Albert L. Busza

Diffusion-weighted imaging studies of cerebral ischemia in gerbils. Potential relevance to energy failure.

Background and Purpose: Diffusion-weighted magnetic resonance imaging has been shown to be particularly suited to the study of the acute phase of cerebral ischemia in animal models. The studies reported in this paper were undertaken to determine whether this technique is sensitive to the known ischemic thresholds for cerebral tissue energy failure and disturbance of membrane ion gradients. Methods: Diffusion-weighted images of the gerbil brain were acquired under two sets of experimental conditions: as a function of cerebral blood flow after controlled graded occlusion of the common carotid arteries (partial ischemia), as a function of time following complete bilateral carotid artery occlusion (severe global ischemia), and on deocclusion after 60 minutes of ischemia. Results: During partial cerebral ischemia, the diffusion-weighted images remained unchanged until the cerebral blood flow was reduced to 15–20 ml · 100 g−1 · min−1 and below, when image intensity increased as the cerebral blood flow was lowered further. This is similar to the critical flow threshold for maintenance of tissue high-energy metabolites and ion homeostasis. After the onset of severe global cerebral ischemia, diffusion-weighted image intensity increased gradually after a delay of approximately 2.5 minutes, consistent with complete loss of tissue adenosine triphosphate and with the time course of increase in extracellular potassium. This hyperintensity decreased on deocclusion following 60 minutes of ischemia. Conclusions: The data suggest that diffusion-weighted imaging is sensitive to the disruption of tissue energy metabolism or a consequence of this disruption. This raises the possibility of imaging energy failure noninvasively. In humans, this could have potential in visualizing brain regions where energy metabolism is impaired, particularly during the acute phase following stroke.

Acute Cerebral Ischaemia: Concurrent Changes in Cerebral Blood Flow, Energy Metabolites, pH, and Lactate Measured with Hydrogen Clearance and 31P and 1H Nuclear Magnetic Resonance Spectroscopy. II. Changes during Ischaemia

CBF has been measured with the hydrogen clearance technique in the two cerebral hemispheres of the gerbil under halothane anaesthesia. At the same time, intracellular pH and the concentrations of lactate and high-energy phosphates were measured in the brain using 1H and 31P nuclear magnetic resonance spectroscopy. Flow and metabolism have been followed during either a 15- or a 30-min ischaemic period (induced by bilateral carotid occlusion) and for up to 1 h of recovery. There was no significant difference between the flow characteristics of the two experimental groups. High-energy phosphate levels and pH returned to control within ∼20 min of the end of the ischaemic period. Lactate clearance, following a 30-min occlusion, was slower than the recovery of pH. The concentration of free ADP, calculated from the creatine kinase equilibrium, was lower during the recovery phase than under control conditions.

Early changes in water diffusion, perfusion, T1, and T2 during focal cerebral ischemia in the rat studied at 8.5 T †

The time evolution of water diffusion, perfusion, T1, and T2 is investigated at high magnetic field (8.5 T) following permanent middle cerebral artery occlusion in the rat. Cerebral blood flow maps were obtained using arterial spin tagging. Although the quantitative perfusion measurements in ischemic tissue still pose difficulties, the combined perfusion and diffusion data nevertheless distinguish between a “moderately affected area,” with reduced perfusion but normal diffusion; and a “severely affected area,” in which both perfusion and diffusion are significantly reduced. Two novel magnetic resonance imaging observations are reported, namely, a decrease in T2 and an increase in T1, both within the first few minutes of ischemia. The rapid initial decrease in T2 is believed to be associated with an increase in deoxyhemoglobin levels, while the initial increase in T1 may be related to several factors, such as flow effects, an alteration in tissue oxygenation, and changes in water environment. Magn Reson Med 41:479–485, 1999.

Acute changes in MRI diffusion, perfusion, T1, and T2 in a rat model of oligemia produced by partial occlusion of the middle cerebral artery

Oligemic regions, in which the cerebral blood flow is reduced without impaired energy metabolism, have the potential to evolve toward infarction and remain a target for therapy. The aim of this study was to investigate this oligemic region using various MRI parameters in a rat model of focal oligemia. This model has been designed specifically for remote‐controlled occlusion from outside an MRI scanner. Wistar rats underwent remote partial MCAO using an undersize 0.2 mm nylon monofilament with a bullet‐shaped tip. Cerebral blood flow (CBFASL), using an arterial spin labeling technique, the apparent diffusion coefficient of water (ADC), and the relaxation times T1 and T2 were acquired using an 8.5 T vertical magnet. Following occlusion there was a decrease in CBFASL to 35 ± 5% of baseline throughout the middle cerebral artery territory. During the entire period of the study there were no observed changes in the ADC. On occlusion, T2 rapidly decreased in both cortex and basal ganglia and then normalized to the preocclusion values. T1 values rapidly increased (within approximately 7 min) on occlusion. In conclusion, this study demonstrates the feasibility of partially occluding the middle cerebral artery to produce a large area of oligemia within the MRI scanner. In this region of oligemic flow we detect a rapid increase in T1 and decrease in T2. These changes occur before the onset of vasogenic edema. We attribute the acute change in T2 to increased amounts of deoxyhemoglobin; the mechanisms underlying the change in T1 require further investigation. Magn Reson Med 44:706–712, 2000.

In vivo GABA+ measurement at 1.5T using a PRESS-localized double quantum filter.

A point‐resolved spectroscopy (PRESS)‐localized double quantum filter was implemented on a 1.5T clinical scanner for the estimation of γ‐amino butyric acid (GABA) concentrations in vivo. Several calibrations were found to be necessary for consistent results to be obtained. The apparent filter yield was approximately 38%; filter strength was sufficient to reduce the singlet metabolite peaks in vivo to below the level of the noise. Metabolite‐nulled experiments were performed, which confirmed that significant overlap occurred between macromolecule signals and the GABA resonance at 3.1 ppm. Although the multiplet arm at 2.9 ppm was confirmed to be relatively free of contamination with macromolecules, some contribution from these and from peptides is likely to remain; therefore, the term GABA+ is used. GABA+ concentrations were estimated relative to creatine (Cr) at the same echo time (TE) in a group of controls, studied on two occasions. The GABA+ concentration in 35‐ml regions of interest (ROIs) in the occipital lobe was found to be 1.4 ± 0.2 mM, with scan‐rescan repeatability of 38%. Magn Reson Med 48:233–241, 2002.

Hypothermic perfusion preservation of liver : the role of phosphate in stimulating ATP synthesis studied by 31P NMR

Hypothermic perfusion of rat livers was investigated by 31phosphorus nuclear magnetic resonance (31P NMR) spectroscopy using a temperature-controlled module that allowed data acquisition at various time points during a 48-h period. The livers were perfused with an oxygenated lactobionate/raffinose-based solution containing adenosine and inorganic phosphate, and changes in tissue oedema were monitored by direct on-line measurements of liver weight changes. Liver tissue ATP concentrations, determined by fluorimetric assay, were low immediately after organ removal, probably reflecting metabolic stress during the removal period, and these increased slightly during the next 3 h. This was reflected by changes in the 31P NMR spectra. However, by 24 h ATP levels had increased significantly, and these were maintained for up to 48 h, suggesting a shift in the balance between energy production and consumption. When inorganic phosphate was replaced by another anion (citrate), ATP was maintained at a constant lower level during perfusion for 48 h. Tissue weight changes were similar in both groups, suggesting that volume control was not affected by the different ATP contents of the livers. By combining the temperature-controlled module with a separate perfusion circuit, NMR spectroscopy can provide a sensitive method for following energy metabolism in the same organ over long periods during hypothermic perfusion.

Applications of magnetic resonance spectroscopy and diffusion-weighted imaging to the study of brain biochemistry and pathology

The first practical demonstration that nuclear magnetic resonance (NMR) spectroscopy could be applied to the study of brain biochemistry in vivo came in 1980, with the studies of the rat brain using a surface coil. Since then the technique has been rapidly and extensively developed into a versatile, non-invasive tool for the investigation of various aspects of brain biochemistry, physiology and disease. NMR is non-destructive and can be used to examine a wide variety of samples, ranging from localized regions within the whole brain in humans or animals, through tissue preparations (perfused organ, tissue slices and homogenates), to isolated cells and aqueous solutions, such as tissue extracts. 31P and 1H NMR spectra deriving from endogenous compounds of the brain in situ allow assessment of tissue metabolites and provide information about high-energy phosphates, lactate, certain amino acids, intracellular pH and ionic concentrations. Exogenous substrates or probes labelled with stable isotopes can also be introduced into the brain and used to monitor metabolism. Animal models of brain diseases have given some impetus to rapid progress in clinical NMR spectroscopy and also magnetic imaging techniques. The purpose of this article is to highlight the type of information available from these NMR techniques, and to present this in a neuroscience context, emphasizing the biochemical, physiological and pathological information that can be obtained using these methods.

STUDIES ON CRYOPROTECTANT EQUILIBRATION IN THE INTACT RAT-LIVER USING NUCLEAR MAGNETIC-RESONANCE SPECTROSCOPY - A NONINVASIVE METHOD TO ASSESS DISTRIBUTION OF DIMETHYL-SULFOXIDE IN TISSUES

Nuclear magnetic resonance (NMR) spectroscopy was used in the study of rat livers following flushing with a clinically used preservation solution containing either 12 or 30% (v/v) Me2SO. The extent of equilibration of Me2SO in the tissue after 10-15 min of perfusion with Me2SO and again after subsequent washout with Me2SO-free medium was assessed by 1H NMR spectroscopy. 31P NMR spectroscopy was used to follow the changes in ATP, ADP, inorganic phosphate, and tissue pH. The data show that 1H NMR spectroscopy can be used as a sensitive and rapid method of assessing the equilibration and concentration of compounds such as Me2SO, since these compounds are likely to be present at concentrations greatly in excess of other constituents of the medium and will therefore give rise to strong, easily detected signals. At the same time, 31P NMR spectroscopy can be used to monitor the metabolic status of the tissue reflected in the levels of ATP, ADP, and inorganic phosphate, as well as being a noninvasive monitor of intracellular pH. The possibility of determining the tissue pH in the presence of solutes such as Me2SO is discussed.

The relationship between magnetic resonance diffusion imaging and autoradiographic markers of cerebral blood flow and hypoxia in an animal stroke model

This study examined the relationship between magnetic resonance diffusion imaging and autoradiographic markers of cerebral blood flow (99mTc‐hexamethylpropylene amine oxime) and cerebral hypoxia (125I‐iodoazomycin arabinoside) in a rat model of stroke. Middle cerebral artery occlusion in the rat was performed using an intraluminal suture approach. Diffusion, hypoxia, and blood flow maps were acquired 2 hr following occlusion, and were compared with T2 images and histology at 7 hr. Two hours following middle cerebral artery occlusion the lesion distributions from the diffusion maps and hypoxic autoradiographs were similar. The blood flow threshold for increased uptake of the hypoxic marker was approximately 34 ± 7% of the normal flow. The combination of diffusion or hypoxic images with perfusion maps allowed differentiation between four regions: 1) normal tissue; 2) a region of decreased perfusion but normal diffusion and normal uptake of hypoxic marker; 3) a region of decreased perfusion, decreased diffusion and increased uptake of hypoxic marker; 4) a region of decreased perfusion, decreased diffusion and low uptake of hypoxic marker. The areas for increased uptake of hypoxic marker and decreased diffusion are equivalent, indicating similar blood flow thresholds. Regions of oligaemic misery perfusion, ischaemic misery perfusion and lesion core may be delineated with the combination of diffusion or hypoxic images and perfusion maps. Magn Reson Med 41:706–714, 1999.

The application of nuclear magnetic resonance spectroscopy to assess viability in stored tissues and organs

The use of nuclear magnetic resonance (NMR) spectroscopy to assess metabolic viability in organ preservation is discussed. A brief coverage of the physical principles involved and the biochemical information available from NMR spectroscopy is given. We also present the advantages and disadvantages of the method and outline the future possibilities of the technique in relation to organ preservation.