Cerebral perfusion during anesthesia with fentanyl, isoflurane, or pentobarbital in normal rats studied by arterial spin-labeled MRI.

The influence of anesthetic agents on cerebral blood flow (CBF) was tested in normal rats. CBF is quantified with arterial spin-labeled MRI in rats anesthetized with either an opiate (fentanyl), a potent inhalation anesthetic agent (isoflurane), or a barbiturate (pentobarbital) using doses commonly employed in experimental paradigms. CBF values were found to be about 2.5-3 times lower in most regions analyzed during anesthesia with either fentanyl (with N(2)O/O(2)) or pentobarbital vs. isoflurane (with N(2)O/O(2)), in agreement with findings utilizing invasive measurement techniques. CBF was heterogeneous in rats anesthetized with isoflurane (with N(2)O/O(2)), but relatively homogeneous in rats anesthetized with either fentanyl (with N(2)O/O(2)) or pentobarbital, also in agreement with studies using other techniques. Magn Reson Med 46:202-206, 2001.

Assessment of the effect of 2-chloroadenosine in normal rat brain using spin-labeled MRI measurement of perfusion.

Adenosine analogs such as 2-chloroadenosine are potent cerebrovasodilators. Spin-labeled MRI was used to investigate the spatial distribution, dose-response, and timing of the effect of 2-chloroadenosine on cerebral blood flow (CBF) after intraparenchymal injection into rat brain. Sprague-Dawley rats (N = 10) were injected with 2-chloroadenosine at doses of 0.3, 6.0, or 12 nmoles, or saline vehicle (2-4 microL). CBF was serially quantified in a slice through the injection site in a circular (3.6 mm diameter) region of interest (ROI) around the injection and in ipsilateral hemispheric ROIs at approximately 90 min and approximately 180 min. Marked 3.77- and 3.93-fold increases in CBF (vs. vehicle) were seen in the circular ROI at approximately 90 min and approximately 180 min after 12-nmol injection, respectively. Similarly, 2.92- and 2.78-fold increases in hemispheric CBF were observed at approximately 90 min and approximately 180 min, respectively, after injection of 12 nmoles. Linear dose-response relationships were observed at both times after injection in both ROIs (all P < 0.01). Spin-labeling MRI assessment revealed that parenchymal injection of 2-chloroadenosine produces potent, dose-dependent, and sustained vasodilation over large areas of brain. This treatment and imaging paradigm should facilitate investigation of the effect of CBF promotion in models of traumatic and ischemic brain injury.

Magnetic resonance imaging detection of rat renal transplant rejection by monitoring macrophage infiltration.

BACKGROUND: A rat renal transplantation model was studied by noninvasive magnetic resonance imaging (MRI) with an infusion of ultrasmall superparamagnetic iron oxide (USPIO) particles to test whether the accumulation of immune cells, such as macrophages, could be detected in vivo while the kidney transplant was being rejected. METHODS: Major histocompatibility disparate DA to BN male rat renal transplantation recipients were infused with USPIO particles, with magnetic resonance (MR) images acquired before, immediately after, and one day following infusion. RESULTS: When the USPIO infusion was on the fourth day post-transplantation, some rejecting allografts showed a decrease of MR signal intensity one day later. Isografts and allografts with triple immunosuppressant treatment had no MR signal reduction. Immunohistologic staining for ED1+ macrophages and CD4+ and CD8+ T cells in allogeneic transplanted kidneys indicated the accumulation of these immune cells as acute rejection occurred. Morphological studies by electron microscopy confirmed the existence of iron inside the lysosomes of macrophages of rejecting kidneys, while Prussian blue staining detected the presence of iron plaques in macrophages. Isografts and allografts with a triple immunosuppressant treatment exhibited smaller MR signal reductions with minimal histologic changes. CONCLUSIONS: The concurrence of MR signal reduction following USPIO infusion with pathological manifestation in a rat renal allograft model suggests the possibility that renal transplantation status may be assessed by MRI using USPIO particles as markers for the accumulation of immune cells, such as macrophages.

Assessment of 2-chloroadenosine treatment after experimental traumatic brain injury in the rat using arterial spin-labeled MRI: a preliminary report.

Adenosine is a putative endogenous neuroprotectant. Its action at A1 receptors mitigates excitotoxicity while action at A2 receptors increases cerebral blood flow (CBF). We hypothesized that cerebral injection of the adenosine analog, 2-chloroadenosine, would decrease swelling and increase CBF early after experimental traumatic brain injury (TBI). To test this hypothesis, rats were anesthetized and subjected to TBI using a controlled cortical impact (CCI) model (n = 5/group). Immediately after injury, 2-chloroadenosine (0.3 nmole in 2 microliters) or an equal volume of vehicle were stereotactically injected lateral to the area of contusion. Using magnetic resonance imaging (MRI), in vivo spin-lattice relaxation time of tissue water (Tlobs) and CBF (arterial spin labeling) were measured in a 2-mm thick slice in the injured and non-injured hemispheres at 3-4 h after CCI. In a separate, preliminary experiment, the effect of 2-chloroadenosine injection in normal rat brain was studied. Rats (n = 2) were anesthetized and a burr hole was made for injection of 2-chloroadenosine into the same site as in the TBI model. One rat received the standard dose of 0.3 nmole and one rat received a 6 nmole injection. Tlobs and CBF studies were obtained 1.5-3.5 h after injection, using the same MRI methods as in the TBI study. In rats subjected to TBI, treatment with 2-chloroadenosine attenuated the increase in Tlobs after injury (p < 0.05 for treatment vs vehicle) in both hippocampus and cortex ipsilateral to injury. However, treatment with 2-chloroadenosine did not improve post-traumatic hypoperfusion. In normal rats, injection of 0.3 nmole of 2-chloroadenosine did not increase CBF, but the higher dosage of 6 nmole dramatically increased hemispheric CBF by 1.5-2.0-fold. The effect of local injection of 2-chloroadenosine at a dose of 0.3 nmole after experimental TBI on Tlobs presumably represents a reduction in post-traumatic edema. This reduction in edema, along with the augmentation of CBF seen in normal rats at higher dosage (6 nmole), supports a role for adenosine in neuroprotection following TBI.

Early perfusion after controlled cortical impact in rats: quantification by arterial spin-labeled MRI and the influence of spin-lattice relaxation time heterogeneity.

Early posttraumatic cerebral hypoperfusion is implicated in the evolution of secondary damage after experimental and clinical traumatic brain injury (TBI). This is the first report of cerebral blood flow (CBF) measurement by continuous arterial spin-labeled magnetic resonance imaging (MRI) early after TBI in rats using the controlled cortical impact (CCI) model. CCI reduced CBF globally at approximately 3 hr (versus normal), with 85% and 49% reductions in a contused cortical region and contralateral cortex, respectively. In contrast, a prior MRI study from this laboratory showed at 24 hr post trauma a focal CBF reduction restricted to the injury site. In vivo spin-lattice relaxation time (T(1obs)), which is used in CBF quantification, was spatially heterogeneous early after CCI, a time when edema is developing in injured brain tissue. At 4.7 T, T(1obs) values are increased 29% in the contusion (versus normal), consequently reducing CBF quantification to a similar degree. MRI should facilitate coupling posttraumatic CBF with long-term functional outcome. Magn Reson Med 42:673-681, 1999.

Perfusion quantitation in transplanted rat kidney by MRI with arterial spin labeling.

The purpose of this study was to determine the feasibility of using quantitative magnetic resonance imaging (MRI) with non-invasive arterial spin labeling to assess perfusion of transplanted kidneys in rats. MRI studies were performed on five groups of rats: normal Fisher 344 rats, Fisher 344 rats that had received a syngeneic kidney transplant either 3 or seven days prior to study, and Fisher 344 rats that had received an allogeneic kidney (ACI rat as the donor) either three or seven days prior to study. The contralateral native kidney remained in place for comparison. Cortical perfusion was quantitated from a slice through the center of each kidney in anesthetized rats at 4.7 Tesla with a fast gradient-echo MRI sequence following the arterial spin labeling. The spin-lattice relaxation time was measured within the cortex, and the cross sectional area of the kidney was also determined within the same MRI plane. Immediately after the perfusion imaging measurement, transplanted kidneys were removed and scored for rejection using the Banff histological criteria. Renal cortical perfusion in normal kidneys was 7.5 +/- 0.8 ml/g/min (N = 12 rats, 24 kidneys). At the third day post-transplantation, that is, before marked acute rejection, the renal cortical perfusion rate was similar in both syngeneic and allogeneic kidneys [3.3 +/- 1.7 (N = 6) and 3.0 +/- 2.4 ml/g/min (N = 6), respectively]. In contrast, at the seventh day post-transplantation, that is, during severe rejection, the renal cortical perfusion rate in allogeneic kidneys was very low (undetectable) compared to the value in syngeneic kidneys [that is, < or = 0.3 (N = 6) versus 5.2 +/- 2.0 ml/g/min (N = 6), respectively]. Moreover, the renal cortical perfusion rate determined by MRI was significantly (P < 0.05, r = -0.82) correlated with histological rejection. We conclude that the quantitative measurement of renal cortical perfusion by MRI with arterial spin-labeling could provide a non-invasive diagnostic method for monitoring the status of renal transplants without requiring the administration of a contrast agent.

Assessment of cerebral blood flow and CO2 reactivity after controlled cortical impact by perfusion magnetic resonance imaging using arterial spin-labeling in rats.

We measured CBF and CO2 reactivity after traumatic brain injury (TBI) produced by controlled cortical impact (CCI) using magnetic resonance imaging (MRI) and spin-labeled carotid artery water protons as an endogenous tracer. Fourteen Sprague-Dawley rats divided into TBI (CCI; 4.02 +/- 0.14 m/s velocity; 2.5 mm deformation), sham, and control groups were studied 24 hours after TBI or surgery. Perfusion maps were generated during normocarbia (Paco2 30 to 40 mm Hg) and hypocarbia (PaCO2 15 to 25 mm Hg). During normocarbia, CBF was reduced within a cortical region of interest (ROI, injured versus contralateral) after TBI (200 +/- 82 versus 296 +/- 65 mL.100 g-1.min-1, P < 0.05). Within a contusion-enriched ROI, CBF was reduced after TBI (142 +/- 73 versus 280 +/- 64 mL.100 g-1.min-1, P < 0.05). Cerebral blood flow in the sham group was modestly reduced (212 +/- 112 versus 262 +/- 118 mL.100 g-1.min-1, P < 0.05). Also, TBI widened the distribution of CBF in injured and contralateral cortex. Hypocarbia reduced cortical CBF in control (48%), sham (45%), and TBI rats (48%) versus normocarbia, P < 0.05. In the contusion-enriched ROI, only controls showed a significant reduction in CBF, suggesting blunted CO2 reactivity in the sham and TBI group. CO2 reactivity was reduced in the sham (13%) and TBI (30%) groups within the cortical ROI (versus contralateral cortex). These values were increased twofold within the contusion-enriched ROI but were not statistically significant. After TBI, hypocarbia narrowed the CBF distribution in the injured cortex. We conclude that perfusion MRI using arterial spin-labeling is feasible for the serial, noninvasive measurement of CBF and CO2 reactivity in rats.

Transmural bioenergetic responses of normal myocardium to high workstates.

The response of myocardial high-energy and inorganic phosphates (HEP and Pi, respectively) and associated changes in myocardial blood flow, lactate uptake, and O2 consumption (MVo2) rates were examined in an open-chest canine model during progressively increasing workloads achieved by catecholamine infusion. HEP and Pi levels (measured with transmurally localized 31P-nuclear magnetic resonance spectroscopy) were unaffected by moderate increases in the level of energy expenditure but were significantly altered by high workloads, especially in the subepicardium. The MVo2 and HEP data from three different protocols that utilized pharmacological augmentation of blood flow demonstrated that the maximal rate of myocardial energy production during inotropic stimulation was dictated by perfusion limitation. This limitation was more severe in the subepicardial layer at the high workloads despite equivalent or even higher increases in blood flow to this layer, reflecting a preferential enhancement of demand in the outer layer by catecholamines. In contrast, under basal conditions, existence of a marginal perfusion limitation was evident in the inner but not in the outer layer.

Effects of dobutamine on myocardial blood flow, contractile function, and bioenergetic responses distal to coronary stenosis: implications with regard to dobutamine stress testing.

To determine the effects of dobutamine stimulation on myocardium distal to a coronary stenosis, transmural spatially localized phosphorus 31 nuclear magnetic resonance measurements of myocardial high-energy phosphate compounds (adenosine triphosphate and phosphocreatine), inorganic phosphate, and blood flow and systolic wall thickening were made in 8 open-chested dogs. Data were collected under (1) control conditions, (2) after the application of a moderate coronary stenosis, (3) during infusion of dobutamine with continuing stenosis, and (4) after the release of the stenosis with continuing dobutamine. Stenosis was associated with concordant reductions of subendocardial blood flow, wall thickening, and high-energy phosphate, and mild elevation of inorganic phosphate; subepicardial measurements were essentially unchanged. During dobutamine infusion, blood flow increased in all myocardial layers. Wall thickening returned to control values in the subendocardium and increased nonsignificantly in the subepicardium. Additional loss of high-energy phosphate occurred only in the subepicardium. The data suggest that improved contractile function associated with dobutamine infusion resulted from the inotropic effects of dobutamine and was made possible by the improved blood flow it produced. The data indicate that measurements of blood flow and contractile function do not reliably predict the transmural myocardial metabolic responses to inotropic perturbations in the hypoperfused heart. Taken together, the present findings yield insights with regard to the interpretation of diagnostic dobutamine stimulation testing with single photon emission tomography, radionuclide angiography, and echocardiography.

An in vivo 31P magnetic resonance spectroscopy study of uridine excess in rats fed orotic acid.

Spatially localized 31P NMR spectroscopy was used to assay in vivo the liver of intact rats fed orotic acid (OA) in a diet which produces hepatic steatosis. Twenty-three sets of multiple volume spectra were obtained from twenty-one 265- to 315-g female rats after 0-9 days of feeding either a 1% OA/64% sucrose diet (12 rats) or a 65% sucrose control diet (9 rats). The intensity of the in vivo diphosphodiester resonance ascribed to UDP-hexos(amin)es increased and the phosphomonoester resonance decreased in intensity prior to fatty infiltration. High resolution NMR spectroscopy of extracts of these livers indicated that the UDP-hexos(amin)e peak included four different UDP-sugars including UDP-N-acetylglucosamine (UDP-glcNAc), and that lower phosphocholine (P-Cho) accounted for the lower phosphomonoester resonance in vivo. Increased UDP-glcNAc is thought to reflect impaired lipoprotein glycosylation as a mechanism for hepatic steatosis in orotic acid feeding. P-Cho deficiency has been shown to be due to an increased rate of phosphatidylcholine synthesis. Low P-Cho concentration has been shown to be associated with lipid accumulation in a choline-deficient diet, but was not previously associated with hepatic steatosis in OA feeding. Changes in phosphorus metabolites were observed 2 days prior to development of fatty liver. HPLC assay of uridine nucleotides showed a good correlation between magnetic resonance spectroscopy and HPLC quantitation. In this study there were two biochemical correlates of impaired hepatic lipid secretion detectable by in vivo assay with 31P NMR spectroscopy. This method has application for noninvasive assays in ornithine transcarbamylase-deficient patients.

Spectroscopic imaging of circular voxels with a two-dimensional Fourier-series window technique.

Spatial localization with the spectroscopic imaging technique is normally implemented with the Fourier-transform approach, yielding rectangular voxels, with potentially significant cross-voxel contamination. Multidimensional Fourier-series window (FSW) is an alternate approach that generates single voxels of predetermined shape, with minimal out-of-voxel contamination. The spatial location of the voxel is shifted by means of postacquisition processing. A two-dimensional circular voxel is introduced, which for many in vivo applications is a good match of the region of interest. Phantom images illustrate the spatial distribution of signal intensity within the circular FSW voxels. Phantom spectroscopic studies show excellent spatial localization, with no detectable out-of-voxel contamination. The circular FSW voxel approach is implemented in human and animal model studies, demonstrating the technique's utility. This arbitrary shape approach can be extended to three dimensions, defining, for example, cylinders, spheres, or ellipsoids.

Surface coil cardiac tagging and 31P spectroscopic localization with B1-insensitive adiabatic pulses.

A technique is presented for MRI tagging in the presence of inhomogeneous B1 fields. A rectangular tagging grid is produced with B1-insensitive adiabatic pulses in a magnetization preparation period that precedes image acquisition. Phantom results demonstrate that the method is well-suited to surface coil experiments. The technique is applied to a canine model of myocardial ischemia to track the spatially dependent wall motion of the left ventricle during the cardiac cycle. Transmural 31P spectra are acquired from the same double-tuned surface coil, with tagging and spectroscopy performed for the first time, during normal, ischemic, and recovery conditions for the same animal.

High-energy phosphate responses to tachycardia and inotropic stimulation in left ventricular hypertrophy.

Spatially localized nuclear magnetic resonance (NMR) spectroscopy was used to examine the effect of tachycardia and inotropic stimulation on myocardial ATP, creatine phosphate (CrP), and inorganic phosphate (Pi) in animals with left ventricular hypertrophy (LVH). Studies were performed in eight normal dogs and seven dogs with moderate LVH produced by banding the ascending aorta. 31P-NMR spectra were obtained from five layers across the LV wall, while blood flow (BF) was measured with microspheres during control conditions, pacing at 200 and 240 beats/min, and during dobutamine infusion (Dob). Myocardial ATP and CrP levels were normal in the LVH hearts during control conditions. Pacing did not alter the transmural distribution of perfusion or the levels of CrP, ATP, and Pi in normal hearts. In contrast, in four of seven LVH hearts, pacing decreased the subendocardial/subepicardial (ENDO/EPI) BF ratio and caused depletion of CrP and appearance of Pi characteristic of ischemia in the subendocardium. Dob produced greater increases in the heart rate x LV systolic pressure product (RPP) and greater increases of Pi and decreases of CrP in LVH than in normal hearts; however, at comparable elevations of RPP the alterations of Pi and CrP were similar in both groups. Although Dob decreased the ENDO/EPI in LVH hearts, Dob-induced alterations in CrP and Pi were uniform across the LV wall. Increasing myocardial BF with adenosine or carbochromen did not reverse the alterations in Pi or CrP produced by Dob. We conclude that 1) ENDO perfusion abnormalities during tachycardia in LVH do produce ENDO subendocardial ischemia; 2) when the degree of augmentation of mechanical performance is considered, the metabolic changes induced by Dob were similar in normal and LVH hearts; 3) Dob-induced alterations in Pi and CrP were not related to inadequate perfusion, since increasing coronary BF did not reverse these changes; and 4) alterations of Pi and CrP during Dob infusion were not more prominent in the ENDO, indicating that the decreased ENDO/EPI flow did not cause ENDO ischemia but may reflect relatively lower O2 demands in this region during inotropic stimulation.

Potential pitfalls of functional MRI using conventional gradient-recalled echo techniques.

The conventional gradient-recalled echo technique, FLASH, has widely been used for functional MRI. FLASH results at 4 T with short TEs of 10-20 ms mimic those at 1.5 T with TEs of 25-50 ms or longer. Under these conditions, large venous vessels dominate the activated area; however, the use of longer TEs at 4 T reveals activation in gray matter areas as well as large vessels. Inflow effects of large vessels can be greatly reduced with centric-reordering of phase-encoding steps and inter-image delay. Finger and toe movement paradigms show that functional activation maps are consistent with classical somatotopic maps, and are specific to the tasks. Navigator-based motion correction generates functional maps with larger activation areas by reducing physiological noise.

Imaging at high magnetic fields: initial experiences at 4 T.

This article reviews the preliminary experiences and the results obtained on the human brain at 4 T at the University of Minnesota. Anatomical and functional images are presented. Contrary to initial expectations and the early results, it is possible to obtain high-resolution images of the human brain with exquisite T1 contrast, delineating structures especially in the basal ganglia and thalamus, which were not observed clearly in 1.5-T images until now. These 4-T images are possible using a new approach that achieves maximal contrast for different T1 values at approximately the same repetition time and has built-in tolerance to variations in B1 magnitude. For functional images, the high field provides increased contribution from the venuoles and the capillary bed because the susceptibility-induced alterations in 1/T2* from these small-diameter vessels increase quadratically with the magnitude of the main field. Images obtained with short echo times at 4 T, and by implication at lower fields with correspondingly longer echo times, are expected to be dominated by contributions from large venous vessel or in-flow effects from the large arteries; such images are undesirable because of their poor spatial correspondence with actual sites of neuronal activity.

Bioenergetic abnormalities associated with severe left ventricular hypertrophy.

Transmurally localized 31P-nuclear magnetic resonance spectroscopy (NMR) was used to study the effect of severe pressure overload left ventricular hypertrophy (LVH) on myocardial high energy phosphate content. Studies were performed on 8 normal dogs and 12 dogs with severe left ventricular hypertrophy produced by banding the ascending aorta at 8 wk of age. Spatially localized 31P-NMR spectroscopy provided measurements of the transmural distribution of myocardial ATP, phosphocreatine (CP), and inorganic phosphate (Pi); spectra were calibrated from measurements of ATP content in myocardial biopsies using HPLC. Blood flow was measured with microspheres. In hypertrophied hearts during basal conditions, ATP was decreased by 42%, CP by 58%, and the CP/ATP ratio by 32% in comparison with normal. Increasing myocardial blood flow with adenosine did not correct these abnormalities, indicating that they were not the result of persistent hypoperfusion. Atrial pacing at 200 and 240 beats per min caused no change in high energy phosphate content in normal hearts but resulted in further CP depletion with Pi accumulation in the inner left ventricular layers of the hypertrophied hearts. These changes were correlated with redistribution of blood flow away from the subendocardium in LVH hearts. These findings demonstrate that high energy phosphate levels and the CP/ATP ratio are significantly decreased in severe LVH. These abnormalities are proportional to the degree of hypertrophy but are not the result of persistent abnormalities of myocardial perfusion. In contrast, depletion of CP and accumulation of Pi during tachycardia in LVH are closely related to the pacing-induced perfusion abnormalities and likely reflect subendocardial ischemia.

Functional magnetic resonance imaging of motor cortex: hemispheric asymmetry and handedness.

A hemispheric asymmetry in the functional activation of the human motor cortex during contralateral (C) and ipsilateral (I) finger movements, especially in right-handed subjects, was documented with nuclear magnetic resonance imaging at high field strength (4 tesla). Whereas the right motor cortex was activated mostly during contralateral finger movements in both right-handed (C/I mean area of activation = 36.8) and left-handed (C/I = 29.9) subjects, the left motor cortex was activated substantially during ipsilateral movements in left-handed subjects (C/I = 5.4) and even more so in right-handed subjects (C/I = 1.3).

31P NMR spectroscopy of the human heart at 4 T: detection of substantially uncontaminated cardiac spectra and differentiation of subepicardium and subendocardium.

31P NMR spectroscopy of the human heart was undertaken at 4 T to investigate whether spectra localized exclusively to the myocardium can be obtained. Utilizing a Fourier series window approach to spectroscopic imaging, we find that at least two layers across the anterior left ventricle wall can be detected, with voxel sizes of about 8 cm3.

Spatially localized in vivo 1H magnetic resonance spectroscopy of an intracerebral rat glioma.

Surface coil MRI combined with spatially localized spectroscopy was used to noninvasively detect 1H signals from metabolites within an intracerebral malignant glioma in rats. The MRS pulse sequence was based upon two-dimensional ISIS, which restricted 1H signals to a column-shaped volume, combined with one-dimensional spectroscopic imaging, which further resolved the signals into 8 or 16 slices along the major axis of the column. All experiments were executed with adiabatic pulses which induced uniform spin excitation despite the inhomogeneous radiofrequency field distribution produced by the surface coil transmitter. Surface coil MRI and MRS experiments were performed on phantom samples, normal rat brains, and rat brains harboring malignant gliomas. Spatially resolved in vivo 1H spectra of intracerebral gliomas revealed significantly decreased concentrations of N-acetyl-aspartate and creatine and increased lactic acid (or lipids) as compared to the contralateral hemisphere. These results demonstrate that metabolic abnormalities in intracerebral rat gliomas can be spatially resolved in a noninvasive manner using localized in vivo 1H MRS.