Arterial spin labeling perfusion magnetic resonance imaging of non-human primates

Magnetic resonance imaging in canine idiopathic epilepsy: a mini-review

Magnetic resonance imaging (MRI) in an integral part of the diagnostic workup in canines with idiopathic epilepsy (IE). While highly sensitive and specific in identifying structural lesions, conventional MRI is unable to detect changes at the microscopic level. Utilizing more advanced neuroimaging techniques may provide further information on changes at the neuronal level in the brain of canines with IE, thus providing crucial information on the pathogenesis of canine epilepsy. Additionally, earlier detection of these changes may aid clinicians in the development of improved and targeted therapies. Advances in MRI techniques are being developed which can assess metabolic, cellular, architectural, and functional alterations; as well alterations in neuronal tissue mechanical properties, some of which are currently being applied in research on canine IE. This mini-review focuses on novel MRI techniques being utilized to better understand canine epilepsy, which include magnetic resonance spectroscopy, diffusion-weighted imaging, diffusion tensor imaging, perfusion-weighted imaging, voxel based morphometry, and functional MRI; as well as techniques applied in human medicine and their potential use in veterinary species.

Longitudinal evaluation of the functional connectivity changes in the secondary somatosensory cortex (S2) of the monkey brain during acute stroke

Background

Somatosensory deficits are frequently seen in acute stroke patients and may recover over time and affect functional outcome. However, the underlying mechanism of function recovery remains poorly understood. In the present study, progressive function alteration of the secondary somatosensory cortex (S2) and its relationship with regional perfusion and neurological outcome were examined using a monkey model of stroke.

Methods and materials

Rhesus monkeys (n = 4) were induced with permanent middle cerebral artery occlusion (pMCAo). Resting-state functional MRI, dynamic susceptibility contrast perfusion MRI, diffusion-weighted, T₁ and T₂ weighted images were collected before surgery and at 4-6, 48, and 96 h post stroke on a 3T scanner. Progressive changes of relative functional connectivity (FC), cerebral blood flow (CBF), and CBF/Tmax (Time to Maximum) of affected S2 regions were evaluated. Neurological deficits were assessed using the Spetzler approach.

Results

Ischemic lesion was evidently seen in the MCA territory including S2 in each monkey. Relative FC of injured S2 regions decreased substantially following stroke. Spetzler scores dropped substantially at 24 h post stroke but slightly recovered from Day 2 to Day 4. Relative FC progressively increased from 6 to 48 and 96 h post stroke and correlated significantly with relative CBFand CBF/Tmax changes.

Conclusion

The present study revealed the progressive alteration of function connectivity in S2 during acute stroke. The preliminary results suggested the function recovery might start couple days post occlusion and collateral circulation might play a key role in the recovery of somatosensory function after stroke insult. The relative function connectivity in S2 may provide additional information for prediction of functional outcome in stroke patients.

Magnetic resonance imaging of the monkey fetal brain in utero

Non-human primates (NHPs) are the closest living relatives of the human and play a critical role in investigating the effects of maternal viral infection and consumption of medicines, drugs, and alcohol on fetal development. With the advance of contemporary fast MRI techniques with parallel imaging, fetal MRI is becoming a robust tool increasingly used in clinical practice and preclinical studies to examine congenital abnormalities including placental dysfunction, congenital heart disease (CHD), and brain abnormalities non-invasively. Because NHPs are usually scanned under anesthesia, the motion artifact is reduced substantially, allowing multi-parameter MRI techniques to be used intensively to examine the fetal development in a single scanning session or longitudinal studies. In this paper, the MRI techniques for scanning monkey fetal brains in utero in biomedical research are summarized. Also, a fast imaging protocol including T2-weighted imaging, diffusion MRI, resting-state functional MRI (rsfMRI) to examine rhesus monkey fetal brains in utero on a clinical 3T scanner is introduced.

Effects of Anesthesia on Cerebral Blood Flow and Functional Connectivity of Nonhuman Primates

Nonhuman primates (NHPs) are the closest living relatives of humans and play a critical and unique role in neuroscience research and pharmaceutical development. General anesthesia is usually required in neuroimaging studies of NHPs to keep the animal from stress and motion. However, the adverse effects of anesthesia on cerebral physiology and neural activity are pronounced and can compromise the data collection and interpretation. Functional connectivity is frequently examined using resting-state functional MRI (rsfMRI) to assess the functional abnormality in the animal brain under anesthesia. The fMRI signal can be dramatically suppressed by most anesthetics in a dose-dependent manner. In addition, rsfMRI studies may be further compromised by inter-subject variations when the sample size is small (as seen in most neuroscience studies of NHPs). Therefore, proper use of anesthesia is strongly demanded to ensure steady and consistent physiology maintained during rsfMRI data collection of each subject. The aim of this review is to summarize typical anesthesia used in rsfMRI scans of NHPs and the effects of anesthetics on cerebral physiology and functional connectivity. Moreover, the protocols with optimal rsfMRI data acquisition and anesthesia procedures for functional connectivity study of macaque monkeys are introduced.

Brain perfusion magnetic resonance imaging using pseudocontinuous arterial spin labeling in 314 dogs and cats

Background

Arterial spin labeling (ASL) is a noninvasive brain perfusion magnetic resonance imaging (MRI) technique that has not been assessed in clinical veterinary medicine.

Hypothesis/Objectives

To test the feasibility of ASL using a 1.5 Tesla scanner and provide recommendations for optimal quantification of cerebral blood flow (CBF) in dogs and cats.

Animals

Three hundred fourteen prospectively selected client‐owned dogs and cats.

Methods

Each animal underwent brain MRI including morphological sequences and ≥1 ASL sequences using different sites of blood labeling and postlabeling delays (PLD). Calculated ASL success rates were compared. The CBF was quantified in animals that had morphologically normal brain MRI results and parameters of ASL optimization were investigated.

Results

Arterial spin labeling was easily implemented with an overall success rate of 95% in animals with normal brain MRI. Technical recommendations included (a) positioning of the imaging slab at the foramen magnum and (b) selected PLD of 1025 ms in cats and dogs <7 kg, 1525 ms in dogs 7 to 38 kg, and 2025 ms in dogs >38 kg. In 37 dogs, median optimal CBF in the cortex and thalamic nuclei were 114 and 95 mL/100 g/min, respectively. In 28 cats, median CBF in the cortex and thalamic nuclei were 113 and 114 mL/100 g/min, respectively.

Conclusions and Clinical Importance

Our survey of brain perfusion ASL‐MRI demonstrated the feasibility of ASL at 1.5 Tesla, suggested technical recommendations and provided CBF values that should be helpful in the characterization of various brain diseases in dogs and cats.

Effects of alfaxalone on cerebral blood flow and intrinsic neural activity of rhesus monkeys: A comparison study with ketamine

OBJECTIVE

Alfaxalone has been used increasingly in biomedical research and veterinary medicine of large animals in recent years. However, its effects on the cerebral blood flow (CBF) physiology and intrinsic neuronal activity of anesthetized brains remain poorly understood.

METHODS

Four healthy adult rhesus monkeys were anesthetized initially with alfaxalone (0.125 mg/kg/min) or ketamine (1.6 mg/kg/min) for 50 min, then administrated with 0.8% isoflurane for 60 min. Heart rates, breathing beats, and blood pressures were continuously monitored. CBF data were collected using pseudo-continuous arterial spin-labeling (pCASL) MRI technique and rsfMRI data were collected using single-shot EPI sequence for each anesthetic.

RESULTS

Both the heart rates and mean arterial pressure (MAP) remained more stable during alfaxalone infusion than those during ketamine administration. Alfaxalone reduced CBF substantially compared to ketamine anesthesia (grey matter, 65 ± 22 vs. 179 ± 38 ml/100g/min, p<0.001; white matter, 14 ± 7 vs. 26 ± 6 ml/100g/min, p < 0.05); In addition, CBF increase was seen in all selected cortical and subcortical regions of alfaxalone-pretreated monkey brains during isoflurane exposure, very different from the findings in isoflurane-exposed monkeys pretreated with ketamine. Also, alfaxalone showed suppression effects on functional connectivity of the monkey brain similar to ketamine.

CONCLUSION

Alfaxalone showed strong suppression effects on CBF of the monkey brain.The residual effect of alfaxalone on CBF of isoflurane-exposed brains was evident and monotonous in all the examined brain regions when used as induction agent for inhalational anesthesia. In particular, alfaxalone showed similar suppression effect on intrinsic neuronal activity of the brain in comparison with ketamine. These findings suggest alfaxalone can be a good alternative to veterinary anesthesia in neuroimaging examination of large animal models. However, its effects on CBF and functional connectivity should be considered.

Evaluation of prolonged administration of isoflurane on cerebral blood flow and default mode network in macaque monkeys anesthetized with different maintenance doses

OBJECT

Isoflurane is a commonly used volatile anesthetic agent in clinical anesthesia and biomedical research. Prior study suggested the cerebral blood flow (CBF) and default mode network (DMN) could be changed after prolonged administration of isoflurane. The normal maintenance doses of isoflurane may vary from light (∼0.75%) to deep (∼1.5 or 2%) anesthesia. However, it is not clear how the duration effects are affected by the altered doses. The present study is aimed to examine if the duration effects are affected when isoflurane concentration is altered within normal maintenance doses.

MATERIALS AND METHODS

Adult rhesus monkeys (n=5, 8-12 years old, 8-10kg) were anesthetized and maintained at isoflurane levels 0.89±0.03%, 1.05±0.12%, or 1.19±0.08%. CBF and DMN of monkeys were examined using arterial spin-labeling perfusion and resting state functional MRI techniques.

RESULTS

the functional connectivity (FC) in the dominant DMN (posterior cingulate cortex (PCC) to anterior cingulated cortex (ACC) or media prefrontal cortex (MPFC)) decreased substantially and similarly during 4-h administration of isoflurane at any given maintenance dosage. CBF changes varied with isoflurane dosage. At the low dose (∼0.89%), CBF decreased in most brain regions. In contrast, no obvious changes was seen in those regions (except for the subcortex) when higher doses of isoflurane were applied.

CONCLUSION

FC in DMN was reduced substantially during prolonged administration of isoflurane. The FC reduction was not varying significantly with maintenance doses of isoflurane but the duration effect on CBF was dose-dependent. Such duration effects of isoflurane administration on DMN and CBF should be considered in the interpretation of the outcome in related neuroimaging studies of anesthetized subjects.