M. Lenox

The ECAT HRRT: performance and first clinical application of the new high resolution research tomograph

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.

FPGA based front-end electronics for a high resolution PET scanner

A high resolution PET scanner requiring processing electronics for 936 block technology channels and just under sixty-thousand crystal elements has been developed. With the advances in flexibility, number of gates, lower costs and speed of Field Programmable Gate Arrays (FPGA), an FPGA implementation of the front-end processing electronics was chosen over the traditional discrete logic or Application Specific Integrated Circuit (ASIC). The FPGA architecture reduced the development time and risks compared to a mask-based ASIC architecture while keeping costs and electronics packing density comparable. The extensive use FPGAs enables much faster circuit realization and a very efficient logic utilization by allowing re-configuration of the electronics functionality to support system setup, self-diagnostics, and several calibration modes for detector setup. Logic realized within the FPGAs performs the crystal selection, energy qualification, time correction, depth of interaction determination, and event counting functions. Since the FPGAs are in-circuit re-configurable (ICR), the functionality of the electronics is easily modified to support the different modes of operation. Thus the development time is reduced as well as the amount of electronics required, saving board area, power consumption and costs.

Characterization of a single LSO crystal layer High Resolution Research Tomograph

The purpose of this study was to determine the performance of a single lutetium oxy-orthosilicate (LSO) crystal layer High Resolution Research Tomograph (HRRT) positron emission tomography (PET) scanner. The HRRT is a high resolution PET scanner designed for human brain and small animal imaging. The scanner consists of eight panel detectors, which have one layer of 2.1 x 2.1 x 7.5 mm thick LSO crystals. Several phantom studies were performed to determine scanner characteristics, such as resolution, scatter fraction, count rate and noise equivalent count rates (NECR). NECR curves were measured according to both NEMA NU2-1994 and NU2-2001 for three different energy windows, i.e. lower level discriminators (lld) of 350, 400 and 450 keV and an upper level discriminator (uld) of 650 keV. Accuracy of scatter and single photon attenuation corrections was evaluated according to NU2-1994. Data were acquired using a ring difference of 67 and a span of 9. Reconstructions were performed using FORE + 2D FBP or OSEM. Transaxial resolution varied from 2.7 to 2.9 mm FWHM between I and 10 cm off centre locations, and axial resolution varied from 3.2 to 4.4 mm FWHM. Scatter fractions (NU2-1994) equalled 0.31, 0.42 and 0.54 for lld of 450, 400 and 350 keV, respectively. NECR data were highest for an lid of 400 keV and showed a maximum of 46 kcps at 38 kBq cm(-3). Lower NECR values were observed according to NU2-2001, but were still optimal for an lld of 400 keV. After scatter and attenuation corrections, pixel values within water, air and teflon inserts of the NU2-1994 phantom were 14, 4 and 35% of the background activity, respectively. The single layer LSO HRRT scanner shows excellent spatial resolution, making it suitable for small animal studies. The low count rate performance, due to the small amount of LSO, prohibits studies of the human brain, but is sufficient for studies in small laboratory animals.

The ECAT HRRT: NEMA NEC evaluation of the HRRT system, the new High Resolution Research Tomograph

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 HRRTs 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.

SPMD cluster-based parallel 3D OSEM

This study empirically compares two approaches to parallel 3D OSEM that differ as to whether calculations are assigned to nodes by projection number or by transaxial plane number. For projection space decomposition (PSD), the forward projection is completely parallel, but backprojection requires a slow image synchronization. For image space decomposition (ISD), the communication associated with forward projection can be overlapped with calculation, and the communication associated with backprojection is more efficient. To compare these methods, an implementation of 3D OSEM for three PET scanners is developed that runs on an experimental, 9-node, 18-processor cluster computer. For selected benchmarks, both methods exhibit speedups in excess of 8 for 9 nodes, and comparable performance for the tested range of cluster sizes.

Performance evaluation of a new LSO high resolution research tomograph-HRRT

In order to improve the capability for investigating the living human brain using positron emission tomography with regard to blood flow, metabolism and receptor characteristics for small structures such as cortical sublayers and nuclei, the spatial resolution has to be improved relative to what is available today. A spatial resolution of 2 mm or less in all three dimensions may be necessary to reach these research goals. In order to meet this goal a new next generation high resolution 3D-only LSO brain PET has been developed at CTI. The HRRT (High Resolution Research Tomograph) is not only the first LSO PET for human studies it is also the first PET with full DOI capability over an extended FOV. The HRRT has been delivered to the MPI Cologne in February 1999 and is presently being optimized in terms of energy discrimination, crystal positioning, coincidence timing and PSD to achieve optimal system performance with respect to resolution, count rate efficiency and scatter. The panel detector setup with its new setup challenges is still under development and investigations. First evaluation measurements are presently obtained. The measurements promise an excellent high resolution performance with a high count rate capability of the HRRT. The high random rates and the SF fraction measurements underline the necessity for a short coincidence time window and an improved energy resolution for such an in the images reveal a not yet optimized system setup and reconstruction parameters. The authors believe the results demonstrate the high capability of the new brain tomograph and justify the excitement for the first LSO tomograph with DOI capability for human brain investigations and the new scintillator LSO, superior for coincidence timing and energy resolution.

Development of a daily quality check procedure for the high resolution research tomograph (HRRT) using natural LSO background radioactivity

A daily quality check (DQC) plays an important role for clinical medical systems like a Positron Emission Tomograph (PET). To guarantee image quality a constant monitoring of the scanner integrity is essential. On present PET scanners the built-in transmission sources are being used for the measurement of a uniform blank scan and the computation of detector-block efficiencies. These efficiencies can be compared to reference data and deviations indicate changes of the scanner condition. On the High Resolution Research Tomograph (ECAT HRRT) the design of the transmission source makes this method impractical. For that reason we investigated the use of the natural background radioactivity from the new scintillator material LSO as a uniform source. A procedure was developed to measure detector-block sensitivities and energy spectra directly. An initial quality check (QC) scan is the basis for comparison of daily scans, so that PMT gain shifts and specific hardware defects can be detected. In early stage of development of the DQC, the low LSO count rate results in long acquisition times. In the meantime, the listmode based data acquisition was changed so that it was possible to reduce the acquisition time to around 3 hours. With that this routine is now as practical as previous procedures in clinical routine.

Coincidence time alignment of high resolution planar detectors

Planar Quadrant Shared Array PET detectors are highly dependent upon accurate timing to improve their coincidence efficiency. Techniques are described to determine the timing characteristics of the planar detectors used in the CPS HRRT PET tomograph, and to compensate for manufacturing tolerances.

The electronics system for the LBNL positron emission mammography (PEM) camera

Describes the electronics for a high-performance positron emission mammography (PEM) camera. It is based on the electronics for a human brain positron emission tomography (PET) camera (the Siemens/CTI HRRT), modified to use a detector module that incorporates a photodiode (PD) array. An application-specified integrated circuit (ASIC) services the photodetector (PD) array, amplifying its signal and identifying the crystal of interaction. Another ASIC services the photomultiplier tube (PMT), measuring its output and providing a timing signal. Field-programmable gate arrays (FPGAs) and lookup RAMs are used to apply crystal-by-crystal correction factors and measure the energy deposit and the interaction depth (based on the PD/PMT ratio). Additional FPGAs provide event multiplexing, derandomization, coincidence detection, and real-time rebinning. Embedded PC/104 microprocessors provide communication, real-time control, and configure the system. Extensive use of FPGAs make the overall design extremely flexible, allowing many different functions (or design modifications) to be realized without hardware changes. Incorporation of extensive onboard diagnostics, implemented in the FPGAs, is required by the very high level of integration and density achieved by this system.

Performance characteristics of the high resolution research tomograph comparison of three prototypes

The high resolution research tomograph (HRRT) is the first PET scanner with depth of interaction (DOI) capability. Presently three different versions have been developed: one equipped with two 7.5 mm thick crystal layers (HRRT-DC), the second with only a single 7.5 mm crystal layer (HRRT-S) and the latest HRRT with two 10 mm thick crystal layers (HRRT-D). In this study the performance of the new HRRT-D was assessed and compared with the other two HRRTs. The characteristics were measured according to the NEMA NU2 standards. Similar scatter fractions between all three scanners were observed. NEC rates of the HRRT-D were about 8 and 3 times higher than those of HRRT-S and HRRT-DC, respectively. However, spatial resolution of the new HRRT-D is somewhat lower than that of HRRT-DC and HRRT-S. Use of thicker crystals in the new HRRT-D improved the sensitivity and NECR performance significantly at the cost of only a small deterioration of the spatial resolution compared with the other HRRT designs.