Elena Rota Kops

Performance characteristics of an eight-ring whole body PET scanner.

The technical characteristics of the multislice whole-body positron emission tomographic scanner (model PC4096-15WB Scanditronix) and its performance parameters are described. Spatial resolution at the center of the field of view was found to be 4.9 mm in-plane and 4.6 mm (cross slices) and 6.0 mm (direct slices) in the axial direction. The sensitivity for true and scattered coincidences is approximately 5,000 cps for direct slices and 7,100 cps for cross slices. At an activity concentration of 37 kBq/ml the system deadtime was approximately 5%. By measuring a uniform phantom with a cold cylindrical insert (5.0 cm diameter), the scatter fraction was found to be approximately 5%. The mean global uniformity over all 15 slices was 6.5%, whereas the local uniformity was found to be 4.3%. No systematic nonuniformities were observed. Finally, various methods for attenuation correction (transmission scan, contour finding, ellipse) were utilized to test their effects on the resulting reconstructed images.

Comparison of cerebral blood flow acquired by simultaneous [15O]water positron emission tomography and arterial spin labeling magnetic resonance imaging.

Until recently, no direct comparison between [15O]water positron emission tomography (PET) and arterial spin labeling (ASL) for measuring cerebral blood flow (CBF) was possible. With the introduction of integrated, hybrid magnetic resonance (MR)-PET scanners, such a comparison becomes feasible. This study presents results of CBF measurements recorded simultaneously with [15O]water and ASL. A 3T MR-BrainPET scanner was used for the simultaneous acquisition of pseudo-continuous ASL (pCASL) magnetic resonance imaging (MRI) and [15O]water PET. Quantitative CBF values were compared in 10 young healthy male volunteers at baseline conditions. A statistically significant (P<0.05) correlation was observed between the two modalities; the whole-brain CBF values determined with PET and pCASL were 43.3 ±6.1 mL and 51.9 ± 7.1 mL per 100 g per minute, respectively. The gray/white matter (GM/WM) ratio of CBF was 3.0 for PET and 3.4 for pCASL. A paired t-test revealed differences in regional CBF between ASL and PET with higher ASL-CBF than PET-CBF values in cortical areas. Using an integrated, hybrid MR-PET a direct simultaneous comparison between ASL and [15O]water PET became possible for the first time so that temporal, physiologic, and functional variations were avoided. Regional and individual differences were found despite the overall similarity between ASL and PET, requiring further detailed investigations.

Alternative methods for attenuation correction for PET images in MR-PET scanners

This paper describes and compares procedures to obtain attenuation maps used for the absorption correction (AC) of PET brain scans if a transmission scan is not available as in the case of future MR-PET scanners. A previously reported approach called MBA (MRT-based attenuation correction) used Tl- weighted MR images which were segmented into four tissue types representing brain tissue, bone, other tissue and sinus to which appropriate attenuation coefficients were assigned. In this work a template-based attenuation correction (TBA) is presented which applies an attenuation template to single subjects. A common attenuation template was created from transmission scans of 10 normal volunteers and spatially normalized to the SPM2 standard brain shape. For each subject the Tl-MR template of SPM2 was warped onto the subjects individual MR image. The resulting warping matrix was applied to the common attenuation template so that an attenuation map matching the subjects brain shape was obtained. The attenuation maps of MBA and TBA were forward projected into attenuation factors which were alternatively used for AC. FDG scans of four subjects were reconstructed after AC with MBA and TBA and compared to images whose ACs were based on conventional attenuation maps (PBA=PET-based attenuation correction). Using PBA as reference in a region of interest analysis, MBA and TBA showed similar under- and overestimation of the reconstructed radioactivity up to -10% and 9%, respectively. The procedure to obtain the attenuation template needs still some improvements. Nevertheless, the TBA method of attenuation correction is a promising alternative to MBA with its still complex and not yet resolved accurate segmentation of MR images.

Template based attenuation correction for PET in MR-PET scanners

This work investigates a template based procedure for attenuation correction (TBA) of PET scans acquired in future hybrid MR-PET scanners which will not offer a measured attenuation correction. A previous report of our group described a method (TBA-SPM) how individual attenuation maps can be obtained from an attenuation template which is spatially normalized to the SPM2 standard brain shape. Attenuation maps of females and males obtained from PET transmission scans were used as input for this template. The present study replaces the template referring to SPM2 by a female and a male attenuation template (fAT and mAT), each based on four measured attenuation images (TBA-f&m). The corresponding T1-MR templates (fMR and mMR) were also available. Thus, possible morphological gender-related differences, not considered when the standardized SPM2 1brain shape is used, may be taken into account. To examine this approach PET scans of 15 female and 15 male subjects of an ongoing study were attenuation corrected using the templates fAT and mAT. For this purpose and depending on the subject’s gender the fMR or mMR templates were warped onto the individual MR image. The resulting warping matrix was applied to fAT or mAT, respectively. The individualized attenuation maps were used to reconstruct the PET emission data. These were compared to PET images attenuation corrected with the conventional PET based transmission data (PBA). While the relative differences between PBA and TBA=f&m reconstructed images averaged over each group and all regions of interest were 0.57% ± 3.76% for females and −0.59% ± 3.56% for males, the corresponding values obtained with the TBA-SPM method showed an overestimation with similar standard deviations (2.39% ± 3.76% for females and 2.42% ± 3.37% for males). In conclusion, the alternative gender-related template method TBA-f&m gives acceptable results with no significant differences between the genders.

MR-based PET motion correction procedure for simultaneous MR-PET neuroimaging of human brain

Positron Emission Tomography (PET) images are prone to motion artefacts due to the long acquisition time of PET measurements. Recently, simultaneous magnetic resonance imaging (MRI) and PET have become available in the first generation of Hybrid MR-PET scanners. In this work, the elimination of artefacts due to head motion in PET neuroimages is achieved by a new approach utilising MR-based motion tracking in combination with PET list mode data motion correction for simultaneous MR-PET acquisitions. The method comprises accurate MR-based motion measurements, an intra-frame motion minimising and reconstruction time reducing temporal framing algorithm, and a list mode based PET reconstruction which utilises the Ordinary Poisson Algorithm and avoids axial and transaxial compression. Compared to images uncorrected for motion, an increased image quality is shown in phantom as well as in vivo images. In vivo motion corrected images show an evident increase of contrast at the basal ganglia and a good visibility of uptake in tiny structures such as superior colliculi.

Attenuation correction in MR-PET scanners with segmented T1-weighted MR images

Attenuation correction of PET data acquired in new hybrid MR-PET scanners which do not offer the possibility of a measured attenuation correction can be done in different ways. A previous report of our group described a method which used attenuation templates. The present study utilizes a new knowledge-based segmentation approach applied on T1-weighted MR images. It examines the position and the tissue membership of each voxel and segments the head volume into attenuation-differing regions: brain tissue, extracerebral soft tissue, skull, air-filled nasal and paranasal cavities as well as the mastoid process. To examine this new approach three groups of subjects having MRI and PET were chosen, the selection criterion being the different MR scanners, while the PET scanner was the ECAT HR+ in all cases: 1) four subjects with 1.5T MR images and CPFPX PET scans, 2) four subjects with 3T MR images and Altanserin PET scans, and 3) three brain tumor patients with 3T MR images from the hybrid MR-BrainPET scanner and FET PET scans. Furthermore, a single subject had 3T MR images, a FDG PET scan, and an additional CT scan. All segmented T1-weighted MR images were converted into attenuation maps for 511 KeV photons with coefficients of 0.096 1/cm for brain tissue, 0.146 1/cm for skull, 0.095 1/cm for soft tissue, 0.054 1/cm for the mastoid process, and 0.0 1/cm for nasal and paranasal cavities. The CT volume was also converted from the Hounsfield units into attenuation coefficients valid for 511 keV photons. The 12 segmented-based attenuation (SBA) maps as well as the CT-based attenuation (CBA) map were first filtered by a 3D Gaussian kernel of 10 mm filter width and then used to reconstruct the corresponding PET emission data. These were compared to the PET images attenuation corrected using the conventional PET-based transmission data (PBA). Relative differences (RD) were calculated from ROIs. For the single subject the RD of CBA data exhibit a mean of 1.66%?0.84% with a range from -0.88% to 3.42%, while the RDs mean of SBA data is 1.42%?2.61% (range from -4.12% to 4.66%). Comparing the results obtained with the SBA correction only, the RD for 1) range from -6.10% to 2.56% for cortical regions and from -6.99% to 5.64% for subcortical regions; for 2) they range from -7.33% to 2.33% for the cortical regions, subcortical ones being not drawn due to the not significant tracer uptake; for 3) the mean over the three subjects resulted in 0.89%?1.10% for ROIs at 48% threshold of the images maximum and in 2.25%?1.50% for ROIs at 72% threshold. ROIs on the healthy contra-lateral grey matter show a mean of -3.24%?0.87%. In conclusion, the first attenuation correction results obtained with the new segmented-based method on a strongly heterogeneous collective are very promising. Further improvements of the method will be focused on the delineation of the skull.

Effects of Magnetic Fields of up to 9.4 T on Resolution and Contrast of PET Images as Measured with an MR-BrainPET

Simultaneous, hybrid MR-PET is expected to improve PET image resolution in the plane perpendicular to the static magnetic field of the scanner. Previous papers have reported this either by simulation or experiment with simple sources and detector arrangements. Here, we extend those studies using a realistic brain phantom in a recently installed MR-PET system comprising a 9.4 T MRI-scanner and an APD-based BrainPET insert in the magnet bore. Point and line sources and a 3D brain phantom were filled with 18F (low-energy positron emitter), 68Ga (medium energy positron emitter) or 120I, a non-standard positron emitter (high positron energies of up to 4.6 MeV). Using the BrainPET insert, emission scans of the phantoms were recorded at different positions inside and outside the magnet bore such that the magnetic field was 0 T, 3 T, 7 T or 9.4 T. Brain phantom images, with the ‘grey matter’ compartment filled with 18F, showed no obvious resolution improvement with increasing field. This is confirmed by practically unchanged transaxial FWHM and ‘grey/white matter’ ratio values between at 0T and 9.4T. Field-dependent improvements in the resolution and contrast of transaxial PET images were clearly evident when the brain phantom was filled with 68Ga or 120I. The grey/white matter ratio increased by 7.3% and 16.3%, respectively. The greater reduction of the FWTM compared to FWHM in 68Ga or 120I line-spread images was in agreement with the improved contrast of 68Ga or 120I images. Notwithstanding elongations seen in the z-direction of 68Ga or 120I point source images acquired in foam, brain phantom images show no comparable extension. Our experimental study confirms that integrated MR-PET delivers improved PET image resolution and contrast for medium- and high-energy positron emitters even though the positron range is reduced only in directions perpendicular to the magnetic field.

MRI Based Attenuation Correction for Brain PET Images

This work describes a procedure to yield attenuation maps from MR images which are used for the absorption correction (AC) of brain PET data. Such an approach could be mandatory for future combined PET and MRI scanners, which probably do not include a transmission facility. T1-weighted MR images were segmented into brain tissue, bone, soft tissue, and sinus; attenuation coefficients corresponding to elemental composition and density as well as to 511 keV photon energy were respectively assigned. Attenuation maps containing up to four compartments were created and forward projected into sinograms with attenuation factors which then were used for AC during reconstruction of FDG-PET data. The commonly used AC based on a radioactive (68Ge) transmission scan served as reference. The reconstructed radioactivity values obtained with the MRI-based AC were about 20% lower than those obtained with PET-based AC if the skull was not taken into account. Considering the skull the difference was still about 10%. Our investigations demonstrate the feasibility of a MRI-based AC, but revealed also the necessity of a satisfying delineation of bone thickness which tends to be underestimated in our first approach of T1-weighted MR image segmentation.

Relationship of Regional Cerebral Blood Flow and Kinetic Behaviour of O-(2-18F- Fluoroethyl)-L-Tyrosine Uptake in Cerebral Gliomas.

ObjectivesO-(2-18F-fluoroethyl)-L-tyrosine (18F-FET) is an established tracer for brain tumour imaging. 18F-FET kinetics in gliomas appear to have potential for tumour grading, but the mechanisms remain unclear. The aim of this study was to explore the relationship between regional cerebral blood flow (rCBF) as measured by arterial spin labelling MRI and the kinetic behaviour of 18F-FET PET in cerebral gliomas. Materials and methodsTwenty patients with cerebral gliomas were investigated using arterial spin labelling MRI and dynamic 18F-FET PET. Time–activity curves (TACs) of 18F-FET uptake were analysed in 33 different tumour regions. The slopes of TAC during the early (0–5 min; slopeup) and late phases of tracer uptake (17–50 min; slopedown) were fitted using linear regression lines. In addition, TACs of each lesion were assigned to different curve patterns. Furthermore, we calculated tumour-to-brain ratios of 18F-FET uptake. The relationship between 18F-FET parameters and rCBF was determined. Results18F-FET uptake in the early phase (slopeup) showed a significant correlation with rCBF (r=0.4; P=0.02). In contrast, both slopedown and TAC patterns showed no significant correlation with rCBF. Furthermore, a significant correlation was found between rCBF and tumour-to-brain ratio (r=0.53; P=0.002). ConclusionThere is a relationship between rCBF and 18F-FET uptake in cerebral gliomas in the initial uptake phase, but the kinetic behaviour of 18F-FET uptake in the late phase is not significantly influenced by rCBF. Thus, the differential kinetic pattern of 18F-FET uptake in high-grade and low-grade gliomas appears to be determined by factors other than rCBF.

CT-based evaluation of segmented head regions for attenuation correction in MR-PET systems

Attenuation correction (AC) is an important prerequisite for quantitative brain PET in MR-BrainPET systems. The new knowledge-based method segments attenuation-differing head regions solely based on the routinely acquired T1-weighted MR data set of the patients head. The original approach (O) was extended (E1-E3) with regard to the MR image quality at different bandwidth/voxel (130 HZ/voxel, 610 Hz/voxel) obtained at a 3T MR TimTrio system with BrainPET insert installed. Based on the Dice coefficient, the automatically obtained MR-based segmentation results for bone and soft tissue were compared with segmented CT data as gold standard modality data. So far, registered multi-modality data (MR, CT, PET) are available for one female volunteer F and two tumor patients T1, T2 with CT data of different image quality. Best results were obtained for BW130-E3 and BW610-E2. For F, the Dice coefficient for bone is up to 0.776 for BW130-E3 and up to 0.723 for BW610-E2 in the best part of the cranial region. The Dice coefficient for soft tissue is 0.867 for BW130-E3 and 0.868 for BW610-E2 in the whole data set used for AC. The SegMR-(SBA) and CT-based AC (CBA) were compared w.r.t. PET-based AC (PBA) for a HR+ PET device. AC with SBA yields very similar results to the gold standard CBA.