Christof Knoess

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.

Correction methods for missing data in sinograms of the HRRT PET scanner

The High Resolution Research Tomograph (HRRT) is a 3D PET scanner designed for brain imaging and small animal imaging. The HRRT consists of 8 panel detector heads that are separated by a gap of 17 mm resulting in data gaps in the sinogram. Furthermore, data gaps can result from detector-block failure. To prevent artifacts in the reconstruction when using FORE, filling in of the data gaps is required. The purpose of this study was to evaluate the accuracy of several gap filling methods. Two gap-filling methods were investigated: a) bilinear interpolation, b) a model-based method: an intermediate volume is reconstructed (2D) based on only direct planes, after which this image is forward projected towards the data gaps. In addition, an improved model-based method is introduced: c) first fill the gaps using interpolation, then reconstructing using FORE and forward projecting to fill the gaps. Detector gaps and block failures were mimicked by zeroing LORs in simulated and experimentally acquired sinograms. The gaps were filled using the different methods, reconstructed using FORE+2DOSEM and compared with reconstruction of the original sinogram. From the variance of the reconstructions and from difference images it could be concluded that for homogeneous objects which are large as compared to the extent of data gaps all methods give similar results, although the interpolation methods requires significant less computation time. For objects with dimensions comparable to the size of a data gap the interpolation method falls short. The simple model-based method however suffers from artifacts in the intermediate direct planes reconstruction. The latter is overcome by the improved model-based method. In conclusion, the improved model-based method might outperform the interpolation method, but due to the long computation times usage of this method is only justified in case of small objects.

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.

Evaluation of single photon transmission for the HRRT

A dedicated whole human brain positron emission tomograph (PET), the High Resolution Research Tomograph (ECAT HRRT) is utilized to evaluate attenuation correction using single photon based transmission scanning. The patented transmission procedure uses a 740 MBq Cs-137 point source, which is extended into the FOV and collimated to flood the opposing detectors only. An attenuation map is then calculated iteratively using a blank and transmission scan, scaled to 511 keV, and re-projected using inverse Fourier rebinning to estimate the 3D attenuation correction. We have evaluated the accuracy of the single-based transmission procedure and attenuation correction process. In particular, we compare variance weighted OSEM and a dedicated TR algorithm with regularization (MAP-TR) for the reconstruction of the /spl mu/-image. Contamination from emission is estimated from a mock scan without moving the source. Results of a measurement of the patient dose during HRRT transmission scans show a 4 times lower dose compared to patient transmission scans on the ECAT HR.

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.

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.

Evaluation of the depth of interaction (DOI) for the High Resolution Research Tomograph (HRRT) - a comparison between scanners with and without DOI

The High Resolution Research Tomograph (HRRT) is the first clinical LSO scanner with depth of interaction (DOI) capability. At the moment two HRRTs exist: one is located at the Max-Planck-Institute in Cologne and is equipped with two 7.5 mm thick crystal layers and DOI (HRRT-D); the second HRRT is located at the VUme in Amsterdam with a single crystal layer (HRRT-S). This constellation allows evaluation of scanner characteristics with and without DOI. The effects of DOI on resolution, sensitivity, scatter fraction (SF) and noise equivalent count (NEC) rates were studied. These characteristics were measured according to the NEMA NU2-1994 or 2001 standards. Emission scans were acquired in 64 bit list mode. For HRRT-D emission data were sorted into files with coincidences between the layers nearest to the center of the scanner only and into files with all layer combinations providing DOI. Resolution data were analyzed with different data compressions (span 3 and 9) and for FORE+2D-HOSP and 3D-OSEM. Different scatter fractions between both scanners were observed for lower level discriminator values (lids) of higher energy. NEC rates were about 3 times higher for HRRT-D. Best resolution values of 2.0 mm FWHM at 0 cm radial offset from the center and 2.5 mm FWHM at 10 cm radial offset were found using span 3 and 3D-OSEM for HRRT-S. Equal values were found for HRRT-D using DOI. For span 9 data and FORE+2D-HOSP the axial resolution decreased at off center locations to 4 mm FWHM or more. The higher NECR of HRRT-D compared with HRRT-S is explained by the higher sensitivity, obtained with the additional crystal layer. Differences in scatter fractions might be explained by the difference in crystal energy spectra of both scanners. Comparison of resolution data of both scanners showed that high spatial resolution was preserved with DOI.

Derivation of the input function from dynamic PET images with the HRRT

The High Resolution Research Tomograph (HRRT) with its improved spatial resolution (<2.5 mm), extended axial FOV (25 cm) and high sensitivity offers the possibility to obtain an image derived input function for quantitative PET studies. From the carotid artery in the region of the neck and the petrous bone, the time course of the blood activity can be derived with sufficient counting statistics in human studies. However an online motion correction system is a must. With medium sized animals like cats, the FOV of the HRRT covers both, the brain and the heart, which allows to determine the input function from the blood pool in the heart In cat studies with /sup 15/O-tracers the time course of the tracer activity in arterial blood has been measured with an arteriovenous shunt connected to a blood sampling system. The direct comparison to the input function derived from the blood pool in the left heart chamber in kinetic PET images reconstructed with OSEM3D in short time frames demonstrates the high accuracy and precision of the image derived input function.

NEMA count-rate evaluation of the first and second generation of the Ecat Exact and Ecat Exact HR family of scanners

The first and second generation of the Exact and Exact HR family of scanners has been evaluated in terms of noise equivalent count rate (NEC) and count-rate capabilities. The new National Electrical Manufacturers Association standard was used for the evaluation. In spite of improved electronics and improved count-rate capabilities, the peak NEC was found to be fairly constant between the generations. The results are discussed in terms of the different electronic solutions for the two generations and its implications on system dead time and NEC count-rate capability.

Inducible gene expression by use of an HET-amplicon vector

Objective: To investigate whether non-invasive assessment of regulated expression of a PET marker gene is possible.