Electronic Collimation

Abstract: Owing to the development of high resolution scanners and the new possibility of whole body scanning in recent years positron emission tomography has become more and more important in diagnostic imaging, especially for the diagnosis of malignant tumors and their metastases. Positron emission tomography (PET) is a special nuclear medical imaging technique, which is quantitative (correcting for radiation absorption by transmission measurements), and tomographic (imaging primarily a transversal body section as a tomographic field of view, i.e. a volume in time (4D) for a kinetic analysis or a sequence of fields of view to obtain whole body data sets). The radionuclides used emit a positron which is converted into a pair of photons after a short path of a few millimeters in the tissue (‘annihilation event’). The coincidence detection of the two photons, which are travelling on a line in opposite directions permits a sensitive electronic collimation as a first step in the localisation of the site of the radionuclide decay (see Fig. 1). With positron emitters of biochemically interesting elements such as carbon-11 (C), oxygen-15 (O), European Journal of Endocrinology (1998) 138 492–496 ISSN 0804-4643

Abstract: An evaluation of solid-state photomultiplier (SSPM) has been conducted for Positron Emission Tomography (PET) applications. The single-channel PET detector has been measured for its performance with respect to linearity of light detection, energy resolution, coincidence timing resolution, and depth-of-interaction detection capability. The SSPMs used have a 1×1 mm 2 active detection area. At nominal bias, it has a peak sensitivity around 470 nm, typical single photon detection efficiency around 20%, gain about 600,000, dark current 25 μA, and excess noise factor 2 cross-sectional area, where the light loss could reach 75%. Measured energy and coincidence timing resolutions are 23% and 1.8 ns, respectively, all within the SSPM linear region of photon detection up to ∼250 photoelectrons. The depth-of-interaction (DOI) resolution was measured with two SSPMs detecting lights at both ends of a 1.8×2×20 mm 3 LSO crystal, using a conventional electronic collimation method to localize the DOI positions. The measured DOI resolution was 4.5 (+/−0.3) mm, sufficient to develop a PET detector for the measurement of 3D interaction locations. These preliminary measurements have demonstrated the feasibility of using SSPMs for PET applications.

Abstract: The combination of a mechanically collimated gamma-ray camera with an electronically collimated gamma camera offers both the high-efficiency and good angular resolution typical in a mechanically collimated camera for lower photon energies and the uncoupling of spatial resolution and efficiency provided by an electronically collimated camera at higher energies. The Hybrid Portable Gamma Camera (HPGC) combines a MURA coded aperture camera with a Compton scatter camera to provide a broad range of energy response suitable for industrial imaging situations (50 keV–2 MeV). This paper compares the relative information content per photon for the three imaging modalities possible with the HPGC: mechanical collimation, electronic collimation and hybrid collimation, which combines information from the spatially coded aperture with Compton aperture information. The analysis is done for point sources at two incident energies (412 and 662 keV) in the medium-energy range of operation for the HPGC (400– 800 keV) where neither mechanical collimation nor electronic collimation performs particularly well acting independently. A resolution–variance analysis is used to compare the three modalities. Results show that hybrid collimation is superior to mechanical and electronic collimation at both 412 and 662 keV over the resolution range likely to be used for such a camera.

Abstract: Electronically collimated gamma cameras based on Compton scattering are gaining increased attention as the associated hardware and physical principles are further developed. To date, however, there has not been a direct, simultaneous experimental comparison between electronic collimation and mechanical collimation. This paper examines the relative performance of these two techniques at medium gamma-ray energies (0.1 – 1 MeV). A mechanically collimated (pinhole) camera was built and its performance was compared to an electronically collimated camera. Planar radioactive sources were imaged simultaneously by both cameras for identical periods of real time. Data are presented for several radioactive sources of various energies and shapes, including a multi-energy source. Results using an iterative image reconstruction techniques are presented. Comparing critical performance measures such as spatial resolution and efficiency for the two cameras may highlight differences between them, but does not provide an unambiguous basis for comparison. A tool from estimation theory, the resolution-variance curve, was applied to analyze their relative performance. Additionally, data from the two cameras were combined, creating a ‘dual-collimated camera’, and its resolution-variance performance was also examined. All data sets were also compared on a ‘per detected photon’ basis by reconstructing an equal number of events. Results showed that the pinhole camera performed better at the lowest energy examined, 279 keV. At 412 keV, the electronically collimated camera performed best on a ‘per detected photon’ basis, but the higher efficiency of the pinhole camera caused both cameras to have nearly identical performance when all the data was included. At 811 keV, the highest energy used, the electronically-collimated camera demonstrated superior performance. At all energies tested, the combined data set performed as well as, or better than, the best camera operating individually.