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ABSTRACT: The High Resolution Research Tomograph (HRRT) is a dedicated 3D brain positron emission tomograph (PET), designed for a resolution of 3 mm or less. Recent improvements in image reconstruction strategies, such as the implementation of ordinary Poisson OSEM, improved the quantitative accuracy of HRRT PET. Further improvement of its accuracy might be expected using a new randoms estimation method based on coincidence histograms. The purpose of the present study was therefore to further evaluate the accuracy of HRRT studies using these new reconstruction methods. Moreover, data will be compared with those measured on a clinical HR+ PET scanner (Siemens), which has been used most frequently for human brain applications so far. To this end, a number of phantom experiment using, for example, NEMA scatter and attenuation, homogeneous (linearity and uniformity) and anthropomorphic brain phantoms, have been performed on both scanners. When using the new randoms estimation method, HRRT experiments showed a remnant scatter fraction <5%, uniformity <5% and linearity <3% up to 136 MBq. These results were similar of better than those obtained with the HR + HRRT brain phantom studies provided accurate results within 10 and 15% for grey and white matter areas, respectively, for high statistics (>1800 s) scans only. Large under- and overestimations of 20 and 50% in grey and white matter areas, respectively, were observed in case of short acquisition frames (10-30 s). As short acquisition frames of 10-30 s are normally applied in dynamic brain studies, it is concluded that further refinement of image reconstruction strategies [Boellaard, R, et. al., 2004] is required to obtain more accurate results, which are comparable with those of the HR+, for dynamic HRRT PET brain studies.
Nuclear Science Symposium Conference Record, 2006. IEEE; 12/2006
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V. Sossi, H.W.A.M. de Jong,
W.C. Barker,
P. Bloomfield,
Z. Burbar,
M.-L. Camborde,
C. Comtat,
L.A. Eriksson,
S. Houle,
D. Keator, [......],
A.A. Lammertsma,
A. Rahmim,
M. Sibomana,
M. Teras,
C.J. Thompson,
R. Trebossen,
J. Votaw,
M. Walker,
K. Wienhard,
D.F. Wong
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ABSTRACT: The high resolution research tomograph (HRRT) is one of the most complex existing positron emission tomographs: it is the only human size scanner capable of decoding the depth of the γ-ray interaction in the crystal, using a lutetium LSO/LYSO phoswich detector arrangement. In this study we determined basic scanner hardware characteristics, such as scanner data acquisition stability, and their variability across eleven centres. In addition a subset of the NEMA NU-2001 standards measurements was performed. We found (i) significant variability in the DOI decoding results between centres, (ii) a trend toward an increasing number of detected true coincident events as a function of elapsed time from scanner calibration likely due to a shifting energy spectrum, (iii) a count-rate dependent layer identification, (iv) scatter fraction ranging from ∼ 42% to 54% where the variability was partly related to the shifting of the energy spectrum, (v) sensitivity ranging from ∼5.5% to 6.5% across centres, (vi) resolution of ∼(2.5 mm)<sup>3</sup>, fairly consistent across centres, (vii) image quality which is very comparable to other scanners.
Nuclear Science Symposium Conference Record, 2005 IEEE; 11/2005
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ABSTRACT: In this study the imaging characteristics influencing the quantitative accuracy of <sup>68</sup>Ga, <sup>124</sup>I and <sup>89</sup>Zr were determined and compared to those of <sup>18</sup>F using a 3D high resolution PET (high resolution research tomograph, HRRT) scanner. Although there were large discrepancies found in the sensitivity of these isotopes, which can be explained by their positron abundancy, none of the assessed imaging characteristic prevents the isotopes for usage in (high resolution) quantitative PET imaging. Care has to be taken, however, that accurate correction methods are used for dead time, background and scatter, and partial volume.
Nuclear Science Symposium Conference Record, 2005 IEEE; 11/2005
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ABSTRACT: 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.
Nuclear Science Symposium Conference Record, 2004 IEEE; 11/2004
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ABSTRACT: The high resolution research tomograph (HRRT) is the first PET scanner with depth of interaction (DOT) 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.
Nuclear Science Symposium Conference Record, 2004 IEEE; 11/2004
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ABSTRACT: The high resolution research tomograph (HRRT) is a brain-dedicated scanner manufactured by CPS Innovations using LSO panel detectors. Transmission is measured using a <sup>137</sup>Cs point source, which is moved axially and rotated to cover the FOV. The point source is collimated axially and transaxially to illuminate only a few planes on the heads opposite to the point source location. Pseudo-coincidence events are generated using a given crystal and the source location. The transmission system was previously validated for cold transmissions. For post-injection (hot) transmission, it is not possible to eliminate the emission contamination by raising the lower energy threshold. Since real mock scan is unpractical on HRRT and fake mock scan requires additional data, we developed a new technique to simultaneously measure the transmission and the mock scans. The technique uses a virtual source, axially located at a distance equal to the half of the axial FOV and illuminates a fan separated from the real transmission fan. We validated the shifted-mock scan technique by comparing it to real mock scan one with a <sup>68</sup>Ge phantom and examined its effectiveness with a hot 20 cm phantom filled with <sup>18</sup>F decaying over several half-lives. Local residual bias in μ-map was attributed to transmission scatter and corrected by using partial segmentation in the MAP-TR algorithm, μ-maps from cold and hot transmissions were compared on several clinical patients and a Hoffman brain phantom for which their influence on emission quantification was studied.
Nuclear Science Symposium Conference Record, 2004 IEEE; 11/2004
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ABSTRACT: The count rate performance of the single LSO crystal layer high-resolution research tomograph (HRRT-S) PET scanner is limited by the processing speed of its electronics. Therefore, the feasibility of using an in-field-of-view (in-FOV) shield to improve the noise equivalent count rates (NECR) for small animal brain studies was investigated. The in-FOV shield consists of a lead tube of 12 cm length, 6 cm inner diameter and 9 mm wall thickness. It is large enough to shield the activity in the body of a rat or mouse. First, the effect of this shield on NECR was studied. Secondly, a number of experiments were performed to assess the effects of the shield on the accuracy of transmission scan data and, next, on reconstructed activity distribution in the brain. For activities below 150 MBq NECR improved only by 5-10%. For higher activities NECR maxima of 1.2E4 cps at 200 MBq and 2.2E4 cps at 370 MBq were found without and with shield, respectively. Listmode data taken without shield, however, were corrupted for activities above 75 MBq due to data overrun problems (time tag losses) of the electronics. When the shield was used data overrun was avoided up to activities of 150 MBq. For the unshielded part of the phantom, transmission scan data were the same with and without shield. The estimated scatter contribution was approximately 8.5% without and 5.5% with shield. Reconstructed emission data showed a difference up to 5% in the unshielded part of the phantom at 5 mm or more from the edge of the shielding. Of this 5% about 3% results from the difference in the uncorrected scatter contribution. In conclusion, an in-FOV shield can be used successfully in an HRRT PET scanner to improve NECR and accuracy of small animal brain studies. The latter is especially important when high activities are required for tracers with low brain uptake or when multiple animals are scanned simultaneously.
Physics in Medicine and Biology 01/2004; 48(23):N335-42. · 2.83 Impact Factor
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ABSTRACT: The High Resolution Research Tomograph PET scanner is equipped with a moving Cs-137 transmission point source emitting 662 keV single photons. During postinjection transmission imaging 511 keV emission photons can be detected in the transmission window leading to gross underestimation of the reconstructed mu-value. Using cylindrical phantoms, methods to compensate for this emission contamination (EC) were investigated including histogram-based scaling and segmentation with varying transmission window settings. Furthermore, the effects of subtracting a uniform or nonuniform EC estimation prior to transmission reconstruction were quantified. In conclusion, EC during HRRT transmission scans can lead to gross underestimation of mu-values, and the most accurate way to compensate for this is to combine nonuniform EC subtraction with image segmentation.
Nuclear Science Symposium Conference Record, 2003 IEEE; 11/2003
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ABSTRACT: The high resolution research tomograph (HRRT) is a 3-D PET scanner designed for human brain and small animal imaging. The HRRT consists of eight panel detector heads that are separated by gaps of 17 mm resulting in gaps in the sinogram. Furthermore, gaps can result from detector-block failure. To prevent artifacts in the reconstruction when using Fourier rebinning (FORE), filling the data gaps is required. The purpose of this study was to evaluate the accuracy of three gap filling methods: a) bilinear interpolation of sinogram data; b) a model-based method in which an intermediate volume is reconstructed [2-D ordered subsets expectation maximization (2-D OSEM)] based on direct planes only, after which this image is forward projected to fill the gaps; c) an improved model-based method in which gaps are first filled using interpolation, then reconstructed using FORE + 2-D OSEM and forward projected. The improved model-based method outperforms interpolation, but requires more computation time.
IEEE Transactions on Nuclear Science 11/2003; · 1.45 Impact Factor
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ABSTRACT: 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.
Physics in Medicine and Biology 03/2003; 48(4):429-48. · 2.83 Impact Factor
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IEEE Transactions on Nuclear Science, v.50, 1452-1456 (2003).
