Michel Décorps

A new approach for analyzing proton magnetic resonance spectroscopic images of brain tumors: nosologic images

A new approach for analyzing proton magnetic resonance spectroscopic images of brain tumors: nosologic images

In vivo measurement of the size of lipid droplets in an intracerebral glioma in the rat

Pulsed field gradient NMR was used to measure the root mean square displacement λ of the NMR visible lipid molecules in C6 brain tumors in the rat at different diffusion times. For a distribution of spherical droplets of diameter with volume fraction ξ(Φi), the mean characteristic droplet diameter Φc = was shown to be related to the root mean square displacement at long diffusion times by the simple relationship Φ  2c = 10λ2. In the range of diffusion times 100–530 msec, λ was found to be independent of the diffusion time and equal to 1.35 ± 0.22 μm and Φc to 4.27 ± 0.71 μm. The data reinforce the notion that the presence of lipid resonances in NMR spectra of tumors is due to lipid droplets. Light microscopy of histologic slices showed the presence of lipid droplets mainly in the necrotic region and in a layer of tumor cells surrounding the necrosis. Magn Reson Med 45:409–414, 2001.

In vivo 129Xe NMR in rat brain during intra-arterial injection of hyperpolarized 129Xe dissolved in a lipid emulsion

Hyperpolarized 129Xe was dissolved in a lipid emulsion and administered to anaesthetized rats by manual injections into the carotid (approximately 1-1.5 mL in a maximum time of 30 s). During injection, 129Xe NMR brain spectra at 2.35 T were recorded over 51 s, with a repetition time of 253 ms. Two peaks assigned to dissolved 129Xe were observed (the larger at 194 +/- 1 ppm assigned to intravascular xenon and the smaller at 199 +/- 1 ppm to xenon dissolved in the brain tissue). Their kinetics revealed a rapid intensity increase, followed by a plateau (approximately 15 s duration) and then a decrease over 5 s. This behaviour was attributed to combined influences of the T1 relaxation of the tracer, of radiofrequency sampling, and of the tracer perfusion rate in rat brain. Similar kinetics were observed in experiments carried out on a simple micro-vessel phantom. An identical experimental set-up was used to acquire a series of 2D projection 129Xe images on the phantom and the rat brain.

fMRI assessment of hemispheric language dominance using a simple inner speech paradigm.

Hemispheric language dominance (HLD) has been determined by means of functional MRI (fMRI) using a simple, inner speech, word fluency paradigm. During the task periods, subjects perform mental imagery of visual scenes and generate silently the nouns of all objects visualized. During the control periods, subjects attend to the scanner noise. Activated areas have been identified by means of cross‐correlation analysis. HLD indices have been determined by comparing the number of activated pixels detected in both hemispheres within predefined cortical areas (Brodmann areas 6, 9, 10, 39, 40 and 44–47). The paradigm has been assessed on 10 healthy, right‐handed volunteers. A volume 35 mm thick, centered on the inferior frontal gyrus, was imaged. A conventional GRE MR sequence was used on a 1.5 T clinical MR scanner. HLD indices were compared with those determined for overt speech. Robust fMRI reponses were obtained. HLD indices indicated left hemispheric language dominance for all subjects examined. They correlated well with those obtained for overt speech (R2  =  0.93, regression coefficient  =  0.998, with p < 10−4). Thus, an inner speech paradigm based on visual imagery is well adapted for assessment of HLD by means of fMRI. Copyright

Lactate Accumulation During Moderate Hypoxic Hypoxia in Neocortical Rat Brain

Neocortical metabolism was studied during moderate hypoxic hypoxia, reoxygenation, and postmortem periods in anesthetized normocapnic rats using 1H nuclear magnetic resonance (NMR) spectroscopic imaging. Rats were prepared with unilateral common carotid occlusion to determine the ipsilateral metabolic effects of inadequate cerebral blood flow (CBF) response to hypoxia. No difference in brain metabolism between the two hemispheres was found during the control period. Hypoxic hypoxia (Pao2 = 54.1 ± 5.8 mm Hg) resulted in a significant rise in neocortical lactate peak in both hemispheres, with an additional marked rise in the clamped side compared to the unclamped side (53 ± 27 vs. 22 ± 13% of postmortem value, p < 0.001). These lactate changes were not reversible within 30 min of reoxygenation in the clamped hemisphere. No changes in neocortical lactate peak were observed while elevating arterial lactate via intravenous lactate infusion without hypoxia. In addition, hypoxic hypoxia resulted in an apparent decrease in neocortical water and N-acetyl aspartate (NAA) signals, which were related to a shortening in T2 relaxation times. It is concluded that neocortical lactate is an early metabolic indicator during moderate hypoxic hypoxia in normocapnic conditions.

A new gadolinium‐based contrast agent for magnetic resonance imaging of brain tumors: Kinetic study on a C6 rat glioma model

T1‐weighted magnetic resonance imaging (MRI) was used to evaluate the potential interest of a new Gd‐based contrast agent, termed P760, to characterize brain tumor heterogeneity and vascularization and to delineate regions containing permeable vessels. The C6 rat glioma model was used as a model of high‐grade glioblastoma. The signal enhancement was measured as a function of time in the vascular compartment and in different regions of interest (ROIs) within the tumor after the injection of 0.02 mmol kg−1 of P760. The results were compared to those obtained after the injection of 0.1 mmol kg−1 of Gd‐DOTA. We showed that P760, in spite of a Gd concentration five times smaller, produces an enhancement in the blood pool similar to that produced by Gd‐DOTA. It was shown that P760 makes possible an excellent delineation of regions containing vessels with a damaged blood‐brain barrier (BBB). Images acquired 5–10 minutes after P760 injection showed the location of permeable vessels more accurately than Gd‐DOTA‐enhanced images. The enhancement produced in the tumor by P760 was, however, less than that produced by Gd‐DOTA. The extravasation and/or diffusion rate of P760 in the interstitial medium were found to be strongly reduced, compared to those found with Gd‐DOTA. This study suggests that the new contrast agent has promising capabilities in clinical imaging of brain tumors. J. Magn. Reson. Imaging 2001;14:97–105.

NMR relaxation times from serial stereotactic biopsies in human brain tumors.

Tissue samples were obtained by serial stereotactic biopsies in 32 patients with glial brain tumors localized by CT. On each sample, several tissue parameters, such as water content, specific gravity of the dry and wet tissue, electrical impedance, histopathological and cytological examinations, as well as nuclear magnetic resonance (NMR) relaxation times T1 and T2 were measured and correlated. Comparisons were made between both normal and tumorous tissues and between the various types of neoplasms. Differences of the measured parameters between tumors and normal brain tissue and their intercorrelations are analyzed and discussed. Comparison of the NMR results with the corresponding densitorhetric data from CT images suggests that NMR may add useful information in brain tumor imaging. The authors propose an index of malignancy for gliomas dependent on relaxation time Tl and proton density measured by NMR imaging.

1H in Vivo NMR spectroscopy with surface coils. Hard-pulse water suppression sequences and spatial localization

Abstract The use of binomial pulse sequences to achieve water suppression in the inhomogeneous B1 field of surface coils is analyzed. It is shown that the frequency response to the binomial excitation sequences is dependent on the spatial coordinates. Therefore the shape and the location of the sensitive volume can be strongly dependent on the chemical shift, and the frequency response cannot be accurately known. The drawbacks of the binomial excitation sequences can be overcome by using a pulse angle small enough so that the spin system behaves as a linear system. However, this is at the cost of a reduction in the signal-to-noise ratio, and of the loss of the ability of surface coils to obtain spectra from tissues located at some depth beneath the surface of the sample. The use of phase-cycled binomial refocusing pulses is then analyzed. It is shown that they never introduce an additional phase shift. Moreover, the particularly simple response obtained with the 2 2 and 2 4 2 refocusing pulses when used with the Exorcycle phase-cycling scheme is emphasized. It is shown that the use of spin-echo sequences greatly improves the spatial localization properties of binomial excitation sequences. Finally, spin-echo sequences which allow water suppression without any phase correction and which give a frequency response independent of the shape of the sample are proposed.

Out-and-in spiral spectroscopic imaging in rat brain at 7 T

With standard spectroscopic imaging, high spatial resolution is achieved at the price of a large number of phase‐encoding steps, leading to long acquisition times. Fast spatial encoding methods reduce the minimum total acquisition time. In this article, a k‐space scanning scheme using a continuous series of growing and shrinking, or “out‐and‐in,” spiral trajectories is implemented and the feasibility of spiral spectroscopic imaging for animal models at high B0 field is demonstrated. This method was applied to rat brain at 7 T. With a voxel size of about 8.7 μl (as calculated from the point‐spread function), a 30 × 30 matrix, and a spectral bandwidth of 11 kHz, the minimum scan time was 9 min 20 sec for a signal‐to‐noise ratio of 7.1 measured on the N‐acetylaspartate peak. Magn Reson Med 50:1127–1133, 2003.

Regional cerebral blood volume response to hypocapnia using susceptibility contrast MRI.

We used steady‐state susceptibility contrast MRI to evaluate the regional cerebral blood volume (rCBV) response to hypocapnia in anesthetised rats. The rCBV was determined in the dorsoparietal neocortex, the corpus striatum, the cerebellum, as well as blood volume in extracerebral tissue (group 1). In addition, we used laser‐Doppler flow (LDF) measurements in the left dorsoparietal neocortex (group 2), to correlate changes in CBV and in cerebral blood flow. Baseline values, expressed as a percentage of blood volume in each voxel, were higher in the brain regions than in extracerebral tissue. Hypocapnia (PaCO2 ≈ 25 mmHg) resulted in a significant decrease in CBV in the cerebellum (−17 ± 9%), in the corpus striatum (−15 ± 6%) and in the neocortex (−12 ± 7%), compared to the normocapnic CBV values (group 1). These changes were in good agreement with the values obtained using alternative techniques. No significant changes in blood volume were found in extracerebral tissue. The CBV changes were reversed during the recovery period. In the left dorsoparietal neocortex, the reduction in LDF (group 2) induced by hypocapnia (−21 ± 8%) was in accordance with the values predicted by the Poiseuilles law. We conclude that rCBV changes during CO2 manipulation can be accurately measured by susceptibility contrast MRI. Copyright