Gwenael Herigault

Dual-echo Dixon imaging with flexible choice of echo times.

In this work, a new two‐point method for water–fat imaging is described and explored. It generalizes existing two‐point methods by eliminating some of the restrictions that these methods impose on the choice of echo times. Thus, the new two‐point method promises to provide more freedom in the selection of protocol parameters and to reach higher scan efficiency. Its performance was studied theoretically and was evaluated experimentally in abdominal imaging with a multigradient‐echo sequence. While depending on the choice of echo times, it is generally found to be favorable compared to existing two‐point methods. Notably, water images with higher spatial resolution and better signal‐to‐noise ratio were attained with it in single breathholds at 3.0 T and 1.5 T, respectively. The use of more accurate spectral models of fat is shown to substantially reduce observed variations in the extent of fat suppression. The acquisition of in‐ and opposed‐phase images is demonstrated to be replaceable by a synthesis from water and fat images. The new two‐point method is finally also applied to autocalibrate a multidimensional eddy current correction and to enhance the fat suppression achieved with three‐point methods in this way, especially toward the edges of larger field of views. Magn Reson Med, 2010.

Swallowing, arterial pulsation, and breathing induce motion artifacts in carotid artery MRI

To identify and quantify the potential sources of motion in carotid artery imaging.

Feasibility of fast MR-thermometry during cardiac radiofrequency ablation.

Online MR temperature monitoring during radiofrequency (RF) ablation of cardiac arrhythmias may improve the efficacy and safety of the treatment. MR thermometry at 1.5 T using the proton resonance frequency (PRF) method was assessed in 10 healthy volunteers under normal breathing conditions, using a multi‐slice, ECG‐gated, echo planar imaging (EPI) sequence in combination with slice tracking. Temperature images were post‐processed to remove residual motion‐related artifacts. Using an MR‐compatible steerable catheter and electromagnetic noise filter, RF ablation was performed in the ventricles of two sheep in vivo. The standard deviation of the temperature evolution in time (TSD) was computed. Temperature mapping of the left ventricle was achieved at an update rate of approximately 1 Hz with a mean TSD of 3.6 ± 0.9 °C. TSD measurements at the septum showed a higher precision (2.8 ± 0.9 °C) than at the myocardial regions at the heart–lung and heart–liver interfaces (4.1 ± 0.9 °C). Temperature rose maximally by 9 °C and 16 °C during 5 W and 10 W RF applications, respectively, for 60 s each. Tissue temperature can be monitored at an update rate of approximately 1 Hz in five slices. Typical temperature changes observed during clinical RF application can be monitored with an acceptable level of precision. Copyright

Diffusion-weighted magnetic resonance imaging of the liver using tracking only navigator echo: feasibility study.

Purpose:To introduce and assess the TRacking Only Navigator echo (TRON) technique for diffusion-weighted magnetic resonance imaging (DWI) of the liver. Subjects and Methods:A total of 10 volunteers underwent TRON, respiratory triggered (RT), and free breathing (FB) DWI of the liver. Scan times of TRON and RT DWI were measured, and image sharpness in TRON, RT, and FB DWI was assessed and compared using nonparametric tests. Furthermore, 14 patients with liver metastasis who had undergone TRON and RT DWI of the liver were retrospectively assessed. Relative contrast ratios (RCRs) and apparent diffusion coefficients (ADCs) of the largest hepatic metastasis in TRON and RT DWI were measured. RCRs were compared using a parametric test and agreement in ADCs was assessed using the Bland-Altman method. Results:In the volunteers, mean scan times of TRON and RT relative to FB DWI were 110% to 112% and 261% to 290%, respectively. On axial images, there were no significant differences in images sharpness among TRON, RT, and FB DWI, but on coronal images image sharpness in TRON was nearly always significantly better (P < 0.05) than in RT and FB DWI. In the patients, mean RCRs between TRON and RT DWI were not significantly different (P = 0.9091). Mean difference in ADC ± limits of agreement (in 10−3 mm2/s) between TRON and RT DWI was −0.16 ± 0.79. Conclusion:TRON offers sharp diffusion-weighted images of the liver using an efficient scan time, making it an excellent alternative to RT and FB DWI. The moderate to poor agreement in ADCs of liver metastases between TRON and RT DWI requires further investigation.

Three-dimensional contrast-enhanced hepatic MR imaging: comparison between a centric technique and a linear approach with partial Fourier along both slice and phase directions.

To compare the image quality of two variants of a three‐dimensional (3D) gradient echo sequence (GRE) for hepatic MRI.

Multi-spin-echo J-resolved spectroscopic imaging without water suppression: application to a rat glioma at 7 T

Two-dimensional J-resolved spectroscopy may be used to separate resonances which overlap in 1D NMR spectra. Coupled with spectroscopic imaging (SI), it would give unequivocal information on the distribution of such resonances. Multi-echo acquisition decreases the minimum experimental time of such 4D experiments. The water peak may be used for phase and chemical-shift reference. This study aimed to demonstrate the feasibility of J-resolved SI based on a multi-echo sequence and without water suppression, and its ability to separate the peaks for lactate and mobile lipid in a rat glioma. Experiments were performed on rat brain, without water suppression, at 7 T. The water signal was used for correcting the phase of the echoes. A FOCSY-like acquisition was used to collect the first part of the echoes at short echo times. Two different data processing methods were tested to overcome the problem of contaminations of metabolite signals by the intense water signal. Maps of N-acetylaspartate, choline, creatine, lactate and mobile lipids were obtained in vivo on a rat glioma in 70 min. The in-plane resolution was 2 mm2. The 2D spatially resolved, 2D J-resolved spectra enabled the separate mapping of lactate and mobile lipids.

Modified electrocardiograph-triggered black-blood turbo spin-echo technique to improve T1-weighting in contrast-enhanced MRI of atherosclerotic carotid arteries

To assess the efficacy of a modified electrocardiograph (EKG)‐triggered black‐blood T1W (T1W) spin‐echo sequence in improving contrast on post‐gadolinium high‐resolution carotid plaque imaging by implementing heart‐rate–independent contrast preparation.

A Method for Large Vessels/Brain Activity Colocalization

FMRI is a technique using BOLD contrast to identify areas of cerebral activity. Large vessels contribution to this signal induces a delay in the measured response. It is therefore imperative to precisely colocalize brain activity and large veins. MR venography is an acquisition procedure using phase variations to reinforce cerebral vasculature on anatomical data. In this article, we present a method to register brain large veins and activation map in order to achieve an accurate interpretation of measured activity.