Brice Fernandez

Free‐breathing imaging of the heart using 2D cine‐GRICS (generalized reconstruction by inversion of coupled systems) with assessment of ventricular volumes and function

To assess cardiac function by means of a novel free‐breathing cardiac magnetic resonance imaging (MRI) strategy.

Real valued diffusion-weighted imaging using decorrelated phase filtering.

Because of the intrinsic low signal‐to‐noise ratio in diffusion‐weighted imaging (DWI), magnitude processing often causes an overestimation of the signals 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.

Subject-specific bone attenuation correction for brain PET/MR: can ZTE-MRI substitute CT scan accurately?

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.

Adaptive black blood fast spin echo for end-systolic rest cardiac imaging†

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 T1 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. Magn Reson Med, 2010.

Multi-echo EPI of human fear conditioning reveals improved BOLD detection in ventromedial prefrontal cortex

&NA; Standard Symbol 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 (12 ms), standard (29 ms) and long (46 ms) 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. Symbol. No caption available.

Bias and Precision Analysis of Diffusional Kurtosis Imaging for Different Acquisition Schemes

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.

Exploring the relation between MR ZTE intensity and tissue density: Application to MR attenuation correction in PET/MR

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.