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The ECAT HRRT (high-resolution research tomograph) is a three-dimensional(3-D)-only dedicated brain positron emission tomograph (PET) with LSO and GSO scintillators. The system is based on eight panels of detectors. The HRRT's imaging performance has previously been tested with phantoms and FDG scans performed in animal and human brains that showed significantly improved spatial resolution, below 2.5 mm for animal studies and below 3 mm for brain studies. The NEC count rate performance has been evaluated based on the NU 2-2001 protocol. The results show a peak NEC approximately 30% higher than the performance of the Ecat HR+, this in spite of a more shallow detector for the HRRT, only 15 mm relative to the 30 mm for the HR+. However, peak NEC as derived from the 70-cm line source phantom is not optimized for the performance of dedicated brain scanner. NEC data derived with a 20-cm diameter, 20-cm long phantoms show a peak NEC more than 60% higher than for the HR+. The reasons for the high performance are due to several factors, the large axial coverage, the short timing window of 6 ns, and the low detector dead time, all factors that imply the use of fast LSO and GSO scintillators. Index Terms--LSO and GSO scintillators, Monte Carlo simulations, NEC count rate measurements, panel detectors, positron emission tomography.

The ECAT HRRT is a three-dimensional (3-D) only dedicated brain tomograph employing the new scintillator lutetium-oxy-orthosilicate (LSO) and using depth of interaction (DOI) information to achieve uniform isotropic resolution across a 20-cm diameter volume. With its unique technological innovations it represents the prototype of a new generation of high-resolution brain tomographs. The physical performance with respect to count rate, live time, scatter, sensitivity, and resolution was evaluated with phantom studies and measurements with a point source. The HRRTs imaging performance was tested with phantoms and fluorodeoxyglucose (FDG) scans performed in animal and human brains. We find that due to the significantly improved resolution and the large solid angle covered by the panel detectors, several issues that have been adequately solved for older generation scanners demand new attention for the HRRT, like acquiring and handling large amounts of data effectively, strategies for optimal reconstruction, shielding, and correction of random coincidences. Index Terms--Brain tomograph, depth of interaction (DOI), lutetium-oxy-orthosilicate (LSO), phoswitch detector, positron emission tomography (PET).

Recently new high resolution brain PET and PET/SPECT tomographs have been discussed using phoswich detectors. The detector design is based on two layers of scintillation crystals in the z-axis with different physical properties, such as light decay time or light yield. This makes it feasible to assign the depth of interaction (DOI) information in order to be able to correct for spatial degradation in PET or to operate the detector in PET or SPECT mode. The pixel DOI detectors are cost effectively arranged in a block design [1]. In this work we discuss the possibility of separating events from the two different layers in the time domain using pulse shape discrimination or in the energy domain using pulse height discrimination. Furthermore, we have investigated the feasibility of interdetector scatter suppression using pulse shape discrimination. The measurements have been done using a LSO/LSO high resolution PET detector, a LSO and GSO crystal in coincidence and a NaI(Tl)/LSO combined PET/SPECT detector. The investigations show, that the pulse shape discrimination technique has a significant higher identification probability compared to the pulse height discrimination and does make a interdetector scatter detection feasible.

To improve the spatial resolution and uniformity in modern high resolution brain PET systems over the entire field of view (FOV), it is necessary to archive the depth of interaction (DOI) information and correct for spatial resolution degradation. In this work we present the performance results of a high resolution LSO/GSO phoswich block detector with DOI capability. This detector design will be used in the new CTI High Resolution Research Tomograph, ECAT HRRT. The two crystal layers (19 x 19 x 7.5 [mm.sup.3]) and a light guide are stacked on each other and mounted on a (2 x 2) PMT set, so that the corners of the phoswich are positioned over the PMT centers. The crystal phoswich is cut into an 8 x 8 matrix of discrete crystals. The separation of the LSO and the GSO layer by pulse shape discrimination allows discrete DOI information to be obtained. The high light output and the light guide design results in an accurate identification of the 128 single crystals per block. Flood source measurements document a very good homogeneity of events, energy centroid stability and energy resolution (14-20% FWHM) per single crystal. An intrinsic resolution of [approximately] 1.3 mm and the DOI feasibility is extracted by coincidence measurements with a single GSO crystal. Keywords - Depth of interaction, LSO, phoswich detector, block detector, pulse shape discrimination, positron emission tomography, high resolution PET.

The performance of a phoswich array detector module for possible use in a combined single and coincidence photon imaging system has been evaluated. The detector consist of a linear array of discrete 4x4x15 [mm.sup.3] YSO elements coupled to a combined detector array/light guide of LSO, 10 mm thick. Since the scintillation light decay time is different in YSO and LSO (70 and 40 ns, respectively), events originating from the two detector materials can be separated by pulse shape discrimination. The front layer of YSO could then be used for detection of low energy, single photon events and the LSO layer for coincidence detection of annihilation radiation. The light collection of the PMTs coupled to the detector was found to be adequate to accurately identify each detector element in the array using the same positioning logic used in conventional BGO block detectors. The average energy resolution of the YSO elements at 140 keV for the block detector was found to be 14.5% FWHM, ranging from 13.8 to 15.4%. Spatial resolution of the detector block in single photon mode, using a high resolution collimator (geometric resolution 6.5 mm at 10 cm) was measured by scanning a 99mTc line source. The resolution at 5 and 10 cm from the collimator face was found to be 5.9 and 8.5 mm FWHM, respectively.

In order to achieve high sensitivity and maintain good uniform spatial resolution over the field of view in high resolution PET systems, adequate depth of interaction information must be extracted from the crystal. A phoswich detector can supply one solution to the depth of interaction problem. In this approach, two or more scintillators exhibiting different light decay times are positioned on top of each other and separated by pulse shape discrimination. Initially, our experiments focused on separating different types of scintillators such as LSO and GSO or LSO and YSO. These combinations were all well separated as expected. During the investigation, a shift in the time distribution of different samples of LSO was noticed. Further investigation showed two groups of LSO. The shift in the zero cross time was more than twice the FWHM of the time distribution. A single photon experiment revealed that the decay time of the 'fast' crystal was 33.4 nanoseconds while the decay of the 'slow' crystal was 42.2 nanoseconds. A spectral plot revealed that the spectral output of the 'slow' crystal was skewed to the longer wavelengths as compared to the 'fast' crystal. Further investigation on other crystal samples revealed decay times between the two extremes, suggesting a continuum in the light decay.

Quantitative imaging of the human thorax poses one of the most difficult challenges for three-dimensional (3-D) (septaless) positron emission tomography (PET), due to the strong attenuation of the annihilation radiation and the large contribution of scattered photons to the data. In [18F] fluorodeoxyglucose (FDG) studies of the heart with the patient's arms in the field of view, the contribution of scattered events can exceed 50% of the total detected coincidences. Accurate correction for this scatter component is necessary for meaningful quantitative image analysis and tracer kinetic modeling. For this reason, we have implemented a single-scatter simulation technique for scatter correction in positron volume imaging. In this paper we describe this algorithm and present scatter correction results from human and chest phantom studies. Index Terms - 3-D PET, cardiac, Compton scattering, heart, model-based scatter correction, positron emission tomography, positron volume imaging, scatter correction, single-scatter simulation.