Marc A N Korevaar

Multi-scale algorithm for improved scintillation detection in a CCD-based gamma camera

Gamma cameras based on charge-coupled devices (CCDs) and micro-columnar CsI scintillators can reach high spatial resolutions. However, the gamma interaction probability of these scintillators is low (typically <30% at 141 keV) due to the limited thickness of presently available micro-columnar scintillators. Continuous scintillators can improve the interaction probability but suffer from increased light spread compared to columnar scintillators. In addition, for both types of scintillators, gamma photons incident at an oblique angle reduce the spatial resolution due to the variable depth of interaction (DOI). To improve the spatial resolution and spectral characteristics of these detectors, we have developed a fast analytic scintillation detection algorithm that makes use of a depth-dependent light spread model and as a result is able to estimate the DOI in the scintillator. This algorithm, performing multi-scale frame analysis, was tested for an electron multiplying CCD (EM-CCD) optically coupled to CsI(Tl) scintillators of different thicknesses. For the thickest scintillator (2.6 mm) a spatial resolution of 148 microm full width half maximum (FWHM) was obtained with an energy resolution of 46% FWHM for perpendicularly incident gamma photons (interaction probability 61% at 141 keV). The multi-scale algorithm improves the spatial resolution up to 11%, the energy resolution up to 36% and the signal-to-background counts ratio up to 46% compared to a previously implemented algorithm that did not model the depth-dependent light spread. In addition, the multi-scale algorithm can accurately estimate DOI. As a result, degradation of the spatial resolution due to the variable DOI for gamma photons incident at a 45 degrees angle was improved from 2.0 10(3) to 448 microm FWHM. We conclude that the multi-scale algorithm significantly improves CCD-based gamma cameras as can be applied in future SPECT systems.

A pinhole gamma camera with optical depth-of-interaction elimination

The performance of pinhole single photon emission computed tomography (SPECT) depends on the spatial resolution of the gamma-ray detectors used. Pinhole cameras suffer from strong resolution loss due to the varying depth-of-interaction (DOI) of gamma quanta that enter the detector material at an angle. We eliminate DOI effects in a scintillation gamma camera via a dedicated optic fiber bundle that acts as a focusing collimator for light generated in a scintillation crystal. A curved crystal is connected to a concavely shaped fiber-optic bundle such that the fibers connect perpendicularly to the crystals convex surface and point straight at the pinhole opening. Limiting the fiber numerical apertures can be used to suppress resolution losses due to light spread. Here we demonstrate experimentally that this prototype position-sensitive gamma sensor successfully eliminates DOI effects, and has an intrinsic resolution of better than 280 microm full width at half maximum with an interaction probability of 67% for 140 keV photons. Therefore, the detector has great potential for increasing the resolution of pinhole SPECT.

An enhanced high-resolution EMCCD-based gamma camera using SiPM side detection

Electron-multiplying charge-coupled devices (EMCCDs) coupled to scintillation crystals can be used for high-resolution imaging of gamma rays in scintillation counting mode. However, the detection of false events as a result of EMCCD noise deteriorates the spatial and energy resolution of these gamma cameras and creates a detrimental background in the reconstructed image. In order to improve the performance of an EMCCD-based gamma camera with a monolithic scintillation crystal, arrays of silicon photon-multipliers (SiPMs) can be mounted on the sides of the crystal to detect escaping scintillation photons, which are otherwise neglected. This will provide a priori knowledge about the correct number and energies of gamma interactions that are to be detected in each CCD frame. This information can be used as an additional detection criterion, e.g. for the rejection of otherwise falsely detected events. The method was tested using a gamma camera based on a back-illuminated EMCCD, coupled to a 3 mm thick continuous CsI:Tl crystal. Twelve SiPMs have been mounted on the sides of the CsI:Tl crystal. When the information of the SiPMs is used to select scintillation events in the EMCCD image, the background level for (99m)Tc is reduced by a factor of 2. Furthermore, the SiPMs enable detection of (125)I scintillations. A hybrid SiPM-/EMCCD-based gamma camera thus offers great potential for applications such as in vivo imaging of gamma emitters.

Maximum-likelihood scintillation detection for EM-CCD based gamma cameras.

Gamma cameras based on charge-coupled devices (CCDs) coupled to continuous scintillation crystals can combine a good detection efficiency with high spatial resolutions with the aid of advanced scintillation detection algorithms. A previously developed analytical multi-scale algorithm (MSA) models the depth-dependent light distribution but does not take statistics into account. Here we present and validate a novel statistical maximum-likelihood algorithm (MLA) that combines a realistic light distribution model with an experimentally validated statistical model. The MLA was tested for an electron multiplying CCD optically coupled to CsI(Tl) scintillators of different thicknesses. For (99m)Tc imaging, the spatial resolution (for perpendicular and oblique incidence), energy resolution and signal-to-background counts ratio (SBR) obtained with the MLA were compared with those of the MSA. Compared to the MSA, the MLA improves the energy resolution by more than a factor of 1.6 and the SBR is enhanced by more than a factor of 1.3. For oblique incidence (approximately 45°), the depth-of-interaction corrected spatial resolution is improved by a factor of at least 1.1, while for perpendicular incidence the MLA resolution does not consistently differ significantly from the MSA result for all tested scintillator thicknesses. For the thickest scintillator (3 mm, interaction probability 66% at 141 keV) a spatial resolution (perpendicular incidence) of 147 µm full width at half maximum (FWHM) was obtained with an energy resolution of 35.2% FWHM. These results of the MLA were achieved without prior calibration of scintillations as is needed for many statistical scintillation detection algorithms. We conclude that the MLA significantly improves the gamma camera performance compared to the MSA.

SiPM side detectors for performance enhancements of an EMCCD-based gamma camera

To achieve ultra-high spatial resolution gamma ray imaging, the use of charge-coupled devices (CCDs) has become of great interest. However, these detectors still suffer from performances degradations due to their high intrinsic noise level. In our developped electron-multiplying CCD based gamma camera, we have introduced the use of solid state photomultipliers in side detection of a continuous crystal to improve the signal-to-background ratio and energy resolution. The so-called silicon photomultipliers (SiPMs) are individually read-out to measure the previously lost optical photons escaping through the crystal sides to give an a-priori evaluation of the energy and position of the γ-rays interactions to a statistical algorithm for EM-CCD image analysis. In this work, we show the results obtained using 12 SiPMs optically coupled to a 3 mm thick CsI:Tl crystal where the prelocated events are reconstructed using a maximum likelihood algorithm. This algorithm uses Look-Up Tables (LUTs) evaluated by Monte-Carlo simulations to retrieve the interaction positions via the individual SiPMs signals. The spatial resolution obtained with this process is evaluated at 0.8 mm using a 200 µm pinhole collimator with a 99mTc source. The energy resolution is then measured at 19% for the 140 keV γ-rays emission.