Hervé Saint-Jalmes

Transient decrease in water diffusion observed in human occipital cortex during visual stimulation

Using MRI, we report the observation of a transient decrease of the apparent diffusion coefficient (ADC) of water in the human brain visual cortex during activation by a black and white 8-Hz-flickering checkerboard. The ADC decrease was small (<1%), but significant and reproducible, and closely followed the time course of the activation paradigm. Based on the known sensitivity of diffusion MRI to cell size in tissues and on optical imaging studies that have revealed changes in the shape of neurons and glial cells during activation, the observed ADC findings have been tentatively ascribed to a transient swelling of cortical cells. These preliminary results suggest a new approach to produce images of brain activation with MRI from signals directly associated with neuronal activation, and not through changes in local blood flow.

Low-coherence in-depth microscopy for biological tissue imaging: design of a real-time control system

We describe the design of a versatile electronic system performing a lock-in detection in parallel on every pixel of a 2D CCD camera. The system is based on a multiplexed lock- in detection method that requires accurate synchronization of the camera, the excitation signal and the processing computer. This device has been incorporated in an imaging setup based on the optical coherence tomography principle, enabling to acquire a full 2D head-on image without scanning. The imaging experiment is implemented on a modified commercial microscope. Lateral resolution is on the order of 2 micrometers , and the coherence length of the light source defines an axial resolution of approximately 8 micrometers . Images of onion cells a few hundred microns deep into the sample are obtained with 100 dB sensitivity.

MR monitoring of tumour thermal therapy

Thermal therapy of tumour including hyperthermia and thermal ablation by heat or cold delivery requires on line monitoring. Due to its temperature sensitivity, Magnetic Resonance Imaging (MRI) allows thermal mapping at the time of the treatment. The different techniques of MR temperature monitoring based on water proton resonance frequency (PRF), longitudinal relaxation time Tl, diffusion coefficient and MR Spectroscopic Imaging (MRSI) are reviewed and debated. The PRF method appears the most widely used and the most efficient at high magnetic field in spite of important drawbacks. The Tl method is the easiest method of visualisation of qualitative temperature distribution and quantitative measurement seems possible in the tissue surrounding the tumour up to a temperature of 45–65°C. Despite its high temperature sensitivity, application of the diffusion method in vivo is restricted due to its high motion sensitivity. The recent MRSI technique seems very promising provided acquisition times can be reduced. Results from the literature indicate that MR temperature monitoring in vivo can be achieved in vivo with a precision of about 3°C in 13 s for a voxel of 16 mm3 (1.5 × 1.5 × 7 mm) in 1.5 T scanners.

Description and time reduction of a Monte Carlo code to simulate propagation of polarized light through scattering media

Propagation of polarized light through a scattering medium has been studied with a Monte Carlo code to obtain polarized backscattered images. Studies of these backscattered patterns obtained with polarized illumination can be used as a technique to characterize the medium anisotropy factor g. First we present the different steps of the Monte Carlo simulation that describe polarized light propagation in a turbid medium. Monte Carlo is a good tool to simulate the backscattered polarized light but is time-consuming. Therefore, we consider two ways to decrease the computation time. The first way deals with angle sampling of the light direction. The second takes advantage of backscattered image symmetry to divide the simulation time by a factor of 4. By combining these two techniques we significantly decrease the code computation time.

MR temperature measurement in liver tissue at 0.23 T with a steady-state free precession sequence.

MRI can be used for monitoring temperature during a thermocoagulation treatment of tumors. The aim of this study was to demonstrate the suitability of a 3D steady‐state free precession sequence (3D Fast Imaging with Steady‐State Precession, 3D TrueFISP) for MR temperature measurement at 0.23 T, and to compare it to the spin‐echo (SE) and spoiled 3D gradient‐echo (3D GRE) sequences. The optimal flip angle for the TrueFISP sequence was calculated for the best temperature sensitivity in the image signal from liver tissue, and verified from the images acquired during the thermocoagulation of excised pig liver. Factors influencing the accuracy of the measured temperatures are discussed. The TrueFISP results are compared to the calculated values of optimized SE and 3D GRE sequences. The accuracy of TrueFISP in the liver at 0.23 T, in imaging conditions used during thermocoagulation procedures, is estimated to be ±3.3°C for a voxel of 2.5 × 2.5 × 6 mm3 and acquisition time of 18 s. For the SE and GRE sequences, with similar resolution and somewhat longer imaging time, the uncertainty in the temperature is estimated to be larger by a factor of 2 and 1.2, respectively. Magn Reson Med 47:940–947, 2002.

Implantable planar rf microcoils for NMR microspectroscopy

This communication reports the design, the fabrication and preliminary tests of planar microcoils used as nuclear magnetic resonance (NMR) radio frequency (rf) detection coils. The magnetic coupling between a rf coil and a sample is maximized by physically designing the coil as small as possible as to just accommodate the sample. Maximum coupling corresponds directly to an optimal sensitivity for the signal reception. Designed for a biological application, these coils have to be as less invasive as possible and they must allow studying small quantities of metabolites surrounding the microcoil. NMR microspectroscopy experiments have been performed in water and butter samples using a 500mm � 1000 mm planar microcoil tuned and matched at 85.13 MHz (proton’s frequency at 2 T). # 2002 Elsevier Science B.V. All rights reserved.

Can Dynamic Contrast-Enhanced Magnetic Resonance Imaging Combined with Texture Analysis Differentiate Malignant Glioneuronal Tumors from Other Glioblastoma?

An interesting approach has been proposed to differentiate malignant glioneuronal tumors (MGNTs) as a subclass of the WHO grade III and IV malignant gliomas. MGNT histologically resemble any WHO grade III or IV glioma but have a different biological behavior, presenting a survival twice longer as WHO glioblastomas and a lower occurrence of metastases. However, neurofilament protein immunostaining was required for identification of MGNT. Using two complementary methods, dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and texture analysis (MRI-TA) from the same acquisition process, the challenge is to in vivo identify MGNT and demonstrate that MRI postprocessing could contribute to a better typing and grading of glioblastoma. Results are obtained on a preliminary group of 19 patients a posteriori selected for a blind investigation of DCE T1-weighted and TA at 1.5 T. The optimal classification (0/11 misclassified MGNT) is obtained by combining the two methods, DCE-MRI and MRI-TA.

Two-point method for T1 estimation with optimized gradient-echo sequence

Relaxation times estimation methods play a central role in various problems, such as magnetic resonance (MR) hardware calibration, tissue characterization, or temperature measurement. Previous studies have proposed optimization criteria to estimate the relaxation time T1 faster than with a multipoint method leading to two-point methods. In this paper, the class of optimized two-point methods is extended to gradient-echo (GE) sequence offering new advantages over spin-echo (SE) or inversion recovery (IR) sequences. Two GE acquisitions, with optimal flip angles theta1 and theta2 minimizing both the total scan time and the variance in the computed T1 image were applied to estimate T1, and the results were compared with those of SE sequence with optimized paired repetition times T(R1) and T(R2). First, phantom studies were carried out with five tissue-like samples on a 0.5T scanner. Then in vivo, human brain T1 image were calculated using both optimized GE and SE two-point methods. More precise T1 GE estimates than those for SE were found thanks to high signal-to-noise ratio (SNR) per unit of time, but with a small bias. These results also concern the temperature variation measurement methods, based on T1 estimation. Preliminary experimental data for temperature measurement are given.

MR monitoring of laser-induced lesions of the liver in vivo in a low-field open magnet: temperature mapping and lesion size prediction

The aims of this study were, firstly, to monitor temperature with magnetic resonance (MR) during laser ablations performed in pig livers in vivo in a low‐field open scanner (0.23T) and, secondly, to study the feasibility of lesion size prediction. Spin‐echo (SE) images of 29 sec acquired during laser applications allowed calculation of temperature maps using T1 and M0 temperature sensitivity. Temperature was also measured with thermocouples. Images of prediction of tissue damage were calculated using temperature maps and Arrhenius model. T2W sequences were acquired after the ablations. Animals were sacrificed immediately. Lesions were photographed macroscopically. Lesion surfaces were measured and compared in T2W images, temperature images, damage prediction images, and macroscopic pictures. A correlation exists between temperature measured with MR and with thermocouples (ρ = 0.878; P < 0.001, Spearman test). Mean surface of predicted damaged tissue is consistent with mean early necrosis measured in macroscopic pictures. Early T2W images underestimate mean necrosis size. J. Magn. Reson. Imaging 2001;13:42–49.

Optimization of intra‐voxel incoherent motion imaging at 3.0 Tesla for fast liver examination

Optimization of multi b‐values MR protocol for fast intra‐voxel incoherent motion imaging of the liver at 3.0 Tesla.