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.

In vivo assessment of tumoral angiogenesis.

Vessel size imaging (VSI) for brain tumor characterization was evaluated and the vessel size index measured by MRI (VSIMRI) was correlated with VSI obtained by histology (VSIhisto). Blood volume (BV) and VSI maps were obtained on 12 rats by simultaneous measurements of R2* and R2, before and after the injection of a macromolecular contrast agent, AMI-227. Immunostaining of collagen IV in vessels was performed. An expression was derived for evaluating VSI from stereologic measurements on histology data (VSIhisto). On BV and VSI images obtained from large-size tumors (n = 9), three regions could be distinguished and correlated well with histological sections: a high BV region surrounding the tumor, a necrotic area where BV is very low, and a viable tumor tissue region showing lower BV but higher VSI than the normal rat cortex, with the presence of larger vessels. The quantitative analysis showed a good correlation (Spearman rank's rho = 0.74) between VSIhisto and VSIMRI with a linear regression coefficient of 1.17. The good correlation coefficient supports VSI imaging as a quantitative method for tumor vasculature characterization.

Pimonidazole binding in C6 rat brain glioma: relation with lipid droplet detection.

In C6 rat brain glioma, we have investigated the relation between hypoxia and the presence of lipid droplets in the cytoplasm of viable cells adjacent to necrosis. For this purpose, rats were stereotaxically implanted with C6 cells. Experiments were carried out by the end of the tumour development. A multifluorescence staining protocol combined with digital image analysis was used to quantitatively study the spatial distribution of hypoxic cells (pimonidazole), blood perfusion (Hoechst 33342), total vascular bed (collagen type IV) and lipid droplets (Red Oil) in single frozen sections. All tumours (n=6) showed necrosis, pimonidazole binding and lipid droplets. Pimonidazole binding occurred at a mean distance of 114 microm from perfused vessels mainly around necrosis. Lipid droplets were principally located in the necrotic tissue. Some smaller droplets were also observed in part of the pimonidazole-binding cells surrounding necrosis. Hence, lipid droplets appeared only in hypoxic cells adjacent to necrosis, at an approximate distance of 181 microm from perfused vessels. In conclusion, our results show that severe hypoxic cells accumulated small lipid droplets. However, a 100% colocalisation of hypoxia and lipid droplets does not exist. Thus, lipid droplets cannot be considered as a surrogate marker of hypoxia, but rather of severe, prenecrotic hypoxia.

Regional response of cerebral blood volume to graded hypoxic hypoxia in rat brain.

BACKGROUND: The response of cerebral blood flow to hypoxic hypoxia is usually effected by dilation of cerebral arterioles. However, the resulting changes in cerebral blood volume (CBV) have received little attention. We have determined, using susceptibility contrast magnetic resonance imaging (MRI), changes in regional CBV induced by graded hypoxic hypoxia. METHODS: Six anaesthetized rats were subjected to incremental reduction in the fraction of inspired oxygen: 0.35, 0.25, 0.15, and 0.12. At each episode, CBV was determined in five regions of each hemisphere after injection of a contrast agent: superficial and deep neocortex, striatum, corpus callosum and cerebellum. A control group (n = 6 rats) was studied with the same protocol without contrast agent, to determine blood oxygenation level dependent (BOLD) contribution to the MRI changes. RESULTS: Each brain region exhibited a significant graded increase in CBV during the two hypoxic episodes: 10-27% of control values at 70% SaO2, and 26-38% at 55% SaO2. There was no difference between regions in their response to hypoxia. The mean CBV of all regions increased from 3.6 (SD 0.6) to 4.1 (0.6) ml (100 g)-1 and to 4.7 (0.7) ml (100 g)-1 during the two hypoxic episodes, respectively (Scheffé F-test; P < 0.01). Over this range, CBV was inversely proportional to SaO2 (r2 = 0.80). In the absence of the contrast agent, changes due to the BOLD effect were negligible. CONCLUSIONS: These findings imply that hypoxic hypoxia significantly raises CBV in different brain areas, in proportion to the severity of the insult. These results support the notion that the vasodilatory effect of hypoxia is deleterious in patients with reduced intracranial compliance.

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 x kg(-1) of P760. The results were compared to those obtained after the injection of 0.1 mmol x 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.

High glycolytic activity in rat glioma demonstrated in vivo by correlation peak 1H magnetic resonance imaging.

High-grade brain tumors are known to have a high rate of glucose (Glc) consumption. Postmortem measurements have suggested that Glc content in experimental brain tumors is relatively low. We used magnetic resonance spectroscopy to investigate this, in vivo, in the brains of seven rats bearing intracerebral C6 gliomas. We combined the high spectral resolution allowed by two-dimensional proton nuclear magnetic resonance with spatial encoding by magnetic field gradient pulses to obtain in vivo maps of Glc, alanine, hypotaurine, aspartate, phosphoethanolamine, Glu/Gln, N-acetylaspartate (NAA), phosphocreatine/creatine (PCr/Cr), choline-containing compounds, and lactate (Lac) (some of which are involved in energy metabolism). Compared with normal brain tissue, the main differences found in the gliomas were that Glc, NAA, PCr/Cr, and aspartate concentrations were much lower, whereas concentrations of alanine, hypotaurine, phosphoethanolamine, and Lac were higher, whatever the extent of necrosis. A striking observation is the similarity of the NAA and Glc images: the concentrations of both metabolites are lower in the tumor than they are in the contralateral brain. If Glc was completely absent from the tumor tissue, and if the residual Glc level was due only to a partial volume effect like that for NAA, a neuronal marker, the ratio [Glc]tumor/[Glc]contralateral tissue, should be similar to that found for NAA. The ratio for Glc was 0.48 +/- 0.22 (+/- SD; n = 6), a ratio similar to that found for PCr/Cr (0.50 +/- 0.19) but significantly higher than that obtained for NAA (0.29 +/- 0.07). This observation indicates that a measurable Glc concentration is still present in the tumor tissue. Intense glycolysis in tumor cells may explain the increased production of Lac and alanine and decreased amount of Glc. These nuclear magnetic resonance measurements of metabolite concentrations are complementary to positron emission tomography, which measures Glc consumption.

Rat lung MRI using low-temperature prepolarized helium-3.

The purpose of this study was to evaluate the recently proposed technique of 3He prepolarization at low temperature and high field (Kober et al. Magn Reson Med 1999; 41:1084-1087) for fast imaging of the lung. Helium-3 was cooled to 2.4 K in a magnetic field of 8 Tesla to obtain a polarization of 0.26%. The polarized 3He was warmed up to room temperature and transferred to a rat, with a final polarization of about 0.1%, large enough for acquiring a 3D image of the rat lung in 30 s.

Methodology of brain perfusion imaging.

Numerous techniques have been proposed in the last 15 years to measure various perfusion-related parameters in the brain. In particular, two approaches have proven extremely successful: injection of paramagnetic contrast agents for measuring cerebral blood volumes (CBV) and arterial spin labeling (ASL) for measuring cerebral blood flows (CBF). This review presents the methodology of the different magnetic resonance imaging (MRI) techniques in use for CBV and CBF measurements and briefly discusses their limitations and potentials.

Vessel size imaging.

Vessel size imaging is a new method that is based on simultaneous measurement of the changes Delta R(2) and Delta R(2)(*) in relaxation rate constants induced by the injection of an intravascular superparamagnetic contrast agent. Using the static dephasing approximation for Delta R(2)(*) estimation and the slow-diffusion approximation for Delta R(2) estimation, it is shown that the ratio Delta R(2)/Delta R(2)(*) can be expressed as a function of the susceptibility difference between vessels and brain tissue, the brain water diffusion coefficient, and a weighted mean of vessel sizes. Comparison of the results with 1) the Monte Carlo simulations used to quantify the relationship between tissue parameters and susceptibility contrast, 2) the experimental MRI data in the normal rat brain, and 3) the histologic data establishes the validity of this approach. This technique, which allows images of a weighted mean of the vessel size to be obtained, could be useful for in vivo studies of tumor vascularization. Magn Reson Med 45:397-408, 2001.

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 lambda 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 xi(Phi(i)), the mean characteristic droplet diameter Phi(c) = square root of Sigma(i)xi(Phi(i)Phi(i)(2) was shown to be related to the root mean square displacement at long diffusion times by the simple relationship Phi(c)(2) = 10 lambda(2). In the range of diffusion times 100--530 msec, lambda was found to be independent of the diffusion time and equal to 1.35 +/- 0.22 microm and Phi(c) to 4.27 +/- 0.71 microm. 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.

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 (P(a)CO(2) approximately 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 Poiseuille's law. We conclude that rCBV changes during CO(2) manipulation can be accurately measured by susceptibility contrast MRI. Abbreviations used: ANOVA analysis of variance CBF cerebral blood flow CBV cerebral blood volume CPMG Carr-Purcell-Meiboom-Gill FiO(2) fractional inspired oxygen ICP intracranial pressure LDF laser-Doppler flow MABP mean arterial blood pressure MRI magnetic resonance imaging MTT mean transit time PaCO(2) arterial partial pressure of carbon dioxide PaO(2) arterial partial pressure of oxygen PET positron emission tomography rCBV regional cerebral blood volume SPECT single-photon emission computed tomography

[NMR perfusion imaging: applications to the study of brain tumor angiogenesis]. / L'imagerie RMN de perfusion: applications a l'étude de l'angiogenèse tumorale en pathologie cérébrale.

NMR imaging allow specific study of contrast variations due to intravascular agents. It is possible to measure regional cerebral blood volume (rCBV). In brain tumor, this parameter allow to characterize tumoral vascularisation and blood brain barrier lesions. We use today 1st pass bolus technic. Easy to perform in clinical practice, it is useful for differential diagnosis, prebiopsic planning and follow up of lesions. That should be particularly interesting to evaluate the anti-angiogenic treatment efficiency.

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.

Laser-polarized (3)He as a probe for dynamic regional measurements of lung perfusion and ventilation using magnetic resonance imaging.

Magnetic resonance imaging (MRI) using laser-polarized noble gases, such as (129)Xe and (3)He, allows unparalleled noninvasive information on gas distribution in lung airways and distal spaces. In addition to pulmonary ventilation, lung perfusion assessment is crucial for proper diagnosis of pathological conditions, such as pulmonary embolism. Magnetic resonance perfusion imaging usually can be performed using techniques based on the detection of water protons in tissues. However, lung proton imaging is extremely difficult due to the low proton density and the magnetically inhomogeneous structure of the lung parenchyma. Here we show that laser-polarized (3)He can be used as a noninvasive probe to image, in a single MRI experiment, not only the ventilation but also the perfusion state of the lungs. Blood volume maps of the lungs were generated based on the (3)He signal depletion during the first pass of a superparamagnetic contrast agent bolus. The combined and simultaneous lung ventilation and perfusion assessments are demonstrated in normal rat lungs and are applied to an experimental animal model of pulmonary embolism. Magn Reson Med 44:1-4, 2000.

2D-spatial/2D-spectral spectroscopic imaging of intracerebral gliomas in rat brain.

1H-MR spectroscopy in vivo is often hampered by poor spectral resolution. Spectral overlap can be avoided with two-dimensional spectroscopic techniques. Correlation peak imaging has been implemented to measure unambiguously the distribution of several metabolites in a rat brain glioma model. Acquisition-weighted spectroscopic imaging reduced the experimental time and provided excellent spatial localization. The choice of an appropriate spectral acquisition window granted good sensitivity. Spectroscopic images presenting a full two-dimensional spectrum in every image pixel were acquired in seven rats at 7 Tesla in 195 min, with a nominal voxel volume of 75 microl. Among other metabolites, the distribution of hypotaurine, phosphoethanolamine, alanine, and even glucose could be visualized both in the C6-glioma and in the unaffected brain.

Necrotic tumor versus brain abscess: importance of amino acids detected at 1H MR spectroscopy--initial results.

PURPOSE: To assess the usefulness of the 0.9-ppm peak from amino acids (-CH3 moieties from valine, leucine, and isoleucine) for the differentiation of brain abscesses and tumors at in vivo hydrogen 1 magnetic resonance (MR) spectroscopy. MATERIALS AND METHODS: Amino acid concentrations were determined in vitro in 13 purulent samples from brain and nonbrain tissues and in nine aseptic fluids from necrotic brain tumors at two-dimensional (2D) 1H MR spectroscopy and liquid chromatography. Thirty-four patients with cystic intracerebral mass lesions (28 tumors, six abscesses) were examined at 1H MR spectroscopy in vivo. RESULTS: Amino acids were identified in vitro in both purulent and aseptic samples. Amino acid concentrations measured in the aseptic fluids at both liquid chromatography and 2D MR spectroscopy were far below the detection threshold of in vivo 1H MR spectroscopy. Quantitative results obtained at 2D MR spectroscopy showed no overlap in the ranges of amino acid concentrations in purulent and aseptic samples. In vivo, the proton spectra obtained with a 136-msec echo time (TE) revealed amino acids (inverted peak at 0.9 ppm) in only the abscesses. CONCLUSION: The detection of amino acid resonance at 0.9 ppm at in vivo 1H MR spectroscopy (136-msec TE) is a promising tool for distinguishing bacterial abscesses and cystic brain tumors.

Use of T(2)-weighted susceptibility contrast MRI for mapping the blood volume in the glioma-bearing rat brain.

The aim of this work was to evaluate the potential of T(2)-weighted, steady-state susceptibility-enhanced contrast magnetic resonance imaging (MRI), to characterize brain tumor heterogeneity and tumor vascularization. In vivo T(2)-weighted MRI experiments were carried out on normal rats (n = 11) and rats bearing C6 glioma (n = 17), before and after the injection of a remanent superparamagnetic contrast agent. The DeltaR(2) variations of the transverse relaxation rate due to the injection of the contrast agent were used to generate relative cerebral blood volume (CBV) maps. Contrast enhancement of the tumor was shown to reflect tissue vascularization rather than leakage of the blood-brain barrier. The quantitative results clearly show the heterogeneity of tumor vascularization and reveal a high vessel density in the peripheral area (CBV(per) approximately 17.2 +/- 2.3 sec(-1)) and a low vessel density in the central area of the tumor (CBV(cen) approximately 2.5 +/- 0.5 sec(-1)). Magn Reson Med 42:754-761, 1999.