Yves Mauss

Distinct patterns of active and non-active plaques using texture analysis on brain NMR images in multiple sclerosis patients: preliminary results

The benefits of texture analysis of magnetic resonance images have been assessed in multiple sclerosis (MS) patients. Out of thirty-two lesions identified in eight MS patients, nine were considered active, judging from their gadolinium uptake. Texture analysis allowed to obtain forty-two characterizing parameters for each lesion. Using discriminant analysis as a statistical method allowed to classify the lesions into two groups: active or non-active. An attempt to classify their level of activity by using only co-occurrence matrices was unsuccessful. Alternately, the same type of analysis performed on runlength analysis criteria allowed the accurate classification of 88% of active lesions and 96% of non-active lesions. Using incremental discriminate analysis can reduce the number of useful parameters. This method showed that among the 42 parameters, 8 only were highly significant and permitted an accurate classification. Five of these parameters are runlength parameters, and three others are more directly related to the global distribution. The main interest of runlength parameters is that they allowed to demonstrate that the lesion structure was different in active and non-active plaques. This preliminary work suggests that using texture analysis could be of interest in the follow-up of MS patients because it provides an opportunity to identify active lesions without frequent gadolinium injections.

Existence of contralateral abnormalities revealed by texture analysis in unilateral intractable hippocampal epilepsy

We selected 23 patients with unilateral temporal lobe epilepsy characterized by ipsilateral hippocampal sclerosis and an apparently normal contralateral hippocampus on MR imaging. Images were acquired on a 0.28 T MR scanner using a conventional Carr-Purcell Meiboom Gill sequence in all patients and in 9 healthy subjects. Texture analysis was applied to axial MR images of the first and tenth echoes. Texture analysis detects macroscopic lesions and microscopic abnormalities that can not be observed visually. The presence of texture differences in the between normal (controls) and sclerotic hippocampi was ascertained by statistical discriminant analysis. The apparently normal contralateral hippocampi can be classified into three categories in terms of texture: 4 apparently healthy, 8 similar to sclerosis, and 11 different from either healthy or sclerosis. These findings are related to a certain degree of hippocampal alteration, which further investigation might help better characterize.

Neural substrates of animal mental imagery: calcarine sulcus and dorsal pathway involvement: an fMRI study

Neural response was measured using fMRI in six healthy volunteers, performing a mental imagery task, using verbal cues exclusively. They listened to a list of animal names from which to generate a mental image, and listened passively to a list of abstract words. They were tested twice, using the same protocol. SPM99-processed results showed for both sessions activation in the calcarine sulcus and local activation foci, mainly in the occipito-parietal region. Other studies involving figurative mental imagery using verbal cues, have shown activation in the occipito-temporal area, but none in the calcarine sulcus or in the dorsal route. We account for the discrepancies relative to previous mental imagery studies using verbal cues, in terms of differences in the experimental conditions. In our opinion, restricting the stimuli to a single semantic category (animals) and increasing the time dedicated to the production of MI, may have enhanced the components of the pictures. This mental imagery generation protocol shows the importance of the design of experimental tasks on anatomo-functional responses.

Multi-exponential analysis of T2 images

Clinical images show that T2 contrast allows in certain cases a better discrimination between the healthy and pathological tissues. For instance, in Fig. 1 it can be seen that multiple sclerosis’ lesions are more clearly visible on the images obtained with late echoes than those obtained with the early echoes. On the other hand in vitro studies on perfused organs’ have shown that the T2 decay curve depends on the interval between 180” pulses: The decay curve is mono-exponential for 4-ms time intervals between 180” pulses, and bi-exponential for 0.4-ms time intervals (Fig. 2). For these reasons, we have tried to analyze the T2 decay curve by multi-exponential decomposition in order to improve tissue characterization. To obtain the T2 decay curve, we used the classical CPMG pulse sequence. By adding linear magnetic field gradients to this sequence it is possible to reconstruct a sample image for each echo. The T2 decay curve is constructed by taking the corresponding pixel from the echo image series, for instance, the first point corresponds to the first image, the second point to the second images, etc. Thus we obtain the T2 decay curve of the elementary volume defined by the pixel whose dimensions are the slice thickness (7 mm) and the spatial resolution (2-2.5 mm). It suffices, therefore, to repeat this operation pixel by pixel in order to obtain parametric T2 decay curves over the whole slice. A slice of the head is represented by about 6000 pixels, which means that we need to repeat the decomposition operation 6000 times. We therefore had to find an algorithm sufficiently rapid and not too memory consuming. The algorithm chosen was of the noniterative, Prony (algebraic) type. 3 It was then necessary to verify this method of analysis using first simulated image signals and thereafter simple test objects. Classically, the multi-exponential decomposition is an ill-posed problem. The “solution” is obtained by nonlinear least-squares optimization. With a noniterative algorithm we transform the nonlinear leastsquares optimization into two linear least-squares problems. The decomposition method may be presented in four steps:

The Role of Mycobacterium tuberculosis in Experimental Allergic Encephalomyelitis

Various detergents used in preparative membrane protein chemistry were added to a complete Freunds adjuvant/water emulsion in order to increase the solubility and/or immunologic availability of the Mycobacterium tuberculosis membrane and to explain its role in blood-brain barrier (BBB) permeability. Magnetic resonance imaging was used for in vivo determination of the BBB breakdown and cerebral edema. The results showed that with 1% 10 tridecyl ether, which increases emulsion stability, abundant BBB breakdown and cerebral edema were observed, similar to those encountered in experimental allergic encephalomyelitis (EAE). We suggest that the immunologic response triggered off by M. tuberculosis largely contributes to the BBB permeability changes observed during EAE, probably by an action on the endothelial cells of the cerebral blood vessels.

Detection of late epilepsy by the texture analysis of MR brain images in the lithium-pilocarpine rat model

The aim of the study was to detect by texture analysis non easily visible anomalies of magnetic resonance (MR) images of piriform and entorhinal cortices relevant to the lithium-pilocarpine (Li-Pilo) model of temporal lobe epilepsy in rats. Status epilepticus was induced by Li-Pilo in twenty male rats 21 day-old. T(2)-weighted MR images of their brain, were obtained before injection of Li-Pilo and one day after status epilepticus. An hyperintense signal was found in the piriform and entorhinal cortices of six rats, which developed chronic epilepsy after a latent period of one to three months. Among the 14 other rats which displayed images similar to those obtained before injection, four remained healthy but 10 rats developed late epileptic symptoms, raising the problem of hidden cortical damage which may be too subtle to be detected by classic MRI examination. A numeric treatment of digital images was then undertaken by texture analysis, to derive image information from a purely computational point of view. The combined texture and discriminant analyses based on pixels pattern anomalies, selected 3 texture parameters derived from co-occurrence matrix which characterized structural abnormalities relevant to the hyperintense signal, not only in the modified images of 6 rats but also in images of 10 rats with apparently non modified images. These three textures parameters allowed to classify the twenty rats of our experiment as follows: sixteen epileptic rats were effectively classified with cortical lesions, two non epileptic were correctly classified with healthy cortex, but two healthy rats were not correctly classified. This misclassification is discussed on the basis of the time dependence of the onset of seizure in the Li-Pilo model. These promising results suggest to apply this method to MRI examinations for an improvement of the early diagnostic of human epilepsy.

An evaluation of the significance of areas of intense signal in the MR brain images of patients with multiple sclerosis

This study evaluates the distribution of areas of intense signal (AIS) in 189 patients with multiple sclerosis (MS) and 83 patients presenting high-risk factors of cerebrovascular injuries. Two multivariate statistical analyses (multifactorial discriminant analysis and logistic regression analysis) with two AIS scores and several subpopulations of patients (according to age and/or the number of AIS) were tested. The results of these analyses were expressed with the usual screening test. The results obtained in this study even without the help of any clinical information are very promising, since they established that the specificity of MRI could be improved by using the distribution of the AIS in the various anatomical areas as a criterion. Five regions of the brain display were particularly significant in the discrimination between MS and non-MS patients: In decreasing order we found that the temporal, occipital, brain stem, and parietal regions were more specific with respect to the diagnosis of MS, while only the basal ganglia could account for non-MS patients. With multifactorial discriminant analysis and logistic regression analysis respectively, 78.9 (+/- 2.8)% and 85.1 (+/- 2.8)% of the patients were correctly classified by MRI. The results obtained on the main group were confirmed by a predictive test carried out on an other population of 40 patients, which produced similar results. The comparison between our method and Fazekass imaging criteria showed a 20% improvement in favour of our approach and it is hoped that it will contribute to make the most of MRI as a tool for the diagnosis of MS.

Proton nuclear magnetic resonance relaxation rates in aqueous solutions of amino acids

Variations of water-proton NMR relaxation rates induced by amino acids are very different for longitudinal and transverse relaxations. The presence of amino acids increases the longitudinal relaxation rate only slightly, when compared with the increase induced by the oxygen content of the solution. The measured transverse relaxation rate is dependent on the pulse delay of a Carr-Purcell-Meiboom-Gill sequence. Transverse proton relaxation dispersions are shown to be consistent with rapid proton exchange between water and the NH+ 3 groups of the amino acids. The mean exchange time is about 1 ms. The variation of the exchange time with temperature yields the enthalpy and the entropy of the apparent first-order reaction of this exchange. Changes of pH, temperature and oxygen content give rise to opposite effects on relaxation rates of amino acid solutions. The discussion here underlines the fact that, even in such a simple system, relaxation-rate measurements give valuable information only if the physico-chem...

Effect of interpulse delay on NMR transverse relaxation rate of tissues

INTRODUCTION The nuclear spin systems have relaxation times varying from a few microseconds to several seconds. The time during which the precession maintains phase memory has been called spin-spin or transverse relaxation time ( T, ). The Carr-Purcell-Meiboom-Gil1 (CPMG) sequence’p2 provides a good method for measuring the spin-spin relaxation time with the classical sequence 90” (7, 180”, 7, record), . While Hahn’s method’ (90, 7, 90, 7, record, 5Tl), is sensitive to diffusion effects in nonviscous samples and permits accurate measurements of diffusion constants, the CPMG sequence partially eliminates the influence of diffusion on echo decay. As shown by Carr and Purcell’ the echo amplitude, detected at time t = 2nr is given by quencies. The exchange process will transfer nuclei from one environment to the other, with an accompanying switch of their resonance frequency. These frequency changes increase the linewidth of the NMR spectrum. An increase of the exchange rate causes the complex broad pattern to narrow to a single line. Finally, if the jumping frequency reaches values comparable to the Larmor frequency, the jumping becomes a mechanism for the spin relaxation. The general theory for calculating the effect6s7 of chemical exchange and spin coupling on echo amplitudes in CPMG sequences was developed by Gutowsky et a/.* As a first-order approximation Luz and Meiboom’ calculated the decay rate of the echoes:

Chemical and molecular exchange effects on T2 relaxation of living tissues: a pulse spacing dependence study.

Contrast in magnetic resonance imaging depends principally on the longitudinal relaxation (R1) and the transverse relaxation rate (R2) of the observed nuclei, most often the protons. The spin-spin relaxation rate (R2) is the result of several mechanisms. The dependence of the interpulse delay of the Carr-Purcell-Meiboom-Gill sequence on the transverse relaxation rate of the water was studied in rat organs in vitro. It gives an insight into the exchange mechanisms involved. The increase of the interpulse delay from 0.2 ms to 5 ms gives an R2 increase of 23, 15, 3, and 2 s-1 for the heart, the liver, the spleen and the brain, respectively. These increases are compared to the R2 increases obtained in 17O-enriched water, amino acid and albumin solutions atomic exchange takes place. The concentration of these materials in organs cannot explain the R2 increase of the organs with the interpulse delay. Water exchange between intra and extracellular compartments is proposed to explain the R2 increase with interpulse delays in organs like the heart and the liver.