Noninvasive assessment of the functional neovasculature in 9L-glioma growing in rat brain by dynamic 1H magnetic resonance imaging of gadolinium uptake.

Pathophysiologic parameters of the functional neovasculature and the blood-brain barrier of 9L-glioma in rat brain were measured noninvasively by dynamic 1H magnetic resonance imaging studies of gadolinium (Gd)-DTPA uptake. Changes of apparent [Gd-DTPA] uptake in time (CT[t]) were analyzed in a slice through the center of 10 9L-gliomas using fast T1 measurements. The distribution of the contrast agent was spatially correlated with the distribution of perfused microvessels as determined by immunohistochemical analysis. This method permits a distinction between perfused and nonperfused microvessels with a disrupted blood-brain barrier. In transverse slices of the whole tumor, a spatial correlation was observed between CT maps and the two-dimensional distribution of perfused microvessels. In the next step, Gd-DTPA uptake rates were spatially related to the perfused microvessel density (Np) or vascular surface area (Sp). In tumor voxels with perfused microvessels, a linear correlation was found between Gd-DTPA uptake rate constants (k values) and Np or Sp. No correlation was observed between k values and the total microvessel density. These are the first data that show a relation between Gd-DTPA uptake rates and parameters of the functional neovasculature in 9L-glioma growing in rat brain. Now that Gd-DTPA uptake studies can be related to parameters of the functional neovasculature, they may be used more efficiently as a prognostic tool before or during therapy.

Determination of in vivo rat muscle Gd-DTPA relaxivity at 6.3 T.

For the in vivo relaxivity of Gd-DTPA at 6.3 T in rat muscle a value of 2.7+/-0.5 (mM s)(-1) was found, and for the in vitro value in water 3.00+/-0.56 (mM s)(-1) at 37 degrees C. The temperature dependence of the in vitro relaxivity was -0.087 (mM s degrees C)(-1). The relation between 1/T1 and the tissue Gd-DTPA concentration is linear for the normally used in vivo Gd-DTPA concentration range.

Parameter estimation from Rician-distributed data sets using a maximum likelihood estimator: application to T1 and perfusion measurements.

General expressions are presented to calculate the maximum likelihood (ML) estimator and corresponding Fisher matrix for Rician-distributed data sets. This estimator results in the most precise, unbiased estimations of T1 from magnitude data sets, even when low signal-to-noise ratios (<6) are present. By optimizing the sample point distributions for inversion-recovery experiments, a 32% increase in precision of the estimated T1 is obtained, compared with a linear sampling scheme. Perfusion rates are estimated from combined data sets of the slice- and nonslice-selective inversion-recovery experiments, as obtained with the flow-sensitive alternating inversion recovery (FAIR) technique. The ML estimator for the combined data set results in the most precise, unbiased estimations of the perfusion rate. Error analysis shows that very high signal-to-noise ratios are required for precise estimation of perfusion rates from FAIR experiments.

Absolute metabolite quantification by in vivo NMR spectroscopy: I. Introduction, objectives and activities of a concerted action in biomedical research.

By utilizing achievements and results of two previous concerted research projects on magnetic resonance imaging and spectroscopy (MRS), the EU BIOMED 1 Concerted Action on "Cancer and brain disease characterization and therapy assessment by quantitative MRS" was specifically aimed at: 1) developing at a multicentre level harmonized methodologies and protocols for quantitative and reproducible MRS measurements, as a basis for validating these procedures in well controlled clinical and experimental conditions; and 2) providing multicentre critical reviews on the present understanding of the significance of MRS parameters as possible new markers of diagnosis, prognosis and response to therapy. The programme comprised the following main areas of collaborative research and multicentre evaluation: a) development of methods and protocols for quality assessment, calibration and absolute metabolite quantification in in vivo localized, volume-selective MRS; b) design and validation of a new method for assessing localization performance in spectroscopic imaging (MRSI); c) interlaboratory comparison of different methods of signal processing and data analysis, for improving signal quantification in vivo and in vitro MRS spectra; d) quality assessment of high resolution MRS analyses of biological fluids; e) protocol for assembling a pilot data base of MR spectra of tumour extracts for pattern recognition analysis; f) multicentre review on evaluation of the significance of MRS parameters in monitoring lipid metabolism and function in cancer; and g) multicentre review on evaluation of drug pharmacokinetics and metabolism using MRS. The main results and conclusions of four multi-centre trials on items (a), (b) and (c), which involved 24 teams, are reported in the accompanying papers of this series.

Influence of the pharmacokinetic model on the quantification of the Gd-DTPA uptake rate in brain tumours using direct T1 measurements.

After a bolus injection of Gd-DTPA, a fast tracer uptake in rat brain tumours was already observed during the tracer bolus passage. For the quantification of the uptake rate constant, pharmacokinetic models are commonly used. For a good quantification, the changes of the plasma tracer concentration directly after the bolus injection must be incorporated into these models as prior knowledge. The aim of this paper is to investigate whether or not it is necessary to include the bolus passage into the description for the plasma tracer concentration. The result of this study indicates that the best quantification of the uptake rate constant is achieved by using only the data points after the bolus passage. Using all the data points and incorporating the bolus passage in the pharmacokinetic models results in a less accurate estimation.

NMR perfusion and metabolic imaging and some examples on brain tumours.

The potentials and limitations of proton Magnetic Resonance to map the spatial distribution of perfusion parameters and of metabolite concentrations in the brain are demonstrated and discussed. Some examples of applications to brain tumours are given.

In vitro evaluation of a novel bioreactor based on an integral oxygenator and a spirally wound nonwoven polyester matrix for hepatocyte culture as small aggregates.

BACKGROUND/AIMS: The development of custom-made bioreactors for use as a bioartificial liver (BAL) is considered to be one of the last challenges on the road to successful temporary extracorporeal liver support therapy. We devised a novel bioreactor (patent pending) which allows individual perfusion of high density cultured hepatocytes with low diffusional gradients, thereby more closely resembling the conditions in the intact liver lobuli. METHODS: The bioreactor consists of a spirally wound nonwoven polyester matrix, i.e. a sheet-shaped, three-dimensional framework for hepatocyte immobilization and aggregation, and of integrated hydrophobic hollow-fiber membranes for decentralized oxygen supply and CO2 removal. Medium (plasma in vivo) was perfused through the extrafiber space and therefore in direct hepatocyte contact. Various parameters were assessed over a period of 4 days including galactose elimination, urea synthesis, lidocaine elimination, lactate/pyruvate ratios, amino acid metabolism, pH, the last day being reserved exclusively for determination of protein secretion. RESULTS: Microscopic examination of the hepatocytes revealed cytoarchitectural characteristics as found in vivo. The biochemical performance of the bioreactor remained stable over the investigated period. The urea synthesizing capacity of hepatocytes in the bioreactor was twice that of hepatocytes in monolayer cultures. Flow sensitive magnetic resonance imaging (MRI) revealed that the bioreactor construction ensured medium flow through all parts of the device irrespective of its size. CONCLUSIONS: The novel bioreactor showed encouraging efficiency. The device is easy to manufacture with scale-up to the liver mass required for possible short-term support of patients in hepatic failure.

L-ornithine vs. L-ornithine-L-aspartate as a treatment for hyperammonemia-induced encephalopathy in rats.

BACKGROUND/AIMS: The effect of L-ornithine (ORN) and L-ornithine-L-aspartate (OA) therapy on "extracerebral" nitrogen metabolism, brain metabolism and neurotransmission has been investigated in portacaval shunted rats with hyperammonemia-induced encephalopathy. METHODS: One day before ammonium-acetate infusion, a portacaval shunt was performed in three experimental groups: 1-control rats, 2-ORN-treated rats and 3-OA-treated rats. Ammonium-acetate was given as an intravenous bolus injection (0.4 mmol.kg bw-1) followed by a constant infusion (1.9 mmol.kg bw-1.h-1) so that steady-state blood ammonia concentrations (500-800 microM) were obtained in the course of 5 h. After 1 h, ammonium-acetate infusion, either L-ornithine or L-ornithine-L-aspartate, was infused for the next 4 h (3.0 mmol.kg bw-1.h-1) in the treated groups. The following parameters were measured: clinical grade of encephalopathy, EEG activity (n = 10 - 20/group), amino acids in plasma (n = 10 - 20/group) and brain dialysate (n = 5 - 9/group), and brain metabolites obtained by in vivo cerebral 1H-MRS (n = 4 - 6/group). RESULTS: ORN and OA treatment resulted in significantly lower blood (34% and 39%) and brain (42% and 22%) ammonia concentrations, significantly higher urea production (39% and 86%) and significantly smaller increases in brain glutamine and lactate concentrations than in controls. These changes were associated with a significantly smaller increase in clinical grade of encephalopathy in ORN- and OA-treated rats, and a significant improvement in EEG activity in ORN-treated rats. OA-treated rats showed a significant increase in aspartate and glutamate concentrations in brain dialysate. CONCLUSIONS: The beneficial effects of both treatments on the manifestations of hyperammonemia-induced encephalopathy can be explained by a reduction in blood and brain ammonia concentrations. It is suggested that when OA is administered, the effect of ornithine is partly counteracted by aspartate, inducing high brain extracellular concentrations of the two excitatory amino acids glutamate and aspartate, and perhaps causing overstimulation of NMDA receptors.

Spatial resolution and spectral information content of proton NMR spectroscopic imaging of tumour metabolism.

The value of metabolic proton NMR spectroscopic imaging to detect and classify tumours increases with the spatial resolution and the information content of the spectra. Several factors influencing these quantities are discussed.

Analysis of micellar and vesicular lecithin and cholesterol in model bile using 1H- and 31P-NMR.

The distribution of phospholipid and cholesterol between the vesicular and micellar phases in bile plays an important role in the formation of cholesterol gallstones. Conventional analytical procedures to determine the distribution are potentially unreliable because they disturb the distribution of these compounds between the two phases. In this work, we circumvent this problem by using NMR. 31P-NMR is used to quantify directly the micellar and the vesicular amount of lecithin. The previously described 1H-NMR method to determine directly the micellar lecithin (Groen et al., J Lipid Res 31: 1315-1321) has been optimized by the implementation of a spectral quantification method. The agreement between the 31P and 1H methods was excellent. In our hands, the method of Ellul et al. (FEBS Lett 300: 30-32) did not allow quantification of micellar cholesterol, although our fitting procedure offered the possibility of quantifying overlapping spectral peaks by the use of prior knowledge about all the parameters of the compounds visible in the spectrum. The micellar cholesterol concentration was so low compared to the overlapping lecithin peaks that no reliable quantification was possible. The same problem was encountered when using other characteristic cholesterol resonances for quantification. Our data suggest that cholesterol present in the vesicular phases is too immobile to give rise to high-resolution peaks and the amount of cholesterol present in the micellar phase is too low to allow quantitation by NMR. We conclude that 1H-NMR can be used to determine micellar lecithin, and 31P-NMR to determine micellar as well as vesicular lecithin in model bile.

Quality assessment in in vivo NMR spectroscopy: II. A protocol for quality assessment. EEC Concerted Research Project.

A protocol has been developed for quality assessment in in vivo NMR spectroscopy (MRS) on whole body and animal systems. The protocol, which has been evaluated in a series of international trials, defines procedures, objects and substances to evaluate the localisation of common slice and volume selection sequences, and to assess other relevant aspects of system performance, including: signal-to-noise; signal loss; relation between signal strength, concentration and volume; selective suppression of spectral lines; spectrometer stability.

Quality assessment in in vivo NMR spectroscopy: I. Introduction, objectives, and activities.

By enabling noninvasive measurements of tissue biochemistry, nuclear magnetic resonance spectroscopy (MRS) provides a unique means of characterizing tissues. Differences in equipment, techniques, and methodology between different laboratories cause major difficulties when comparing results, whether from measurements of tissue metabolism, or from the effects of different therapies. This is of concern in critically evaluating work from different laboratories and centres, causing potential difficulties in reproducing results, limiting the establishment of MRS as a standard method of diagnosis and of characterising disease and response to therapy in the laboratory and clinic. It also poses particular problems in establishing the multicentre clinical trials of MRS that are now required to provide adequate statistical power in confirming the encouraging preliminary clinical observations. These difficulties arise principally from imperfect localization of signal from selected regions of interest in the body, and from the subsequent analyses of the MRS spectra. Improvement is possible by establishing agreed procedures for test measurements and for data analysis, and by using appropriate test objects and test substances to establish the quality of measurements. A concerted research project on characterisation of biological tissues by NMR, principally concerned with MR imaging (MRI), was activated in 1984 by the European Economic Community as part of its third Medical and Health Research Programme, under the auspices of the Biomedical Engineering Concerted Actions' Committee (COMAC-BME). In 1988, this project was prolonged for 5 years, when the programme was expanded to encompass MRS.(ABSTRACT TRUNCATED AT 250 WORDS)

Quality assessment in in vivo NMR spectroscopy: III. Clinical test objects: design, construction, and solutions.

Based on the requirements of test protocols developed to evaluate clinical MRS single slice and volume localisation sequences, two clinical test objects, STO1 and STO2 have been developed. The properties of a range of potential construction materials have been assessed, demonstrating that the water/Perspex interface produced minimum susceptibility effects. The design of the objects has been evaluated in trials on different magnetic resonance instruments, with size and loading being adjusted to allow use on currently available equipment. Appropriate test solutions for 31P and 1H measurements have been developed and their properties evaluated.

Quality assessment in in vivo NMR spectroscopy: V. Multicentre evaluation of prototype test objects and protocols for performance assessment in small bore MRS equipment.

This paper reports the results of multicentre studies aimed at designing, constructing, and evaluating prototype test objects for performance assessment in small-bore MRS systems, by utilizing the test protocols already proposed by the EEC COMAC-BME Concerted Action for clinical MRS equipment. Three classes of test objects were considered: (1) a multicompartment test object for 31P MRS measurements performed with slice-selective sequences; (2) a two-compartment test object for volume-selection 1H MRS; and (3) two-compartment test objects for assessing the performance of experimental systems using ISIS as volume localization sequence in 31P MRS. The results suggested the interest of adopting some of these prototypes for improving the comparison of spectroscopy data obtained from different sites, for providing useful means of quality assurance in experimental MRS, and facilitating the validation of new localization sequences.

Quality assessment in in vivo NMR spectroscopy: VI. Multicentre quantification of MRS test signals.

In this paper the results are presented of a joint European quantitative data-analysis study on a series of in vivo NMR time-domain test signals. The purpose of this study was to investigate, whether the quantitative MRS results from the various European biomedical research-centres can be made more comparable and reproducible. From the results of the study it can be concluded that in case of heavily overlapping NMR lines the quantifications should be done by some form of model function fitting and that, whenever possible, prior knowledge on the quantitative parameters should be incorporated into the quantification algorithms.

Spatially selective RF pulses and the effects of digitization on their performance.

Spectrometers make use of D/A converters to generate RF and gradient shapes. This paper examines by exact simulations the time and amplitude digitization effects, inherent to the use of D/A converters, on the performance of amplitude modulated (AM) frequency selective RF pulses. By making use of Fourier theory and the small tip angle approximation, an approximate model of these effects on the magnetization slice profiles is derived and verified for several pulse types by computer simulations. This approximate model will be used to derive requirements for D/A converters with respect to spatial localization. The dynamics of the spin system allows pulse width modulation (PWM) as an alternative to AM for pulse envelope encoding. The effects of PWM on the slice profile are examined and compared with conventional AM pulses. It is shown by simulation and measurement that adiabatic PWM pulses can be found. In contrast to AM modulated adiabatic pulses, adiabatic PWM pulses have side bands with the same slice quality as the main slice and might therefore be useful as multislice selective pulses.

Lactate imaging of the human brain at 1.5 T using a double-quantum filter.

The use of a double-quantum filtered 1H NMR spectroscopic imaging technique is described to detect the spatial distribution of lactate in the human brain. In two patients the feasibility of this technique is shown and compared with existing single-quantum spectroscopic imaging and single voxel techniques. Single-slice double-quantum filtered lactate images were obtained showing the lactate distribution over the entire slice in the brain. The lipid signal suppression was sufficient for the unambiguous detection of lactate. The signal loss of the lactate signal due to the incorporation of the double-quantum filter was 50-70% relative to the single-quantum signal.

Diffusion in red blood cell suspensions: separation of the intracellular and extracellular NMR sodium signal.

It is shown that the signal of intracellular and extracellular sodium of red blood cells can be separated by a difference in diffusion. Comparison with proton diffusion experiments conducted in parallel showed that this difference was caused by restriction to the cell volume. The measured proton and sodium root mean square displacements agreed well with the cell dimensions. However, this experiment is limited to use in vitro by the required gradient strength.

Tissue characterization by magnetic resonance spectroscopy and imaging: Results of a concerted research project of the European Economic Community. Introduction, objectives, and activities.

The multi-parameter dependence of magnetic resonance (MR) images allows a unique flexibility of soft tissue contrast and gives access to peculiar sources of in vivo tissue characterization, mainly associated with magnetic relaxation properties. However, MR methodologies have not yet expressed their full potential in terms of tissue characterization for several reasons: a) problems of quality control and quantitation have generally not been addressed by centers using MRI equipment, nor in most of the published literature; and b) data scattering of quantitative measurements obtained from tissues in vitro and in vivo appear to be a major factor in inhibiting or limiting the clinical utility of MRI, for a possible in vivo characterization of pathological tissues. An international project, aimed at evaluating the clinical significance of tissue characterization by MR, was activated in 1984 by the Biomedical Engineering Advisory Committee of the European Communities (EC COMAC-BME) within the 3rd EC Medical and Health Research Programme (MHRP). The scientific achievements of this first project (Magn. Reson. Imaging, 6:171-222; 1988) represented the basis for launching and performing a second Concerted Action, in the frame of the 4th MHRP (1988-1992). Main areas of research of this second project were: a) development of standard methodologies for quantitative measurements of MR parameters and correlation with histo- and physiopathology; b) performance assessment and calibration of MR clinical equipment; c) harmonization of test procedures with other centers and industry; and d) pilot multi-center collections of data.(ABSTRACT TRUNCATED AT 250 WORDS)