fMRI study of olfaction in the olfactory bulb and high olfactory structures of rats: Insight into their roles in habituation.

Cerebral blood volume (CBV) fMRI with ultrasmall superparamagnetic iron oxide particles (USPIO) as a contrast agent was used to investigate olfactory processing in rats. fMRI data were acquired in sixteen 0.75-mm coronal slices covering the olfactory bulb (OB) and higher olfactory regions (HOR), including the anterior olfactory nucleus and piriform cortex. For each animal, multiple consecutive fMRI measurements were made during a 3-h experiment session, with each measurement consisting of a baseline period, an odorant stimulation period, and a recovery period. Two different stimulation paradigms with a stimulation period of 40s or 80s, respectively, were used to study olfactory processing. Odorant-induced CBV increases were robustly observed in the OB and HOR of each individual animal. Olfactory adaptation, which is characterized by an attenuation of responses to continuous exposure or repeated stimulations, has different characteristics in the OB and HOR. For adaptation to repeated stimuli, while it was observed in both the OB and HOR, CBV responses in the HOR were attenuated more significantly than responses in the OB. In contrast, within each continuous 40-s or 80-s odor exposure, CBV responses in the OB were stable and did not show adaptation, but the CBV responses in the HOR were state dependent, with no adaptation during initial exposures, but significant adaptation during following exposures. These results support previous reports that HOR plays a more significant role than OB in olfactory habituation. The technical approach presented in this study should enable more extensive fMRI studies of olfactory processing in rats.

Functional imaging of olfaction by CBV fMRI in monkeys: insight into the role of olfactory bulb in habituation.

Cerebral blood volume (CBV) fMRI with superparamagnetic iron oxide nanoparticles (USPIO) as contrast agent was used to investigate the odorant-induced olfaction in anesthetized rhesus monkeys. fMRI data were acquired in 24 axial slices covering the entire brain, with isoamyl-acetate as the odor stimulant. For each experiment, multiple fMRI measurements were made during a 1- or 2-h period, with each measurement consisting of a baseline period, a stimulation period, and a recovery period. Three different stimulation paradigms with a stimulation period of 1 min, 2 min, or 8 min, respectively, were used to study the olfactory responses in the olfactory bulb (OB). Odorant-induced CBV increases were observed in the OB of each individual monkey. The spatial and temporal activation patterns were reproducible within and between animals. The sensitivity of CBV fMRI in OB was comparable with the sensitivities reported in previous animal fMRI studies. The CBV responses during the 1-min, 2-min, or 8-min odor stimulation period were relatively stable, and did not show attenuation. The amplitudes of CBV response to the repeated stimuli during the 1- or 2-h period were also stable. The stable CBV response in the OB to both continuous and repeated odor stimuli suggests that the OB may not play a major role in olfactory habituation. The technical approach described in this report can enable more extensive fMRI studies of olfactory processing in OB of both humans and non-human primates.

Characteristic time courses of cortical and medullary sodium signals measured by noninvasive (23) Na-MRI in rat kidney induced by furosemide.

BACKGROUND: To characterize regional kidney sodium response by MRI following NKCC2 inhibition. METHODS: Regional renal sodium signals were monitored noninvasively using (23) Na-MRI at 9.4T with a temporal resolution of 1.5 min in anesthetized rats (N = 14). A mild NKCC2 inhibition was induced using a slow intravenous furosemide infusion. Time course of sodium signal was modeled as an exponential transient with a single characteristic time constant. RESULTS: Under normal physiological conditions, the renal sodium signals in medullary and cortical regions were stable and found to respond differently to furosemide challenge. Furosemide infusion at 1.2 mg/kg/h (N = 7) increased sodium signal in the cortex by 40 ± 6% (P < 7 × 10(-5) ) whereas decreased in the medulla by 29 ± 2% (P < 3 × 10(-6) ) with different temporal kinetics. The characteristic time constants of the change were determined to be: 8 ± 2 and 70 ± 10 min for medulla and cortex. Also, the medullary change occurred 9(±3) times faster than cortical independent of furosemide infusion rate up to 35 mg/kg/h. CONCLUSION: The pharmacological effects in terms of regional kidney sodium signal changes induced by NKCC2 inhibition are region-specific and highly predictable. Using noninvasive sodium MRI, we obtained regional renal sodium kinetics data sets in response to a low dose furosemide infusion in normal rats.

Qualification of fMRI as a biomarker for pain in anesthetized rats by comparison with behavioral response in conscious rats.

fMRI can objectively measure pain-related neural activities in humans and animals, providing a valuable tool for studying the mechanisms of nociception and for developing new analgesics. However, due to its extreme sensitivity to subject motion, pain fMRI studies are performed in animals that are immobilized, typically with anesthesia. Since anesthesia could confound the nociceptive processes, it is unknown how well nociceptive-related neural activities measured by fMRI in anesthetized animals correlate with nociceptive behaviors in conscious animals. The threshold to vocalization (VT) in response to an increasing noxious electrical stimulus (NES) was implemented in conscious rats as a behavioral measure of nociception. The antinociceptive effect of systemic (intravenous infusion) lidocaine on NES-induced fMRI signals in anesthetized rats was compared with the corresponding VT in conscious rats. Lidocaine infusion increased VT and suppressed the NES-induced fMRI signals in most activated brain regions. The temporal characteristics of the nociception signal by fMRI and by VT in response to lidocaine infusion were highly correlated with each other, and with the pharmacokinetics (PK) of lidocaine. These results indicate that the fMRI activations in these regions may be used as biomarkers of acute nociception in anesthetized rats. Interestingly, systemic lidocaine had no effect on NES-induced fMRI activations in the primary somatosensory cortex (S1), a result that warrants further investigation.

fMRI of pain processing in the brain: a within-animal comparative study of BOLD vs. CBV and noxious electrical vs. noxious mechanical stimulation in rat.

This study aims to identify fMRI signatures of nociceptive processing in whole brain of anesthetized rats during noxious electrical stimulation (NES) and noxious mechanical stimulation (NMS) of paw. Activation patterns for NES were mapped with blood oxygen level dependent (BOLD) and cerebral blood volume (CBV) fMRI, respectively, to investigate the spatially-dependent hemodynamic responses during nociception processing. A systematic evaluation of fMRI responses to varying frequencies of electrical stimulus was carried out to optimize the NES protocol. Both BOLD and CBV fMRI showed widespread activations, but with different spatial characteristics. While BOLD and CBV showed well-localized activations in ipsilateral dorsal column nucleus, contralateral primary somatosensory cortex (S1), and bilateral caudate putamen (CPu), CBV fMRI showed additional bilateral activations in the regions of pons, midbrain and thalamus compared to BOLD fMRI. CBV fMRI that offers higher sensitivity compared to BOLD was then used to compare the nociception processing during NES and NMS in the same animal. The activations in most regions were similar. In the medulla, however, NES induced a robust activation in the ipsilateral dorsal column nucleus while NMS showed no activation. This study demonstrates that (1) the hemodynamic response to nociception is spatial-dependent; (2) the widespread activations during nociception in CBV fMRI are similar to what have been observed in (14)C-2-deoxyglucose (2DG) autoradiography and PET; (3) the bilateral activations in the brain originate from the divergence of neural responses at supraspinal level; and (4) the similarity of activation patterns suggests that nociceptive processing in rats is similar during NES and NMS.

Pain fMRI in rat cervical spinal cord: an echo planar imaging evaluation of sensitivity of BOLD and blood volume-weighted fMRI.

Objective measure of pain is valuable in drug discovery research and development of analgesics. Spinal cord is an important relay of the pain pathway, and fMRI offers an excellent opportunity to quantify pain using activation in the spinal cord induced by painful stimuli. fMRI literature of cervical spinal cord with regard to the spatial extent, in both longitudinal and cross-sectional directions, of neuronal activation induced by noxious stimulation is ambiguous. This study investigates the feasibility of developing a robust pain assay using fMRI in the cervical spinal cord in alpha-chloralose anesthetized rats subjected to transcutaneous noxious electrical stimulation of the forepaw. Blood oxygenation level dependent (BOLD) and blood volume (BV)-weighted fMRI data were acquired without and with intravenous injection of ultra small superparamagnetic iron oxide particles (USPIO), respectively. BOLD data were acquired by gradient-echo (GE) and spin-echo (SE) echo planar imaging (EPI), while BV-weighted fMRI data were acquired only by GE EPI. Cervical spinal cord activity was robustly detected by all three fMRI techniques. The sensitivity of the fMRI signal was highest in GE BV-weighted fMRI followed in order by GE BOLD, and SE BOLD, respectively. Spatially, the fMRI signal extended approximately 9 mm in the longitudinal direction, covering C(4)-C(8) segments, coinciding with the synapse location of afferent terminals from the stimulated site. In the cross-sectional direction, the signal change is localized predominantly to the ipsilateral dorsal region. This study demonstrates that cervical spinal cord fMRI can be performed reliably in anesthetized rats offering it as a potential tool for analgesic drug development.

Long-term treatment with odanacatib maintains normal trabecular biomechanical properties in ovariectomized adult monkeys as demonstrated by micro-CT-based finite element analysis.

The cathepsin K inhibitor odanacatib (ODN) is a potent and reversible inhibitor of osteoclastic resorption activity. This drug is currently under development for the treatment of postmenopausal osteoporosis. Previously, we described data on the treatment efficacy of ODN in a preclinical estrogen-deficient model of an ovariectomized (OVX) rhesus monkey using HR-pQCT based finite element analysis (FEA) in vivo estimates of bone strength on the distal radius. To support the bone safety profile of ODN, we report ex vivo data on the apparent and hard tissue biomechanical properties of the trabecular bone of vertebrae of animals after 20 months of dosing in three treatment groups: Vehicle (VEH), ODN (2 mg/kg/day), and ALN (30 µg/kg/week). Biomechanical axial compression tests were performed on cylindrical trabecular samples cored out of the third lumbar vertebra of each animal at the end of the study. The biomechanical test results demonstrated that a normal (positive correlation between bone mineral density and bone strength) apparent material property relationship was maintained in the lumbar spine of ODN and ALN treated non-human primates (NHP). Trabecular bone hard tissue Young's modulus value was estimated using experimentally measured stiffness combined with FEA. The FEA and experimental results demonstrated that ODN treatment for 20 months maintained normal trabecular bone material hard tissue properties in the OVX-monkeys and was comparable to ALN.

The role of MRI and PET/SPECT in Alzheimer's disease.

Alzheimer's disease (AD) is difficult to diagnose in its early stages, and even if detected early, there is no preventative treatment. Imaging modalities such as MRI, PET, and SPECT have the potential to contribute to both the diagnosis of Alzheimer's disease, as well as assist in the search for more effective treatments. A number of AD-related biomarkers have been proposed and evaluated. The use of PET imaging to detect alterations in regional brain metabolism using [(18)F]FDG has enabled more sensitive and accurate early diagnosis of AD, especially in conjunction with traditional medical evaluation. Additionally, magnetic resonance imaging and spectroscopy provide a wide range of biomarkers that have been shown to correlate with the progression of AD. Some of these markers have been pursued in clinical trials. Progress has been made toward the evaluation of other more AD-specific biomarkers. However, many questions remain concerning the validity and sensitivity of these imaging biomarkers to aid in the assessment of potential new treatments, especially those related to increased levels of amyloid peptides in the brain.

Differences between arterial occlusive and cortical photothrombosis stroke models with magnetic resonance imaging and microtubule-associated protein-2 immunoreactivity.

The differences between two models of cerebral ischemia [middle cerebral arterial transection (MCAT) and cortical photothrombosis (PT)] were explored with multiparametric MRI of apparent diffusion coefficient trace (ADCtr), cerebral blood flow (CBF) and T1. Microtubule-associated protein-2 (MAP2) immunoreactivity sections aligned with the MR images in the same coronal plane were used to map the infarct and to guide region-of-interest selection. In ischemic cortex, the larger T1 increase in PT versus MCAT (42+/-7% vs. 16+/-5%) is related to the different character of edema between these models; yet, neither CBF nor ADCtr discriminated between them at 3.5 h, suggesting that different mechanisms of ischemic damage to the brain cells resulted in the same ADCtr value. CBF and ADCtr were depressed in immediately adjacent ischemic border by 27+/-7% and 47+/-10%, respectively, in MCAT but not in PT, suggesting marginal perfusion in MCAT. CBF in homotopic normal cortex in the opposite hemisphere was higher for PT compared with MCAT (199+/-20 and 134+/-10 ml/100 g/min, respectively). Different pathological processes in the two models affect CBF, ADCtr and T1 in a unique, regionally specific manner. The PT model differs substantially from the MCAT and is not a model of cortical ischemia with an appreciable border zone.

Quantitative perfusion imaging using arterial spin labeling.

MRI-based perfusion imaging techniques can be classified into those that use exogenously administered contrast agents and those that use an endogenous material that reflects blood flow. This chapter focuses on the technique of arterial spin labeling (ASL), in which endogenous water is made a freely diffusible perfusion tracer by perturbing the magnetization of blood water in arteries prior to their entry into tissue of interest. The technique is totally noninvasive and allows repeated quantitative blood flow measurements in a time scale limited only by the spin lattice relaxation time (T1). Absolute quantitation requires measurement of T1, transit time, and labeling efficiency, as well as careful control for magnetization transfer effects. Two main variants of the ASL technique are in use: continuous ASL (CASL) and pulsed ASL (PASL). This chapter describes basic theory for CASL, and experimental and computational procedures for obtaining quantitative perfusion maps of the brain. Extension of the technique for renal perfusion imaging is outlined.

Cerebral blood flow at one year after controlled cortical impact in rats: assessment by magnetic resonance imaging.

Progressive tissue loss and delayed cognitive deficits are seen in rats during the initial year after experimental traumatic brain injury (TBI). As much as 10% of parenchymal volume is lost even in the contralateral hemisphere by 1 year after controlled cortical impact (CCI) in rats. Progressive declines in cerebral blood flow (CBF) are also associated with advanced age and neurodegenerative diseases. Surprisingly, the long-term effects of TBI on CBF remain undefined. CBF was quantified by continuous arterial spin-labeled magnetic resonance imaging (MRI) and measurements of spin-lattice relaxation time in a slice through the plane of injury at 1 year after experimental TBI produced by CCI (n = 4) or sham surgery (n = 4) in rats. CBF was quantified in six regions of interest (ROIs) that were anatomically identified on the control images in each hemisphere and included a medial cortical segment (contusion-enriched, beneath the impact site, on the ipsilateral side) cortex, hippocampus, thalamus, amygdala/pyriform cortex, and hemisphere. At 1 year after injury, CBF was dramatically (96%) reduced in structures within the large cystic lesion that was seen in three of four rats and variably included cortex and hippocampus. Overall, there was an 80% reduction in CBF in the ipsilateral medial cortical segment comparing CCI and sham groups. Similarly, 52% and 67% reductions were seen in CBF in the cortical and hippocampal ROIs ipsilateral to impact (CCI vs. sham), respectively. These are regions both with marked CBF disturbances early after injury and that ultimately suffer considerable tissue loss over the 1-year interval. However, at 1 year after CCI, CBF was not different from sham in other ROIs, including ipsilateral thalamus, or either contralateral hippocampus or hemisphere. We conclude that, at 1 year after CCI, CBF is reduced in anatomic structures at or near the impact site, including injured cortex and hippocampus, and this translates into a reduction in hemispheric CBF. However, despite both significant occult tissue loss ipsilateral and contralateral to the injury and delayed cognitive deficits, widespread reductions in CBF are not observed. This suggests the possibility of remodeling or repackaging of the brain that preserves CBF outside of the cystic lesion.

Investigation of cross-species translatability of pharmacological MRI in awake nonhuman primate - a buprenorphine challenge study.

BACKGROUND: Pharmacological MRI (phMRI) is a neuroimaging technique where drug-induced hemodynamic responses can represent a pharmacodynamic biomarker to delineate underlying biological consequences of drug actions. In most preclinical studies, animals are anesthetized during image acquisition to minimize movement. However, it has been demonstrated anesthesia could attenuate basal neuronal activity, which can confound interpretation of drug-induced brain activation patterns. Significant efforts have been made to establish awake imaging in rodents and nonhuman primates (NHP). Whilst various platforms have been developed for imaging awake NHP, comparison and validation of phMRI data as translational biomarkers across species remain to be explored. METHODOLOGY: We have established an awake NHP imaging model that encompasses comprehensive acclimation procedures with a dedicated animal restrainer. Using a cerebral blood volume (CBV)-based phMRI approach, we have determined differential responses of brain activation elicited by the systemic administration of buprenorphine (0.03 mg/kg i.v.), a partial µ-opioid receptor agonist, in the same animal under awake and anesthetized conditions. Additionally, region-of-interest analyses were performed to determine regional drug-induced CBV time-course data and corresponding area-under-curve (AUC) values from brain areas with high density of µ-opioid receptors. PRINCIPAL FINDINGS: In awake NHPs, group-level analyses revealed buprenorphine significantly activated brain regions including, thalamus, striatum, frontal and cingulate cortices (paired t-test, versus saline vehicle, p<0.05, n = 4). This observation is strikingly consistent with µ-opioid receptor distribution depicted by [6-O-[(11)C]methyl]buprenorphine ([(11)C]BPN) positron emission tomography imaging study in baboons. Furthermore, our findings are consistent with previous buprenorphine phMRI studies in humans and conscious rats which collectively demonstrate the cross-species translatability of awake imaging. Conversely, no significant change in activated brain regions was found in the same animals imaged under the anesthetized condition. CONCLUSIONS: Our data highlight the utility and importance of awake NHP imaging as a translational imaging biomarker for drug research.

High-resolution peripheral quantitative computed tomography and finite element analysis of bone strength at the distal radius in ovariectomized adult rhesus monkey demonstrate efficacy of odanacatib and differentiation from alendronate.

Translational evaluation of disease progression and treatment response is critical to the development of therapies for osteoporosis. In this study, longitudinal in-vivo monitoring of odanacatib (ODN) treatment efficacy was compared to alendronate (ALN) in ovariectomized (OVX) non-human primates (NHPs) using high-resolution peripheral computed tomography (HR-pQCT). Treatment effects were evaluated using several determinants of bone strength, density and quality, including volumetric bone mineral density (vBMD), three-dimensional structure, finite element analysis (FEA) estimated peak force and biomechanical properties at the ultradistal (UD) radius at baseline, 3, 6, 9, 12, and 18 months of dosing in three treatment groups: vehicle (VEH), low ODN (2 mg/kg/day, L-ODN), and ALN (30 µg/kg/week). Biomechanical axial compression tests were performed at the end of the study. Bone strength estimates using FEA were validated by ex-vivo mechanical compression testing experiments. After 18months of dosing, L-ODN demonstrated significant increases from baseline in integral vBMD (13.5%), cortical thickness (24.4%), total bone volume fraction BV/TV (13.5%), FEA-estimated peak force (26.6%) and peak stress (17.1%), respectively. Increases from baseline for L-ODN at 18 months were significantly higher than that for ALN in DXA-based aBMD (7.6%), cortical thickness (22.9%), integral vBMD (12.2%), total BV/TV (10.1%), FEA peak force (17.7%) and FEA peak stress (11.5%), respectively. These results demonstrate a superior efficacy of ODN treatment compared to ALN at the UD radii in ovariectomized NHPs.

Effect of odanacatib on bone turnover markers, bone density and geometry of the spine and hip of ovariectomized monkeys: a head-to-head comparison with alendronate.

Odanacatib (ODN) is a selective and reversible Cathepsin K (CatK) inhibitor currently being developed as a once weekly treatment for osteoporosis. Here, effects of ODN compared to alendronate (ALN) on bone turnover, DXA-based areal bone mineral density (aBMD), QCT-based volumetric BMD (vBMD) and geometric parameters were studied in ovariectomized (OVX) rhesus monkeys. Treatment was initiated 10 days after ovariectomy and continued for 20 months. The study consisted of four groups: L-ODN (2 mg/kg, daily p.o.), H-ODN (8/4 mg/kg daily p.o.), ALN (15 µg/kg, twice weekly, s.c.), and VEH (vehicle, daily, p.o.). L-ODN and ALN doses were selected to approximate the clinical exposures of the ODN 50-mg and ALN 70-mg once-weekly, respectively. L-ODN and ALN effectively reduced bone resorption markers uNTx and sCTx compared to VEH. There was no additional efficacy with these markers achieved with H-ODN. Conversely, ODN displayed inversely dose-dependent reduction of bone formation markers, sP1NP and sBSAP, and L-ODN reduced formation to a lesser degree than ALN. At month 18 post-OVX, L-ODN showed robust increases in lumbar spine aBMD (11.4%, p<0.001), spine trabecular vBMD (13.7%, p<0.001), femoral neck (FN) integral (int) vBMD (9.0%, p<0.001) and sub-trochanteric proximal femur (SubTrPF) int vBMD, (6.4%, p<0.001) compared to baseline. L-ODN significantly increased FN cortical thickness (Ct.Th) and cortical bone mineral content (Ct.BMC) by 22.5% (p<0.001) and 21.8% (p<0.001), respectively, and SubTrPF Ct.Th and Ct.BMC by 10.9% (p<0.001) and 11.3% (p<0.001) respectively. Compared to ALN, L-ODN significantly increased FN Ct. BMC by 8.7% (p<0.05), and SubTrPF Ct.Th by 7.6% (p<0.05) and Ct.BMC by 6.2% (p<0.05). H-ODN showed no additional efficacy compared to L-ODN in OVX-monkeys in prevention mode. Taken together, the results from this study have demonstrated that administration of ODN at levels which approximate clinical exposure in OVX-monkeys had comparable efficacy to ALN in DXA-based aBMD and QCT-based vBMD. However, FN cortical mineral content clearly demonstrated superior efficacy of ODN versus ALN in this model of estrogen-deficient non-human primates.

Evaluation of high-resolution peripheral quantitative computed tomography, finite element analysis and biomechanical testing in a pre-clinical model of osteoporosis: a study with odanacatib treatment in the ovariectomized adult rhesus monkey.

This study aimed to validate finite element analysis (FEA) estimation of strength, identify high-resolution peripheral quantitative computed tomography (HR-pQCT) measures correlating with strength, and evaluate the precision of HR-pQCT measurements to longitudinally monitor effects of osteoporosis treatment in ovariectomized (OVX) non-human primates (NHPs). HR-pQCT images were acquired in three groups of NHPs: Intact (n=10), OVX-odanacatib treated (OVX-ODN 30 mg/kg, n=10) and OVX-vehicle treated (OVX-Veh, n=10) at the ultradistal (UD) and distal 1/3 radii and tibia at 12, 16 and 20 months. FEA estimates of bone strength using the Pistoia criterion were validated by ex-vivo mechanical compression (r(2)=0.95) of the UD radius. Single linear regressions of FEA-determined ultimate stress showed high correlation with HR-pQCT derived parameters: integral vBMD (r(2)=0.86), bone volume fraction (r(2)=0.84) and cortical thickness (r(2)=0.79). Precision of HR-pQCT measurements, obtained from an excised radius and tibia, showed low variation (CV=0.005%-5.6%) and helped identify possible sources of error. Comparison of OVX-Veh and Intact groups showed decreases in bone parameters demonstrating trends consistent with bone loss. Comparison of OVX-ODN and OVX-Veh groups showed a treatment effect with increases in bone parameters: integral vBMD (477±27 vs. 364±22 mgHA/cm(3)) and cortical thickness (Ct.Th) (0.90±0.07 vs. 0.64±0.04 mm) at the UD radius, Ct.Th (2.15±0.28 vs. 1.56±0.08 mm) at the distal 1/3 radius. Axial compression peak stress calculated and obtained experimentally showed the OVX-ODN group was 33% stronger than the OVX-Veh group. We conclude that HR-pQCT and FEA serve as robust techniques to longitudinally monitor bone parameters and strength in NHP's.

fMRI investigation of the effect of local and systemic lidocaine on noxious electrical stimulation-induced activation in spinal cord.

Spinal cord fMRI offers an excellent opportunity to quantify nociception using neuronal activation induced by painful stimuli. Measurement of the magnitude of stimulation-induced activation, and its suppression with analgesics can provide objective measures of pain and efficacy of analgesics. This study investigates the feasibility of using spinal cord fMRI in anesthetized rats as a pain assay to test the analgesic effect of locally and systemically administered lidocaine. Blood volume (BV)-weighted fMRI signal acquired after intravenous injection of ultrasmall superparamagnetic iron oxide (USPIO) particles was used as an indirect readout of the neuronal activity. Transcutaneous noxious electrical stimulation was used as the pain model. BV-weighted fMRI signal could be robustly quantified on a run-by-run basis, opening the possibility of measuring pharmacodynamics (PD) of the analgesics with a temporal resolution of approximately 2 min. Local administration of lidocaine was shown to ablate all stimulation-induced fMRI signals by the total blockage of peripheral nerve transmission, while the analgesic effect of systemically administered lidocaine was robustly detected after intravenous infusion of approximately 3mg/kg, which is similar to clinical dosage for human. This study establishes spinal cord fMRI as a viable assay for analgesics. With respect to the mode of action of lidocaine, this study suggests that systemic lidocaine, which is clinically used for the treatment of neuropathic pain, and believed to only block the peripheral nerve transmission of abnormal neural activity (ectopic discharge) originating from the damaged peripheral nerves, also blocks the peripheral nerve transmission of normal neural activity induced by transcutaneous noxious electrical stimulation.

BOLD and blood volume-weighted fMRI of rat lumbar spinal cord during non-noxious and noxious electrical hindpaw stimulation.

Spinal cord fMRI is a useful tool for studying spinal mechanisms of pain, hence for analgesic drug development. Its technical feasibility in both humans and rats has been demonstrated. This study investigates the reproducibility, robustness, and spatial accuracy of fMRI of lumbar spinal cord activation due to transcutaneous noxious and non-noxious electrical stimulation of the hindpaw in alpha-chloralose-anesthetized rats. Blood oxygenation level-dependent (BOLD) and blood volume-weighted fMRI data were acquired without and with intravenous injection of ultra small superparamagnetic iron oxide particles (USPIO), respectively, using a gradient echo (GE) echo planar imaging (EPI) technique at 4.7 T. Neuronal activation in the spinal cord induced by noxious stimulation to the hindpaw (2 ms wide, 5 mA amplitude, known to activate C-fibers) can be robustly detected by both fMRI techniques with excellent reproducibility and peaked at the stimulus frequency of 40 Hz. However, both fMRI techniques were not sensitive to neuronal activation in spinal cord induced by non-noxious stimulation (0.3 ms, 1.5 mA, known only to activate A-fibers). Spatially, the fMRI signal extended approximately 5 mm in the longitudinal direction, covering L(3)-L(5) segments. In the cross-sectional direction, the highest signal change of blood volume-weighted fMRI was in the middle of the ipsilateral dorsal horn, which roughly corresponds to laminae V and VI, while the highest signal change of BOLD fMRI was in the ipsilateral dorsal surface. This study demonstrates that spinal cord fMRI can be performed in anesthetized rats reliably and reproducibly offering it as a potential tool for analgesic drug discovery.

Interleaved water and fat imaging and applications to lipid quantitation using the gradient reversal technique.

PURPOSE: To implement and evaluate the gradient reversal-based chemical shift imaging technique to obtain qualitative and quantitative spatially-registered fat and water images with high imaging efficiency at very high field. MATERIALS AND METHODS: A multiecho gradient reversal-based sequence allowing interleaved water-fat imaging during a single acquisition and quantitation of fat/water content is presented. The sequence was optimized and implemented at 11.7T. The quantitation was verified with water-fat phantoms and applied to lipid measurement in an in vivo mouse model. RESULTS: Results from phantoms, in vivo lipid measurement in mouse liver and hind limb muscle, and ex vivo rat knee imaging experiments demonstrated the robustness and high selectivity of this technique for interleaved and quantitative water and fat imaging at very high field. CONCLUSION: The proposed MRI technique permits interleaved water and fat imaging, with which spectrally well-separated water and fat images at the identical slice locations could be obtained in a single acquisition without increasing scan time. The technique could be used for in vivo quantitative mapping of lipid content and applied to investigations using small animal experiment models.

Normal and transplanted rat kidneys: diffusion MR imaging at 7 T.

PURPOSE: To investigate the feasibility of obtaining reproducible apparent diffusion coefficient (ADC) maps of normal rat kidneys by using respiratory-triggered spin-echo diffusion-weighted magnetic resonance (MR) imaging, to investigate the sensitivity of ADC maps in the evaluation of renal blood flow, and to use this technique to monitor acute graft rejection in transplanted rat kidneys. MATERIALS AND METHODS: Spin-echo diffusion-weighted MR imaging measurements were performed in 20 normal rats and nine rats that had undergone transplantation (six rats had received allografts; three had received isografts) at 7 T. To evaluate the effect of alteration in blood flow and water transport function, angiotensin II was infused in six normal rats and a series of spin-echo diffusion-weighted MR images was obtained at five time points. Transplanted kidneys were monitored by obtaining spin-echo diffusion-weighted MR images and gradient-echo MR images every 2 hours for 8 hours on postoperative day 4. Statistical analysis was performed with repeated-measures multivariate analysis of variance and the paired t test. RESULTS: No significant differences in ADC values were observed between right and left kidneys in all three orthogonal directions; however, a small difference was observed between the cortex and medulla. ADC values in the cephalocaudal and mediolateral directions were higher than those in the anteroposterior direction (P <.01 for all). ADC values in the cortex and medulla decreased significantly (by >35%, P <.01) during angiotensin II-induced reduction in renal blood flow. No significant signal intensity change was observed between native and transplanted kidneys on gradient-echo MR images. Allografts exhibited decreased ADC values (P <.01) and isografts exhibited similar ADC values compared with native kidneys. CONCLUSION: These findings suggest that reproducible renal ADC maps can be obtained in rats by using spin-echo diffusion-weighted MR imaging at 7 T. Spin-echo diffusion-weighted MR imaging may have potential as a noninvasive tool for monitoring early graft rejection after kidney transplantation.

MRI of lungs using partial liquid ventilation with water-in-perfluorocarbon emulsions.

A novel (1)H-MRI contrast modality for rat lungs has been developed using water-in-perfluorocarbon (PFC) emulsions for partial liquid ventilation (PLV). The feasibility of the new ventilation protocol for (1)H-MRI studies of lungs has been demonstrated. (1)H-MR images of lungs have been obtained with sensitivity and spatial resolution higher than those of the (19)F-MRI of lungs previously reported. Diffusion-weighted MRI measurements of lungs showed that the results obtained are related to the pulmonary architecture and functional properties of lungs. Although the methodology needs further improvement and evaluation, it appears to have great potential in a wide range of new applications in the field of lung MRI, such as in vivo detection of lung cancer, emphysema, and allograft rejection following lung transplantation. The ability of this technique to achieve high-quality MR images of lungs, together with its technical simplicity, stability, and low cost, makes this method a promising imaging technique for the lungs.