number 1061, 2005. Madrid, Spain.
“Tutorial Xilinx MicroBlaze-uCLinux ”, Aguayo E., Gonzlez I. y Boemo E., IV Jornadas de Computaci´on Reconfigurable y Aplica- ciones, JCRA 2004. Barcelona, Spain.
As well as this accepted to be published work;
“Run-time coprocessor self-change in embedded systems based on low- cost FPGAs”, Ivan Gonzalez, Estanislao Aguayo and Sergio Lopez-Buedo, IEEE micro 2007.
And this work submitted for publication;
“Dose assessment for high energy radioisotope tomography using an electronic collimated gamma camera.”, E. Aguayo, M. Ca˜nadas, J.
Carrascal, P. Rato, P. arce, J. M. P´erez and E. Boemo, Nu- clear Instruments And Methods In Physics Research Section A: Accel- erators, Spectrometers, Detectors And Associated Equipment, Elsevier B.V. 2007.
Electronic preprints are available on the Internet at the following URL:
ABSTRACT
Gamma ray detection techniques for radioisotope imaging purposes are quickly evolving. Monte Carlo simulations show the possibility of achieving an outstand- ing image spatial resolution in the images obtained with techniques using electronic collimation. The great advantage of using electronic collimation is the increased efficiency of the gamma camera with respect to the usage of the mechanical colli- mation technique. These new imaging techniques require radiation detectors with very specific features, such as low noise, desired stooping power and compact- ness. In this thesis project, an apparatus for detection of a Compton deposition of gamma rays, capable of differentiating the position of interaction of the gamma ray in the active volume of the detector, has been developed. The design and manufacture of such an apparatus implies the selection and characterization of the radiation sensitive material and its calibration, as well as its associated electronics, in order to achieve the critical requirements to be used as part of the electronic collimated gamma camera. Along with the detector development, the electronic collimation requires a very specific control system. The required logic system that makes possible the usage of the apparatus as part of the the electronic collimation set-up, has also been developed as part of this thesis work. This electronic system is meant to work in coordination with other sensors, and its final output is to give exact information of the photon-electron interaction points, in order for an image to be deduced. The result of this thesis work is a radiation detector ready to be used as the tracker component in the application of the electronic collima- tion technique. Its control systems allows it to be used by simply substituting the mechanical collimator of a regular gamma-camera with the detector developed, to make it an electronic collimated gamma camera.
ABSTRACT (SPANISH)
Las t´ecnicas de detecci´on de rayos gamma para realizar im´agenes de concentraci´on de isotopos est´an evolucionando. Las posibilidades de conseguir una imagen con una resoluci´on espacial mejorada usando t´ecnicas de colimaci´on electr´onica, han sido demostradas por simulaciones Monte Carlo. La gran ventaja de la utilizaci´on de la t´ecnica de colimaci´on electr´onica es su mejorada eficiencia respecto de la colimaci´on mec´anica. Estas nuevas t´ecnicas de acquisici´on de imagenes requieren detectores con una caracteristicas espec´ıficas, tales como bajo ruido, poder de fre- nado suficiente y un dise˜no compacto. En este trabajo de tesis, un aparato de detecci´on de una deposici´on por efecto Compton de rayos gamma, capaz de difer- enciar la posici´on donde se ha llevado a cabo la interacci´on del photon incidente, ha sido desarrollado. El dise˜no y la construcci´on de tal aparato conlleva la selecci´on y caracterizaci´on del material sensible a radiaci´on y su calibraci´on, as´ı como el completo dise˜no de su electr´onica asociada, con el objetivo de conseguir alcanzar los valores de los par´ametros cr´ıticos para poder utilizar dicho aparato como col- imador electr´onico en una c´amara gamma comercial. Junto con el desarrolo del detector, el colimador electr´onico requiere un sistema de control muy espec´ıfico.
El sistema l´ogico que hace posible el uso del aparato como parte del ambiente de aplicaci´on de la t´ecnica de la colimaci´on electr´onica, tambi´en ha sido desarrollado como parte de este trabajo de tesis. Este sistema electr´onico ha de trabajar en coordinaci´on con otros sensores, y su salida final ha de dar informaci´on del punto de interacci´on entre el fot´on y electr´on, para deducir una imagen. El resultado de este trabajo de tesis es un detector de radiaci´on listo para ser usado como el com- ponente seguidor de fotones en la aplicaci´on t´ecnica de la colimaci´on electr´onica.
El sistema de control desarrollado en esta proyecto de tesis lo hace capaz de operar simplemente sustituyendo el colimador mec´anico de una c´amara gamma comercial.
ACKNOWLEDGMENTS
This thesis project has been financially supported by CIEMAT (Centro de Investi- gaciones Energ´eticas, Medio Ambientales y Tecnol´ogicas). The sensor unit of the Technology Department of this research center bought the detector crystal and paid for all the trips and material that was needed to complete this thesis work.
Thanks are certainly in order to the department for providing all the required re- sources. One member of this department in particular, Jos´e Manuel P´erez Morales, gave up a great amount of his time and energies, and I would like to thank him for his support. He has been the main contributor of the scientifically innovative contents exposed in this thesis. I would also like to acknowledge the Computer Science Department at the UAM (Universidad Aut´onoma de Madrid), in par- ticular thank Eduardo Boemo Scalvinoni, director of this thesis work. Eduardo supported me the first years of my doctorate career, his instruction was pivitol in helping me develop high speed digital embedded systems, which has been a key issue in this thesis work. Thank you also to Raquel Gonz´alez de Ordu˜na, who helped directly and indirectly in many of the experiments carried out in this work.
Thanks to Gustavo Sutter, Iv´an Gonzalez, Juan Gonzalez, El´ıas Todorovich and Sergio L´opez-Buedo, members of the Computer Science Department at the UAM.
Thanks to Eduardo de Burgos, Carlos Willmott, Jorge Carrascal, Mario Canadas, Oscar Vela, Gustavo Mart´ınez, Cristina Fern´andez, Jesus Mar´ın, Jorge Carrascal, Pedro M´endez, Antonio Arana, David Fr´ancia, Alfonso and Manolo Corral (in the shop works) and Gaspar L´opez, all members of the technological department at CIEMAT. Thanks to Mar´ıa Garc´ıa, Raquel Portela, Cristina De La Rua, Ruben Mart´ın, Sete, Antonio, Juan, and all the personnel in the studying phase of the doctorate, they all have been a great support. I would like to give a special thanks to Bjørn S¨undal CTO at IDEAS, for his instruction, without them this thesis work would not have turn out as well as it has. And thanks to Michael Friederle, Alex Fauler and Andrea Zweger at the university of Freiburg, for their support in the most critical moments of the project. Thanks, Margit Sperling for the editing and
Sarah Anderson, in this work there is much more of you than you think, your love, kindness and support could not have been better.
CONTENTS
Table of Contents . . . xv List of figures . . . xviii List of tables . . . xxiii 1. Introduction . . . 1
1.1 Structure of this thesis . . . 4 1.2 Radiation usage in medical applications . . . 7 1.3 Recent works in the field . . . 8 1.4 Detector development collaborators . . . 10 1.5 A remark on notation . . . 13 2. Detector material and electrode design research . . . 15 2.1 Gamma rays and their interaction with matter . . . 16 2.2 Gamma ray detection techniques . . . 21 2.3 Semiconductor Materials . . . 23 2.3.1 Electron and Hole mobilities . . . 24 2.3.2 Fano Factor . . . 27 2.3.3 Room temperature operation . . . 28 2.4 Metal-Semiconductor contact behavior . . . 28 3. CZT crystal characteristics and anode design for electronic collimation us-
age . . . 33 3.1 Semiconductor Crystal growth process . . . 34 3.2 Single polarity charge sensing . . . 36 3.3 Pixellated anode pattern design and charge collection efficiency . . 38 3.3.1 Charge cloud focalization . . . 39 3.3.2 Field and charge transport effects due to Edge Discontinuity 41
3.4.1 Bulk leakage current . . . 44 3.4.2 Surface leakage current . . . 45 3.5 Interaction Depth Sensing . . . 46 3.6 Bi-parametric correction . . . 47 3.7 Large volume CZT related issues . . . 48 3.7.1 Temperature Effects on large volume CZT detectors . . . 48 3.7.2 CZT crystal aging . . . 49 3.7.3 CZT crystal asymmetry . . . 50 4. Detector calibration . . . 51 4.1 Experimental set up . . . 52 4.2 Charge carriers mobility and drift time measurement . . . 53 4.3 Voxel Gain variation correction . . . 55 4.4 Leakage current . . . 56 4.4.1 Leakage current versus cathode bias voltage . . . 57 4.4.2 Leakage current versus steering grid bias voltage . . . 58 4.5 Charge sharing . . . 58 5. Front End Electronics Design . . . 65 5.1 Basic circuitry for a detector readout Channel . . . 66 5.2 ASIC technology usage for reading out detector channels . . . 67 5.2.1 VA Rich2 preamplifier ASIC performance measurements . . 72 5.2.2 TA cg2 trigger ASIC performance measurements . . . 75 5.3 Front end card design . . . 76 5.3.1 Pitch adapter design . . . 78 5.3.2 ASIC connection and placement . . . 82 5.3.3 Board Discrete components . . . 83 5.3.4 Noise figures . . . 85 5.4 Signal digitalization circuitry . . . 90 5.4.1 Anode channels . . . 91 5.4.2 Cathode and steering grid channels . . . 92
6. Data Acquisition system electronic design . . . 95 6.1 Electronic collimation Control system requirements . . . 95 6.2 Electronic collimator data acquisition board prototype . . . 97 6.3 Reconfigurable hardware based control system . . . 98 6.3.1 Dead Time optimization . . . 100 6.3.2 Resource usage optimization . . . 101 6.4 Embedded system design for electronic collimation application . . . 102 6.4.1 Hardware: Control logic design based on a soft-processor . . 103 6.4.2 Software: Image construction . . . 105 6.4.3 Software: Soft-processor based Scheduler system . . . 107 6.4.4 Dynamic reconfiguration for resource utilization optimization 109 7. Applications of electronic collimation . . . 113 7.1 Detector performance figures . . . 114 7.2 Gamma ray imaging techniques overview . . . 114 7.2.1 Positron emission tomography (PET) basics . . . 116 7.2.2 Single Photon Emission Computed Tomography (SPECT)
basics . . . 118 7.2.3 Compton Scatter Imaging . . . 119 7.2.4 Advanced positron emission tomography technology with
three photons. . . 120 7.3 Electronic collimation in SPECT . . . 121 7.3.1 Point of Spread Function Evaluation . . . 123 7.3.2 Data counting . . . 123 7.3.3 System triggering . . . 124 7.4 Electronic collimation in PET . . . 125 7.4.1 Mechanical setup . . . 126 7.5 Proposed applications . . . 127 7.5.1 3D imaging . . . 128 7.5.2 Detector tune-ups for specific applications . . . 129 7.6 Known errata in the prototype development . . . 130 Conclusions . . . 132 Conclusions (Spanish) . . . 135
B. Electronic collimator data acquisition board schematics and layout . . . 151 C. Electronic collimator high voltage filter schematics and layout . . . 165 D. Anode field simulation code . . . 169 Bibliography . . . 173 Author0s vita . . . 188
LIST OF FIGURES
1.1 Detector configuration schema for electronic collimator application. 2 1.2 Electronic collimation prototype set-up and software tasks distribu-
tion over its components using an embedded system architecture . . 6 2.1 The energy spectrum of photons observed by humans. . . 16 2.2 Gamma ray interaction cross-section versus its energy, for a certain
material density (18.95 g/cm3) . . . 17 2.3 Photo-electric interaction of an incident gamma ray with an atom. . 19 2.4 Energy levels on a reverse biased Schottky contact. . . 29 2.5 Reverse biased Schottky contact schema. . . 30 3.1 Barium 133 spectrum performed with a low quality CZT crystal. . . 35 3.2 Spectrometric performance degradation versus applied voltage at
the cathode, for the semiconductor material used to build the elec- tronic collimator prototype. . . 37 3.3 Anode Pixels pattern, corner detail. A total of 1024 pixels are
deposited in the 15x15 mm2 anode side of the crystal. . . 39 3.4 Anode field distribution using a steering grid contact around the
collecting contact. . . 40 3.5 The field distribution lines along the bulk of the crystal, a) Ideal
dielectric surface b) Surface conductivity one order of magnitude smaller than bulk resistivity. . . 41 3.6 Bulk leakage current theoretical results. . . 45 3.7 Depth sensing experiment sketch. . . 47 3.8 Electron µτ change versus working temperature of the large volume
CZT crystal. . . 49 4.1 CZT detector for the electronic collimator prototype. . . 52
4.3 Anode channels gain variation figure for the experimental set-up.
Only channels 8,9,36,40,41 and 42 are connected to the crystal in this measurement. . . 56 4.4 Detector leakage current measurement set up, at the electronics
division of CIEMAT. Picture taken on September 2006. . . 57 4.5 Experimental Result for the measurement of leakage current on the
bulk of the 15x15x5 mm3 CZT crystal. Two different ranges shown a) 0-400V b) 0-1000V . . . 59 4.6 Experimental Result for the measurement of leakage current on the
surface of the 15x15x5 mm3 CZT crystal. . . 60 4.7 Spectrums taken with the prototype active crystal without any volt-
age applied to the steering grid contact. In the figure a), b), c) cor- respond to spectra taken on three neighboring pixels in the center of the crystal, and d) is the spectrum of the back radiation. . . 61 4.8 Experimental Set up for the charge sharing experiment. . . 62 4.9 One pixel scan. XY plane movement step of 50 µm. 100 µm colli-
mated X-ray beam on the test pixellated CZT crystal. . . 63 4.10 Two pixel scan. XY plane movement step of 50 µm. . . 64 5.1 Basic readout channel circuitry schema. . . 66 5.2 VA32 RICH2 v0.90 component picture, approximated die size 3500
µm x 3400 µm. Picture taken from the component’s datasheet. . . . 69 5.3 TA32 cg2 component picture, approximated die size 4100 µm x 3400
µm. Picture taken from the component’s datasheet. . . 71 5.4 Anode channel gain experiments. Figure shows one channel gain
doing a shaper polarization voltage snapping. . . 73 5.5 Anode channels gain experiment. Figure shows all channel gain. . . 74 5.6 Anode channel linearity ensuring experiment. . . 75 5.7 Single channel TA cg2 ASIC trigger threshold snapping as a function
of the injected charge. . . 76 5.8 Front end electronics board. . . 77
5.9 Pitch adapter design, the result of a collaboration between Jos´e Manuel P´erez Morales and Estanislao Aguayo from CIEMAT, and Michael Fiederle, Alex Fauler and Andrea Zwerger at the Mate- rial reseach and detector technology research center (FMF) at the Albert-Ludwigs Universitat Freiburg on April 2006. All distances given in millimeters. . . 79 5.10 Pitch adapter assembler. Designed by Alex Fauler, Estanislao Aguayo
and Andrea Zwerger at the Material reseach and detector technology research center (FMF) at the Albert-Ludwigs Universitat Freiburg on February 2007. The piece was manufactured at the physics school workshops. . . 80 5.11 Pitch adapter final layout. Designed by Alex Fauler on the second
quarter of 2006. The piece was manufactured at the Material re- seach and detector technology research center (FMF) at the Albert- Ludwigs Universitat Freiburg. . . 81 5.12 VA32 RICH2 v0.90 bonding to the front end card. . . 84 5.13 TA32 cgd2 bonding to the front end card. . . 85 5.14 Front end electronics card side view, showing the Pair of ASICs ,
the pitch adapter and the aluminum cap screw. . . 86 5.15 Bias generation component of the electronic collimator data acqui-
sition prototype board acting as a wave generator, resulting wave shown. . . 87 5.16 VA Rich and TA cg2 manufacturer bias signals recommended values. 88 5.17 Anode channels noise figures, a) Preamplifier polarization voltage
snapping b)Shaper polarization voltage snapping. . . 89 5.18 Anode readout components chain schema. Upper square: Signal
magnitude in each stage. Lower square: Components Timing Per- formance. . . 92 5.19 Anode channels digitalization component linearity measurements. . 93 5.20 Anode readout components chain schema. Upper square: Signal
magnitude in each stage. Lower square: Components Timing Per- formance. . . 94 5.21 Cathode (ChA) and steering grid (ChB) channels digitalization com-
ponent linearity measurements. . . 94
6.3 Space optimal design results. . . 101 6.4 Dead time optimal design results. . . 102 6.5 Electronic collimation electronic circuitry prototype block diagram,
two different architecture approaches, a)Daisy chained readout cir- cuitry b)Parallel implementation of the readout circuitry . . . 104 6.6 Real time operation system diagram for the electronic collimator
control . . . 108 6.7 FPGA connection diagram for reconfigurable computing capability
of the data acquisition system . . . 110 7.1 Cs137 Spectrum, taken with the electronic collimator experimental
set-up. FWHM = 3.1 % . . . 115 7.2 Theoretical detector configuration schema for electronic collimation
application. Taken from (1) . . . 116 7.3 Positron annihilation, the base for PET imaging. Taken form (2) . 117 7.4 Angle uncertainty determination, depending on how big the angle
of deflection of the incoming photon. Taken from (3). . . 120 7.5 Experimental simulation geometry to evaluate the electronic colli-
mated gamma camera to medical applications. . . 127 7.6 Proposed gamma ray micro scope application scenario. . . 130 A.1 ASICs 1 to 8 schematics. . . 140 A.2 ASICs 9 to 16 schematics. . . 141 A.3 Interface and bias schematics. . . 142 A.4 Top silkscreen. . . 143 A.5 Top soldermask. . . 144 A.6 Top side, used for routing. . . 145 A.7 Top layer. . . 146 A.8 bonding diagram of the ASICs. . . 147 A.9 Cover Cap. . . 148 A.10 Front End Electronics board picture. . . 149 A.11 Detailed view of the ASIC pair footprint. . . 150 B.1 Anode channels analog to digital conversion circuitry. . . 152
B.2 Cathode an steering grid channels analog to digital conversion cir- cuitry. . . 153 B.3 Digital to analog conversion of the control signals for the VA-TA
ASICs. . . 154 B.4 Front end card connector and bias signal for the VA-TA ASICs. . . 155 B.5 FPGA device banks connections. . . 156 B.6 FPGA miscellaneous components and external connectors schematics.157 B.7 FPGA device power connections. . . 158 B.8 Data acquisition card to PCI interface card connection circuitry. . . 159 B.9 Data acquisition card power components. . . 160 B.10 Component positioning in the prototype board. . . 161 B.11 Top routing layer. . . 162 B.12 Prototype board power planes. . . 163 C.1 Schematics of the high voltage filter and decoupling capacitor. . . . 166 C.2 Layout of the high voltage filter component. . . 167 D.1 Page 1 . . . 170 D.2 Page 2 . . . 171 D.3 Page 3 . . . 172
LIST OF TABLES
1.1 Medical imaging techniques . . . 7 2.1 Different detector types characterization table, along with their per-
formance. . . 21 2.2 Band gaps figure for Common materials operating at room temper-
ature. . . 24 3.1 Comparison table between CZT and HPGe materials in terms of
charge carrier mobility and trapping length. . . 34 5.1 VA rich32 length measurements, performed by Jos´e Manuel P´erez
at CIEMAT using a Mitutoyo caliper on 20th September 2006. All lengths given in µm. . . 70 5.2 Wire bonding processes maximum parameters table. Measurements
taken at CERN in the electronics division with Manuel Sanchez Suarez on September 19th 2006 . . . 83 5.3 Digital to analog component required performance set-up parameters. 87 5.4 Noise measurements for the cathode and steering grid contacts. . . 91 6.1 Resource utilization and performance of the electronic collimator
control logic for the different experimental architectures. . . 103 6.2 Software tasks performance results . . . 108 7.1 Electronic collimator specification parameters. . . 115 7.2 Performance characteristics. . . 115
1. INTRODUCTION
Modern radiation imaging techniques using gamma rays are evolving pursuing better image resolution. The challenge nowadays is to do so without having to increase the radiation dose administrated to the subject in study, required to obtain an image. Electronic collimation has been proven theoretically to do so.
This technique achieves an increase of gamma ray detection efficiency in the energy range that do interest medical imaging techniques. The idea behind this technique is quite simple. Using two detectors, each one having deposited part of the energy of the photon of interest, along with the use of complex algorithms to analyze the fractions of energy deposited in each detector, it is possible to reconstruct the path followed by the incoming gamma ray photon with a better resolution than actual apparatus for this purpose do. This task of reconstructing the incoming gamma ray path is nowadays performed using a mechanical collimation technique.
This technique forces to detect only photons that follow a straight path from the radiation source. The use of this method means that only a small fraction of the emitted gamma rays are detected, loosing a great number of photons that could provide even more information about the disintegration events localization point.
Electronic collimators, since they utilize as many emitted photons as possible from all directions, improve the solid angle of detection and therefore provide improved detection efficiency and sensitivity over mechanical collimators.
As it has been mentioned, and it is discussed along this thesis work, elec- tronic collimation is done using two detectors, a Compton scatter tracking detec- tor, referred as Compton tracker, and a second detector acting as a calorimeter, as sketched in Figure 1.1. The first detector can be thought as a pinhole, with an hole diameter equal to the spatial resolution of the tracker. A photon emitted from the source undergoes Compton scatter at the first detector, where the recoil electron is absorbed and its energy and the location of interaction are determined. The scat- tered photon leaves the scatter detector and is absorbed in the second detector, where the energy deposited and position are determined. From this information
Fig. 1.1: Detector configuration schema for electronic collimator application.
the source of the incident photon is found to be on the surface of a cone, the so called backprojected cone. The greater the number of channels of the detectors in the prototype, the better the space resolution.
Is somewhat clear the great dependency for this technique to achieve an im- provement in the image resolution on the data acquisition and trigger coordination of the detectors versus other medical imaging techniques. This thesis work has been motivated by the need of such a radiation detector with the very specific features of a radiation detector, in order to be used as an Compton tracker in the electronic collimation technique application. There is not a commercial part available that meets the specifications required for this detector. This is due to the thigh link between the cost and the spatial resolution of a detector. This fact together with the complexity of such a detector arrangement, to apply the technique makes the
3
commercialization costs hard to be faced by a private gamma ray medical imaging company. And specially when competitors in this field are still managing to im- prove their gamma scanners using other techniques, such as photon time of flight computation, with an affordable budget, so far.
The research involved the development of this thesis work covers several scien- tific as well as technological disciplines, it goes from the radiation sensitive material research and characterization, to ensure a final prototype construction without crit- ical performance on its base component, all the way to the design of an embedded real time digital system and dedicated micro mechanical board manufacturing, to ensure the detector to have the required characteristics for its application in the electronic collimation technique. The optimal balance between performance in energy an spatial resolution and the detection efficiency must be achieved in the prototype design. And in order to ensure it, every engineering aspect of this design is carefully studied in detail thought this thesis work. The design has been carried out from a research approach, going in each step of the development of the prototype in an unusual engineering detail level. An example of this level of detail is shown in issues such as the track resistance computations from the anode to the readout electronics, or processor power distribution figures of the acquisition electronics, just to give a couple of examples.
The prototype apparatus product of this research work, the electronic collima- tor, will be the base for new scanner designs. Each one of the components of the electronics developed in this thesis work is well known by the experts in radiation detector development scientists in the art, but the implementation described as well as the results obtained, up to our knowledge, has not been carried out by any other research group in the field. This thesis work results has been evaluated using among others, the point of spread function, witch evaluates the spatial resolution of a radiation detector. This parameter takes into account the collimator resolu- tion, intrinsic resolution of the detector used, scattered radiation and collimator septal penetration. This is the function that will be used to weight the possible improvement achieved by electronic collimation together with a scintillator based calorimeter.
1.1 Structure of this thesis
This thesis falls naturally into a classification relating the different design stages of the electronic collimator. These stages are relatively independent and each one conforms a different sets of chapters, as they are ordered in the following list;
• Research of radiation sensitive materials. This first part of this thesis work exposes a very short exposition of the theoretical bases of radiation detectors, from a physical point of view. It focuses in the different materials that are used for radiation detection and that conform the list of candidates for the construction of the electronic collimator prototype. There is a wide variety of materials used on radiation detectors construction, having a clear differentiation between commercially and for research purposes (See chapter 2, mainly due to the cost of certain materials. Once the radiation sensitive material has been identified as the proper one to achieve the performance figures required for the construction of the electronic collimator prototype, the interaction of radiation with this material is discussed in detail, since its going to be the base of the whole thesis work (See chapter 3). But not only the material elected for the detector is a key issue, the electrode design is as important as the material, since it will give completely different features to the detector once chosen, this issue is exposed and studied in detail in this same chapter. The main focus of this chapter is the CZT semiconductor as a radiation sensitive material, since it has been the one chosen to construct the electronic collimator prototype. Due to its complexity from a material manu- facturing point of view, as well as from the radiation signal acquisition usage point of view, the literature is extensive, and requires a detailed study and experimentation, exposed throughout the chapter. Aspects, such as charge recollection efficiency and its specific causes of degradation are studied, since they define the filters that will enhance the readout measurements, used in order to get coherent results. Also of key interest, is the position interaction deduction from the crystal electrodes. The process of readout of a single channel is explained theoretically and studying its drawbacks carefully, since this aspect is one of the compton tracker features that represent a technical advance in compton detector design.
• Detector calibration This design stage does englobe the experimental test-
1.1. Structure of this thesis 5
ing in terms of spectrometric performance of the device manufactured with the pixellated cadmium-Zinc -Telluride crystal, as described in the previous chapters. This calibration process is meant to be with two objectives. The first one of these objectives is to perform a proof of concept. Although all the theoretical base has been taken from state of the art models and the most recent scientific research in the field published in internationally cred- ited congresses, is a must to confirm all the suppositions and expected results that will enhance the electronic collimator technique. In the second place, a detector behavior characterization, in order to be used properly in the subse- quent development, without loosing any of its expected performance figures.
Issues concerning spectrometric performance, charge sharing effects, leakage current, optimal shaping time and charge gain for the front end electronics, and so on, are the focus of this chapter 4. This detector calibration has been carried out using components that have been characterized in previous works by prestigious research groups, so the results represent the performance fig- ures to achieve during the design stage of the electronic collimator.
• The design of the readout electronic chains The prototype of the elec- tronic collimator has been theoretical based and tested in the previous parts of this thesis work. The following chapter exposes the complete manufacture of the prototype. The data acquisition of the different output signals of the CZT crystal is classified to belong to this part of this thesis work. It begins with a research on how to connect the great number of electrodes of the crys- tal to the electronic system, maintaining the electronic noise of the readout as low as possible is the task exposed in chapter 5. The dedicated readout ASICs components and their control circuitry, as well as its implementation description and performance characterization is described in detail. These components are research parts, so they need a great amount of engineer work in order to have them working at their optimum performance. This thesis part ends with a performance comparison between several interconnection schemes for the readout chains, and the chosen one for the prototype con- struction (See Chapter 6). Characterization of the components used in the discrimination process of the acquired data are also carefully studied, in or- der to correct possible lacks of linearity of the acquisition circuitry, in order to be compensated in subsequent data management stages.
Fig. 1.2: Electronic collimation prototype set-up and software tasks distribution over its components using an embedded system architecture
An effort of applying embedded system design, based on an FPGA device, is presented for the electronic collimator prototype. The control logic in charge of the data digitalization synchronization and preparation for it to be used along with other detectors readouts, as shown on figure 1.2. This feature will allow the application of the electronic collimation technique. A much more versatile architecture for the control of the prototype is described using a soft-processor built inside the reconfigurable device. The improve- ments of the performance using such an architecture are presented, in terms of data throughput and system complexity, always maintaining the timing performance that the readout circuitry achieves. Software related tasks of the electronic collimation technique are developed for such an embedded system, and its performance is validated for such an application.
• Mechanical set up and data analysis for three photon event lo- calization The description of the electronic collimation technique and the required set up, how is mounted along with the calorimeter, is covered in this last part of this thesis work, in chapter 7. The applications of this technique in gamma imaging protocols in medical applications are presented. Along with them, the modification proposed for the usage of an electronic colli- mator along with those techniques. The imaging resolution performance of these techniques with an electronic collimator is evaluated, using the spread
1.2. Radiation usage in medical applications 7
Medical imaging technique Physical parameter sensitivity Transmission computed tomography Density and average atomic number
Emission computed tomography Concentration of radionuclides
Magnetic resonance Chemical form
Ultrasound Acoustic impedance mismatches Tab. 1.1: Medical imaging techniques
function to measure the advantage, if any, of using this technique. To finish up, the applications in different platforms, not specifically medical applica- tions, and ideas about configurations of the detectors to perform gamma ray detection, for space observation, light behavior experiments and many things that could make use of such a prototype as the one developed in thesis work. Possible improvements in the apparatus developed in this work are the closing notes of this thesis work.
1.2 Radiation usage in medical applications
Gamma ray imaging is an active research field nowadays, thanks in part to its medical applications. Radiation has been used for medical imaging since the be- ginning of the century (4), in the first applications using artificially generated x-rays or using the photons produced by the decay properties of matter. Those techniques using x-rays fall into the Transmission Computed Tomography (See Ta- ble 1.1), and the ones using those decay properties of matter, fall into the Emission Computed Tomography. This thesis work is oriented towards the improvement of image resolution in this second field of medical imaging. The electronic collimation technique applied to medical imaging has the ultimate objective of determining with the most accuracy achievable the location of the radionuclide concentration inside a studied object or body.
The detectors used for this kind of imaging have evolved rapidly in the past fifty years. From the early Anger cameras that used a radio sensitive film, to modern digital gamma cameras. And as electronic technology evolves towards lower noise levels, and higher speed data acquisition devices, these complex detectors are the perfect field of application for such technological advantages. The electronic design implied in this thesis work follows that trend, incorporating the ultimate electronic
technology, such as embedded system design, for the electronics of the detector developed.
Radiation can induce harmful effects in the human body due to its power of interaction with matter and its structure. The level of risk taken by a patient who use medical imaging radiation techniques has to be kept at a minimum. Although this thesis just studies a prototype, the future practical application will depend on factors such as irradiated dose for a patient necessary for a good image, and factors such as detector dead time. The electronic design of the detector used for the image acquisition can induce a variation in the required dose, that is why in the development of the electronic collimator technique, the spectrometric performance is not the only aspect to focus on, but also the efficiency of detection of the developed prototype.
1.3 Recent works in the field
The gamma ray imaging has been done traditionally using Anger cameras (5).
But they have reached their optimal compromise between sensitivity, as well as their energy and spatial resolution. An spectral resolution of 10% of the acquired energy could not be improved (6). This fact states the need for new detectors in the medical imaging research field.
The first work in the application of the Compton Scatter aperture (7) to imag- ing in nuclear medicine was first proposed by Todd and Everett (8) in 1974. This camera used an innovative component, the first electronic collimator, based on the Compton scattering effect. 22 years later the actual development into a practi- cal prototype, of this first theoretical work was published (9) , and a two orders of magnitude sensitivity over a mechanical calorimeter has been reported (10).
These facts by themselves, justify the work effort put into this research area, and in particular in supporting this thesis work.
New medical imaging techniques, mostly based on PET, are being described in the literature (11) and they require low spatial resolution in the event localization, witch implies high energy resolution in the energy range of interest, as well as the feature of room temperature working conditions. This feature avoids the need of complex external components and the use of dangerous cooling chemicals for the apparatus involved on the image acquisition. The required features are much better
1.3. Recent works in the field 9
than the ones achieved by commercial detectors available nowadays. And also it needs to be mentioned that these commercial detectors do not have the flexibility to incorporate newly developed image reconstruction algorithms. Support for this kind of algorithms, without a compromise of the detector performance, is what is pursued with the electronic collimator design based on a pixellated CZT.
Regarding the detector material, it needs to be mentioned that large volume CZT crystals with pixellated anode patterns have been developed in previous works (12), (13). These works have shown the feasibility of using such detectors for high energy resolution applications. The particularities of working with this type of detectors is sketched in the referred works. The difference of the detector developed in this work is the volume of the crystal, and the number of readout channels. This implies a completely new design approach, specially of the electronics involved in the design of an electronic collimator. This increased volume on the detector active component ensures a improvement in the detection efficiency of the gamma photons.
There are, also, current research covering the exact same challenge as the one faced in this thesis work, to develop an electronic collimator for medical purposes (14). This work specifically is using silicon as the material used as the detector active material, with the drawback of having to use extra components to cool down the active material.
The one work that arouses when talking about detectors electronics designed for medical application is the medipix collaboration (15),(16),(17) and (18). This collaboration shares with the electronic collimator developed in this thesis work the usage of application specific integrated circuitry to hold the front end electronics.
There are some major differences, the medipix chip does only count events with a certain energy, it does not have the ability to discern different energy depositions ion all the working energy range. This facts makes it so this chip is not applicable for a compton tracker. Other differences are that the medipix chip has a much smaller pitch for the detection channels, achieving a number of 256x256 channels acquired in the same chip. This is done by connecting the semiconductor material directly to the chip.
The applications of electronic collimation is not restricted to medical imaging techniques. They are as well used as a primary component in gamma camera set ups of where high spatial sensitivity is required, such as the one described in (19).
This work deals with a development where a gamma telescope is based on compton scatter interaction in a detector with lower sensitivity, such as the one expected from the one developed in this thesis work. Is to be mentioned that the work referred uses the same readout electronics, as well as in the studies performed by He et al. (20). This fact ensures that the design carried out is based on a well proven front end electronics platform. What this work faces regarding this issue, is the usage of a large number of ASICs, 16, to achieve a larger number of channel readouts. The readout system is completely newly developed in a close collaboration with the component manufacturer (21).
The data acquisition system is an electronic control design where many archi- tectures have been specifically engineered for the purpose of the electronic collima- tion application. The large amounts of tasks and the execution speed that these tasks require, makes the design of a data acquisition system for gamma ray imaging a complete research field by itself. Research work for small animal PET applica- tion, such as in (22), has been carried out by other research groups. This specific application is suited to research activities, since the prototypes are mechanically smaller and the results can be transposed to human application without much effort. The assembly and testing work of a somewhat alike platforms, are actually being developed in the research group supporting this thesis. Unfortunately, this means that data from it is not to be compared yet with other results of the inte- gration of the electronic collimator. The comparison of the prototype developed in this thesis with a somewhat alike architecture and usage as the one presented in the last referred article will be made when the mounting stage of the complete machine is finished.
1.4 Detector development collaborators
A large effort has been made and many research laboratories have been involved in a technical development such as the one carried out. The design of such an apparatus as the one faced in this thesis work requires technical resources in certain design stages that are very specific and not available usually in a single lab. At least in those labs visited where this research work took place. A total number of three research labs and four private companies, participated at some stage on the research work presented in this thesis. These institutions are listed along with
1.4. Detector development collaborators 11
their specific collaboration in the following list;
• eV-products. Private company based in the United States where the crystal growth was carried out. eV PRODUCTS develops and manufactures solid- state room temperature X-ray Detectors and Gamma-ray Detectors. In many industrial, medical, and laboratory applications, their CZT Detectors are replacing alternative Radiation Sensors and Radiation Detectors including:
NaI Detectors, CsI Detectors and Photodiode arrays. With more than ten thousand CZT Detectors sold annually, eV PRODUCTS has made eV-CZT a high volume commercially viable detector technology (23).
• Baltic Scientific Instruments. Private company based in Latvia where the anode pattern for the crystal was deposited. Encapsulation of the crystal for the detector characterization stage was also carried out in this lab. Baltic Scientific Instruments (BSI) was established in 1994 as a private company on the basis of Riga Research and Development Institute for Radio-Isotope Apparatus (RNIIRP) est. 1966. The majority of BSI was acquired by the German Bruker AXS GmbH in the early 2003 (24).
• Gamma Medica - Ideas Norway. Private company where the front end ASICs where developed, manufactured and mounted for the prototype board (21). But the collaboration with them has not been only of ASIC provider, they have participated as well in some stages of the mechanical assembly of the front end electronics card, and constant consultancy with them has been a key part of the results presented. Several visits from them to Madrid, two, and from the author to Oslo, three, have prove this close collaboration.
Gamma Medica-Ideas designs, develops and manufactures next-generation imaging systems for both clinical and pre-clinical applications. The company invented MicroSPECT and its X-SPECT system is the market leader in the small animal SPECT category. Gamma Medica X-PET system has the highest sensitivity and largest axial field of view among small animal PET systems. The company X-O system is a stand alone Micro CT device that can perform full body scans on a wide range of test subjects in under one minute, making it one of the fastest such systems in the industry. All three systems share a common gantry and are part of the company FLEX platform of pre- clinical imaging systems. Any two systems can be combined in the common
gantry, as can all three. Gamma Medica pre-clinical imaging systems are used by medical researchers and drug companies that use in-vivo imaging techniques and molecular markers to dramatically speed up studies of disease progression and therapy.
• Freiburger Materialforschungszentrum (FMF) Albert-Ludwigs-Universitt Freiburg. The detector pitch adapter was manufactured in the material re-
search center of this university. The specific variation of electrical, magnetic, thermal, chemical and optical features of either organic or inorganic ma- terials in order to realice certain specific solutions lead to innovations in the fields of (bio-)sensor technology, medical diagnosis, biological analysis, biotechnology, bio-optics, solar-energy technologies and the technology of Microsystems. New fields of application in the diagnosis and analysis could have been found when producing specific nanoparticles, their derivatives and composites. The sophisticated techniques in the field of semiconductor tech- nology developed at the FMF are internationally in great demand.
• Electronics Division Laboratory at CERN. The board for the front end electronics was manufactured in this lab. CERN is the European Or- ganization for Nuclear Research, the world’s largest particle physics centre.
It sits astride the Franco-Swiss border near Geneva. CERN is a laboratory where scientists unite to study the building blocks of matter and the forces that hold them together. CERN exists primarily to provide them with the necessary tools. These are accelerators, which accelerate particles to almost the speed of light and detectors to make the particles visible. Founded in 1954, the laboratory was one of Europe’s first joint ventures and includes now 20 Member States.
• Technological Department at CIEMAT. Central base of operations dur- ing the development of this research work. In its labs, all the characteriza- tion, and final assembly has been carried out. CIEMAT is a research center associated to the education and science ministry of Spain. It was created in 1951, and it is focused on the development of research projects in technical fields. Is meant to represent Spain in technical discussions in international forums, and to help Spanish administration when coming to technological decisions.
1.5. A remark on notation 13
• Escuela Polit´ecnica Superior at Universidad Aut´onoma de Madrid.
In its labs the software development and embedded system design was carried out. The autonomous University of Madrid was founded in 1971 and is one of the public institutions of reference in the university context of Spain.
• Lab-circuits This is the laboratory where the acquisition card prototype was manufactured. With over 30 years experience and a policy that remains true to its origins, Lab Circuits has a main plant with 4000 square me- ters of facilities located in the municipality of Santa Maria de Palautordera (Barcelona), and is made up of a team of 80 highly specialized professionals committed to the companys service philosophy. It allows them to produce pcb HDI, RoHS, microvias-laser, buried routes, impedance certification and special materials pcb boards, with maximum quality.
1.5 A remark on notation
Before ongoing further in this thesis, is to be mentioned the notation with which the developed apparatus is referred to. Due to its multiple applications, is hard to find an exact term that defines the apparatus developed in this thesis work. In the context of medical imaging, for example, many times the apparatus is referred to as the tracker, since it will be used as a tracker for electronic collimation. In certain contexts, where the detectors are identified by their physical interaction in witch the apparatus is based, the tracker is referred as compton aperture detector. In the applications contexts, this component is referred as electronic collimator because the compton detector is referred relating it to its usage in the big picture. These three terms are closely related, since a tracker, in the energy range of interest in witch this apparatus will be used, is a compton detector, and as well, a compton detector used in imaging techniques is referred as electronic collimator. When referring to the literature, many entries referred on this thesis work might talk about spectrometer, radiation counting, Compton scatter detector or the one most widely used on the literature Compton Camera. These terms define a radiation detector, once again related to its usage, and some aspects of the different definitions are shared with the work developed in this thesis, while others might be just completely different. In those cases where the reader cannot find a straightforward connection between the referred work and its relationship
with the contents of this thesis work, do not hesitate on looking for not such straightforward links with it.
2. DETECTOR MATERIAL AND ELECTRODE DESIGN RESEARCH
The knowledge of physics acquired in the past fifty years has enormously broaden the way of looking at the world trough the eyes of physics. The impressive advance, mainly in the theoretical sense of theoretical physics, and specially in such fields related as the ones related with this thesis work. Fields such as relativistic physics, solid state physics, new materials research and a long etcetera, have improve so much their ability to develop new devices and explain what goes on in these new materials structure, that is absolutely unquestionable the need for research work to apply these theoretical advances to real working scenarios. This means that the advances in those mentioned fields, has also made possible the broadening of the research of the technical engineering of this knowledge, to get it to applied to real applications.
The standard model of particles is one of the major parts of these theoretical advantages. The application of this theory to different human dilemmas, brings up an extensive field of research, and this thesis work is a result from it. In particular, this thesis work focuses on the behavior of photons and their interaction with matter. This phenomena is studied in order to use them as information carriers for medical purposes. That is, studying how photons interact with different materials, from complex crystals to human tissues, along with the the study of the production of such particles, ensures an accurate way of diagnosing certain human illnesses with improved accuracy. All this knowledge is in use nowadays, but it has room to improve, and is expected to yield to a better usage of these properties to improve their application in nuclear medicine. The engineering of such an apparatus that achieves this improvement, requires the maximum information and the closest knowledge of the different factors affecting the basic principles in witch the gamma ray imaging technique is based on.
In this chapter this knowledge is exposed, focusing in those issues related to
Fig. 2.1: The energy spectrum of photons observed by humans.
the development of the prototype that this work is intended to construct. Is important before facing the design to name the achievable goals that the apparatus is intended to reach. In particular, this first chapter deals with radiation detection techniques and the explanation from the physics science of the principles that are the base on which the electronic collimator is based. Gamma ray physics, along with their detection methods are the first issues treated in the chapter. The detection method, related to the specific material used in the manufacturing of the electronic collimator is what conforms the rest of the chapter.
2.1 Gamma rays and their interaction with matter
Gamma rays are photons that have an energy in the range 1Kev-8Mev, the mist energetic ones in between those observed by humans. Photons are a kind of parti- cles with very specific behavior, as explained and demonstrated by Albert Einstein and Max Plank (25). These particles have certain features that distinguish them from the rest of the particles. The one that is exclusive to photons is that they do not have mass at rest, so they are always traveling at the speed of light. This feature makes them hard to imagine and some authors have exclusively identified them as waves, because it has been observed that they can interact with each other in a similar way than waves do. A photon can be described as a wave. As a re- sult, their traditional description as waves have left the characterization of photon energy with the wavelength of their associated wave, as shown in figure 2.1.
Due to their high energy and traveling speed of the gamma rays, relative to other particles in our universe, and their lack of electric charge, the interaction of particles with mass at rest with them, is different from any other particle. And this fact is shown observing their penetration power, witch means that a gamma ray can for example, travel thought concrete walls, such as observed in nuclear reactors, where the walls have be really thick, in order to effectively contain these particles.
2.1. Gamma rays and their interaction with matter 17
Fig. 2.2: Gamma ray interaction cross-section versus its energy, for a certain material density (18.95 g/cm3)
Gamma rays are produced in nuclear reactions, such as radioactive decaying of some elements or, also, in subatomic processes. The elements are composed by a certain number and kind of particles. One way to classify elements is to identify the number of electrons, neutrons or protons that their structure holds. They produce different gamma rays when these subatomic structures interact, making possible to characterize elements using their emitted gamma ray as their finger print. So, knowing the gamma photon emitted by a certain element and using their penetration power, the utility of gamma rays is obvious. The photons can be used to identify materials that might be either really far away from us or that have other materials between them and the observer. These features are the base of medical imaging using gamma rays, as well as universe observation, material identification and characterization techniques.
The probability of interaction of a gamma ray with other particles is described in the literature in terms of cross sections1. Figure 2.2 shows the different prob- ability of a certain interaction of a photon, as a function of its energy. This is a way of going around the fact that is impossible, yet, to observe and characterize the conditions in which a certain photon interaction takes place, instead of any
1Defined as figure likelihood of an event involving a certain type of interaction between two particles
other one. The probability of a certain event taking place in the active volume of a material, is the ratio between the intensity of flux of the radiation hitting that volume and the radiation flux intensity leaving that volume (26), and its given by Beers law and its represented in equation 2.1.1, where I0 is the intensity of radiation hitting the active volume, I(x) represents the intensity leaving the de- tector material as a function of x, which is the thickness of such detector volume.
And µ is the linear attenuation coefficient. So talking in terms of probability is our only choice. These probabilities are experimentally obtained, and the linear attenuation coefficient is the parameter that characterizes materials in terms of their capability of interacting with gamma rays.
I(x)/I0 = e−µx (2.1.1)
The different types of interactions of the gamma ray with matter are listed and explained below2;
Coherent Scattering: this type of interaction happens when a gamma-ray or X-ray photon undergoes a change in its traveling direction without loss of energy. The interaction occurs between the gamma ray and all the electrons of an absorber atom, coherently. In the idealistic situation of the interac- tion being between a photon and a single free electron the process is called Thomson Scattering (28). The realization of the scattering of electromag- netic radiation by a charged particle happens. This should be contrasted with the real world situation where photons are scattered by bound elec- trons. The electrons are set vibrating by the oscillating electromagnetic field associated with the photon. Subsequently, a photon of radiation is emit- ted with the same wavelength as the incident radiation leaving the atom in its original undisturbed state. The waves from electrons within the atom combine with each other to form the scattered wave. That is, the coherent scattering is a cooperative phenomenon between all the bounded electrons in an element. There is no net ionization in the process, a property which dis- tinguishes coherent scattering from other photon interactions. The process occurs mainly at low energies and large values of Z and is typically a just small proportion of the total number of interactions. Since no net energy is
2Mainly extracted from (27)
2.1. Gamma rays and their interaction with matter 19
Fig. 2.3: Photo-electric interaction of an incident gamma ray with an atom.
transformed this type of interaction is hard to observe and is not useful for element identification, therefore useless for medical applications.
Photoelectric Effect: When a gamma-ray or an X-ray collides with an orbital electron of an atom that conforms the structure of the material through which it is passing, The photoelectric effect takes place. It can be a transfer of all the energy of the traveling photon to the electron, therefore, the photon ceases to exist. This interaction is sketched in figure 2.3. On the basis of the Principle of Conservation of Energy, we can deduce that the electron will leave the atom with a kinetic energy equal to the energy of the gamma-ray minus the orbital binding energy. This electron is called a photoelectron.
The probability of an interaction is greatest if the electron is deeply bound.
That is, the larger the atomic number, Z, the greater is the probability of a photoelectric process (29). This probability is given by equation 2.1.2, where NA is the number of atoms in a cubic centimeter, hν is the gamma photon energy in MeV and λγ represents the absorbtion depth of the material.
1
λγ = NAZ5(hν)−3.510−31m−1 (2.1.2) The photoelectric effect is the interaction that the electronic collimation technique uses to determine a photon energy deposited in the calorimeter.
By annihilating the photon all its energy is transferred to the active volume of this detector and this information is used to reconstruct the path of the
incoming photon.
Compton Effect: This is the name given to the event that occurs when an X-ray or gamma-ray transfers only part of its energy to a valance electron, which is essentially free. Notice that the electron leaves the atom and may act like a beta-particle, and that the gamma-ray deflects off in a different direction to that with which it approached the atom. The probability of the Compton effect happening, when a photon travels trough a material, is related to the number of electrons per unit mass. With the exception of hydrogen, all elements contain approximately the same number of electrons per unit mass.
Therefore, to a first order, the number of Compton reactions is independent of atomic number. However, for tissues of biological interest, the probability of an interaction does decrease slowly with increasing photon energy above about 50 keV.
Es= E
1 + E(1+cos Θ)mc2
(2.1.3)
Equation 2.1.3 shows the relationship between the energy of the scattered photon, Es, the energy of the incident photon E, and Θ, is the scattering angle of the incoming photon. In this equation c and m are constants, which represent the speed of light and the electron mass respectively.
The electronic collimation technique is based on this effect. When a compton scatter happens in the electronic collimator and the outgoing photon under- goes a photoelectric interaction on the second detector, the path followed by the gamma ray can be determined.
Pair Production: When the energy of the incident photon into the active vol- ume of a material is greater than 1022 keV, the photon may be absorbed through the process of pair Production. What happens is that as soon as such a photon passes near the nucleus of the atom it experiences the strong field of the nucleus and may be absorbed with the creation of a positive and negative electron pair. This is an example of conversion of energy to mass as exposed by Albert Einstein. No electronic charge is created since the positron and electron are equal and oppositely charged. Such an energetic process has no interest to medical imaging, since such high energy is hard
2.2. Gamma ray detection techniques 21
γ Ray detection technique Density (g/cm3) Energy Resolution (%)
Gas-filled detectors 0.001 Low
Scintillation detectors 2-7.5 > 6
Semiconductor detectors 2-6 1-2
Track-etch detectors High Medium
Tab. 2.1: Different detector types characterization table, along with their performance.
to detect in a somehow small detector, these energetic particles are detected with huge detector volumes.
Nuclear Photodisintegration: At extremely high energies (> 8 MeV), a pho- ton may interact directly with the nucleus of an atom and eject a neutron, proton or on rare occasions even an alpha particle. This interaction is of no interest at all for the electronic collimation technique.
2.2 Gamma ray detection techniques
As it has been mentioned in the previous section, gamma ray interaction with matter obeys particular processes and the probability of these happening, is the lowest since photons travel faster of those in between all particles. The detection of gamma rays is based on their ionization power (photoelectric and compton interactions) and implies mechanical and material research to make possible a high detection efficiency with a reasonable energy resolution. These two parameters, detection efficiency, related to the gamma ray stopping power of the detector, and the energy resolution, are related to these parameters; electronic signal to noise ratio (SNR), charge carrier creation required energy and charge collection efficiency (µτ product). These parameters characterize radiation detection techniques (30).
A small detector characterization, based on its principle of operation and the accuracy to determine the energy deposited by a photon interacting in its active volume, is shown in table 2.1. The density of the active material is the physical parameter of the active material that is directly identified with the stopping power of the detector.
Gas filled detectors are the simplest ones. Their working principle is the acceler- ation of charged particles in its active volume by generating a huge electromagnetic field inside of them. When a photon generates a photoelectron it gets accelerated
and produces a cascade effect hitting others electrons in the active volume. But their energy resolution is almost none and the gas volume required to achieve a reasonable energy efficiency, makes this type of detectors not applicable to develop an electronic collimator, the first detector in the electronic collimation technique.
Track-etch detectors are based in a destructive principle, the damage of dielectric materials exposed to radiation, where this radiation modifies somehow its structure forever. They are one time readable radiation detectors. These two techniques are not suitable for the application for witch the detector developed in this work is to be used.
Scintillation counters and semiconductor detectors are the ones that gather most of the research attention of the medical imaging area nowadays. Any of these detection principles could be thought at first sight, to build the electronic collimator, because they have both stopping power and energy resolution. And in fact, other works are using scintillator detectors for similar purposes than the ones pursued in this thesis work. The comparison of the chosen material to build the electronic collimator and a last generation scintillator base detectors have been carried out and reported in the literature (31). The one aspect that traditionally differentiated these two kind of detectors is their energy resolution. Scintillation detectors, due to the chain of events that must take place for converting the in- coming radiation to light, involving potentially inefficient steps, show a poorer performance in terms of energy resolution than semiconductor detectors. This fact is denoted by looking at the energy information carrier creation efficiency fig- ures. A scintillator needs around 100 eV to create a photoelectron, resulting in a poor spectrometry performance of 6% FWHM 3(30), compared with the 1.2%
FWHM claimed by some semiconductor based detectors. Semiconductor detectors use the electron-hole pairs created along the path taken by the charged particle created by the photoelectron created by the incident photon. And as well, these ionizing particles can be a primary or a secondary product from the radiation of interest. The motion of the electron-hole pairs in an applied electric field gener- ates the basic signal that the detector produces. interpreting this signal gives the energy deposited by the incident photon.
Another factor involving the material choice decision for the design of an elec- tronic collimator that can beneficiate semiconductor detector usage versus scintil-
3Full width at half maximum of the peak in the spectra
