Jeff F. Dunn

The effects of ketamine–xylazine anesthesia on cerebral blood flow and oxygenation observed using nuclear magnetic resonance perfusion imaging and electron paramagnetic resonance oximetry

Ketamine-xylazine is a commonly used anesthetic for laboratory rats. Previous results showed that rats anesthetized with ketamine-xylazine can have a much lower cerebral partial pressure of oxygen (P(t)O(2)), compared to unanesthetized and isoflurane anesthetized rats. The underlying mechanisms for the P(t)O(2) reduction need to be elucidated. In this study, we measured regional cerebral blood flow (CBF) using nuclear magnetic resonance (NMR) perfusion imaging and cortical P(t)O(2) using electron paramagnetic resonance (EPR) oximetry in the forebrain of rats under isoflurane, ketamine, ketamine-xylazine and isoflurane-xylazine anesthesia. The results show that in ventilated rats ketamine at a dose of 50 mg/kg does not induce significant changes in CBF, compared to isoflurane. Ketamine-xylazine in combination causes 25-65% reductions in forebrain CBF in a region-dependent manner. Adding xylazine to isoflurane anesthesia results in similar regional reductions in CBF. EPR oximetry measurements show ketamine increases cortical P(t)O(2) while xylazine decreases cortical P(t)O(2). The xylazine induced reduction in CBF could explain the reduced brain oxygenation observed in ketamine-xylazine anesthetized rats.

Changes in Oxygenation of Intracranial Tumors With Carbogen: a BOLD MRI and EPR Oximetry Study

To examine, using blood oxygen level dependent (BOLD) MRI and EPR oximetry, the changes in oxygenation of intracranial tumors induced by carbogen breathing.

Micro-imaging of articular cartilage: T2, proton density, and the magic angle effect

RATIONALE AND OBJECTIVES This study was designed to determine the relative influences of proton density versus collagen fiber orientation (through its influence on T2) in defining the layers of articular cartilage as seen in long-repetition-time magnetic resonance (MR) images. The authors mapped the T2 and proton densities of articular cartilage at 0 degree and 55 degrees with respect to the main magnetic field (B0) to determine the influence of T2 and water content on the normal laminar appearance of hyaline cartilage. MATERIALS AND METHODS Six patellae of white-tailed deer were imaged at 7 T. T2 and proton densities were calculated from echo time versus signal intensity plots obtained with a multiecho, composite pulse sequence. Regions of interest in the radial and transitional zones were compared with the articular facets at 0 degree and 55 degrees relative to B0. Transmission electron microscopy was performed for correlation. RESULTS At 0 degree, T2 was longer in the transitional than in the radial zone (29 vs 11 msec). AT 55 degrees, T2 increased in both radial and transitional zones, although the difference between the zones decreased (37 vs 29 msec). There was no difference in proton density between the two layers. CONCLUSION Collagen fiber orientation, through T2 effects, is the dominant influence on the appearance of layers in hyaline cartilage in long-repetition-time MR images; proton density is not a major factor, and the collagen fiber orientation in the transitional zone is not totally random.

Assessment of cerebral pO2 by EPR oximetry in rodents: effects of anesthesia, ischemia, and breathing gas

This report describes experiments designed to assess and illustrate the effectiveness of a new method for the measurement of cerebral interstitial pO2 in conscious rodents. It is based on the use of low frequency electron paramagnetic resonance (EPR) spectroscopy with lithium phthalocyanine as the oxygen sensitive probe. Magnetic resonance imaging was used to document placement of the probe in the brain, and to assess potential cerebral changes associated with the placement. The technique provided accurate and reproducible measurements of localized pO2 in the brains of conscious rodents under a variety of physiological conditions and for time periods of at least 2 weeks. Using this approach we quantitated the depressing effects on cerebral pO2 of three representative anesthetics, isoflurane, ketamine/xylazine, and sodium pentobarbital. The effects of changing the content of oxygen in the breathing gas was investigated and found to change the cerebral pO2. In experiments with gerbils, crystals of lithium phthalocyanine were implanted in each side of the brain and using a one-dimensional magnetic field gradient, simultaneous measurement of pO2 values from normal and ischemic (ischemia induced by unilateral ligation of a carotid artery) hemispheres of the brain were obtained. These results demonstrate that EPR oximetry with lithium phthalocyanine is a versatile and useful method in the measurement of cerebral pO2 under various physiological and pathophysiological conditions.

Near-infrared imaging in the small animal brain: optimization of fiber positions

We investigate fiber placement issues associated with a hybrid magnetic resonance imaging (MRI) near-infrared (NIR) imaging technique for small animal brain studies. Location of the optical fibers on the cranium is examined, with an emphasis on maximizing the recovered resolution and contrast in the region of interest, which in this case is the murine brain. In a series of simulation studies, singular value decomposition of the Jacobian is used in order to determine the measurement sites that provide the most information about the region of interest. The modeling results indicate that data collected using fibers arranged on one side of the head near the brain contain as much information about optical changes within the brain as those positioned equally spaced around the entire periphery of the head. Practical space limitation considerations favor the one-sided fiber array geometry in the case where the NIR acquisition is expected to occur simultaneously with MRI.

Endotoxin-induced changes in intrarenal pO2, measured by in vivo electron paramagnetic resonance oximetry and magnetic resonance imaging.

Electron Paramagnetic Resonance (EPR) oximetry was used to measure tissue oxygen tension (pO2-partial pressure of oxygen) simultaneously in the kidney cortex and outer medulla in vivo in mice. pO2 in the cortex region was higher compared to that in the outer medulla. An intravenous injection of endotoxin resulted in a sharp drop in pO2 in the cortex and an increase in the medulla region, resulting in a transient period of equal pO2 in both regions. In control kidneys, functional Magnetic Resonance (MR) images showed the cortex region to have high signal intensity (T2*-weighted images), indicating that this region was well supplied with oxygenated hemoglobin, whereas the outer medulla showed low signal intensity. After administration of endotoxin, we observed an immediate increase in signal intensity in the outer medulla region, reflecting an increased level of oxygenated blood in this region. Pretreatment of mice with NG-monomethyl-L-arginine prevented both the changes in tissue pO2 and distribution of oxygenated hemoglobin, suggesting that localized production of nitric oxide has a critical role to play in renal medullary hemodynamics. In combining in vivo EPR with MR images of kidneys, we demonstrate the usefulness of these techniques for monitoring renal pO2 and changes in the distribution of oxygen.

In vivo electron paramagnetic resonance oximetry with particulate materials

Electron paramagnetic resonance (EPR) methods can be used to study tissue pO(2) (PtO(2)) in anesthetized or awake animals (EPR oximetry). The method takes advantage of the fact that some paramagnetic materials have an EPR linewidth that is sensitive to the pO(2) in which the material is located. This article provides an overview of the method of EPR oximetry using implanted particulate materials as the sensors of pO(2). Characteristics of these materials are described to help the reader understand the factors involved in choosing the optimum particulate material. Examples of biological studies are included that show how EPR oximetry may be used on both awake and anesthetized animals.

QUANTITATIVE MAGNETIC RESONANCE IMAGING OF THE mdx MOUSE MODEL OF DUCHENNE MUSCULAR DYSTROPHY

The mdx mouse has a genetically homologous disease to Duchenne muscular dystrophy in humans. The disease progression, however, is not accompanied by the same level of fatty infiltration and muscle degeneration as occurs in humans. Thus, the presence of histological/pathological changes in living mdx mice has been difficult to monitor. We quantified proton density and the T2 relaxation time of protons with a resolution of 195 × 195 × 1000 μm using multiecho magnetic resonance (MR) imaging at 7 Tesla. These relaxation data were correlated with water content in both muscle and brain of mdx and controls. No differences were observed in brain. The mdx muscles had increased water content and proton density and decreased T2 relative to controls. These data indicate that there are intrinsic changes in T2 (opposite to that which would be induced by fatty infiltration) and suggest that T2 imaging could be used to monitor progression and treatment in this animal model.

Monitoring angiogenesis in brain using steady-state quantification of ΔR2 with MION infusion

An MRI method for quantification of cerebral blood volume (CBV) in time‐course studies of angiogenesis is described. Angiogenesis was stimulated by acclimation to hypoxia. The change in relaxation rate, R2, which is relatively sensitive to the microvasculature, was quantified before and after infusion of a superparamagnetic vascular contrast agent (MION). The ΔR2 was measured in serum and brain parenchyma with a multiecho sequence. In vitro and in vivo calibration curves of MION concentration vs. R2 were approximated by a linear function. CBV was 3.14 ± 0.32% (mean ± SE, n = 13) and 6.42 ± 0.54% (n = 4) before and after acclimation. A second acclimated group was hemodiluted to control for polycythemia. CBV was not significantly different between hemodiluted and nonhemodiluted groups. In animals where NMR measurements were taken before and after acclimation, there was a 120% increase in CBV. The NMR technique was validated using quantitative morphometrics, which showed an increase of 147% in CBV with acclimation. We found a linear correlation between MRI and the morphometric results for CBV, as well as demonstrating a quantitative equivalence for relative changes in CBV. This article describes a simple, repeatable method of imaging brain microvascular volume using a plasma‐based contrast agent that can be applied to longitudinal studies of angiogenesis. Magn Reson Med 51:55–61, 2004.

Noninvasive assessment of cerebral oxygenation during acclimation to hypobaric hypoxia.

Factors regulating cerebral tissue Po2 (PtO2) are complex. With the increased use of clinical PtO2 monitors, it has become important to elucidate these mechanisms. The authors are investigating a new methodology (electron paramagnetic resonance oximetry) for use in monitoring cerebral PtO2 in awake animals over time courses of weeks. The authors used this to study cerebral PtO2 in rats during chronic acclimation to hypoxia predicting that such acclimation would cause an increase in PtO2 because of increases that occur in capillary density and oxygen carrying capacity. The average PtO2 between 7 and 21 days was increased by 228% over controls.