Reduced transit-time sensitivity in noninvasive magnetic resonance imaging of human cerebral blood flow

Volumetric cerebral perfusion imaging in healthy adults: regional distribution, laterality, and repeatability of pulsed continuous arterial spin labeling (PCASL)

The regional distribution, laterality, and reliability of volumetric pulsed continuous arterial spin labeling (PCASL) measurements of cerebral blood flow (CBF) in cortical, subcortical, and cerebellar regions were determined in 10 normal volunteers studied on two occasions separated by 3 to 7 days. Regional CBF, normalized for global perfusion, was highly reliable when measured on separate days. Several regions showed significant lateral asymmetry; notably, in frontal regions CBF was greater in the right than left hemisphere, whereas left was greater than right in posterior regions. There was considerable regional variability across the brain, whereby the posterior cingulate and central and posterior precuneus cortices had the highest perfusion and the globus pallidus the lowest gray matter perfusion. The latter may be due to iron-induced T1 shortening affecting labeled spins and computed CBF signal. High CBF in the posterior cingulate and posterior and central precuneus cortices in this task-free acquisition suggests high activity in these principal nodes of the "default mode network."

Estimation of labeling efficiency in pseudocontinuous arterial spin labeling

Pseudocontinuous arterial spin labeling MRI is a new arterial spin labeling technique that has the potential of combining advantages of continuous arterial spin labeling and pulsed arterial spin labeling. However, unlike continuous arterial spin labeling, the labeling process of pseudocontinuous arterial spin labeling is not strictly an adiabatic inversion and the efficiency of labeling may be subject specific. Here, three experiments were performed to study the labeling efficiency in pseudocontinuous arterial spin labeling MRI. First, the optimal labeling position was determined empirically to be approximately 84 mm below the anterior commissure-posterior commissure line in order to achieve the highest sensitivity. Second, an experimental method was developed to utilize phase-contrast velocity MRI as a normalization factor and to estimate the labeling efficiency in vivo, which was founded to be 0.86 +/- 0.06 (n = 10, mean +/- standard deviation). Third, we compared the labeling efficiency of pseudocontinuous arterial spin labeling MRI under normocapnic and hypercapnic (inhalation of 5% CO(2)) conditions and showed that a higher flow velocity in the feeding arteries resulted in a reduction in the labeling efficiency. In summary, our results suggest that labeling efficiency is a critical parameter in pseudocontinuous arterial spin labeling MRI not only in terms of achieving highest sensitivity but also in quantification of absolute cerebral blood flow in milliliters per minute per 100 g. We propose that the labeling efficiency should be estimated using phase-contrast velocity MRI on a subject-specific basis.

Assessment of arterial arrival times derived from multiple inversion time pulsed arterial spin labeling MRI

The purpose of this study was to establish a normal range for the arterial arrival time (AAT) in whole-brain pulsed arterial spin labeling (PASL) cerebral perfusion MRI. Healthy volunteers (N = 36, range: 20 to 35 years) provided informed consent to participate in this study. AAT was assessed in multiple brain regions, using three-dimensional gradient and spin echo (GRASE) pulsed arterial spin labeling at 3.0 T, and found to be 641 +/- 95, 804 +/- 91, 802 +/- 126, and 935 +/- 108 ms in the temporal, parietal, frontal, and occipital lobes, respectively. Mean gray matter AAT was found to be 694 +/- 89 ms for females (N = 15), which was significantly shorter than for men, 814 +/- 192 ms (N = 21; P < 0.0003), and significant after correcting for brain volume (P < 0.001). Significant AAT sex differences were also found using voxelwise permutation testing. An atlas of AAT values across the healthy brain is presented here and may be useful for future experiments that aim to quantify cerebral blood flow from ASL data, as well as for clinical comparisons where disease pathology may lead to altered AAT. Pulsed arterial spin labeling signals were simulated using an identical sampling scheme as the empiric study and revealed AAT can be estimated robustly when simulated arrival times are well beyond the normal range.

Resting-state perfusion in nonmedicated schizophrenic patients: a continuous arterial spin-labeling 3.0-T MR study

PURPOSE

To determine whether well-described patterns of altered perfusion in schizophrenia can be identified by using continuous arterial spin labeling (CASL) with a whole-brain imaging sequence.

MATERIALS AND METHODS

This study was approved by the ethics committee of the local institutional review board, and written informed consent was obtained from all subjects. CASL was used to compare cerebral perfusion between 11 nonmedicated patients with schizophrenia and 25 healthy control subjects. Since antipsychotic medication may affect perfusion, only drug-free subjects were examined. Resting-state perfusion, as measured in terms of regional cerebral blood flow, was compared voxel wise by using an analysis of variance design in a statistical parametric mapping program, with patient age and sex as covariates.

RESULTS

Compared with the healthy control subjects, the schizophrenic patients had extensive areas of hypoperfusion in the frontal lobes bilaterally, in the anterior and medial cingulate gyri, and in the parietal lobes bilaterally. Increased perfusion was observed in the cerebellum, brainstem, and thalamus of the schizophrenic patients as compared with the perfusion in these areas in the control subjects.

CONCLUSION

CASL in schizophrenia revealed patterns of hypo- and hyperperfusion similar to the perfusion patterns in previously published positron emission tomographic and single photon emission computed tomographic studies. The advantages of CASL, including independence from injected contrast agents, no irradiation, and fast acquisition time, may facilitate intensive perfusion studies of the early recognition of schizophrenia and other psychiatric disorders, as well as longitudinal disease-monitoring research of these conditions.

Quantitative analysis of arterial spin labeling FMRI data using a general linear model

Arterial spin labeling techniques can yield quantitative measures of perfusion by fitting a kinetic model to difference images (tagged-control). Because of the noisy nature of the difference images investigators typically average a large number of tagged versus control difference measurements over long periods of time. This averaging requires that the perfusion signal be at a steady state and not at the transitions between active and baseline states in order to quantitatively estimate activation induced perfusion. This can be an impediment for functional magnetic resonance imaging task experiments. In this work, we introduce a general linear model (GLM) that specifies Blood Oxygenation Level Dependent (BOLD) effects and arterial spin labeling modulation effects and translate them into meaningful, quantitative measures of perfusion by using standard tracer kinetic models. We show that there is a strong association between the perfusion values using our GLM method and the traditional subtraction method, but that our GLM method is more robust to noise.

Theoretical and experimental evaluation of continuous arterial spin labeling techniques

Continuous arterial spin labeling is known to be the most sensitive arterial spin labeling technique. To avoid magnetization transfer effects and to overcome hardware limitations, several sequences have been proposed that adiabatically label the inflowing blood. Four of these methods are examined with respect to their sensitivity both theoretically by Bloch equation simulations and experimentally. All sequences were optimized carefully by adjusting their measurement parameters based exclusively on the results of simulations. Perfusion measurements on the human brain obtained at 3 T result in excellent images from all techniques, while differences in sensitivity are similar to those expected from the simulations.

Arterial transit time effects in pulsed arterial spin labeling CBF mapping: insight from a PET and MR study in normal human subjects

Arterial transit time (ATT), a key parameter required to calculate absolute cerebral blood flow in arterial spin labeling (ASL), is subject to much uncertainty. In this study, ASL ATTs were estimated on a per-voxel basis using data measured by both ASL and positron emission tomography in the same subjects. The mean ATT increased by 260 +/- 20 (standard error of the mean) ms when the imaging slab shifted downwards by 54 mm, and increased from 630 +/- 30 to 1220 +/- 30 ms for the first slice, with an increase of 610 +/- 20 ms over a four-slice slab when the gap between the imaging and labeling slab increased from 20 to 74 mm. When the per-slice ATTs were employed in ASL cerebral blood flow quantification and the in-slice ATT variations ignored, regional cerebral blood flow could be significantly different from the positron emission tomography measures. ATT also decreased with focal activation by the same amount for both visual and motor tasks (approximately 80 ms). These results provide a quantitative relationship between ATT and the ASL imaging geometry and yield an assessment of the assumptions commonly used in ASL imaging. These findings should be considered in the interpretation of, and comparisons between, different ASL-based cerebral blood flow studies. The results also provide spatially specific ATT data that may aid in optimizing the ASL imaging parameters.

Reliability and precision of pseudo-continuous arterial spin labeling perfusion MRI on 3.0 T and comparison with 15O-water PET in elderly subjects at risk for Alzheimer's disease

Arterial spin labeling (ASL) offers MRI measurement of cerebral blood flow (CBF) in vivo, and may offer clinical diagnostic utility in populations such as those with early Alzheimer's disease (AD). In the current study, we investigated the reliability and precision of a pseudo-continuous ASL (pcASL) sequence that was performed two or three times within one hour on eight young normal control subjects, and 14 elderly subjects including 11 with normal cognition, one with AD and two with Mild Cognitive Impairment (MCI). Six of these elderly subjects including one AD, two MCIs and three controls also received (15)O-water positron emission tomography (PET) scans 2 h before their pcASL MR scan. The instrumental reliability of pcASL was evaluated with the intraclass correlation coefficient (ICC). The ICCs were greater than 0.90 in pcASL global perfusion measurements for both the young and the elderly groups. The cross-modality perfusion imaging comparison yielded very good global and regional agreement in global gray matter and the posterior cingulate cortex. Significant negative correlation was found between age and the gray/white matter perfusion ratio (r = -0.62, p < 0.002). The AD and MCI patients showed the lowest gray/white matter perfusion ratio among all the subjects. The data suggest that pcASL provides a reliable whole brain CBF measurement in young and elderly adults whose results converge with those obtained with the traditional (15)O-water PET perfusion imaging method. pcASL perfusion MRI offers an alternative method for non-invasive in vivo examination of early pathophysiological changes in AD.

Quantification issues in arterial spin labeling perfusion magnetic resonance imaging

Arterial spin labeling (ASL) perfusion magnetic resonance imaging has gained wide acceptance for its value in clinical and neuroscience applications during recent years. Its capability for noninvasive and absolute perfusion quantification is a key characteristic that makes ASL attractive for many clinical applications. In the present review, we discuss the main parameters or factors that affect the reliability and accuracy of ASL perfusion measurements. Our secondary goal was to outline potential solutions that may improve the reliability and accuracy of ASL in clinical settings. It was found that, through theoretical analyses, flow quantification is most sensitive to tagging efficiency and estimation of the equilibrium magnetization of blood signal (M(0b)). Variations of blood T1 have a greater effect on perfusion quantification than variations of tissue T1. Arterial transit time becomes an influential factor when it is longer than the postlabeling delay time. The T2's of blood and tissue impose minimal effects on perfusion calculation at field strengths equal to or lower than 3.0 T. Subsequently, we proposed various approaches for in vivo estimation or calibration of the above parameters, such as the use of phase-contrast magnetic resonance imaging for calibration of the labeling efficiency as well as the use of inversion recovery TrueFISP (true fast imaging with steady-state precession) sequence for blood T1 mapping. We also list representative clinical cases in which implicit assumptions for ASL perfusion quantification may be violated, such as the venous outflow effect in children with sickle cell disease. Finally, an optimal imaging protocol including in vivo measurements of several critical parameters was recommended for clinical ASL studies.

Overexpression of heat shock protein 27 reduces cortical damage after cerebral ischemia

Heat shock protein 27 (HSP27) has a major role in mediating survival responses to a range of central nervous system insults, functioning as a protein chaperone, an antioxidant, and through inhibition of cell death pathways. We have used transgenic mice overexpressing HSP27 (HSP27tg) to examine the role of HSP27 in cerebral ischemia, using model of permanent middle cerebral artery occlusion (MCAO). Infarct size was evaluated using multislice T(2)-weighted anatomical magnetic resonance imaging (MRI) after 24 h. A significant reduction of 30% in infarct size was detected in HSP27tg animals compared with wild-type (WT) littermates. To gain some insight into the mechanisms contributing to cell death and its attenuation by HSP27, we monitored the effect of induction of c-jun and ATF3 on tissue survival in MCAO and their effects on the expression of endogenous mouse HSP25 and HSP70. It is important that, the c-jun induction seen at 4 h tended to be localized to regions that were salvageable in HSP27tg mice but became infarcted in WT animals. Our results provide support for the powerful neuroprotective effects of HSP27 in cerebral ischemia.

Complex-valued analysis of arterial spin labeling-based functional magnetic resonance imaging signals

Cerebral blood flow-dependent phase differences between tagged and control arterial spin labeling images are reported. A biophysical model is presented to explain the vascular origin of this difference. Arterial spin labeling data indicated that the phase difference is largest when the arterial component of the signals is preserved but is greatly reduced as the arterial contribution is suppressed by postinversion delays or flow-crushing gradients. Arterial vasculature imaging by saturation data of activation and hypercapnia conditions showed increases in phase accompanying blood flow increases.An arterial spin labeling functional magnetic resonance imaging study yielded significant activation by magnitude-only, phase-only, and complex analyses when preserving the whole arterial spin labeling signal. After suppression of the arterial signal by postinversion delays, magnitude-only and complex models yielded similar activation levels, but the phase-only model detected nearly no activation. When flow crushers were used for arterial suppression, magnitude-only activation was slightly lower and fluctuations in phase were dramatically higher than when postinversion delays were used.Although the complex analysis performed did not improve detection, a simulation study indicated that the complex-valued activation model exhibits combined magnitude and phase detection power and thus maximizes sensitivity under ideal conditions. This suggests that, as arterial spin labeling imaging and image correction methods develop, the complex-valued detection model may become helpful in signal detection.

[CT and MRI imaging in tumoral angiogenesis]

Angiogenesis is the process of activating dormant endothelial cells to form new vessels, after stimulation and it is essential in tumor growth. In many types of cancer, angiogenesis results from the activation of oncogenes that stimulate the production of Vascular Endothelial Growth Factor (VEGF). However, these newly formed vessels have a great number of abnormalities: increased density of fragile and hyper-permeable microvessels, arterial-venous shunts, caliber abnormalities and flow instabilities susceptible to flow direction inversion according to interstitial pressure. Anti-angiogenic treatments inhibit VEGF activity, perceived as structural and functional normalization of the microvascular pattern, such as reduced density of microvessels and restored morphology of the remaining ones. Conventional imaging techniques are not sensible to these changes, at best they show tumor size stabilization, hence the need of new techniques. Microvascularization imaging can be achieved by detecting functional disturbances to blood flow and not by showing the microvasculature per se. These techniques are based in quantifying the enhancement in tumor due to the passage of contrast agent after injection or protons labeled by a magnetic field. Through these measurements, one can derive interstitial and blood volumes as well as the tissue perfusion and capillary wall permeability. Microvascular imaging has greatly benefited from the improvements seen in CT and MRI equipment allowing large volume coverage with high spatial and temporal resolutions as from the evolutions in the methods to calculate, present and compare maps of the microcirculation and it's heterogeneity. However, software to analyze microvascularization are still rare, limiting the technique's application and validation in large scale. Nevertheless, imaging of the microcirculation is useful throughout the care of the oncological patient: it can reinforce the suspicious nature of a lesion, suggest anti-angiogenic treatment efficacy in hypervascular lesions, and show early treatment response before morphological changes as in RECIST criteria.

Pathophysiologic evaluation of MELAS strokes by serially quantified MRS and CASL perfusion images

PURPOSE

To clarify the roles of serial MR spectroscopy (MRS) and continuous arterial spin labeling (CASL) perfusion images for evaluating cerebral lesions in patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS).

MATERIALS AND METHODS

Two cases of MELAS followed up serially using MRS and CASL images in addition to routine MR imaging were enrolled.

RESULTS

Newly appeared lesions assessed by MRS revealed increased lactate doublets which correlated well with CSF lactate level, and these showed a decreasing trend after treatment, although conventional T2 weighted images revealed hyper-intensity in both phases. Spectra from normally appearing white matter depicted slight lactate peaks during clinical exacerbation periods with marked elevation of CSF lactate and showed a decreasing NAA concentration during the prolonged course. In CASL images, acute lesions of the disease were clearly visible as hyper-perfusion foci, and chronic lesions were demonstrated as hypo- or iso-perfusion regions.

CONCLUSION

The detection of lactate peaks in the MR spectrum from normally appearing white matter may be considered as systemic lactic acidosis or an exacerbation of MELAS, and active lesions can be distinguished from chronic inactive lesions by the increase of lactate peaks in MRS or the state of hyper-perfusion in CASL images.

Neuroimaging in acute ischaemic stroke: insights into unanswered questions of pathophysiology

The treatment of acute ischaemic stroke is based on the principle that there is ischaemic but still potentially salvageable tissue that could be rescued if blood flow could be restored quickly. It is assumed that salvage might only be possible in the first few hours, and that infarct expansion is a direct result of failed recanalization of the main artery. This concept arose from experimental work in the 1970s, supported more recently by studies using imaging to identify penumbral tissue. However, although magnetic resonance diffusion and perfusion imaging is a way of imaging penumbral tissue and has been around for over a decade, it is not an easy technique to apply in practice and its use has produced conflicting results. Computed tomography perfusion, and any other tissue perfusion imaging technique, is likely to encounter the same difficulties. Indeed many factors, other than the presence of a diffusion-perfusion mismatch acutely, may determine or influence ultimate tissue fate even days after the stroke, and in turn, clinical outcome. Many of these potential influences are beginning to emerge from studies using different forms of imaging at later times after stroke. This review will explore the information now emerging from imaging studies in large artery ischaemic stroke to summarize knowledge to date and indicate unresolved issues for the future.

Arterial spin labeling perfusion MRI at multiple delay times: a correlative study with H(2)(15)O positron emission tomography in patients with symptomatic carotid artery occlusion

Arterial spin labeling (ASL) perfusion magnetic resonance imaging (MRI) with image acquisition at multiple inversion times is a noninvasive ASL technique able to compensate for spatial heterogeneities in transit times caused by collateral blood flow in patients with severe stenosis of the cerebropetal blood vessels. Our aim was to compare ASL-MRI and H(2)(15)O positron emission tomography (PET), the gold standard for cerebral blood flow (CBF) assessment, in patients with a symptomatic internal carotid artery (ICA) occlusion. Fourteen patients (63+/-14 years) with a symptomatic ICA occlusion underwent both ASL-MRI and H(2)(15)O PET. The ASL-MRI was performed using a pulsed STAR labeling technique at multiple inversion times within 7 days of the PET. The CBF was measured in the gray-matter of the anterior, middle and posterior cerebral artery, and white-matter. Both PET and ASL-MRI showed a significantly decreased CBF in the gray-matter of the middle cerebral artery in the hemisphere ipsilateral to the ICA occlusion. The average gray-matter CBF measured with ASL-MRI (71.8+/-4.3 mL/min/100 g) was higher (P<0.01) than measured with H(2)(15)O PET (43.1+/-1.0 mL/min/100 g). In conclusion, ASL-MRI at multiple TIs is capable of depicting areas of regions with low CBF in patients with an occlusion of the ICA, although a systematic overestimation of CBF relative to H(2)(15)O PET was noted.

Separating function from structure in perfusion imaging of the aging brain

The accuracy of cerebral blood flow (CBF) imaging in humans has been impeded by the partial volume effects (PVE), which are a consequence of the limited spatial resolution. Because of brain atrophy, PVE can be particularly problematic in imaging the elderly and can considerably overestimate the CBF difference with the young. The primary goal of this study was to separate the structural decline from the true CBF reduction in elderly. To this end, a PVE-correction algorithm was applied on the CBF images acquired with spin-echo EPI continuous arterial spin labeling MRI (voxel size = 3.4 x 3.4 x 8 mm(3)). Tissue-specific CBF images that were independent of voxels' tissue fractional volume were obtained in elderly (N = 30) and young (N = 26); mean age difference was 43 years. Globally, PVE-corrected gray matter CBF was 88.2 +/- 16.1 and 107.3 +/- 17.5 mL/100 g min(-1) in elderly and young, respectively. The largest PVE contribution was found in the frontal lobe and accounted for an additional 10% and 12% increase in the age-related CBF difference between men and women, respectively. The GM-to-WM CBF ratios were found to be on average 3.5 in elderly and 3.9 in young. Whole brain voxelwise comparisons showed marked CBF decrease in anterior cingulate (bilateral), caudate (bilateral), cingulate gyrus (bilateral), cuneus (left), inferior frontal gyrus (left), insula (left), middle frontal gyrus (left), precuneus (bilateral), prefrontal cortex (bilateral), and superior frontal gyrus (bilateral) in men and amygdala (bilateral), hypothalamus (left), hippocampus (bilateral), and middle frontal gyrus (right) in women.

Characterizing the origin of the arterial spin labelling signal in MRI using a multiecho acquisition approach

Arterial spin labelling (ASL) can noninvasively isolate the MR signal from arterial blood water that has flowed into the brain. In gray matter, the labelled bolus is dispersed within three main compartments during image acquisition: the intravascular compartment; intracellular tissue space; and the extracellular tissue space. Changes in the relative volumes of the extracellular and intracellular tissue space are thought to occur in many pathologic conditions such as stroke and brain tumors. Accurate measurement of the distribution of the ASL signal within these three compartments will yield better understanding of the time course of blood delivery and exchange, and may have particular application in animal models of disease to investigate the relationship between the source of the ASL signal and pathology. In this study, we sample the transverse relaxation of the ASL perfusion weighted and control images acquired with and without vascular crusher gradients at a range of postlabelling delays and tagging durations, to estimate the tricompartmental distribution of labelled water in the rat cortex. Our results provide evidence for rapid exchange of labelled blood water into the intracellular space relative to the transit time through the vascular bed, and provide a more solid foundation for cerebral blood flow quantification using ASL techniques.

Perfusion MRI of brain tumours: a comparative study of pseudo-continuous arterial spin labelling and dynamic susceptibility contrast imaging

INTRODUCTION

The purpose of this study was to compare the non-invasive 3D pseudo-continuous arterial spin labelling (PC ASL) technique with the clinically established dynamic susceptibility contrast perfusion magnetic resonance imaging (DSC-MRI) for evaluation of brain tumours.

METHODS

A prospective study of 28 patients with contrast-enhancing brain tumours was performed at 3 T using DSC-MRI and PC ASL with whole-brain coverage. The visual qualitative evaluation of signal enhancement in tumour was scored from 0 to 3 (0 = no signal enhancement compared with white matter, 3 = pronounced signal enhancement with equal or higher signal intensity than in grey matter/basal ganglia). The extent of susceptibility artefacts in the tumour was scored from 0 to 2 (0 = no susceptibility artefacts and 2 = extensive susceptibility artefacts (maximum diameter > 2 cm)). A quantitative analysis was performed with normalised tumour blood flow values (ASL nTBF, DSC nTBF): mean value for region of interest (ROI) in an area with maximum signal enhancement/the mean value for ROIs in cerebellum.

RESULTS

There was no difference in total visual score for signal enhancement between PC ASL and DSC relative cerebral blood flow (p = 0.12). ASL had a lower susceptibility-artefact score than DSC-MRI (p = 0.03). There was good correlation between DSC nTBF and ASL nTBF values with a correlation coefficient of 0.82.

CONCLUSION

PC ASL is an alternative to DSC-MRI for the evaluation of perfusion in brain tumours. The method has fewer susceptibility artefacts than DSC-MRI and can be used in patients with renal failure because no contrast injection is needed.

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.

Semantic context and visual feature effects in object naming: an fMRI study using arterial spin labeling

Previous behavioral studies reported a robust effect of increased naming latencies when objects to be named were blocked within semantic category, compared to items blocked between category. This semantic context effect has been attributed to various mechanisms including inhibition or excitation of lexico-semantic representations and incremental learning of associations between semantic features and names, and is hypothesized to increase demands on verbal self-monitoring during speech production. Objects within categories also share many visual structural features, introducing a potential confound when interpreting the level at which the context effect might occur. Consistent with previous findings, we report a significant increase in response latencies when naming categorically related objects within blocks, an effect associated with increased perfusion fMRI signal bilaterally in the hippocampus and in the left middle to posterior superior temporal cortex. No perfusion changes were observed in the middle section of the left middle temporal cortex, a region associated with retrieval of lexical-semantic information in previous object naming studies. Although a manipulation of visual feature similarity did not influence naming latencies, we observed perfusion increases in the perirhinal cortex for naming objects with similar visual features that interacted with the semantic context in which objects were named. These results provide support for the view that the semantic context effect in object naming occurs due to an incremental learning mechanism, and involves increased demands on verbal self-monitoring.

Phosphorylation of Tau at S422 is enhanced by Abeta in TauPS2APP triple transgenic mice

Amyloid beta peptides and microtubule-associated protein Tau are misfolded and form aggregates in brains of Alzheimer's disease patients. To examine their specific roles in the pathogenesis of Alzheimer's disease and their relevance in neurodegenerative processes, we have created TauPS2APP triple transgenic mice that express human mutated Amyloid Precursor Protein, presenilin 2 and Tau. We present a cross-sectional analysis of these mice at 4, 8, 12 and 16 months of age. By comparing with single transgenic Tau mice, we demonstrate that accumulation of Abeta in TauPS2APP triple transgenic mice impacts on Tau pathology by increasing the phosphorylation of Tau at serine 422, as determined by a novel immunodetection method that is able to reliably measure phospho-Tau species in transgenic mouse brains. The TauPS2APP triple transgenic mouse model will be very useful for studying the effect of new therapeutic paradigms on amyloid deposition and downstream neurofibrillary tangle development.

Cortical hypoperfusion in the B6.PS2APP mouse model for Alzheimer's disease: comprehensive phenotyping of vascular and tissular parameters by MRI

Function and morphology of the cerebral vasculature were studied in the amyloid (Abeta) plaque-containing double-transgenic (TG) B6.PS2APP Alzheimer's disease (AD) mouse model with MRI at an age range of 10 to 17 months. Perfusion, blood volume, and average vessel geometry were assessed in the brain and compared to age-matched controls (wild-type [WT] C57Bl/6). Additionally, the MR relaxation times T(1), T(2), and T(2)* were measured to detect potential pathological changes that might be associated with Abeta plaque depositions. Both decreased perfusion and decreased blood volume were observed in the occipital cortex in B6.PS2APP mice as compared to controls. A significant decrease in T(1) and T(2) was found in the frontal cortex and in the subiculum/parasubiculum. Immunohistochemistry confirmed plaque depositions in the cortex and in the subiculum/parasubiculum. In summary, our data indicate a reduced blood supply of B6.PS2APP mice in the occipital cortex that parallels the findings in cortical regions of patients with AD.

Noninvasive quantification of whole-brain cerebral metabolic rate of oxygen (CMRO2) by MRI

Cerebral metabolic rate of oxygen (CMRO(2)) is an important marker for brain function and brain health. Existing techniques for quantification of CMRO(2) with positron emission tomography (PET) or MRI involve special equipment and/or exogenous agents, and may not be suitable for routine clinical studies. In the present study, a noninvasive method is developed to estimate whole-brain CMRO(2) in humans. This method applies phase-contrast MRI for quantitative blood flow measurement and T(2)-relaxation-under-spin-tagging (TRUST) MRI for venous oxygenation estimation, and uses the Fick principle of arteriovenous difference for the calculation of CMRO(2). Whole-brain averaged CMRO(2) values in young, healthy subjects were 132.1 +/- 20.0 micromol/100 g/min, in good agreement with literature reports using PET. Various acquisition strategies for phase-contrast and TRUST MRI were compared, and it was found that nongated phase-contrast and sagittal sinus (SS) TRUST MRI were able to provide the most efficient and accurate estimation of CMRO(2). In addition, blood flow and venous oxygenation were found to be positively correlated across subjects. Owing to the noninvasive nature of this method, it may be a convenient and useful approach for assessment of brain metabolism in brain disorders as well as under various physiologic conditions.

Magnetic resonance imaging of the retina

This paper reviews recent developments in high-resolution magnetic resonance imaging (MRI) and its application to image anatomy, physiology, and function in the retina of animals. It describes technical issues and solutions in performing retinal MRI, anatomical MRI, blood oxygenation level-dependent functional MRI (fMRI), and blood-flow MRI both of normal retinas and of retinal degeneration. MRI offers unique advantages over existing retinal imaging techniques, including the ability to image multiple layers without depth limitation and to provide multiple clinically relevant data in a single setting. Retinal MRI has the potential to complement existing retinal imaging techniques.

Strategies for reducing respiratory motion artifacts in renal perfusion imaging with arterial spin labeling

Arterial spin labeling (ASL) perfusion measurements may have many applications outside the brain. In the abdomen, severe image artifacts can arise from motions between acquisitions of multiple signal averages in ASL, even with single-shot image acquisition. Background suppression and respiratory motion synchronization techniques can be used to ameliorate these artifacts. Two separate in vivo studies of renal perfusion imaging using pulsed continuous ASL (pCASL) were performed. The first study assessed various combinations of background suppression and breathing strategies. The second investigated the retrospective sorting of images acquired during free breathing based on respiratory position. Quantitative assessments of the test-retest repeatability of perfusion measurements and the image quality scored by two radiologists were made. Image quality was most significantly improved by using background suppression schemes and controlled breathing when compared to other combinations without background suppression or with free breathing, assessed by test-retests (5% level, F-test), and by radiologists' scores (5% level, Mann-Whitney U-test). Under free breathing, retrospectively sorting images based on respiratory position showed significant improvement. Both radiologists found 100% of the images had preferable image sharpness after sorting. High-quality renal perfusion measurements with reduced respiratory motion artifacts have been demonstrated using ASL when appropriate background suppression and breathing strategies are applied.

Arterial spin-label imaging in patients with normal bolus perfusion-weighted MR imaging findings: pilot identification of the borderzone sign

PURPOSE

To determine whether perfusion abnormalities are depicted on arterial spin-labeling (ASL) images obtained in patients with normal bolus perfusion-weighted (PW) magnetic resonance (MR) imaging findings.

MATERIALS AND METHODS

Institutional review board approval and written informed patient consent were obtained. This study was HIPAA compliant. Consecutive patients suspected or known to have cerebrovascular disease underwent 1.5-T brain MR imaging, including MR angiography, gradient-echo PW imaging, and pseudocontinuous ASL imaging, between October 2007 and January 2008. Patients with normal bolus PW imaging findings were retrospectively identified, and two neuroradiologists subsequently evaluated the ASL images for focal abnormalities. The severity of the borderzone sign-that is, bilateral ASL signal dropout with surrounding cortical areas of hyperintensity in the middle cerebral artery borderzone regions-was classified by using a four-point scale. For each group, the ASL-measured mean mixed cortical cerebral blood flow (CBF) at the level of the centrum semiovale was evaluated by using the Jonckheere-Terpstra test.

RESULTS

One hundred thirty-nine patients met the study inclusion criteria, and 41 (30%) of them had normal bolus PW imaging findings. Twenty-three (56%) of these 41 patients also had normal ASL imaging findings. The remaining 18 (44%) patients had the ASL borderzone sign; these patients were older (mean age, 71 years +/- 11 [standard deviation] vs 57 years +/- 16; P < .005) and had lower mean CBF (30 mL/100 g/min +/- 12 vs 46 mL/100 g/min +/- 12, P < .003) compared with the patients who had normal ASL imaging findings. Five patients had additional focal ASL findings that were related to either slow blood flow in a vascular structure or postsurgical perfusion defects and were not visible on the PW images.

CONCLUSION

Approximately half of the patients with normal bolus PW imaging findings had abnormal ASL findings-most commonly the borderzone sign. Results of this pilot study suggest that ASL imaging in patients who have this sign and are suspected of having cerebrovascular disease yields additional and complementary hemodynamic information.

Estimating cerebral blood volume with expanded vascular space occupancy slice coverage

A model for quantifying cerebral blood volume (CBV) based on the vascular space occupancy (VASO) technique and varying the extent of blood nulling yielding task-related signal changes with various amounts of blood oxygenation level-dependent (BOLD) and VASO weightings was previously described. Challenges associated with VASO include limited slice coverage and the confounding inflow of fresh blood. In this work, an approach that extends the previous model to multiple slices and accounts for the inflow effect is described and applied to data from a multiecho sequence simultaneously acquiring VASO, cerebral blood flow (CBF), and BOLD images. This method led to CBV values (7.9 +/- 0.3 and 5.6 +/- 0.3 ml blood/100 ml brain during activation [CBV(ACT)] and rest [CBV(REST)], respectively) consistent with previous studies using similar visual stimuli. Furthermore, an increase in effective blood relaxation (0.65 +/- 0.01) compared to the published value (0.62) was detected, likely reflecting inflow of fresh blood. Finally, cerebral metabolic rate of oxygen (CMRO(2)) estimates using a multiple compartment model without assumption of CBV(REST) led to estimates (18.7 +/- 17.0%) that were within published ranges.

Skeletal muscle microvascular flow in progressive peripheral artery disease: assessment with continuous arterial spin-labeling perfusion magnetic resonance imaging

OBJECTIVES

We present the novel application of continuous arterial spin-labeling (CASL) magnetic resonance imaging (MRI) for the measurement of calf muscle perfusion in subjects with progressive peripheral arterial disease (PAD).

BACKGROUND

Peripheral arterial disease is largely considered to be a disease of conduit vessels. The impact of PAD upon microvascular flow in the end-organ, muscle, remains unknown. Continuous arterial spin-labeling is a noninvasive MRI method capable of measuring microvascular flow and might assist in our understanding of the impact of PAD upon the microvasculature.

METHODS

Forty subjects with varying degrees of PAD and 17 age-matched PAD-free subjects were recruited and underwent measurement of the ankle-to-brachial index (ABI) and CASL. Peak hyperemic flow (PHF) and time-to-peak (TTP) were computed and assessed as a function of ABI and calf muscle group.

RESULTS

An ABI dependence was found in both PHF (p = 0.04) and TTP (p < 10(-4)). Whereas TTP increased almost immediately with increasing PAD severity, PHF was, in contrast, relatively well preserved until later stages of disease.

CONCLUSIONS

The CASL flow measurements correlate with disease state as measured by ABI and demonstrate preserved microvascular flow reserve in the presence of early to intermediate vascular disease.

Does arterial spin-labeling MR imaging-measured tumor perfusion correlate with renal cell cancer response to antiangiogenic therapy in a mouse model?

PURPOSE

To determine whether arterial spin-labeling (ASL) magnetic resonance (MR) imaging findings at baseline and early during antiangiogenic therapy can predict later resistance to therapy.

MATERIALS AND METHODS

Protocol was approved by an institutional animal care and use committee. Caki-1, A498, and 786-0 human renal cell carcinoma (RCC) xenografts were implanted in 39 nude mice. Animals received 80 mg sorafenib per kilogram of body weight once daily once tumors measured 12 mm. ASL imaging was performed at baseline and day 14, with additional imaging performed for 786-0 and A498 (3 days to 12 weeks). Mean blood flow values and qualitative differences in spatial distribution of blood flow were analyzed and compared with histopathologic findings for viability and microvascular density. t Tests were used to compare differences in mean tumor blood flow. Bonferroni-adjusted P values less than .05 denoted significant differences.

RESULTS

Baseline blood flow was 80.1 mL/100 g/min +/- 23.3 (standard deviation) for A498, 75.1 mL/100 g/min +/- 28.6 for 786-0, and 10.2 mL/100 g/min +/- 9.0 for Caki-1. Treated Caki-1 showed no significant change (14.9 mL/100 g/min +/- 7.6) in flow, whereas flow decreased in all treated A498 on day 14 (47.9 mL/100 g/min +/- 21.1) and in 786-0 on day 3 (20.3 mL/100 g/min +/- 8.7) (P = .003 and .03, respectively). For A498, lowest values were measured at 28-42 days of receiving sorafenib. Regions of increased flow occurred on days 35-49, 17-32 days before documented tumor growth and before significant increases in mean flow (day 77). Although 786-0 showed new, progressive regions with signal intensity detected as early as day 5 that correlated to viable tumor at histopathologic examination, no significant changes in mean flow were noted when day 3 was compared with all subsequent days (P > .99).

CONCLUSION

ASL imaging provides clinically relevant information regarding tumor viability in RCC lines that respond to sorafenib.

Comparison of pulsed arterial spin labeling encoding schemes and absolute perfusion quantification

Arterial spin labeling (ASL) using magnetic resonance imaging (MRI) is a powerful noninvasive technique to investigate the physiological status of brain tissue by measuring cerebral blood flow (CBF). ASL assesses the inflow of magnetically labeled arterial blood into an imaging voxel. In the last 2 decades, various ASL sequences have been proposed which differ in their ease of implementation and their sensitivity to artifacts. In addition, several quantification methods have been developed to determine the absolute value of CBF from ASL magnetization difference images. In this study, we evaluated three pulsed ASL sequences and three absolute quantification schemes. It was found that FAIR-QUIPSSII implementation of ASL yields 10-20% higher signal-to-noise ratio (SNR) and 18% higher CBF as compared with PICORE-Q2TIPS (with FOCI pulses) and PICORE-QUIPSSII (with BASSI pulses). In addition, quantification schemes employed can give rise to up to a 35% difference in CBF values. We conclude that, although all quantitative ASL sequences and CBF calibration methods should in principle result in the similar CBF values and image quality, substantial differences in CBF values and SNR were found. Thus, comparing studies using different ASL sequences and analysis algorithms is likely to result in erroneous intra- and intergroup differences. Therefore, (i) the same quantification schemes should consistently be used, and (ii) quantification using local tissue proton density should yield the most accurate CBF values because, although still requiring definitive demonstration in future studies, the proton density of blood is assumed to be very similar to the value of gray matter.

Reduction in cerebral blood flow in areas appearing as white matter hyperintensities on magnetic resonance imaging

The purpose of this study was to examine cerebral blood flow (CBF) as measured by arterial spin labeling (ASL) in tissue classified as white matter hyperintensities (WMH), normal appearing white matter, and grey matter. Seventeen healthy older adults received structural and ASL MRI. Cerebral blood flow was derived for three tissue types: WMH, normal appearing white matter, and grey matter. Cerebral blood flow was lower in WMH areas relative to normal appearing white matter, which in turn, was lower than grey matter. Regions with consistently lower CBF across individuals were more likely to appear as WMH. Results are consistent with an emerging literature linking diminished regional perfusion with the risk of developing WMH.

Depressive disorders: focally altered cerebral perfusion measured with arterial spin-labeling MR imaging

PURPOSE

To assess focal cerebral perfusion in patients with refractory depressive disorder (RDD), patients with nonrefractory depressive disorder (NDD), and healthy control subjects by using arterial spin-labeling (ASL) magnetic resonance (MR) imaging.

MATERIALS AND METHODS

This study was approved by the local ethical committee, and written informed consent was obtained from all participants. Twenty-four patients with RDD, 37 patients with NDD, and 42 healthy control subjects were included. From February 2006 to July 2007, all participants were imaged with a 3-T MR system. ASL and echo-planar images were subtracted and averaged to give perfusion-weighted images. Voxel-based analysis was performed. Region-of-interest analysis was applied to the bilateral hippocampi, thalami, and lentiform nuclei.

RESULTS

Patients with NDD showed reduced perfusion in the left prefrontal cortex versus control subjects and increased perfusion mainly in the limbic-striatal areas (P < .05). In contrast, patients with RDD had decreased perfusion predominantly in the bilateral frontal and bilateral thalamic regions (P < .05). Compared with patients with RDD, patients with NDD showed higher perfusion mainly in the limbic-striatal areas (P < .05). In region-of-interest analysis, the NDD group showed higher regional cerebral blood flow than both RDD and control groups in the left hippocampus (P = .045), right hippocampus (P = .001), and right lentiform nucleus (P = .049).

CONCLUSION

This study revealed alterations of regional perfusion in the brains of patients with RDD that differed from those in patients with NDD. These results are consistent with the concept that RDD is associated with decreased activity of the bilateral prefrontal areas; and NDD, with decreased activity of left frontal areas in conjunction with overactivity of the bilateral limbic system.

Vasoreactivity and peri-infarct hyperintensities in stroke

OBJECTIVE

It is unknown if impaired cerebral vasoreactivity recovers after ischemic stroke, and whether it compromises perfusion in regions surrounding infarct and other vascular territories. We investigated the regional differences in CO2 vasoreactivity (CO2 VR) and their relationships to peri-infarct T2 hyperintensities (PIHs), chronic infarct volumes, and clinical outcomes.

METHODS

We studied 39 subjects with chronic large middle cerebral artery territory infarcts and 48 matched controls. Anatomic and three-dimensional continuous arterial spin labeling imaging at 3-Tesla MRI were used to measure regional cerebral blood flow (CBF) and CO2 VR during normocapnia, hypercapnia, and hypocapnia in main arteries distributions.

RESULTS

Stroke patients showed a significantly lower augmentation of blood flow at increased CO2 but greater reduction of blood flow with decreased CO2 than the control group. This altered vasoregulatory response was observed both ipsilateral and contralateral to the stroke. Lower CO2 VR on the stroke side was associated with PIHs, greater infarct volume, and worse outcomes. The cases with PIHs (n = 27) had lower CBF during all conditions bilaterally (p < 0.0001) compared to cases with infarct only.

CONCLUSIONS

Perfusion augmentation is inadequate in multiple vascular territories in patients with large artery ischemic infarcts, but vasoconstriction is preserved. Peri-infarct T2 hyperintensities are associated with lower blood flow. Strategies aimed to preserve vasoreactivity after an ischemic stroke should be tested for their effect on long-term outcomes.

Resting cerebral blood flow, attention, and aging

Aging is accompanied by a decline of fluid cognitive functions, e.g., a slowing of information processing, working memory, and division of attention. This is at least partly due to structural and functional changes in the aging brain. Although a decrement of resting cerebral blood flow (CBF) has been positively associated with cognitive functions in patients with brain diseases, studies with healthy participants have revealed inconsistent results. Therefore, we investigated the relation between resting cerebral blood flow and cognitive functions (tonic and phasic alertness, selective and divided attention) in two samples of healthy young and older participants. We found higher resting CBF and better cognitive performances in the young than in the older sample. In addition, resting CBF was inversely correlated with selective attention in the young and with tonic alertness in the elderly participants. This finding is discussed with regard to the neural efficiency hypothesis of human intelligence.

Cerebral Blood Flow Measurement in Children With Sickle Cell Disease Using Continuous Arterial Spin Labeling at 3.0-Tesla MRI

Background and Purpose— Cerebral infarction is an important complication of sickle cell disease (SCD) and occurs in one third of the patients with SCD. The risk of infarction is commonly attributed to the hyperemia that is associated with anemia and reduces the cerebral vascular reserve. We measured regional cerebral blood flow (rCBF) by continuous arterial spin labeling MRI, which is a noninvasive method that does not require ionizing radiation. The purpose of this study was to examine rCBF in children with SCD and compare it with rCBF in healthy children.

Methods— rCBF was measured at 3-T continuous arterial spin labeling MRI in 24 neurological normal patients with SCD and in 12 healthy children matched for ethnicity and age (mean age in both groups 13 years). rCBF was calculated for 6 vascular territories (left and right anterior, middle and posterior cerebral artery). Asymmetry in rCBF was evaluated by measuring differences in flow between left and right hemispheres. The definition of asymmetry (>11.7 mL/100 g/min) was based on a repeatability study performed in 6 healthy adults.

Results— The rCBF was of similar magnitude in patients with SCD and control subjects in the frontal, middle, and posterior territories. The majority of patients with SCD (58%) demonstrated a left–right asymmetry of rCBF in one or more vascular territories, whereas none of the control subjects did.

Conclusion— In contrast to previous studies, we found no difference in cerebral blood flow between patients and control subjects. We did observe an asymmetry in rCBF in the majority of patients with SCD that was not present in healthy control subjects.

Continuous flow-driven inversion for arterial spin labeling using pulsed radio frequency and gradient fields

Continuous labeling by flow-driven adiabatic inversion is advantageous for arterial spin labeling (ASL) perfusion studies, but details of the implementation, including inefficiency, magnetization transfer, and limited support for continuous-mode operation on clinical scanners, have restricted the benefits of this approach. Here a new approach to continuous labeling that employs rapidly repeated gradient and radio frequency (RF) pulses to achieve continuous labeling with high efficiency is characterized. The theoretical underpinnings, numerical simulations, and in vivo implementation of this pulsed continuous ASL (PCASL) method are described. In vivo PCASL labeling efficiency of 96% relative to continuous labeling with comparable labeling parameters far exceeded the 33% duty cycle of the PCASL RF pulses. Imaging at 3T with body coil transmission was readily achieved. This technique should help to realize the benefits of continuous labeling in clinical imagers.

Current trends and challenges in MRI acquisitions to investigate brain function

Functional magnetic resonance imaging (fMRI) studies using the blood oxygenation level dependent (BOLD) response have become a widely used tool for noninvasive assessment of functional organization of the brain. Yet the technique is still fairly new, with many significant challenges remaining. Capitalizing on additional contrast mechanisms available with MRI, several other functional imaging techniques have been developed that potentially provide improved quantification or specificity of neuronal function. This article reviews the challenges and the current state of the art in MRI-based methods of imaging cognitive function.

Protective effect of post-ischaemic viral delivery of heat shock proteins in vivo

Heat shock proteins (HSPs) function as molecular chaperones involved in protein folding, transport and degradation and, in addition, they can promote cell survival both in vitro and in vivo after a range of stresses. Although some in vivo studies have suggested that HSP27 and HSP70 can be neuroprotective, current evidence is limited, particularly when HSPs have been delivered after an insult. The effect of overexpressing HSPs after transient occlusion of the middle cerebral artery in rats was investigated by delivering an attenuated herpes simplex viral vector (HSV-1) engineered to express HSP27 or HSP70 30 mins after tissue reperfusion. Magnetic resonance imaging scans were used to determine lesion size and cerebral blood flow at six different time points up to 1 month after stroke. Animals underwent two sensorimotor tests at the same time points to assess the relationship between lesion size and function. Results indicate that post-ischaemic viral delivery of HSP27, but not of HSP70, caused a statistically significant reduction in lesion size and induced a significant behavioural improvement compared with controls. This is the first evidence of effective post-ischaemic gene therapy with a viral vector expressing HSP27 in an experimental model of stroke.

Arterial spin-labeled perfusion combined with segmentation techniques to evaluate cerebral blood flow in white and gray matter of children with sickle cell anemia

BACKGROUND

Changes in cerebral perfusion are an important feature of the pathophysiology of sickle cell anemia (SCA); cerebrovascular ischemia occurs frequently and leads to neurocognitive deficits, silent infarcts, and overt stroke. Non-invasive MRI methods to measure cerebral blood flow (CBF) by arterial spin labeling (ASL) afford new opportunities to characterize disease- and therapy-induced changes in cerebral hemodynamics in patients with SCA. Recent studies have documented elevated gray matter (GM) CBF in untreated children with SCA, but no measurements of white matter (WM) CBF have been reported.

PROCEDURES

Pulsed ASL with automated brain image segmentation-classification techniques were used to determine the CBF in GM, WM, and abnormal white matter (ABWM) of 21 children with SCA, 18 of whom were receiving hydroxyurea therapy.

RESULTS

GM and WM CBF were highly associated (R(2) = 0.76, P < 0.0001) and the GM to WM CBF ratio was 1.6 (95% confidence interval: 1.43-1.83). Global GM CBF in our treated cohort was 87 +/- 24 mL/min/100 g, a value lower than previously reported in untreated patients with SCA. CBF was elevated in normal appearing WM (43 +/- 14 mL/min/100 g) but decreased in ABWM (6 +/- 12 mL/min/100 g), compared to published normal pediatric controls. Hemispheric asymmetry in CBF was noted in most patients.

CONCLUSIONS

These perfusion measurements suggest that hydroxyurea may normalize GM CBF in children with SCA, but altered perfusion in WM may persist. This novel combined approach for CBF quantification will facilitate prospective studies of cerebral vasculopathy in SCA, particularly regarding the effects of treatments such as hydroxyurea.