Search

The aim of this work was investigating the methods based on coupling cerebral perfusion (ASL) and amino acid metabolism ([18F]DOPA-PET) measurements to evaluate the diagnostic performance of PET/MRI in glioma follow-up. Images were acquired using a 3-T PET/MR system, on a prospective cohort of patients addressed for possible glioma progression. Data were preprocessed with statistical parametric mapping (SPM), including registration on T1-weighted images, spatial and intensity normalization, and tumor segmentation. As index tests, tumor isocontour maps of [18F]DOPA-PET and ASL T-maps were created and metabolic/perfusion abnormalities were evaluated with the asymmetry index z-score. SPM map analysis of significant size clusters and semi-quantitative PET and ASL map evaluation were performed and compared to the gold standard diagnosis. Lastly, ASL and PET topography of significant clusters was compared to that of the initial tumor. Fifty-eight patients with unilateral treated glioma were included (34 progressions and 24 pseudo-progressions). The tumor isocontour maps and T-maps showed the highest specificity (100%) and sensitivity (94.1%) for ASL and [18F]DOPA analysis, respectively. The sensitivity of qualitative SPM maps and semi-quantitative rCBF and rSUV analyses were the highest for glioblastoma. Tumor isocontour T-maps and combined analysis of CBF and [18F]DOPA-PET uptake allow achieving high diagnostic performance in differentiating between progression and pseudo-progression in treated gliomas. The sensitivity is particularly high for glioblastomas. • Applied separately, MRI and PET imaging modalities may be insufficient to characterize the brain glioma post-therapeutic profile. • Combined ASL and [18F]DOPA-PET map analysis allows differentiating between tumor progression and pseudo-progression.

One of the main challenges of integrated PET/MR is to achieve an accurate PET attenuation correction (AC), especially in brain acquisition. Here, we evaluated an AC method based on zero echo time (ZTE) MRI, comparing it with the single-atlas AC method and CT-based AC, set as reference. Fifty patients (70 ± 11 years old, 28 men) underwent FDG-PET/MR examination (SIGNA PET/MR 3.0 T, GE Healthcare) as part of the investigation of suspected dementia. They all had brain computed tomography (CT), 2-point LAVA-flex MRI (for atlas-based AC), and ZTE-MRI. Two AC methods were compared with CT-based AC (CTAC): one based on a single atlas, one based on ZTE segmentation. Impact on brain metabolism was evaluated using voxel and volumes of interest–based analyses. The impact of AC was also evaluated through comparisons between two subgroups of patients extracted from the whole population: 15 patients with mild cognitive impairment and normal metabolic pattern, and 22 others with metabolic pattern suggestive of Alzheimer disease, using SPM12 software. ZTE-AC yielded a lower bias (3.6 ± 3.2%) than the atlas method (4.5 ± 6.1%) and lowest interindividual (4.6% versus 6.8%) and inter-regional (1.4% versus 2.6%) variabilities. Atlas-AC resulted in metabolism overestimation in cortical regions near the vertex and cerebellum underestimation. ZTE-AC yielded a moderate metabolic underestimation mainly in the occipital cortex and cerebellum. Voxel-wise comparison between the two subgroups of patients showed that significant difference clusters had a slightly smaller size but similar locations with PET images corrected with ZTE-AC compared with those corrected with CT, whereas atlas-AC images showed a notable reduction of significant voxels. ZTE-AC performed better than atlas-AC in detecting pathologic areas in suspected neurodegenerative dementia. • The ZTE-based AC improved the accuracy of the metabolism quantification in PET compared with the atlas-AC method. • The overall uptake bias was 21% lower when using ZTE-based AC compared with the atlas-AC method. • ZTE-AC performed better than atlas-AC in detecting pathologic areas in suspected neurodegenerative dementia.

In brain PET/MR applications, accurate attenuation maps are required for accurate PET image quantification. An implemented attenuation correction (AC) method for brain imaging is the single-atlas approach that estimates an AC map from an averaged CT template. As an alternative, we propose to use a zero echo time (ZTE) pulse sequence to segment bone, air and soft tissue. A linear relationship between histogram normalized ZTE intensity and measured CT density in Hounsfield units ([Formula: see text]) in bone has been established thanks to a CT-MR database of 16 patients. Continuous AC maps were computed based on the segmented ZTE by setting a fixed linear attenuation coefficient (LAC) to air and soft tissue and by using the linear relationship to generate continuous μ values for the bone. Additionally, for the purpose of comparison, four other AC maps were generated: a ZTE derived AC map with a fixed LAC for the bone, an AC map based on the single-atlas approach as provided by the PET/MR manufacturer, a soft-tissue only AC map and, finally, the CT derived attenuation map used as the gold standard (CTAC). All these AC maps were used with different levels of smoothing for PET image reconstruction with and without time-of-flight (TOF). The subject-specific AC map generated by combining ZTE-based segmentation and linear scaling of the normalized ZTE signal into [Formula: see text] was found to be a good substitute for the measured CTAC map in brain PET/MR when used with a Gaussian smoothing kernel of [Formula: see text] corresponding to the PET scanner intrinsic resolution. As expected TOF reduces AC error regardless of the AC method. The continuous ZTE-AC performed better than the other alternative MR derived AC methods, reducing the quantification error between the MRAC corrected PET image and the reference CTAC corrected PET image.

Integrated positron emission tomography (PET)/magnetic resonance imaging (MRI) offers promising tools for evaluating brain disorders, including the minimization of exposure to ionizing radiation. Considering the length of scanning time with PET/MRI systems and their high sensitivity, we assumed that the activity could be reduced by one half compared with recommended activity for brain 2-deoxy-2-[We retrospectively simulated the reduction of injected activity (1 vs. 2 MBq/kg, [The intra-operator agreement was high between the PETA reduction of the [

International audience; One of the main technical challenges of PET/MRI is to achieve an accurate PET attenuation correction (AC) estimation. In current systems, AC is accomplished by generating an MRI-based surrogate computed tomography (CT) from which AC-maps are derived. Nevertheless, all techniques currently implemented in clinical routine suffer from bias. We present here a convolutional neural network (CNN) that generated AC-maps from Zero Echo Time (ZTE) MR images. Seventy patients referred to our institution for 18FDG-PET/MR exam (SIGNA PET/MR, GE Healthcare) as part of the investigation of suspected dementia, were included. 23 patients were added to the training set of the manufacturer and 47 were used for validation. Brain computed tomography (CT) scan, two-point LAVA-flex MRI (for atlas-based AC) and ZTE-MRI were available in all patients. Three AC methods were evaluated and compared to CT-based AC (CTAC): one based on a single head-atlas, one based on ZTE-segmentation and one CNN with a 3D U-net architecture to generate AC maps from ZTE MR images. Impact on brain metabolism was evaluated combining voxel and regions-of-interest based analyses with CTAC set as reference. The U-net AC method yielded the lowest bias, the lowest inter-individual and inter-regional variability compared to PET images reconstructed with ZTE and Atlas methods. The impact on brain metabolism was negligible with average errors of -0.2% in most cortical regions. These results suggest that the U-net AC is more reliable for correcting photon attenuation in brain FDG-PET/MR than atlas-AC and ZTE-AC methods.

One of the challenges of PET/MR is replacing CT-measured attenuation map with a MR-based, especially in the absence of bone signal in MR images. Regular MRI cannot distinguish between different tissue types based on electron density, thus, Zero Echo Time (ZTE) has been used to segment bone, air and soft tissue. In this study, we have evaluated the relationship between bone density measured in CT and signal intensity measured in ZTE on an CT-MR database of 16 patients. A linear relationship has been found on bone voxels between normalized ZTE intensity and CT density in Hounsfield Unit (HU). Using this relationship, we can derive patient-specific pseudo-CT from ZTE images.

Objectif La correction d’attenuation (CA) est le principal probleme methodologique en TEP/IRM. En routine, elle est effectuee apres segmentation de l’IRM et classification des tissus. Cette methode n’est pas adaptee aux fantomes composes de plastique et remplis d’eau, leur densite etant differente de celle des tissus humains. Pour le controle qualite (CQ) du SUV, une acquisition TEP d’un fantome rempli de 18F est necessaire. La CA implementee alors par le constructeur utilise un TDM. Par consequent, les SUV mesures en routine ne sont pas entierement controles lors de la realisation du CQ. L’objectif de cette etude etait d’evaluer une nouvelle methode pour valider le SUV sans utiliser de TDM pour la CA en TEP/IRM. Methode L’etude a ete faite sur le SIGNA PET/MR (GE Healthcare). Le fantome SUV a initialement ete rempli d’eau et les coefficients d’attenuation (μ) derives du TDM predefini pour obtenir SUV-TDM comme preconise par le constructeur. Deuxiemement, le protocole clinique a ete utilise : les μ ont ete calcules a partir de l’IRM pour obtenir SUV-IRM-clin. Finalement, le fantome a ete rempli avec un melange d’agent de contraste iode, de sel et d’eau, pour obtenir μ = 0,1 cm − 1 a 511 keV. De la vaseline a ete placee a la surface du fantome pour ameliorer la classification tissulaire. Les μ et le SUV-IRM ont ete calcules comme precedemment. Dans les 3 cas, un TEP de 10 minutes (30 MBq de 18F) a ete acquis et reconstruit. SUV-TDM, SUV-IRM-clin et SUV-IRM ont ete mesures avec une ROI positionnee sur 10 coupes. Resultats La determination de μ avec le protocole constructeur dedie au CQ du SUV est correcte, comme attendu. Avec le protocole de routine, un μ incorrect a ete attribue au liquide (0,086 cm − 1 au lieu de 0,096 cm − 1 pour l’eau). Avec notre nouveau protocole, le systeme a classe le liquide comme tissu mou et lui a attribue un μ correct (0,1 cm − 1). Les erreurs moyennes sur le SUV sont, respectivement, de 3,7 % (SUV-TDM = 1,037 ± 0,005), −14,0 % (SUV-IRM-clin = 0,860 ± 0,004) et 5,4 % (SUV-IRM = 1,054 ± 0,007). Conclusion Le protocole de routine ne peut etre utilise en TEP/IRM pour le CQ du SUV. En modifiant l’attenuation du liquide et en augmentant le contraste de l’IRM, notre protocole pour le CQ du SUV permet l’utilisation de la CA basee sur l’IRM avec des resultats equivalents a la CA derivee du TDM. Notre methode augmente la robustesse du CQ du SUV car la CA est semblable a celle utilisee en routine, ce qui est particulierement important dans le contexte PET/IRM.

Introduction The attenuation correction of PET/MR brain imaging is based on a human atlas. However, this atlas cannot be used for phantoms employed for the usual PET/CT experiments. The method based on the DIXON MRI is used instead to separate water and fat tissues and assign them attenuation coefficients at 511 keV. Nevertheless, the usual PET/CT phantoms generate artefacts on the MR-based attenuation maps. Additionally, their materials have different attenuation coefficients from those of human body. Here, we suggest to substitute the MR derived attenuation correction map with a CT derived attenuation correction map for a brain phantom in PET/MR. Methods Two PET acquisitions of a 3D Hoffman phantom were achieved: one a PET/CT Biograph mCT Flow (Siemens) and one on a Signa PET/MR (GE). Based on these acquisitions, 4 images were reconstructed: 1 with the PET/CT (PETref) and 3 with the PET/MR (one without attenuation correction (PETNAC), one corrected using MR-based attenuation map (PETMRAC) and one corrected using the CT obtained with the PET/CT, registered on the MRI images (PETCTAC). To evaluate the effect of the different attenuation correction of the PET images, activity concentrations ratio between white matter (WM) and gray matter (GM), as well as between anterior and posterior regions (A/P) and between right and left (R/L) regions, were measured on PETNAC, PETMRACand PETCTAC and compared to the ratios obtained with PETref. Results The MR based attenuation correction map of the Hoffman phantom showed artefacts due to segmentation and tissue classification errors. Moreover, for all the reconstructed images, no R/L asymmetry was observed (ratios between 0.96 and 0.99). However, a strong A/P asymmetry with a mean value of 20% was measured in the PETNAC for both WM and GM, versus 1% for the 3 attenuation-corrected images. The WM/GM ratio was underestimated on the PETMRAC (1.84) compared to the CT-based attenuation-corrected ones (1.95 and 1.96 for PETCTAC and PETref, respectively). Conclusions All the attenuation correction methods allowed to correct the A/P asymmetry, which was increased by the presence of MR coil in the PET/MR system. However, MR-based attenuation-correction map suffered from artefacts and was not adapted to correct the PET image of the Hoffman phantom in PET/MR. The CT-based attenuation correction method used to correct the Hoffman phantom PET/MR image showed ratios comparable to the ones measured on a PET/CT considered as reference.

Blood pressure is an important marker for the diagnosis of cardiovascular diseases. It provides information on valve mechanics, flow turbulence and vascular compliance (Karamanoglu et al. 1993). Ad...