Usefulness of pulsed arterial spin labeling MR imaging in mesial temporal lobe epilepsy

Cerebral blood flow measured by arterial-spin labeling MRI: a useful biomarker for characterization of minimal hepatic encephalopathy in patients with cirrhosis

PURPOSE

To investigate the role of arterial-spin labeling (ASL) MRI to non-invasively characterize the patterns of cerebral blood flow (CBF) changes in cirrhotic patients and to assess the potential of ASL MRI to characterize minimal hepatic encephalopathy (MHE).

MATERIALS AND METHODS

This study was approved by the local ethics committee, and written informed consent was obtained from all participants. Thirty six cirrhosis patients without overt hepatic encephalopathy (16 MHE patients and 20 non hepatic encephalopathy (non-HE) patients) and 25 controls underwent ASL MRI, and CBF was measured for each subject. One-way ANOCOVA test with age and gender as covariences was used to compare CBF difference among three groups, and post hoc analysis was performed between each two groups. Region-based correlation analysis was applied between Child-Pugh score, venous blood ammonia level, neuropsychological tests and CBF values in cirrhosis patients. Receiver operator characteristic (ROC) analysis was used for assessing CBF measurements in ASL MRI to differentiate MHE from non-HE patients.

RESULTS

The gray matter CBF of MHE patients (71.09 ± 11.88 mL min(-1)100g(-1)) was significantly higher than that of non-HE patients (55.28 ± 12.30 mL min(-1)100g(-1), P<0.01) and controls (52.09 ± 9.27 mL min(-1)100g(-1), P<0.001). Voxel-wise ANOCOVA results showed that CBFs were significantly different among three groups in multiple gray matter areas (P<0.05, Bonferroni corrected). Post hoc comparisons showed that CBF of these brain regions was increased in MHE patients compared with controls and non-HE patients (P<0.05, Bonferroni corrected). CBF of the right putamen was of the highest sensitivity (93.8%) and moderate specificity (75.0%) for characterization of MHE when using the cutoff value of 50.57 mL min(-1)100g(-1). CBFs in the bilateral median cingulate gyri, left supramarginal gyrus, right angular gyrus, right heschl gyrus and right superior temporal gyrus have both sensitivity and specificity of approximately 80% for the diagnosis of MHE.

CONCLUSION

Higher CBF was found in many brain regions in cirrhotic patients than controls and gradually increased with the progress of disease. CBF measured with ASL MRI can be a useful marker for differentiating MHE from non-HE patients.

Cerebral perfusion alterations in epileptic patients during peri-ictal and post-ictal phase: PASL vs DSC-MRI

Non-invasive pulsed arterial spin labeling (PASL) MRI is a method to study brain perfusion that does not require the administration of a contrast agent, which makes it a valuable diagnostic tool as it reduces cost and side effects. The purpose of the present study was to establish the viability of PASL as an alternative to dynamic susceptibility contrast (DSC-MRI) and other perfusion imaging methods in characterizing changes in perfusion patterns caused by seizures in epileptic patients. We evaluated 19 patients with PASL. Of these, the 9 affected by high-frequency seizures were observed during the peri-ictal period (within 5hours since the last seizure), while the 10 patients affected by low-frequency seizures were observed in the post-ictal period. For comparison, 17/19 patients were also evaluated with DSC-MRI and CBF/CBV. PASL imaging showed focal vascular changes, which allowed the classification of patients in three categories: 8 patients characterized by increased perfusion, 4 patients with normal perfusion and 7 patients with decreased perfusion. PASL perfusion imaging findings were comparable to those obtained by DSC-MRI. Since PASL is a) sensitive to vascular alterations induced by epileptic seizures, b) comparable to DSC-MRI for detecting perfusion asymmetries, c) potentially capable of detecting time-related perfusion changes, it can be recommended for repeated evaluations, to identify the epileptic focus, and in follow-up and/or therapy-response assessment.

Epilepsy as progressive disorders: what is the evidence that can guide our clinical decisions and how can neuroimaging help?

There is evidence that some types of epilepsy progress over time, and an important part of this knowledge has derived from neuroimaging studies. Different authors have demonstrated structural damage more pronounced in individuals with a longer duration of epilepsy, and others have been able to quantify this progression over time. However, others have failed to demonstrate progression possibly due to the heterogeneity of individuals evaluated. Currently, temporal lobe epilepsy associated with hippocampal sclerosis is regarded as a progressive disorder. Conversely, for other types of epilepsy, the evidence is not so clear. The causes of this damage progression are also unknown although there is consistent evidence that seizure is one of the mechanisms. The conflicting data about epilepsy progression can be a challenge for clinical decisions for an individual patient. Studies with homogenous groups and longer follow-up are necessary for appropriate conclusions about the real burden of damage progression in epilepsies, and neuroimaging will be essential in this context.

Application of dynamic susceptibility contrast-enhanced perfusion in temporal lobe epilepsy

BACKGROUND

Accurately locating the epileptogenic focus in temporal lobe epilepsy (TLE) is important in clinical practice. Single-photon emission computed tomography (SPECT) and positron-emission tomography (PET) have been widely used in the lateralization of TLE, but both have limitations. Magnetic resonance perfusion imaging can accurately and reliably reflect differences in cerebral blood flow and volume.

PURPOSE

To investigate the diagnostic value of dynamic susceptibility contrast-enhanced (DSC) perfusion magnetic resonance imaging (MRI) in the lateralization of the epileptogenic focus in TLE.

MATERIAL AND METHODS

Conventional MRI and DSC-MRI scanning was performed in 20 interictal cases of TLE and 20 healthy volunteers. The relative cerebral blood volume (rCBV) and relative cerebral blood flow (rCBF) of the bilateral mesial temporal lobes of the TLE cases and healthy control groups were calculated. The differences in the perfusion asymmetry indices (AIs), derived from the rCBV and rCBF of the bilateral mesial temporal lobes, were compared between the two groups.

RESULTS

In the control group, there were no statistically significant differences between the left and right sides in terms of rCBV (left 1.55 ± 0.32, right 1.57 ± 0.28) or rCBF (left 99.00 ± 24.61, right 100.38 ± 23.46) of the bilateral mesial temporal lobes. However, in the case group the ipsilateral rCBV and rCBF values (1.75 ± 0.64 and 96.35 ± 22.63, respectively) were markedly lower than those of the contralateral side (2.01 ± 0.79 and 108.56 ± 26.92; P < 0.05). Both the AI of the rCBV (AI(rCBV); 13.03 ± 10.33) and the AI of the rCBF (AI(rCBF); 11.24 ± 8.70) of the case group were significantly higher than that of the control group (AI(rCBV) 5.55 ± 3.74, AI(rCBF) 5.12 ± 3.48; P < 0.05). The epileptogenic foci of nine patients were correctly lateralized using the 95th percentile of the AI(rCBV) and AI(rCBF) of the control group as the normal upper limits.

CONCLUSION

In patients with TLE interictal, both rCBV and rCBF of the ipsilateral mesial temporal lobe were markedly lower than that of healthy control subjects. DSC-MRI can provide lateralization for TLE.

Cerebral blood flow quantification in the rat: a direct comparison of arterial spin labeling MRI with radioactive microsphere PET

Background

Arterial spin labeling magnetic resonance imaging (ASL-MRI) has been recognised as a valuable method for non-invasive assessment of cerebral blood flow but validation studies regarding quantification accuracy by comparison against an accepted gold standard are scarce, especially in small animals. We have conducted the present study with the aim of comparing ASL flow-sensitive alternating inversion recovery (FAIR)-derived unidirectional water uptake (K₁) and ⁶⁸Ga/⁶⁴Cu microsphere (MS)-derived blood flow (f) in the rat brain.

Methods

In 15 animals, K₁and f were determined successively in dedicated small animal positron emission tomography and MR scanners. The Renkin-Crone model modified by a scaling factor was used for the quantification of f and K₁.

Results

Below about 1 mL/min/mL, we obtain an approximately linear relationship between f and K₁. At higher flow values, the limited permeability of water at the blood brain barrier becomes apparent. Within the accessed dynamic flow range (0.2 to 1.9 mL/min/mL), the data are adequately described by the Renkin-Crone model yielding a permeability surface area product of (1.53±0.46) mL/min/mL.

Conclusion

The ASL-FAIR technique is suitable for absolute blood flow quantification in the rat brain when using a one-compartment model including a suitable extraction correction for data evaluation.

Trial registration

24-9168.21-4/2004-1 (registered in Freistadt Sachsen, Landesdirektion Dresden)

Applications of arterial spin labeled MRI in the brain

Perfusion provides oxygen and nutrients to tissues and is closely tied to tissue function while disorders of perfusion are major sources of medical morbidity and mortality. It has been almost two decades since the use of arterial spin labeling (ASL) for noninvasive perfusion imaging was first reported. While initial ASL magnetic resonance imaging (MRI) studies focused primarily on technological development and validation, a number of robust ASL implementations have emerged, and ASL MRI is now also available commercially on several platforms. As a result, basic science and clinical applications of ASL MRI have begun to proliferate. Although ASL MRI can be carried out in any organ, most studies to date have focused on the brain. This review covers selected research and clinical applications of ASL MRI in the brain to illustrate its potential in both neuroscience research and clinical care.

Association of chronic vascular changes with functional outcome after traumatic brain injury in rats

We tested the hypothesis that vascular remodeling in the cortex, hippocampus, and thalamus is associated with long-term functional recovery after traumatic brain injury (TBI). We induced TBI with lateral fluid-percussion (LFP) injury in adult rats. Animals were followed-up for 9 months, during which we tested motor performance using a neuroscore test, spatial learning and memory with a Morris water maze, and seizure susceptibility with a pentylenetetrazol (PTZ) test. At 8 months, they underwent structural MRI, and cerebral blood flow (CBF) was assessed by arterial spin labeling (ASL) MRI. Then, rats were perfused for histology to assess the density of blood vessels. In the perilesional cortex, the CBF decreased by 56% (p < 0.01 compared to controls), and vessel density increased by 28% (p < 0.01). There was a negative correlation between CBF in the perilesional cortex and vessel density (r = -0.75, p < 0.01). However, in the hippocampus, we found a 13% decrease in CBF ipsilaterally (p < 0.05) and 20% contralaterally (p < 0.01), and no change in vessel number. In the ipsilateral thalamus, the increase in CBF (34%, p < 0.01) was associated with a remarkable increase in vessel density (78%, p < 0.01). Animals showed motor impairment that was not associated with vascular changes. Instead, poor performance in the Morris water maze correlated with enhanced thalamic vessel density (r = -0.81, p < 0.01). Finally, enhanced seizure susceptibility was associated with reduced CBF in the ipsilateral hippocampus (r = 0.78, p < 0.05) and increased vascular density in the thalamus (r = 0.69, p < 0.05). There was little interaction between the behavioral measures. The present study demonstrates that each of the investigated brain areas has a unique pattern of vascular abnormalities. Chronic alterations in CBF could not be attributed to changes in vascular density. Association of thalamic hypervascularity to epileptogenesis warrants further studies. Finally, hippocampal hypoperfusion may predict later seizure susceptibility in the LFP injury model of TBI, which could be of value for pre-clinical antiepileptogenesis trials.

Arterial spin labeling blood flow MRI: its role in the early characterization of Alzheimer's disease

Arterial spin labeling (ASL) enables the noninvasive, quantitative imaging of cerebral blood flow using standard magnetic resonance imaging (MRI) equipment. Because it requires no contrast injection, ASL can add resting functional information to MRI studies measuring atrophy and signs of ischemic injury. Key features of ASL technology that may affect studies in Alzheimer's disease are described. The existing literature describing ASL blood flow imaging applied to Alzheimer's disease and related dementia is reviewed, and the potential role of ASL in treatment and prevention studies of early Alzheimer's disease is discussed.

Investigation of Linear Coupling Between Single-Event Blood Flow Responses and Interictal Discharges in a Model of Experimental Epilepsy

A successful outcome of epilepsy neurosurgery relies on an accurate delineation of the epileptogenic region to be resected. Functional magnetic resonance imaging (fMRI) would allow doing this noninvasively at high spatial resolution. However, a clear, quantitative description of the relationship between hemodynamic changes and the underlying epileptiform neuronal activity is still missing, thereby preventing the systematic use of fMRI for routine epilepsy surgery planning. To this aim, we used a local epilepsy model to record simultaneously cerebral blood flow (CBF) with laser Doppler (LD) and local field potentials (LFP) in rat frontal cortex. CBF responses to individual interictal-like spikes were large and robust. Their amplitude correlated linearly with spike amplitude. Moreover, the CBF response added linearly in time over a large range of spiking rates. CBF responses could thus be predicted by a linear model of the kind currently used for the interpretation of fMRI data, but including also the spikes’ amplitudes as additional information. Predicted and measured CBF responses matched accurately. For high spiking frequencies (above ∼0.2 Hz), the responses saturated but could eventually recover, indicating the presence of multiple neurovascular coupling mechanisms, which might act at different spatiotemporal scales. Spatially, CBF responses peaked at the center of epileptic activity and displayed a spatial specificity at least as good as the millimeter. These results suggest that simultaneous electroencephalographic and blood flow-based fMRI recordings should be suitable for the noninvasive precise localization of hyperexcitable regions in epileptic patients candidate for neurosurgery.

Reduced ratio of afferent to total vascular density in mesial temporal sclerosis

Mesial temporal sclerosis (MTS) is the most common cause of drug-resistant temporal lobe epilepsy in adults. Despite nearly 2 centuries since the first reports of MTS, relatively little is known about its etiology and pathogenesis. Increasing attention has been directed toward the potential role of vascular abnormalities in MTS. We evaluated the hippocampal microvasculature in 9 MTS cases and 3 non-MTS controls using celloidin tissue sections and markers for total (collagen type IV) and afferent (enzymatic alkaline phosphatase) vessels. Tissue sections were assessed by light microscopy and quantified by threshold analysis of digital images and stereological analysis using the Space Balls probe. Although consistent alterations in the total microvascular density were not found, there was a significant reduction in the density of afferent vessels using both methodologies; these reductions were in areas CA2 and CA3 by image threshold analysis and in area CA3 using stereological measures of the ratio of afferent to total vessels. Increased numbers of string vessels (i.e. remnants of regressing vasculature) were also observed in Ammon's horn, suggesting vascular degeneration in the MTS hippocampus. These findings may help further our understanding of the pathophysiology of MTS.

Arterial spin-labeled perfusion MRI in basic and clinical neuroscience

PURPOSE OF REVIEW

Arterial spin labeling (ASL) provides an endogenous and completely noninvasive tracer for the quantification of regional cerebral blood flow (CBF) with magnetic resonance imaging (MRI). Although the measurement of CBF has obvious utility in cerebrovascular disorders, because CBF is closely coupled to neural metabolism, ASL perfusion MRI has a broad range of potential applications as a biomarker of regional brain function in basic and clinical neuroscience.

RECENT FINDINGS

Over the past few years, ASL technology has improved considerably and the utility of ASL perfusion MRI as a diagnostic and research tool has been demonstrated. This review briefly covers ASL methodologies and clinical applications, while expanding on the use of ASL in human neuroscience research to elucidate patterns of resting brain function that correlate with genotype or phenotype (trait effects), or in response to exogenous manipulations of brain function with pharmacological agents or psychological tasks (state effects).

SUMMARY

ASL perfusion MRI provides a versatile biomarker of regional brain function that can be acquired as part of a multimodal MRI examination. Because ASL quantifies a physiological parameter, it should be useful for multisite or longitudinal studies.