Biomedical subjects

Patrick Le Roux

Publications and source records attributed to Patrick Le Roux.

6 recordsLinked to original sources

PubMed2005

Diffusion tensor imaging of the human optic nerve using a non-CPMG fast spin echo sequence.

PURPOSE: To investigate the diffusion tensor properties of the human optic nerve in vivo using a non-Carr-Purcell-Meiboom-Gill (CPMG) fast spin echo (FSE) sequence. MATERIALS AND METHODS: This non-CPMG FSE sequence, which is based on a quadratic phase modulation of the refocusing pulses, allows diffusion measures to be acquired with full signal and without artifacts from geometric distortions due to magnetic field inhomogeneities, which are among the main problems encountered in the orbital area. RESULTS: Good-quality images were obtained at a resolution of 0.94 x 0.94 x 3 mm. The mean diffusivity (MD) and fractional anisotropy (FA) were respectively 1.1 +/- 0.2 x 10(-3) mm(2)/second and 0.49 +/- 0.06, reflecting the optic nerve anisotropy. CONCLUSION: This non-CPMG-FSE sequence provides reliable diffusion-weighted images of the human optic nerve. This approach could potentially improve the diagnosis and management of optic nerve diseases or compression, such as optic neuritis, orbit tumors, and muscle hypertrophy.

Algorithms↗

PubMed2003

Use of fast spin echo for phase shift magnetic resonance thermometry.

PURPOSE: To propose a modified fast spin echo (FSE) magnetic resonance imaging sequence for MR thermometry, employing the proton resonance frequency (PRF) shift by means of MR phase maps. Despite their obvious advantages of speed and high signal-to-noise ratio (SNR), FSE sequences have not until now been used for this purpose due to the restraints imposed by the Carr-Purcell-Meiboom-Gill (CPMG) conditions. MATERIALS AND METHODS: The new FSE combines a new phase modulation scheme that maintains magnetization that ordinarily is destroyed under CPMG conditions, while employing conventional FSE gradient waveforms. The echoes are read in a single shot using 128 readouts in 650 msec, with a phase sensitive preparation using an optional time shift tau before the start of the refocusing gradient waveforms. This feature allows the quantification of temperature dependent phase shifts. We tested the sequence by imaging a heated agar gel phantom while cooling, using different values for tau. RESULTS: There was good correlation between FSE and fiberoptic-based temperature measurements in the phantom(r(2) >or= 0.95). Temperature sensitivity could be adjusted by varying the tau value. CONCLUSION: With the proposed non-CPMG FSE sequence it is feasible to quantify temperature changes by means of the PRF shift.

Humans↗

PubMed2003

Simplified model and stabilization of SSFP sequences.

Steady-state free precession (SSFP) is used today in a form similar to other rapid sequences like fast spin echo (FSE) where a large longitudinal magnetization is present at the beginning of the train of excitations. This results in a transient behavior which impedes any measurement before the steady state is established. Several solutions have been proposed to stabilize the signals more quickly. Starting from a simplified model of signal generation, and by a suitable change of reference frame, this paper justifies theoretically the linear ramp-up proposed by Nishimura and Vasanawala (p. 301, 8th Annual Proceedings of ISMRM, 2000, Denver). This linear ramp-up can be generalized into a one giving less oscillatory residues. The solution is efficient in the sense that it does not require nutation angles larger than the one used during the stabilized period. Also, this solution is robust because it scales up or down nicely and is thus insensitive to B(1) variations.

Journal Article↗

PubMed2002

Reproducibility of left ventricular mass measurement using a half-Fourier black-blood single-shot fast spin-echo sequence within a single breath hold: comparison with a conventional multiple breath-hold segmented gradient echo technique in patients.

PURPOSE: To compare the reproducibility of left ventricular (LV) mass measurements using a black-blood half-Fourier single-shot fast spin-echo (SSFSE) and a segmented gradient echo magnetic resonance (MR) pulse sequence. MATERIAL AND METHODS: Breath-hold SSFSE and segmented gradient echo cardiac MR examinations were performed twice in 32 patients and manual detection of the LV endocardium and epicardium was applied by two blinded reviewers. The SSFSE pulse sequence allowed whole-heart coverage in a single breath hold, while multiple breath holds were required using the segmented gradient echo sequence. Spatial presaturation slabs were used with the SSFSE pulse sequence to reduce the field of view (FOV) and thereby achieve higher spatial resolution. RESULTS: Intraclass correlation coefficients were higher with the SSFSE pulse sequence than with the segmented gradient echo pulse sequence: intraobserver reproducibility reached 0.999 vs. 0.991; interobserver reproducibility: 0.997 vs. 0.981; and interstudy reproducibility: 0.998 vs. 0.936. These higher levels of reproducibility were confirmed on Bland and Altman plots. CONCLUSION: LV mass measurements can be assessed more reproducibly with the single breath-hold SSFSE technique than with the standard multiple breath-hold segmented gradient echo method.

Adult↗

PubMed2002

On the application of a non-CPMG single-shot fast spin-echo sequence to diffusion tensor MRI of the human brain.

The strong sensitivity of Carr-Purcell-Meiboom-Gill (CPMG) fast spin-echo (FSE) sequences, such as rapid acquisition with relaxation enhancement (RARE), to the phase of the prepared transverse magnetization means that artifact-free single-shot diffusion-weighted images can currently only be obtained with a 30-50% reduction in the signal-to-noise ratio (SNR). However, this phase sensitivity and signal loss can be addressed in FSE sequences that use quadratic phase modulation of the radiofrequency (RF) refocusing pulses to generate a sustained train of stable echoes. Here the first application of such a non-CPMG single-shot FSE (ssFSE) sequence to diffusion tensor MR imaging (DT-MRI) of the human brain is described. This approach provides high SNR diffusion-weighted images that have little or no susceptibility to poor B(0) magnetic field homogeneity and the strong eddy currents typically present in DT-MRI experiments.

Brain↗

PubMed2002

Non-CPMG Fast Spin Echo with full signal.

The standard Fast Spin Echo sequence used in MR imaging relies on the CPMG condition. A consequence of this condition is that only one component of the transverse magnetization can be measured. To counter this, some phase modulation schemes (XY, MLEV.) for the pulse train have been proposed, but they are useful only over a very restricted range, close to pi, of the refocusing pulse rotation angle. Some other solutions not relying on phase modulation have also been suggested, but they destroy one half the available signal. Revisiting the phase modulation approach, J. Murdoch ("Second SMR Scientific Meeting," p. 1145, 1994) suggested that a quadratic phase modulation could generate a train of classical echoes. We show here that indeed a quadratic phase modulation has a very suitable property: after an adequate change of frame, the dynamic of the system composed of all the protons situated in one pixel can be seen as stationary. If the parameter of the quadratic phase modulation is well chosen, it is then possible to put the dynamic system in a combination of two suitable states and obtain a signal identical to the signal of a classical spin echo, at least for nutation of the refocusing pulse higher than, approximately, two radians.

Echo-Planar Imaging↗