INIS-mf—11552 A new gamma camera for Positron Emission Tomography NL89C0813 P. SCHOTANUS A new gamma camera for Positron Emission Tomography A new gamma camera for Positron Emission Tomography PROEFSCHRIFT TER VERKRIJGING VAN DE GRAAD VAN DOCTOR AAN DE TECHNISCHE UNIVERSITEIT DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS, PROF.DRS. P.A. SCHENCK, IN HET OPENBAAR TE VERDEDIGEN TEN OVERSTAAN VAN EEN COMMISSIE, AANGEWEZEN DOOR HET COLLEGE VAN DECANEN, OP DINSDAG 20 SEPTEMBER 1988TE 16.00 UUR. DOOR PAUL SCHOTANUS '$ DOCTORANDUS IN DE NATUURKUNDE GEBOREN TE EINDHOVEN Dit proefschrift is goedgekeurd door de promotor Prof.dr. A.H. Wapstra s ••I Sommige boeken schijnen geschreven te zijn.niet opdat men er iets uit zou leren, maar opdat men weten zal, dat de schrijver iets geweten heeft. Goethe Contents page 1 Introduction 1 2 Nuclear diagnostics as a tool in medical science; principles and applications 2.1 The position of nuclear diagnostics in medical science 2 2.2 The detection of radiation in nuclear diagnostics: 5 standard techniques 2.3 Positron emission tomography 7 2.4 Positron emitting isotopes 9 2.5 Examples of radiodiagnostic studies with PET 11 2.6 Comparison of PET with other diagnostic techniques 12 3 Detectors for positron emission tomography 3.1 The absorption d 5H keV annihilation radiation in solids 15 3.2 Scintillators for the detection of annihilation radiation 21 3.3 The detection of scintillation light 23 3.4 Alternative ways to detect annihilation radiation 28 3-5 Determination of the point of annihilation: detector geometry, 31 scatter and their influence on the image quality 3.6 The use of Time-of-Flight information: TOF PET 35 3.7 The performance of a PET camera 36 3.7-1 Position resolution 36 3.7.2 Sensitivity 39 3.8 Examples of positron camera systems 40 4 The solid scintillator proportional counter, an alternative approach 4.1 History and basic principles 45 4.2 Barium fluoride as a UV scintillator 46 4.3 Photosensitive vapours and liquids: TMAE 47 4.4 Low pressure wire chambers 51 4.5 Solid scintillator proportional counters: operation principles 57 Paper I: Photoelectron production in BaF^-TMAE detectors 58 4.6 The SSPC as a gamma camera for PET 64 5 Detection of 511 keV annihilation radiation with a solid scintillator proportional counter: experiments and results 5.1 Experimental procedures and methods 65 5.1.1 The multiwire chamber 65 5.1.2 Position readout 68 5.1.3 Choice of detector materials 71 5.1.4 Electronics 72 5.1.5 Gas system, TMAE supply 73 5.1.6 Measuring procedures; tests of the MWPC 75 5.2 Experiments with liquid TMAE 77 5.2.1 Design of the test detector 77 5.2.2 UV detection in TMAE 78 5.2.3 Scintillation light detection with liquid 8l TMAE layers 5.3 Detection of annihilation radiation in an SSPC, results at 20 C 82 5.3-1 Energy and time resolution studies 84 5.3-2 Position determination at 20 °C 85 5.4 Experiments at elevated temperatures 88 5.4.1 Experiments with an 8 mm diameter BaF_ crystal 89 5.4.2 Experiments with the 150 mm diameter BaF_ crystal 90 v Paper II: A BaF_-MWPC gamma camera for Positron Emission Tomography 95 f 5.5 Performance of the final detector 103 ' 5.5.1 Gas amplification 103 f 5>5-2 Sensitivity of the detector as a function of energy 105 I 5.5-3 Time resolution 108 | 5-5-4 Position spectra at 55 °C 109 i. ,p 5-6 Efficiency improvement by addition of another scintillation crystal 113 5.6.1 The influence of radiation scattered in the object 116 5.6.2 High count rate behaviour, ageing 117 5.7 Implementation of the SSPC in a PET detector 119 6 Scintillator research 6.1 Luminescence of inorganic solids 122 6.2 Barium fluoride luminescence: "crossover transitions" 127 6.3 Scintillation properties of BaF2 130 6.3-1 Experimental procedures 131 6.3-2 Measuring results 13** Paper III: Temperature dependence of BaF_ 135 scintillation light yield 6.4 La doping of barium fluoride crystals 144 6.5 Pb doping of barium fluoride crystals 150 + Paper IV: The effect of Pb contamination on BaF2 151 scintilation characteristics Paper V: Luminescence of the divalent lead ion in 160 barium fluoride crystals 6.6 Scintillation characteristics of other alkaline-earth fluorides 166 6.7 Ultraviolet luminescence of rare earth doped fluorides 170 6.7-1 Neodymium doped lanthanum fluoride 171 Paper VI: Detection of LaF_:NdJ scintillation light 172 in a photosensitive multiwire chamber 6.7.2 Neodymium doped barium-yttrium-fluoride 182 7 Conclusions 184 Acknowledgements 186 References 187 Sumaary 194 Saaenvatting 197 Curriculua Vitae 200 1 INTRODUCTION Nuclear diagnostics is a non-invasive medical technique to examine the functioning of organs and to detect abnormalities. In it, radioisotopes are introduced in the body, usually in the form of chemical compounds labeled with a suitable radioactive element; the chemical compound is selected on basis of its function in the process of interest. By measuring the radiation emitted by the label outside the body, a functional image can be obtained. The technique is used in most hospitals next to other non-invasive diagnostic techniques such as computerized tomography and ultrasound imaging. The instrument to measure the radioactivity distribution is called a gamma camera, since usually gamma radiation emitting radionuclides are used. The most frequently applied type of gamma camera consists of a crystal, in which the absorption of radiation produces light (scintillations), and a number of light detectors to determine the position of the absorption. Common gamma cameras supply a two dimensional projection of the radioactivity distribution. When two opposing gamma cameras are used to reconstruct a three dimensional image from a number of projections, the technique is called SPECT (Single Photon Emission Computerized Tomography). Positron Emission Tomography (PET) is another in-vivo tracer technique that makes use of the annihilation of positive electrons emitted by radioisotopes. This results in two almost collinear gamma quanta with an energy of 511 keV. By labeling compounds with positron emitters and by measuring the coincident emitted gamma radiation, a true three dimensional image of the distribution of the radioactivity can be obtained. The technique offers the unique possibility to determine quantitatively the uptake of a certain compound. It can be used to measure metabolic processes because of its high sensitivity and because the "biological" elements carbon, nitrogen and oxygen all have convenient positron emitting isotopes. | Gamma cameras for PET have to satisfy special requirements because of the f high energy of the radiation and because the selection of coincident events ; must be possible. Also, most suitable positron emitters have such short half lives that they have to be produced in the hospital itself. This, together with the rather complicated technique of the synthesis of the labeled compounds, makes PET a multidisciplinary technique. The configuration of a set of gamma cameras to register a tomographically . ' - 1 - reconstructed image is called a PET scanner. In the last few years, a considerable improvement in the position resolution of PET scanners has been achieved by reducing the size of the scintillation crystals and by applying high density scintillation materials. This, however, has decreased their detection efficiency and has increased their cost and complexity. Presently, there is a need to increase this efficiency without loss of resolution. This requires a different concept of gamma camera. In this work, a new type of gamma camera consisting of a high density scintillator (barium fluoride (BaF_)) in combination with a light sensitive wire chamber is presented. This type of detector, which is, up to now, only used for the detection of high energy particles, offers several advantages compared to conventional systems. Furthermore, rather inexpensive technology is used. Thus, the cost of a PET scanner can be reduced significantly. The aim of this research is to explore the physical mechanism of gamma radiation detection using this new principle, and to investigate whether this approach is suitable for application in a medical PET scanner. The first part of this work describes the operation and performance of this new gamma camera, the second part is focussed on the scintillation mechanisms in BaF.,. Also a study to investigate ultraviolet scintillation of other fluorides has been conducted. The first part of this work is organized as follows: chapter 2 describes the place of PET in nuclear diagnostics and its advantages and limitations compared to other diagnostic techniques; also some examples of radio- diagnostic studies with PET are presented. Chapter 3 discusses the requirements and performance characteristics of a PET scannner and gives some examples of operational systems. In chapter >\, the theoretical aspects of the above described new way of gamma radiation detection are presented. The results of our experiments with this new gamma camera are presented in chapter 5 together with a list of expected performance characteristics of a future PET scanner. The second part of this thesis (Chapter 6) is dedicated to research on scintillators. The theoretical aspects of solid scintillators together with some recent results on BaF_ are discussed. Finally, a new generation of UV scintillation materials for detection of gamma radiation in combination with light sensitive wire chambers is introduced. - 2 - 2 NUCLEAR DIAGNOSTICS IN MEDICAL SCIENCE; PRINCIPLES AND APPLICATIONS 2.1 THE POSITION OF NUCLEAR DIAGNOSTICS IN MEDICAL SCIENCE Nuclear diagnostics has become an important technique for in-vivo examination of the functioning of organs and for the study of a great number of physiological processes in the human (or animal) body. It comprises the introduction of a radioactive substance into the body after which the spatial distribution, flow or uptake by a certain organ is measured externally by observing the emitted radiation.