J.G. Rogers

Analytic 3D image reconstruction using all detected events

The authors present the results of testing an algorithm for three-dimensional image reconstruction that uses all gamma-ray coincidence events detected by a PET (positron emission tomography) volume imaging scanner. By using two iterations of an analytic filter-backprojection method, the algorithm is not constrained by the requirement of a spatially invariant detector point spread function, which limits normal analytic techniques. Removing this constraint allows the incorporation of all detected events, regardless of orientation, which improves the statistical quality of the final reconstructed image. >

A simulation to model position encoding multicrystal PET detectors

We have developed a simulation to model position encoding multicrystal detectors for positron emission tomography. The simulation is designed to treat the interactions of energetic photons in a scintillator, the geometry of the multicrystal array, as well as the propagation and detection of individual scintillation photons. The simulation is tested with a model of the EXACT HR PLUS block detector manufactured by Siemens-CTI. Position and energy responses derived from the simulation are compared to measured ones. Line-spread-functions, for four columns of crystals, are reproduced with an accuracy of /spl plusmn/0.5 mm. The crystal-by-crystal photopeak pulse heights and FWHMs are also predicted within a range of /spl plusmn/14%, and /sub -6//sup +9/%, respectively.

The DRAGON facility for nuclear astrophysics at TRIUMF-ISAC: design, construction and operation

A facility for measuring cross-sections (resonance strengths) for reactions of astrophysical importance involving short-lived, radioactive reactants has been designed, built and installed at the new TRIUMF-ISAC Radioactive Beams Laboratory in Canada. Named DRAGON (Detector of Recoils And Gamma-rays of Nuclear reactions), it has been successfully commissioned with stable and radioactive heavy ion beams from ISAC. This report presents the main components of the facility, namely, the windowless gas target, the surrounding g detector array, the subsequent electromagnetic recoil mass separator, the focal plane detectors for recoils, the detection system for elastics, and the modular electronics and computer software used for the data acquisition. Examples of the operation of the facility for both stable beam reactions and the first radioactive beam reaction study, 21 Naðp;gÞ 22 Mg are also presented, along with future plans for the program. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.0

Object shape dependent scatter simulations for PET

The correctness of an analytical simulation for single-scattered gamma rays in positron volume imaging was verified by comparing it to Monte Carlo simulation of single scattering. The Monte Carlo simulation was verified using measured tomograph data. The single-scatter analytical simulation was extended to estimate multiple scattering. The multiple-scatter analytical simulation showed good agreement with the Monte Carlo simulation of total object scatter. It is shown using the analytical simulation that the position of a source along the line of response and the shape of the scattering object make a substantial difference in the scatter distribution in the projections. Such projections can be generated in a much shorter time using the analytical simulation than using the Monte Carlo simulation. A scatter correction method which uses the analytical simulation and exploits the inherent smoothness of the scatter distribution to account for three-dimensional effects in scatter distribution and object shape is also proposed. >

Design of an efficient position sensitive gamma ray detector for nuclear medicine

The shortcomings of conventional scintillation cameras are analysed theoretically with a view towards improving performance at gamma ray energies above 140 keV. A camera design is proposed which incorporates several new features to obtain good spatial resolution from thicker crystals of sodium iodide. Computer simulations show that in addition to having good efficiency and spatial resolution, the new design allows parallax error correction and (possibly) Compton scattering correction at gamma energies up to 511 keV.

A practical block detector for a depth encoding PET camera

The depth-of-interaction effect in block detectors degrades the image resolution in commercial PET cameras and impedes the natural evolution of smaller, less expensive cameras. A method for correcting the measured position of each detected gamma ray by measuring its depth-of-interaction was tested and found to recover 38% of the lost resolution at 7.5 cm radius in a tabletop, 50-cm-diameter camera. To obtain the desired depth sensitivity, standard commercial detectors were modified by a simple and practical process that is suitable for mass production of the detectors. The impact of the detector modifications on central image resolution and on the ability of the camera to correct for object scatter were also measured.

The Theory of Three-Dimensional Image Reconstruction for PET

A review is made of selected recent publications on three-dimensional image reconstruction for PET. A new algorithm is proposed which is designed to fully utilize all emitted gamma rays which can be detected in a truncated spherical detector. By such full utilization of emitted rays the new algorithm should produce images of better statistical accuracy than could be produced by previously known algorithms.

Segmented LSO crystals for depth-of-interaction encoding in PET

The authors have developed a method of effecting depth-of-interaction sensitivity for PET detectors. It exploits a significant difference in the index of refraction between adjacent scintillator segments and the compound optically coupling them, to induce discrete and resolved photopeak pulse heights depending on the segment of interaction of /spl gamma/-rays. The approach was put to a first experimental test by manufacturing two prototypes with LSO crystals of dimensions 4/spl times/4/spl times/30 mm and 2.5/spl times/2.5/spl times/20 mm respectively, each comprising three segments along their longitudinal axis. Measurements of their absolute pulse height responses when irradiated by 511 keV photons are presented and show resolved peaks for photoelectric interactions occurring in each longitudinal segment as desired. For the second of the above prototypes, the lowest(highest) of these peaks is measured to correspond to a pulse height of 335(559) photoelectrons with full-width-at-half-maximum resolution of 28%(12%). The expected performances of PET detector units comprising a plurality of such segmented crystals is also discussed.

An improved multicrystal 2-D BGO detector for PET

The authors evaluate and compare two 2-D array detectors for PET (positron emission tomography). Both consist of an 8*8 array of small BGO crystals coupled to a 2*2 array of photomultiplier tubes. The depth of the crystals is 3 cm in one detector and 2 cm in the other. The 2-cm detector is obviously superior in terms of material costs, but it is also superior in energy resolution per crystal, and in its ability to identify clearly the crystal containing the primary interaction. The authors present a flexible and robust algorithm for crystal identification in such array detectors. The prospect of obtaining still better spatial resolution from such block detectors, with increased numbers of crystals, is discussed.<<ETX>>

Three-dimensional image reconstruction in object space

An analytic three-dimensional image-reconstruction algorithm that can utilize the cross-plane gamma rays detected by a wide solid-angle PET (positron-emission tomography) system is presented. Unlike current analytic algorithms, it does not use Fourier transform methods, although mathematical equivalence to Fourier transform methods is proved. The results of implementing the algorithm are briefly discussed. An extension of the algorithm to utilize all measured cross-plane gamma rays is discussed. >