Simone Weber

Comparison of LuYAP, LSO, and BGO as scintillators for high resolution PET detectors

For high resolution positron emission tomographs based on scintillation detectors a fast, dense, and bright scintillator is required. A sample of a new scintillator, Lu/sub 0.8/Y/sub 0.2/AlO/sub 3/:Ce (LuYAP) with a density of 7.7 g/cm/sup 3/ and a scintillation decay time of 20 and 160 ns is compared with LSO and BGO crystals of the same size to estimate the potential of the crystal for high resolution PET detectors. Special attention was paid to use an application specific measurement setup with respect to high resolution PET. The light yield of the crystals using polished crystals as well as PTFE tape and BaSO/sub 4/ as reflector material and the temperature dependence of the light yield are measured using crystals of 2 /spl times/ 2 /spl times/ 10 mm/sup 3/. At room temperature, the light yields of BGO : LuYAP : LSO are 1 : 1.5 : 4.5 with energy resolutions of 27.4% (BGO), 20% (LuYAP) and 15% (LSO), respectively. LuYAP shows an increase in light yield with increasing temperature whereas both BGO and LSO show a decrease.

The design of an animal PET: flexible geometry for achieving optimal spatial resolution or high sensitivity

Presents the design of a positron emission tomograph (PET) with flexible geometry dedicated to in vivo studies of small animals (TierPET). The scanner uses two pairs of detectors. Each detector consists of 400 small individual yttrium aluminum perovskite (YAP) scintillator crystals of dimensions 2/spl times/2/spl times/15 mm/sup 3/, optically isolated and glued together, which are coupled to position-sensitive photomultiplier tubes (PSPMTs). The detector modules can be moved in a radial direction so that the detector-to-detector spacing can be varied. Special hardware has been built for coincidence detection, position detection, and real-time data acquisition, which is performed by a PC. The single-event data are transferred to workstations where the radioactivity distribution is reconstructed. The dimensions of the crystals and the detector layout are the result of extensive simulations which are described in this report, taking into account sensitivity, spatial resolution and additional parameters like parallax error or scatter effects. For the three-dimensional (3-D) reconstruction a genuine 3-D expectation-maximization (EM)-algorithm which can include the characteristics of the detector system has been implemented. The reconstruction software is flexible and matches the different detector configurations. The main advantage of the proposed animal PET scanner is its high flexibility, allowing the realization of various detector-system configurations. By changing the detector-to-detector spacing, the system is capable of either providing good spatial resolution or high sensitivity for dynamic studies of pharmacokinetics.

Small animal PET: aspects of performance assessment.

Dedicated small animal positron emission tomography (PET) systems are increasingly prevalent in industry (e.g. for preclinical drug development) and biological research. Such systems permit researchers to perform animal studies of a longitudinal design characterised by repeated measurements in single animals. With the advent of commercial systems, scanners have become readily available and increasingly popular. As a consequence, technical specifications are becoming more diverse, making scanner systems less broadly applicable. The investigator has, therefore, to make a decision regarding which type of scanner is most suitable for the intended experiments. This decision should be based on gantry characteristics and the physical performance. The first few steps have been taken towards standardisation of the assessment of performance characteristics of dedicated animal PET systems, though such assessment is not yet routinely implemented. In this review, we describe current methods of evaluation of physical performance parameters of small animal PET scanners. Effects of methodologically different approaches on the results are assessed. It is underscored that particular attention has to be paid to spatial resolution, sensitivity, scatter fraction and count rate performance. Differences in performance measurement methods are described with regard to commercially available systems, namely the Concorde MicroPET systems P4 and R4 and the quad-HIDAC. Lastly, consequences of differences in scanner performance parameters are rated with respect to applications of small animal PET.

Evaluation of the TierPET system

The TierPET system, a high resolution 3D positron emission tomograph, has been developed to perform small animal positron emission tomography (PET) studies. High resolution PET opens the possibility to perform animal trials without the need for vivisection. This reduces the expense for such experiments, saving animal lives and cutting costs for example for the development of new radiopharmaceuticals. The scanner uses two orthogonal pairs of detectors consisting of arrays of small individual YAP crystals coupled to position sensitive photomultipliers. The detector-to-detector distance may be varied to improve either sensitivity or spatial resolution. The spatial resolution of the scanner is 2.1 mm.

4.5 tesla magnetic field reduces range of high-energy positrons-potential implications for positron emission tomography

We have theoretically and experimentally investigated the extent to which homogeneous magnetic fields up to 7 Tesla reduce the spatial distance positrons travel before annihilation (positron range). Computer simulations of a noncoincident detector design using a Monte Carlo algorithm calculated the positron range as a function of positron energy and magnetic field strength. The simulation predicted improvements in resolution, defined as full-width at half-maximum (FWBM) of the line-spread function (LSP) for a magnetic field strength up to 7 Tesla: negligible for F-18, from 3.35 mm to 2.73 mm for Ga-68 and from 3.66 mm to 2.68 mm for Rb-82. Also a substantial noise suppression was observed, described by the full-width at tenth-maximum (FWTM) for higher positron energies. The experimental approach confirmed an improvement in resolution for Ga-68 from 3.54 mm at 0 Tesla to 2.99 mm FWHM at 4.5 Tesla and practically no improvement for F-18 (2.97 mm at 0 Tesla and 2.95 mm at 4.5 Tesla). It is concluded that the simulation model is appropriate and that a homogeneous static magnetic field of 4.5 Tesla reduces the range of high-energy positrons to an extent that may improve spatial resolution in positron emission tomography.

Radiation Detectors for Medical Applications

Contents. Preface S. Tavernier.-A Look at Medical Imaging Trends through the Eyes of a Medical Doctor S.S. Makeyev.- Introduction.-Historical Aspect of Nuclear Medicine.-Nowadays in Nuclear Medicine.-Perspectives of Nuclear Medicine Imaging.- New Trends in X-Ray CT Imaging R. Deych and E. Dolazza.- Present Status of X-Ray CT.-Detector Instrumentation in Medical CT.- Scintillator.-Photodetectors.-Future Evolution of Data Measurement Systems.- The Evolution of Spect- from Anger to Today and Beyond W.W. Moses, A. Gektin et al.- Introduction.-General Considerations.-SPECT.- The Anger Camera.-Optimizing Positioning in Anger Cameras.- Collimators.-Scintillators for Spect.- Recently Developed Scintillator Materials.- Conclusion.- New Trends in PET Detector Developments P.Lecoq.- Introduction.-PET Based Molecular Imaging.-Improving Sensitivity.- Improving Spatial and Temporal Resolution.-Multimodaility and Multifunctionality.-New Conversion Materials.- New Photodetectors.-Highly Integrated and Low Noise Electronics.-Intelligent and Triggerable Data Acquisition Systems.-Simulation Software.-New Reconstruction and Visualisation Algorithms.-Conclusion.-Semiconductor Detectors in Radiation Medicine: Radiotherapy and related Applcations A.B. Rosenfeld.- Introduction.-Integral Semiconductor Dosimetry in Radiation Therapy.-Mosfet Detectors.-Semiconductor Radiation Detectors in Hadron Therapy.- Semiconductor Radiation Detectors for Microdosimetry in Radiation Therapy.-Application of Scintillator Based Detector in Radiation Therapy.-Conclusion.-First Results with the ClearPET small Animal PET Scanners S. Tavernier et al.- Introduction.-Description of the ClearPET Scanners.-Measured Performance and Comparison with Monte Carlo Simulations.- Image Reconstruction.-Conclusions.-Investigation of Crystal Identification Methods for ClearPETTM Phoswich Detector D. Wisniewski et al.- Introduction.-Measurement Setup.-Crystal Identification Methods.- Experimental Results.- Conclusions.- Directions in Scintillation Materials Research P. Dorenbos.- Introduction.-Historic Developments.- Fundamental Limits.- Directions in Scintillation Materials Research.-Summary and Conclusions.-Scintillation Detectors for Medical and Biology Applications: Materials, Design and Light Collection Conditions M. Globus, B. Grinyov.- Introduction.-2. Some Features and Regularities of Light Collection in Scintillators.- Medical Diagnostics Instrumentation.- Thin Scintillation Films for Biological Microtomography. Conclusions.- Current and Future Use of LSO: CE Scintillators in PET C.L. Melcher et al.- Introduction.-Physical Properties.- Scintillation Properties.-Crystal Growth.-Detector Design.- Future Uses of LSO: CE in PET.-Conclusion.-Inorganic Scintillators in Positron Emission Tomography C.W.E. van Eijk.- Introduction.-Inorganic Scintillators.- Position Resolution and Depth of Interaction.-Coincidence-Time Resolution, Random Coincidences, Time of Flight and Dead Time.-Conclusion.-Crystal Fibers and thin Films for Imaging Applications C. Pedrini and C. Dujardin.-. Introduction.-Single Crystal Fibers.- Scintillating Thin Films Deposited on Substrate.- Scintillation thin Layers created by Irradiation.-Conclusions. Non-Proportionality and Energy Resolution of Scintillation Detectors M. Moszynski.-Introduction.-Outline of the Problem.Study of Energy Resolution and Non-Proportionality.- Discussion and Conclusions.

High resolution and better quantification by tube of response modelling in 3D PET reconstruction

In high resolution 3D PET it is possible to increase the reconstruction quality by precisely modelling the forward process of the measurement before trying to solve the inverse problem. A general framework is presented how positron range, scatter contributions and attenuation of the /spl gamma/-rays can be integrated into the reconstruction process. Taking the TierPET scanner as an example, algorithms are presented how to derive the necessary properties. The area of the reconstruction volume contributing to a pair of detectors with unscattered events is called Tube of Response (TOR), while the area contributing the scattered events is called Scatter Volume of Response (SVOR). The elements of the system transfer matrix are calculated considering the geometry of the TORs. The scatter fraction is estimated by assuming single scatter and using an analytic derivation using the Klein-Nishina equation. The method was implemented with a 3D-Maximum Likelihood (ML) algorithm. Compared to a simplified modelling of the forward process, this approach yields better reconstructions of simulated point- and phantom sources in terms of quantification and spatial resolution.

Recent results of the TierPET scanner

At the Forschungszentrum Julich a high resolution positron emission tomograph (TierPET) for imaging small laboratory animals, especially rats, has been constructed. The scanner is based on arrays of YAP crystals. As a special feature the detector distances can be varied continuously from 16 to 58 cm. Due to the variable detector distances the performance of the scanner has to be evaluated in various configurations. Special attention was paid to dedicated data acquisition protocols and adequate applications with regard to the system sensitivity.

The KFA TierPET: performance characteristics and measurements

We will present first results of the KFA Tier-PET, a positron emission tomograph with flexible geometry dedicated to in vivo studies of small animals. The flexible geometry allows us to change between measurements with high spatial resolution and measurements with increased sensitivity at the cost of resolution. The detectors consist of yttrium aluminium perovskit scintillator arrays which are glued together from 20/spl times/20 optically isolated crystals, coupled to position sensitive photomultiplier tubes. The fundamental design features concerning crystal dimensions and detector arrangement have been simulated. Based on this data, the definite dimensional outline of the crystals was determined. The YAP:Ce matrix in combination with a position sensitive photomultiplier leads to a detector block with a high spatial resolution. In first measurements a system sensitivity of 1.8 kcps//spl mu/Ci/ml has been evaluated for a detector-to-detector distance of 16 cm.

Preliminary studies of a micro-CT for a combined small animal PET/CT scanner

We are developing an X-ray computed tomography (CT) system which will be combined with a high resolution animal PET system. This permits acquisition of both molecular and anatomical images in a single machine. In particular the CT will also be utilized for the quantification of the animal PET data by providing accurate data for attenuation correction. A first prototype has been built using a commercially available plane silicon diode detector. A cone-beam reconstruction provides the images using the Feldkamp algorithm. First measurements with this system have been performed on a mouse. It could be shown that the CT setup fulfils all demands for a high quality image of the skeleton of the mouse. It is also suited for soft tissue measurements. To improve contrast and resolution and to acquire the X-ray energy further development of the system, especially the use of semiconductor detectors and iterative reconstruction algorithms are planned.