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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.

Introduction The aim of this study was to study the feasibility of a fully integrated multiparametric imaging framework to characterize non-small cell lung cancer (NSCLC) at 3-T PET/MRI. Patients and methods An 18F-FDG PET/MRI multiparametric imaging framework was developed and prospectively applied to 11 biopsy-proven NSCLC patients. For each tumor, 12 parametric maps were generated, including PET full kinetic modeling, apparent diffusion coefficient, T1/T2 relaxation times, and DCE full kinetic modeling. Gaussian mixture model-based clustering was applied at the whole data set level to define supervoxels of similar multidimensional PET/MRI behaviors. Taking the multidimensional voxel behaviors as input and the supervoxel class as output, machine learning procedure was finally trained and validated voxelwise to reveal the dominant PET/MRI characteristics of these supervoxels at the whole data set and individual tumor levels. Results The Gaussian mixture model-based clustering clustering applied at the whole data set level (17,316 voxels) found 3 main multidimensional behaviors underpinned by the 12 PET/MRI quantitative parameters. Four dominant PET/MRI parameters of clinical relevance (PET: k2, k3 and DCE: ve, vp) predicted the overall supervoxel behavior with 97% of accuracy (SD, 0.7; 10-fold cross-validation). At the individual tumor level, these dimensionality-reduced supervoxel maps showed mean discrepancy of 16.7% compared with the original ones. Conclusions One-stop-shop PET/MRI multiparametric quantitative analysis of NSCLC is clinically feasible. Both PET and MRI parameters are useful to characterize the behavior of tumors at the supervoxel level. In the era of precision medicine, the full capabilities of PET/MRI would give further insight of the characterization of NSCLC behavior, opening new avenues toward image-based personalized medicine in this field.

Blood oxygen level-dependent (BOLD) functional MRI (fMRI) is commonly used to measure cerebrovascular reactivity (CVR), which can convey insightful information about neurovascular health. Breath-holding (BH) has been shown to be a practical vasodilatory stimulus for measuring CVR in clinical settings. The conventional BOLD fMRI approach has some limitations, however, such as susceptibility-induced signal dropout at air tissue interfaces and low BOLD sensitivity especially in areas of low T2*. These drawbacks can potentially be mitigated with multi-echo sequences, which acquire several images at different echo times in one shot. When combined with multiband techniques, high temporal resolution images can be acquired. This study compared an advanced multiband multi-echo (MBME) echo planar imaging (EPI) sequence with an existing multiband single-echo (MB) sequence to evaluate the repeatability and sensitivity of BH activation and CVR mapping. Images were acquired from 28 healthy volunteers, of which 18 returned for repeat imaging. Both MBME and MB data were pre-processed using both standard and advanced denoising techniques. The MBME data was further processed by combining echoes using a T2*-weighted approach and denoising using multi-echo independent component analysis. BH activation was calculated using a general linear model and the respiration response function. CVR was computed as the percent change related to the activation. To account for differences in CVR related to TE, relative CVR (rCVR) was computed and normalized to the mean gray matter CVR. Test–retest metrics were assessed with the Dice coefficient, rCVR difference, within subject coefficient of variation, and the intraclass correlation coefficient. Our findings demonstrate that rCVR for MBME scans were significantly higher than for MB scans across most of the gray matter. In areas of high susceptibility-induced signal dropout, however, MBME rCVR was significantly less than MB rCVR due to artifactually high rCVR for MB scans in these regions. MBME rCVR showed improved test–retest metrics compared with MB. Overall, the MBME sequence displayed superior BOLD sensitivity, improved specificity in areas of signal dropout on MBME scans, enhanced reliability, and reduced variability across subjects compared with MB acquisitions. Our results suggest that the MBME EPI sequence is a promising tool for imaging CVR.

BACKGROUND Blood oxygen level-dependent (BOLD) functional MRI (fMRI) has been widely applied to detect brain activations. Recent advances in multiband (MB) and multiecho (ME) techniques have greatly improved fMRI methods. MB imaging improves temporal and/or spatial resolution, while ME imaging has been shown to improve BOLD sensitivity. This study aimed to evaluate the novel MBME echo planar imaging (EPI) sequence utilizing MB and ME simultaneously to determine if the MBME outperform the MB single echo (MBSE) sequence for task fMRI. PURPOSE To compare the performance of MBME with MBSE in a task fMRI study. STUDY TYPE Prospective. POPULATION A total of 29 healthy volunteers aged 20-46 years (9 male, 20 female). FIELD STRENGTH/SEQUENCE MBSE and MBME gradient-echo EPI sequences were applied at 3T. Additional T1 -weighted magnetization-prepared rapid acquisition with gradient echo (MPRAGE) was collected. ASSESSMENT A checkerboard visual task was presented during the functional MBSE and MBME scans. The MBME or MBSE signal was evaluated using the temporal signal-to-noise ratio (tSNR). Task activation was evaluated using the z-score, volume, sensitivity, and specificity. Test-retest metrics of task activation were examined with the Dice coefficient (DC) and intraclass correlation coefficient (ICC) on subjects with repeated scans. STATISTICAL TESTS A linear mixed-effects model was used to compared MBME and MBSE activation at the voxel base. The paired t-test was used to compare tSNR, activation z-score, and volume, along with sensitivity, specificity, and DC between MBSE and MBME. RESULTS While similar task activation was detected in the visual cortex, MBME showed higher activation volume and higher sensitivity compared with MBSE (P

International audience; An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Purpose Because of short signal lifetimes and respiratory motion, 3D lung MRI is still challenging today. Zero-TE (ZTE) pulse sequences offer promising solutions as they overcome the issue of short T 2 ∗ . Nevertheless, as they rely on continuous readout gradients, the trajectories they follow in k-space are not adapted to retrospective gating and inferred motion correction. Theory and methods We propose AZTEK (adaptive ZTE k-space trajectories), a set of 3D radial trajectories featuring three tuning parameters, to adapt the acquisition to any moving organ while keeping seamless transitions between consecutive spokes. Standard ZTE and AZTEK trajectories were compared for static and moving phantom acquisitions as well as for human thoracic imaging performed on 3 volunteers (1 healthy and 2 patients with lung cancer). Results For the static phantom, we observe comparable image qualities with standard and AZTEK trajectories. For the moving phantom, spatially coherent undersampling artifacts observed on gated images with the standard trajectory are alleviated with AZTEK. The same improvement in image quality is obtained in human, so details are more delineated in the lung with the use of the adaptive trajectory. Conclusion The AZTEK technique opens the possibility for 3D dynamic ZTE lung imaging with retrospective gating. It enables us to uniformly sample the k-space for any arbitrary respiratory motion gate, while preserving static image quality, improving dynamic image quality and guaranteeing continuous readout gradient transitions between spokes, which makes it appropriate to ZTE.

Although combined spin- and gradient-echo (SAGE) dynamic susceptibility-contrast (DSC) MRI can provide perfusion quantification that is sensitive to both macrovessels and microvessels while correcting for T1 -shortening effects, spatial coverage is often limited in order to maintain a high temporal resolution for DSC quantification. In this work, we combined a SAGE echo-planar imaging (EPI) sequence with simultaneous multi-slice (SMS) excitation and blipped controlled aliasing in parallel imaging (blipped CAIPI) at 3 T to achieve both high temporal resolution and whole brain coverage. Two protocols using this sequence with multi-band (MB) acceleration factors of 2 and 3 were evaluated in 20 patients with treated gliomas to determine the optimal scan parameters for clinical use. ΔR2 *(t) and ΔR2 (t) curves were derived to calculate dynamic signal-to-noise ratio (dSNR), ΔR2 *- and ΔR2 -based relative cerebral blood volume (rCBV), and mean vessel diameter (mVD) for each voxel. The resulting SAGE DSC images acquired using MB acceleration of 3 versus 2 appeared visually similar in terms of image distortion and contrast. The difference in the mean dSNR from normal-appearing white matter (NAWM) and that in the mean dSNR between NAWM and normal-appearing gray matter were not statistically significant between the two protocols. ΔR2 *- and ΔR2 -rCBV maps and mVD maps provided unique contrast and spatial heterogeneity within tumors.

Objectives To decipher the correlations between PET and DCE kinetic parameters in non-small-cell lung cancer (NSCLC), by using voxel-wise analysis of dynamic simultaneous [18F]FDG PET-MRI. Material and methods Fourteen treatment-naïve patients with biopsy-proven NSCLC prospectively underwent a 1-h dynamic [18F]FDG thoracic PET-MRI scan including DCE. The PET and DCE data were normalized to their corresponding T1-weighted MR morphological space, and tumors were masked semi-automatically. Voxel-wise parametric maps of PET and DCE kinetic parameters were computed by fitting the dynamic PET and DCE tumor data to the Sokoloff and Extended Tofts models respectively, by using in-house developed procedures. Curve-fitting errors were assessed by computing the relative root mean square error (rRMSE) of the estimated PET and DCE signals at the voxel level. For each tumor, Spearman correlation coefficients (rs) between all the pairs of PET and DCE kinetic parameters were estimated on a voxel-wise basis, along with their respective bootstrapped 95% confidence intervals (n = 1000 iterations). Results Curve-fitting metrics provided fit errors under 20% for almost 90% of the PET voxels (median rRMSE = 10.3, interquartile ranges IQR = 8.1; 14.3), whereas 73.3% of the DCE voxels showed fit errors under 45% (median rRMSE = 31.8%, IQR = 22.4; 46.6). The PET-PET, DCE-DCE, and PET-DCE voxel-wise correlations varied according to individual tumor behaviors. Beyond this wide variability, the PET-PET and DCE-DCE correlations were mainly high (absolute rs values > 0.7), whereas the PET-DCE correlations were mainly low to moderate (absolute rs values < 0.7). Half the tumors showed a hypometabolism with low perfused/vascularized profile, a hallmark of hypoxia, and tumor aggressiveness. Conclusion A dynamic “one-stop shop” procedure applied to NSCLC is technically feasible in clinical practice. PET and DCE kinetic parameters assessed simultaneously are not highly correlated in NSCLC, and these correlations showed a wide variability among tumors and patients. These results tend to suggest that PET and DCE kinetic parameters might provide complementary information. In the future, this might make PET-MRI a unique tool to characterize the individual tumor biological behavior in NSCLC.

Objectives: To decipher the interlinks between PET and DCE kinetic parameters in non-small-cell lung cancer (NSCLC), by using voxel-wise analysis of full dynamic simultaneous [18F]FDG PET-MRI. Material and methods: Fourteen treatment-naïve patients with biopsy proven NSCLC prospectively underwent a one-hour dynamic [18F]FDG thoracic PET-MRI scan including DCE. The PET and DCE data were normalized to their corresponding T1-weighted MR morphological space, and tumors were masked semi-automatically. Voxel-wise parametric maps of PET and DCE kinetic parameters were computed by fitting the dynamic PET and DCE tumor data to the Sokoloff and Extended Tofts models respectively, by using in-house developed procedures. Curve fitting errors were assessed by computing the relative root mean square error (rRMSE) of the estimated PET and DCE signals at the voxel level. For each tumor, Spearman correlation coefficients (r s ) between all the pairs of PET and DCE kinetic parameters were estimated on a voxel-wise basis, along with their respective bootstrapped 95% confidence intervals (n = 1000 iterations). Results: Curve fitting metrics provided fit errors under 20% for almost 90% of the PET voxels (median rRMSE,= 10.3, interquartile ranges IQR = 8.1; 14.3), whereas 73.3% of the DCE voxels showed fit errors under 45% (median rRMSE = 31.8%, IQR = 22.4; 46.6). The PET-PET, DCE-DCE and PET-DCE voxel-wise interlinks were heterogeneous, with variations related to individual tumor behaviors. Beyond this wide variability, the PET-PET and DCE-DCE interlinks were mainly high (absolute r s values > 0.7), whereas the PET-DCE interlinks were mainly low to moderate (absolute r s values < 0.7). Half the tumors showed an hypometabolic with low perfused/vascularized profile, a hallmark of hypoxia and tumor aggressiveness. Conclusion: The interlinks between dynamic PET and DCE kinetic parameters assessed simultaneously with [18F]FDG PET-MRI are heterogeneous among NSCLC tumors. In the era of precision medicine, PET-MRI has unique capability to go further insight the individual tumor behavior in NSCLC.

PurposeIn multi-echo fMRI (ME-fMRI), various weighting schemes have been proposed for the combination of the data across echoes. Here we introduce a framework that facilitates a deeper understanding of the weight dependence of temporal SNR measures in ME-fMRI.Theory and MethodsWe examine two metrics that have been used to characterize ME-fMRI performance: temporal SNR (tSNR) and multi-echo temporal (metSNR). Both metrics can be described using the generalized Rayleigh quotient (GRQ) and are predicted to be relatively insensitive to the weights when there is a high degree of similarity between a metric-specific matrix in the GRQ numerator and a metricindependent covariance matrix in the GRQ denominator. The application of the GRQ framework to experimental data is demonstrated using a resting-state fMRI dataset acquired with a multi-echo multi-band EPI sequence.ResultsIn the example dataset, similarities between the covariance matrix and the metSNR and tSNR numerator matrices are highest in grey matter (GM) and cerebrospinal fluid (CSF) voxels, respectively. For representative GM and CSF voxels that exhibit high matrix similarity values, the metSNR and tSNR values, respectively, are both within 4% of their optimal values across a range of weighting schemes. However, there is a fundamental tradeoff, with a high degree of weight sensitivity in the tSNR and metSNR metrics for the representative GM and CSF voxels, respectively. Geometric insight into the observed weight dependencies is provided through a graphical interpretation of the GRQ.ConclusionA GRQ framework can provide insight into the factors that determine the weight sensitivity of tSNR and metSNR measures in ME-fMRI.

Objectives: To decipher the metabolism/vascularization regional interlinks in NSCLC, by using full dynamic simultaneous 18F-FDG PET-MRI voxel-wise analysis. Material and methods: Fourteen treatment-naïve patients with biopsy proven NSCLC prospectively underwent a one-hour dynamic 18F-FDG thoracic PET-MRI including DCE. The PET and DCE data were normalized to a reference space, and masked semi-automatically. Full kinetic voxelwise parametric maps were computed by fitting the PET and DCE tumor data to state-of-the-art Sokoloff and Extended Tofts models respectively, by using in-house procedures. Spearman correlation coefficients (rs) between all the tumors kinetic parameters were estimated on a voxelwise basis, both at the whole dataset and tumor levels.Results: At the whole dataset level, the estimated K1, k2, k3, MRGlu and vb median values (IQR) were 0.25ml.min-1.ml-1 (0.17-0.33), 0.68 min-1 (0.48-1.02), 0.11 min-1 (0.07-0.2), 0.20 μmol.ml-1.min-1 (0.13-0.29) and 0.08 a.u (0.05-0.10). The estimated Ktrans, ve, Kep and vp median values (IQR) were 0.44 min-1 (0.14-0.92), 0.64 a.u (0.24-1.0), 0.79 min-1 (0.44-1.3), and 0.04 a.u (0.0007-0.19). At the whole dataset level, all the PET/DCE regional interlinks were weak (|rs

The goal of this paper was to assess the longitudinal repeatability of a bone identification method, based on the zero-echo-time MRI pulse sequence. The consistency of the bone maps—with potential applications in magnetic resonance-based attenuation correction of positron emission tomography data—was evaluated for a number of clinically realistic variations of the ideal acquisition conditions. The method was shown to be generally repeatable, with differences in the ±1% range. A number of potential issues were identified (e.g., sinus post-processing region placement variability, false positive bone structures near sharp coil sensitivity transitions, and axial coil coverage limitations) and mitigation strategies designed.

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.

Standard T2* weighted functional magnetic resonance imaging (fMRI) performed with echo-planar imaging (EPI) suffers from signal loss in the ventromedial prefrontal cortex (vmPFC) due to macroscopic field inhomogeneity. However, this region is of special interest to affective neuroscience and psychiatry. The Multi-echo EPI (MEPI) approach has several advantages over EPI but its performance against EPI in the vmPFC has not yet been examined in a study with sufficient statistical power using a task specifically eliciting activity in this region. We used a fear conditioning task with MEPI to compare the performance of MEPI and EPI in vmPFC and control regions in 32 healthy young subjects. We analyzed activity associated with short (12ms), standard (29ms) and long (46ms) echo times, and a voxel-wise combination of these three echo times. Behavioral data revealed successful differentiation of the conditioned versus safety stimulus; activity in the vmPFC was shown by the contrast "safety stimulus > conditioned stimulus" as in previous research and proved significantly stronger with the combined MEPI than standard single-echo EPI. Then, we aimed to demonstrate that the additional cluster extent (ventral extension) detected in the vmPFC with MEPI reflects activation in a relevant cluster (i.e., not just non-neuronal noise). To do this, we used resting state data from the same subjects to show that the time-course of this region was both connected to bilateral amygdala and the default mode network. Overall, we demonstrate that MEPI (by means of the weighted sum combination approach) outperforms standard EPI in vmPFC; MEPI performs always at least as good as the best echo time for a given brain region but provides all necessary echo times for an optimal BOLD sensitivity for the whole brain. This is relevant for affective neuroscience and psychiatry given the critical role of the vmPFC in emotion regulation.

International audience; Spirometry is today the gold standard technique for assessing pulmonary ventilatory function in humans. From the shape of a flow-volume loop measured while the patient is performing forced respiratory cycles, the Forced Vital Capacity (FVC) and the Forced Expiratory Volume in one second (FEV1) can be inferred, and the pulmonologist is able to detect and characterize common respiratory afflictions. This technique is non-invasive, simple, widely available, robust, repeatable and reproducible. Yet, its outcomes rely on the patient's cooperation and provide only global information over the lung. With 3D Magnetic Resonance (MR) Spirometry, local ventilation can be assessed by MRI anywhere in the lung while the patient is freely breathing. The larger dimensionality of 3D MR Spirometry advantageously allows the extraction of original metrics that characterize the anisotropic and hysteretic regional mechanical behavior of the lung. Here, we demonstrated the potential of this technique on a healthy human volunteer breathing along different respiratory patterns during the MR acquisition. These new results are discussed with lung physiology and recent pulmonary CT data. As respiratory mechanics inherently support lung ventilation, 3D MR Spirometry may open a new way to non-invasively explore lung function while providing improved diagnosis of localized pulmonary diseases.

PURPOSE Because of the intrinsic low signal-to-noise ratio in diffusion-weighted imaging (DWI), magnitude processing often causes an overestimation of the signal's amplitude. This results in low-estimation accuracy of diffusion models and reduced contrast because of a superposition of the image signal and the noise floor. We adopt a new phase correction (PC) technique that yields real valued diffusion data while maintaining a Gaussian noise distribution. METHODS We conduct simulations of the noise propagation in the echo-planar imaging reconstruction chain to determine the spatial noise correlation in the image. Using the correlation pattern, optimized filter kernels are derived to estimate the true phase of the signal in each voxel. Furthermore, we adopt an outlier detection technique to replace the real value by the magnitude in case of substantial signal loss resulting from incorrect PC. RESULTS The benefits of our method are demonstrated on Monte Carlo simulations, DWI data acquired from healthy volunteer experiments, estimated parameters of the diffusion kurtosis imaging model, and the model-free diffusion spectrum imaging technique. The improved PC approach significantly reduces the noise bias and only slightly increases the sensitivity to local phase variations. CONCLUSION PC can enhance the usefulness of higher b-values, allowing deeper insights into tissue microstructure. Magn Reson Med 77:559-570, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Objectif Evaluer les approches de relaxometrie T1/T2 sur dispositif TEP-IRM 3T applique a l’oncologie thoracique. Materiels et methodes Des approches de relaxometrie T1/T2 ont ete evaluees deux fois a une semaine d’intervalle sur le meme dispositif TEP-IRM 3T. Les erreurs de mesure des differentes methodes ont ete calculees par rapport aux valeurs de reference du fantome. La reproductibilite des methodes a ete evaluee en estimant les coefficients de correlation intra-classe (ICC, modele a effets mixtes, accord absolu). Pour la validation clinique, 8 patients ont realise le meme protocole TEP-IRM thoracique pour suspicion de cancer pulmonaire. Pour tous les patients, le protocole d’imagerie incluait une acquisition TEP, des acquisitions de diffusion et des acquisitions de relaxometrie T1/T2. Les donnees TEP, DWI et T1/T2 ont ete normalisees dans le meme espace de reference et les cartes parametriques multimodales ont ete generees (SUV, ADC, T1 et T2). Les relations multimodales des 4 marqueurs d’imagerie ont ete evaluees a l’echelle regionale intra lesionnelle en calculant les coefficients de Spearman. Resultats Les analyses sur fantome ont montre que l’approche de T1-mapping basee sur la saturation-recuperation (SR) etait superieure a toutes les autres methodes pour les valeurs T1 superieures a 500 ms (erreur SR 0,75 pour tous les couples TEP/IRM. Conclusion L’approche quantitative T1/T2-mapping est realisable en TEP-IRM thoracique oncologique. La majorite des couples de marqueurs TEP/IRM presentaient de faibles correlations, soulignant leur contribution respective pour la caracterisation lesionnelle au niveau regional.

Objectif Evaluer la faisabilite d’une approche TEP/IRM dynamique complete pour la caracterisation des lesions pulmonaires. Materiels et methodes Un total de 9 patients ont realise un protocole d’imagerie TEP-IRM dynamique complet d’une heure pour suspicion de cancer bronchique. Des analyses dynamiques completes metaboliques TEP (modele compartimental de Sokoloff) et perfusionnelles IRM (DCE, modele de Toft etendu), ainsi que des approches standards de diffusion (ADC) et T-mapping (relaxometrie T1/T2) ont ete realisees. Toutes les donnees TEP-IRM ont ete normalisees dans le meme espace de reference avant analyses. Pour chaque lesion pulmonaire, les cartes multiparametriques regionales de 14 caracteristiques biologiques (4 categories principales) ont ete generees a l’aide d’un pipeline de post-traitement multimodal entierement integre et specifiquement developpe a cet effet : perfusion/vascularisation (Ktrans, Kep, Ve et Vp), metabolisme (SUV, k1, k2, k3, Ki, Vb et MRGlu), diffusion (ADC) et caracterisation tissulaire (relaxometrie T1/T2). Pour chaque lesion, des matrices de correlation (Spearman) a l’echelle regionale ont ete generees. Enfin, toutes les lesions ont ete partitionnees par modele de melange gaussien non supervise et les profils de relations multimodales ont ete explores a l’echelle du supervoxel regional. Resultats Les relations entre les categories perfusion/vascularisation, metabolisme, densite cellulaire et caracteristiques tissulaires differaient d’une lesion a l’autre. A l’echelle des caracteristiques, de fortes correlations (valeur r absolue > 0,5) ont ete observees pour les couples perfusion/vascularisation et les couples de caracteristiques metaboliques. A l’echelle de la categorie, les valeurs de r absolues etaient inferieures a 0,5 pour la grande majorite des couples d’entites. En combinant les 14 parametres, le partionnement non supervise fournissait 2 a 4 supervoxels selon la lesion. Aux niveaux des supervoxels, les profils de caracteristiques differaient, ainsi que leurs relations monotones. Conclusion Notre protocole TEP-IRM dynamique complet generait 14 caracteristiques biologiques (4 categories principales) en un seul examen. L’heterogeneite des profils observee a l’echelle du supervoxel illustre la capacite unique de la TEP-IRM a evaluer les comportements biologiques des masses pulmonaires.

Objectif Les distorsions geometriques inherentes a l’approche EPI en imagerie de diffusion limitent les possibilites d’analyses multimodales a l’echelle du voxel en TEP-IRM. La correction des distorsions par inversion de codage de phase n’a jamais ete evaluee en IRM oncologique pulmonaire et, notamment, sur dispositif TEP-IRM 3 T. Materiel et methodes Apres validation de la methode sur fantome, douze patients ayant realise une TEP-IRM pour suspicion de cancer pulmonaire ont ete inclus et ont beneficie du meme protocole TEP-IRM thoracique : une acquisition TEP dynamique d’une heure realisee immediatement apres l’injection intraveineuse de 18F-FDG, avec plusieurs acquisitions IRM simultanees ; une serie d’acquisitions EPI avec polarite de codage de phase anteroposterieur (sens de la phase) et postero-anterieur (inversion du codage de phase) ; une serie d’acquisitions d’imagerie de diffusion (DWI, b = 20, 500, 800 s/mm2) ; une sequence rapide GRE ponderee T1 avec destruction de l’aimantation transversale (3D-T1). Les donnees DWI ont ete normalisees dans leur espace de reference (3D-T1) et corrigees de la distorsion geometrique (DWIcorr) en utilisant la methode d’inversion de codage de phase. Pour chaque patient, la qualite du realignement spatial des images DWI (non et corrigees des distorsions) sur leur image 3D-T1 de reference a ete evaluee quantitativement par information mutuelle et comparee. Les cartes d’ADC issues des images DWI non corrigees et corrigees des distorsions ont ete comparees a l’echelle du voxel. Finalement, pour chaque patient, les cartes parametriques d’ADC corrigees des distorsions et de SUV (derniere frame de 5 minutes de la TEP dynamique) ont ete confrontees a l’echelle du voxel (correlations de Spearman). Resultats Le gain relatif de la qualite de realignement des images DWI et 3D-T1 etait 4–46 % apres correction des distorsions et normalisation, versus 0,4–27 % pour les donnees DWI normalisees mais non corrigees, p Conclusion Les distorsions en imagerie DWI sont significatives sur dispositif TEP-IRM 3 T, et doivent etre corrigees pour des analyses multimodales precises en oncologie thoracique.

Purpose Diffusional kurtosis imaging (DKI) is an approach to characterizing the non-Gaussian fraction of water diffusion in biological tissue. However, DKI is highly susceptible to the low signal-to-noise ratio of diffusion-weighted images, causing low precision and a significant bias due to Rician noise distribution. Here, we evaluate precision and bias using weighted linear least squares fitting of different acquisition schemes including several multishell schemes, a diffusion spectrum imaging (DSI) scheme, as well as a compressed sensing reconstruction of undersampled DSI scheme. Methods Monte Carlo simulations were performed to study the three-dimensional distribution of the apparent kurtosis coefficient (AKC). Experimental data were acquired from one healthy volunteer with multiple repetitions, using the same acquisition schemes as for the simulations. Results The angular distribution of the bias and precision were very inhomogeneous. While axial kurtosis was significantly overestimated, radial kurtosis was underestimated. The precision of radial kurtosis was up to 10-fold lower than axial kurtosis. Conclusion The noise bias behavior of DKI is highly complex and can cause overestimation as well as underestimation of the AKC even within one voxel. The acquisition scheme with three shells, suggested by Poot et al, provided overall the best performance. Magn Reson Med 76:1684–1696, 2016. © 2016 International Society for Magnetic Resonance in Medicine

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.

Introduction Au cours des examens d’imagerie, les lesions tumorales peuvent etre soumises au mouvement respiratoire du patient, qui peut affecter la detection et la quantification des lesions viscerales. L’objectif de ce travail etait d’evaluer l’impact de 2 methodes de correction de mouvement disponibles sur le SIGNA TEP/IRM au cours des bilans oncologiques de « routine ». Materiels et methodes Quarante patients ont beneficie d’une TEP/IRM au FDG (Signa, GE Healthcare, 107 ± 15 min post-injection, 4 MBq/kg, 4–6 pas, 3–4 min/pas, temps de vol, reconstruction OSEM 2 it/28 sous-ensembles, sans post-filtrage), au cours d’un bilan oncologique. Le signal respiratoire etait enregistre a l’aide d’une ceinture abdominale. Deux approches de compensation du mouvement respiratoire ont ete evaluees : Q.Static, exploitant uniquement la phase quiescente, et Q.Freeze, recalant l’ensemble des phases sur une phase de reference. Toutes les lesions sujettes au mouvement respiratoire (thorax et foie), clairement distinctes des lesions adjacentes ou du bruit de fond environnant, ont ete incluses pour analyse visuelle et semi-quantitative (SUVmax et SUVpeak). Le niveau de bruit dans les images TEP a ete evalue par l’ecart-type (SD) du SUV divise par la moyenne du SUV dans une region du parenchyme hepatique. Resultats Quatre-vingt-douze lesions ont ete incluses : 41 nodules pulmonaires, 23 ganglions lymphatiques mediastinaux et 28 lesions hepatiques. Un flou visuel a ete detecte dans 30 lesions (30/92). La methode Q.Freeze s’est averee visuellement satisfaisante dans la quasi-totalite des lesions floues (n = 29/30, soit 97 %), alors que la methode Q.Static dans seulement 27 % (8/30). Lorsque les lesions apparaissaient floues du fait du mouvement respiratoire, Q.Static et Q.Freeze augmentaient significativement les valeurs de SUVpeak par rapport aux images non corrigees : +4,1 % [−3,4 ;10,8] avec Q.Static et + 9,4 % [3,9 ;18,2] avec Q.Freeze (p Conclusion Les methodes Q.Static et Q.Freeze sont utilisables sur des protocoles de routine en TEP/IRM et corrigent significativement du mouvement respiratoire les lesions floues sur les images statiques.

Purpose: To assess cardiac function by means of a novel free-breathing cardiac magnetic resonance imaging (MRI) strategy. Materials and Methods: A stack of ungated 2D steady-state free precession (SSFP) slices was acquired during free breathing and reconstructed as cardiac cine imaging based on the generalized reconstruction by inversion of coupled systems (GRICS). A motion-compensated sliding window approach allows reconstructing cine movies with most motion artifacts cancelled. The proposed reconstruction uses prior knowledge from respiratory belts and electrocardiogram recordings and features a piecewise linear model that relates the electrocardiogram signal to cardiac displacements. The free-breathing protocol was validated in six subjects against a standard breath-held protocol. Results: Image sharpness, as assessed by the image gradient entropy, was comparable to that of breath-held images and significantly better than in uncorrected images. Volumetric parameters of cardiac function in the left ventricle (LV) and right ventricle (RV) were similar, including end-systolic volumes, end-diastolic volumes and mass, stroke volumes, and ejection fractions (with differences of 3% ± 2.4 in the LV and 2.9% ± 4.4 in the RV). The duration of the free-breathing protocol was nearly the same as the breath-held protocol. Conclusion: Free-breathing cine-GRICS enables accurate assessment of volumetric parameters of cardiac function with efficient correction of motion. J. Magn. Reson. Imaging 2012;340-351. © 2011 Wiley Periodicals, Inc.

Black Blood Fast Spin Echo imaging of the heart is usually performed during mid-diastolic rest. This is a direct consequence of the long inversion time required to suppress the blood signal, which is constrained by the T(1) of the blood, and of the heart rate. To overcome these constraints, and to acquire black blood images in the end-systolic rest period, a new approach is introduced aiming at adaptively predicting the best time to prepare and acquire MR signals. It is based on a RR interval prediction algorithm and on a cardiac cycle model. The proposed method was applied to 14 healthy volunteers and is compared to a simple alternative method using a fixed delay and to the standard black blood imaging method for imaging in the mid-diastolic rest period. Results show that the proposed method offers an increased robustness in terms of trigger delay error and image quality compared to the tested simple alternative. Also, it has been shown by qualitative analysis done by an experienced observer that the right ventricle, especially the thin right ventricle free wall, is better depicted with our method than with the standard mid-diastolic rest acquisition.

Diffusional kurtosis imaging (DKI) is an approach to characterizing the non-Gaussian fraction of water diffusion in biological tissue. However, DKI is highly susceptible to the low signal-to-noise ratio of diffusion-weighted images, causing low precision and a significant bias due to Rician noise distribution. Here, we evaluate precision and bias using weighted linear least squares fitting of different acquisition schemes including several multishell schemes, a diffusion spectrum imaging (DSI) scheme, as well as a compressed sensing reconstruction of undersampled DSI scheme.Monte Carlo simulations were performed to study the three-dimensional distribution of the apparent kurtosis coefficient (AKC). Experimental data were acquired from one healthy volunteer with multiple repetitions, using the same acquisition schemes as for the simulations.The angular distribution of the bias and precision were very inhomogeneous. While axial kurtosis was significantly overestimated, radial kurtosis was underestimated. The precision of radial kurtosis was up to 10-fold lower than axial kurtosis.The noise bias behavior of DKI is highly complex and can cause overestimation as well as underestimation of the AKC even within one voxel. The acquisition scheme with three shells, suggested by Poot et al, provided overall the best performance. Magn Reson Med 76:1684-1696, 2016. © 2016 International Society for Magnetic Resonance in Medicine.

The nCPMG sequence is based on a particular phase modulation of the refocusing pulse train, and was originally designed for rendering the spin echo amplitude insensitive to the initial magnetization phase. This pulse sequence has the peculiarity of being easily invertible, which enables perfect driven equilibrium experiments, in the absence of relaxation. This magnetization 'realignment' is effective for all three components. Hence the overall operation is transparent. Supporting theory is presented here, together with the first direct experimental proof of the claim. The experiment shows that, with the present stabilization sequence, perfect realignment is indeed made possible for a range of refocusing pulse nutation angles from 130° to 230°.

International audience; In clinical applications, cardiac gated sequences are commonly used for heart imaging. In practice, we simply wait a fixed time called Trigger Delay (TD) after the R-wave has been detected to acquire data in the chosen cardiac phase. The main problem of this method is that the TD is constant and does not take into account the physiological variability such as instantaneous heart rate changes during breath-hold or free breathing. Another issue is that we cannot use a TD shorter than the preparation time. Consequently when a cardiac gated sequence with a long preparation time is used, only diastolic images are achievable. Double Inversion Recovery Fast Spin Echo (DIR-FSE), resulting in black-blood images of the heart [1], is such a sequence since the inversion time (TI) needed to cancel the blood signal is around 500ms. Systolic view of the heart could also be of clinical interest, especially during end-systolic phase when the cardiac volume is minimum and constant (average TD = 300ms, average duration 60ms [2]). The primary aim of this work was to assess a robust method to acquire black blood FSE in end-systolic phase. For this purpose, the DIR preparation has to be launched before the R-wave in the previous cardiac cycle. A RR interval prediction is then needed and heart rate variability has to be accounted in order to position the acquisition window properly. A new general adaptive method is described here that overcomes the above listed limitations in prospectively cardiac gated sequences with long preparation time using a predictive model. This method has been applied on end-systolic black blood FSE images.

L'imagerie par résonance magnétique (IRM) du coeur reste difficile. Les solutions retenues dans la pratique clinique pour limiter les problèmes liés aux mouvements sont l'apnée et la synchronisation cardiaque. En imagerie cardiaque, la séquence Double Inversion Recovery Fast Spin Echo (DIR-FSE) est couramment utilisée [1]. Cependant, cette séquence, synchronisée avec l'activité cardiaque, nécessite un temps de préparation long (TI). Le temps entre l'onde R et l'acquisition (TD) est donc également long. Par conséquent, cette séquence n'est utilisée qu'en diastole (figure 1-a). De plus, les variations de rythme cardiaque instantanées (temps entre 2 ondes R consécutives) ne sont pas prises en compte. Afin de passer outre ces limitations, nous proposons deux techniques pour acquérir les séquences DIR-FSE en fin de systole (relaxation iso-volumétrique, volume cardiaque minimum et constant). Ces acquisitions en fin de systole devraient nous permettre de mieux voir le ventricule droit en raison de sa contraction. Les deux méthodes proposées seront comparées entre elles en terme de robustesse. Elles seront également comparées à la méthode conventionnelle en terme de visualisation du ventricule droit.

International audience; Double Inversion Recovery Fast Spin Echo (DIR-FSE) sequences, resulting in black-blood images, are widespread in cardiac exams [1] as high resolution and great contrast between heart cavities and walls are achieved. For these sequences, DIR pulses are generally played on the R wave and the inversion time (TI) needed to cancel blood signal (~500ms) only allows diastolic view of the heart. Cardiac contraction is complete at end systole so myocardium can be better observed during this short phase. To perform systolic acquisitions, DIR pulses have to be placed before the R wave in the previous heart cycle and RR interval prediction is mandatory. Moreover, as systole is generally shorter than diastole, shorter echo trains are desirable, resulting in longer acquisition time incompatible with breath hold. A method which combines (i) heart rate prediction [2], (ii) respiratory motion estimation [3] and (iii )motion compensated construction [4] is presented. It allows free breathing black-blood systolic imaging and has been tested on five subjects.

International audience; Cardiac MRI is still challenging due to the perpetual heart motion. In order to avoid cardiac motion artifacts, acquisitions are synchronized with heart activity, generally by triggering on R-waves of the Electrocardiogram (ECG). Double Inversion Recovery Fast Spin Echo (DIR-FSE) sequences, resulting in black-blood images, require furthermore specific timing. For such acquisitions, inversion time (TI) is needed to cancel blood signals (~500ms) [1], making acquisitions in the first 500ms of the cardiac cycle impossible. Black-blood imaging allows then only diastolic view of the heart. Moreover heart rate (HR) variability has been shown to be important during breath-holds [2] making cardiac synchronization even more complicated. In this paper, a RR interval prediction method has been implemented, which permits to launch DIR pulses before R waves and thus enables black blood systolic imaging.

National audience; L'imagerie par résonance magnétique (IRM) du coeur reste difficile. Les solutions retenues dans la pratique clinique pour limiter les problèmes liées aux mouvements sont l'apnée et la synchronisation cardiaque. En imagerie cardiaque, la séquence Double Inversion Récupération Fast Spin Echo (DIR- FSE) est couramment utilisée [1]. Cependant, cette séquence, synchronisée avec l'activité cardiaque, nécessite un temps de préparation long (TI) ce qui contraint le temps entre l'onde R et l'acquisition (TD) à être long. Par conséquent, cette séquence n'est utilisée qu'en diastole. De plus, les variations de rythme cardiaque instantané (temps entre 2 ondes R consécutives) ne sont pas prises en compte (Figure 1-A et 1-B). Afin de passer outre ces limitations, nous désirons mettre en oeuvre une technique adaptative pour utiliser les séquences DIR-FSE en systole (volume cardiaque minimum et constant). Ceci a deux buts : 1. étudier et diagnostiquer certaines maladies en systole et 2. étudier les propriétés magnétiques du coeur en systole et diastole.