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

Low prefrontal perfusion linked to depression symptoms in methadone-maintained opiate-dependent patients

BACKGROUND

Clinically depressed patients without substance use disorders, compared to controls, exhibit significantly lower resting regional cerebral blood flow (rCBF) in the prefrontal cortex (PFC). In this study, we examined the link between resting rCBF in the PFC and current depressive symptoms in methadone-maintained opiate-dependent (MM) patients with or without major depression.

METHODS

Arterial spin labeled perfusion fMRI at 3 Tesla was used to measure resting rCBF in 21 MM patients. Perfusion data were analyzed using SPM2. The relationship between Beck Depression Inventory (BDI) score and resting rCBF was examined in a single regression analysis.

RESULTS

The BDI scores ranged between 0 and 18 (m=7.0, S.D.=4.8), and 30% of the sample had mild to moderate depression symptoms according to BDI scores. A negative correlation was observed between BDI scores and relative rCBF in the bilateral ventrolateral prefrontal cortex, and middle frontal gyri.

CONCLUSIONS

The inverse relationship between prefrontal paralimbic rCBF and depression scores suggests a link between reduced fronto-limbic activity and depressive symptoms in MM patients. A significant subgroup of opiate-dependent patients has clinical or sub-clinical depression that is often undetected; our data identify brain substrates of depression symptoms that may also be a potential marker of relapse in this population. Treatment strategies targeting these brain regions may improve outcomes in depressed substance abusers.

Layer-specific anatomical, physiological and functional MRI of the retina

Most retinal imaging has been performed using optical techniques. This paper reviews alternative retinal imaging methods based on MRI performed with spatial resolution sufficient to resolve multiple well-defined retinal layers. The development of these MRI technologies to study retinal anatomy, physiology (blood flow, blood volume, and oxygenation) and function, and their applications to the study of normal retinas, retinal degeneration and diabetic retinopathy in animal models are discussed. Although the spatiotemporal resolution of MRI is poorer than that of optical imaging techniques, it is unhampered by media opacity and can thus image all retinal and pararetinal structures, and has the potential to provide multiple unique clinically relevant data in a single setting and could thus complement existing retinal imaging techniques. In turn, the highly structured retina with well-defined layers is an excellent model for advancing emerging high-resolution anatomical, physiological and functional MRI technologies.

Hippocampal hyperperfusion in Alzheimer's disease

Many of the regions with the earliest atrophy in Alzheimer's Disease (AD) do not show prominent deficits on functional imaging studies of flow or metabolism. This paradox may provide unique insights into the pathophysiology of AD. We sought to examine the relationship between function and atrophy in AD using MRI blood flow and anatomic imaging. 22 subjects diagnosed with AD, mean Mini Mental State Exam (MMSE) score 22.2, and 16 healthy elderly controls were imaged with a volumetric arterial spin labeling blood flow MRI technique and an anatomical imaging method using the identical spatial resolution, image orientation, and spatial encoding strategy. Cerebral blood flow (CBF) and gray matter (GM) maps derived from the imaging were transformed to a standard anatomical space. GM and CBF maps were tested for significant differences between groups. Additionally, images were tested for regions with significant mismatch of the CBF and GM differences between groups. CBF was significantly lower in the bilateral precuneus, parietal association cortex and the left inferior temporal lobe but was non-significantly increased in the hippocampus and other medial temporal structures. After correction for GM loss, CBF was significantly elevated in the hippocampus and other medial temporal structures. The hippocampus and other regions affected early in AD are characterized by elevated atrophy-corrected perfusion per cm(3) of tissue. This suggests compensatory or pathological elevation of neural activity, inflammation, or elevated production of vasodilators.

Transit delay and flow quantification in muscle with continuous arterial spin labeling perfusion-MRI

PURPOSE

To test the hypothesis that flow measurements using continuous arterial spin labeling (CASL) magnetic resonance imaging (MRI) in muscle depend upon transit delay, and examine the repeatability of CASL measurements.

MATERIALS AND METHODS

A total of 23 healthy subjects underwent CASL imaging of the calf, foot, and forearm with varying postlabeling delay (PLD = 1000, 1500, and 1900 msec). Experiments were conducted on a 3.0T system. An orthopedic tourniquet system was employed to create a five-minute period of ischemia followed by a transient hyperemic flow. Imaging commenced two minutes prior to cuff inflation and ended three minutes after cuff release.

RESULTS

CASL was found able to well resolve the time course of the hyperemic flow pattern with an effective TR of 16 seconds, although we were unable to establish that a plateau had been reached in the flow measurements even at a PLD as long as 1900 msec. Peak hyperemic flow measurements compared favorably with those obtained with contrast-enhanced (CE) MRI following a similar period of ischemia. Region-of-interest (ROI)-based repeated measurements varied by approximately 20% over a period of one hour.

CONCLUSION

In all anatomic regions studied, flow measurements were found to increase with PLD, suggesting the prolonged transit delay in muscle.

Latent state-trait structure of cerebral blood flow in a resting state

Cerebral blood flow (CBF) is an important parameter for the study of brain function. The present paper examines to what extent CBF in a resting state reflects a stable latent trait and to what extent it reflects phasic situational effects. In 38 healthy subjects resting CBF was measured with continuous arterial spin labeling. Data analyses were performed within the methodological framework of latent state-trait theory, which allows the decomposition of the measured variables into temporally stable differences, occasion-specific fluctuations, and measurement errors. For most of the regions of interest, about 70% of the observed variance in resting CBF was determined by individual differences on a latent physiological trait whereas about 20% of the variance was due to occasion-specific influences. Thus, an aggregation across measurement occasions is not necessary in order to yield a stable physiological trait.

Intravascular effect in velocity-selective arterial spin labeling

Arterial spin labeling (ASL) has been developed into a useful tool for measuring local tissue perfusion with magnetic resonance imaging (MRI). Velocity-selective ASL (VS-ASL) tags spins on a basis of flow velocity, instead of the spatial distribution that is commonly used by conventional ASL techniques. Using a 90 degrees-180 degrees-90 degrees radiofrequency pulse train in combination with flow-sensitive gradients, VS-ASL can potentially generate tags that are very close to the imaging plane and whereby avoid the main error source of conventional ASL techniques coming from T1 relaxation during inflow time (TI). In practice, however, TI of VS-ASL should still be chosen with caution with respect to intravascular signal and cutoff velocity (Vc). With higher Vc, the maximum of VS-ASL signal was observed at shorter TI, which is due to intravacular signal in large vessels. For perfusion measurement in human brain, small Vc (=<2cm/s) is recommended. In contrast to the previous studies that suggested CSF suppression for Vc < 5cm/s, this study used lower amplitude and longer separation of diffusion gradients in the VS pulse train, which reduced b-value and eliminated the necessity of CSF suppression for Vc down to 2 cm/s.

Arterial spin labeling of cerebral perfusion territories using a separate labeling coil

PURPOSE

To obtain cerebral perfusion territories of the left, the right, and the posterior circulation in humans with high signal-to-noise ratio (SNR) and robust delineation.

MATERIALS AND METHODS

Continuous arterial spin labeling (CASL) was implemented using a dedicated radio frequency (RF) coil, positioned over the neck, to label the major cerebral feeding arteries in humans. Selective labeling was achieved by flow-driven adiabatic fast passage and by tilting the longitudinal labeling gradient about the Y-axis by theta = +/- 60 degrees .

RESULTS

Mean cerebral blood flow (CBF) values in gray matter (GM) and white matter (WM) were 74 +/- 13 mL . 100 g(-1) . minute(-1) and 14 +/- 13 mL . 100 g(-1) . minute(-1), respectively (N = 14). There were no signal differences between left and right hemispheres when theta = 0 degrees (P > 0.19), indicating efficient labeling of both hemispheres. When theta = +60 degrees , the signal in GM on the left hemisphere, 0.07 +/- 0.06%, was 92% lower than on the right hemisphere, 0.85 +/- 0.30% (P < 1 x 10(-9)), while for theta = -60 degrees , the signal in the right hemisphere, 0.16 +/- 0.13%, was 82% lower than on the contralateral side, 0.89 +/- 0.22% (P < 1 x 10(-10)). Similar attenuations were obtained in WM.

CONCLUSION

Clear delineation of the left and right cerebral perfusion territories was obtained, allowing discrimination of the anterior and posterior circulation in each hemisphere.

Arterial spin-labeling MR imaging measurements of timing parameters in patients with a carotid artery occlusion

BACKGROUND AND PURPOSE

Arterial spin-labeling (ASL) with image acquisition at multiple delay times can be exploited in perfusion MR imaging to visualize and quantify the temporal dynamics of arterial blood inflow. In this study, we investigated the consequences of an internal carotid artery (ICA) occlusion and collateral blood flow on regional timing parameters.

MATERIALS AND METHODS

Seventeen functionally independent patients with a symptomatic ICA occlusion (15 men, 2 women; mean age, 57 years) and 29 sex- and age-matched control subjects were investigated. ASL at multiple delay times was used to quantify regional cerebral blood flow (CBF) and the transit and trailing edge times (arterial timing parameters) reflecting, respectively, the beginning and end of the labeled bolus. Intra-arterial digital subtraction angiography and MR angiography were used to grade collaterals.

RESULTS

In the hemisphere ipsilateral to the ICA occlusion, the CBF was lower in the anterior frontal (31 +/- 4 versus 47 +/- 3 mL/min/100 g, P < .01), posterior frontal (39 +/- 4 versus 55 +/- 2 mL/min/100 g, P < .01), and frontal parietal region (49 +/- 3 versus 61 +/- 3 mL/min/100 g, P = .04) than that in control subjects. The trailing edge of the frontal-parietal region was longer in the hemisphere ipsilateral to the ICA occlusion compared with that in control subjects (2225 +/- 167 versus 1593 +/- 35 ms, P < .01). In patients with leptomeningeal collateral flow, the trailing edge was longer in the anterior frontal region (2436 +/- 275 versus 1648 +/- 201 ms, P = .03) and shorter in the occipital region (1815 +/- 128 versus 2388 +/- 203 ms, P = .04), compared with patients without leptomeningeal collaterals.

CONCLUSION

Regional assessment of timing parameters with ASL may provide valuable information on the cerebral hemodynamic status. In patients with leptomeningeal collaterals, the most impaired territory was found in the frontal lobe.

Pittfalls of MRI measurement of white matter perfusion based on arterial spin labeling

Although arterial spin labeling (ASL) MRI has been successfully applied to measure gray matter (GM) perfusion in vivo, accurate detection of white matter (WM) perfusion has proven difficult. Reported literature values are not consistent with each other or with perfusion measured with other modalities. In this work, the cause of these inconsistencies is investigated. The results suggest that WM perfusion values are substantially affected by the limited image resolution and by signal losses caused by the long transit times in WM, which significantly affect the label. From gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) bolus-tracking experiments (N=6), it is estimated that the transit time can be several seconds long in deep WM. Furthermore, simulations show that even at a spatial resolution of 7 microl voxel size, contamination by the GM signals can exceed 40% of the actual WM signal. From 10-min long flow-sensitive alternating inversion recovery ASL (FAIR-ASL) measurements at 3T in normal subjects (N=7), using highly sensitive detectors, it is shown that single-voxel (7 mul) deep WM perfusion values have an signal-to-noise ratio (SNR) less than 1. The poor sensitivity and heterogeneous transit time limit the applicability of ASL for measurement of perfusion in WM.

Optimal design of pulsed arterial spin labeling MRI experiments

Quantitative measurement of cerebral blood flow (CBF) using arterial spin labeling (ASL) MRI requires the acquisition of multiple inversion times (TIs) and the application of an appropriate kinetic model. The choice of these sampling times will have an impact on the precision of the estimated parameters. Here, optimal sampling schedule (OSS) design techniques, based on the Fisher Information approach, are applied in order to derive an optimal sampling scheme for pulsed arterial spin labeling (PASL) experiments. Such an approach should improve the precision of parameter estimation from experimental data, and provide a formal framework for optimally selecting a limited number of samples. In this study, we aimed to optimize the estimation precision of CBF and bolus arrival time from the PASL data. The performance of OSS was compared to a more standard evenly distributed sampling schedule (EDS) using both simulated and measured experimental data sets. It was found that OSS was able to significantly improve the precision of parameter estimation in PASL studies that sought to estimate either both CBF and bolus arrival time, or CBF alone.

MR measurement of blood flow in the parotid gland without contrast medium: a functional study before and after gustatory stimulation

PURPOSE

To investigate the feasibility of blood flow imaging in the parotid gland using the arterial spin labeling (ASL) technique for assessment of functional changes in the parotid gland after gustatory stimulation.

MATERIALS AND METHODS

Anatomical and ASL imaging of the parotid gland was performed in eight healthy volunteers before and after gustatory stimulation over a period of 17 min. All measurements were carried out in a 1.5 T whole-body MR unit. ASL and data recording were performed with an adapted FAIR TrueFISP (flow-sensitive alternating inversion-recovery true fast imaging with steady precession) technique. Maps of estimated tissue blood inflow in both parotid glands were derived using a simplified model and the extended Bloch equations.

RESULTS

Delineation of the parotid glands was possible on FAIR TrueFISP images in all cases. In the 160 s period immediately after stimulation, a significant (P < 0.01) mean increase of 62% in the estimated parotid blood flow was observed. Estimated baseline blood flow before gustatory stimulation ranged from 226 to 500 mL/min/100 g (mean +/- SD 335 +/- 86). These rates increased in the 160 s immediately after stimulation to 404-772 mL/min/100 g (mean 542 +/- 108). In all volunteers, blood flow returned to near baseline values within the observation period. No statistically significant difference between the right and left parotid was observed in baseline and peak blood flow.

CONCLUSION

ASL FAIR TrueFISP is feasible for functional characterization of the parotid glands. Assessment of changes in blood flow in the parotid gland could serve as a diagnostic tool in patients suffering from xerostomia.

Noninvasive imaging of quantitative cerebral blood flow changes during 100% oxygen inhalation using arterial spin-labeling MR imaging

BACKGROUND AND PURPOSE

Tracer studies have demonstrated that 100% oxygen inhalation causes a small cerebral blood flow (CBF) decrease. This study was performed to determine whether arterial spin-labeling (ASL), a noninvasive MR imaging technique, could image these changes with clinically reasonable imaging durations.

MATERIALS AND METHODS

Continuous ASL imaging was performed in 7 healthy subjects before, during, and after 100% oxygen inhalation. ASL difference signal intensity (DeltaM, control - label), CBF, and CBF percentage change were measured. A test-retest paradigm was used to calculate the variability of the initial and final room air CBF measurements.

RESULTS

During oxygen inhalation, DeltaM decreased significantly in all regions (eg, global DeltaM decreased by 23 +/- 11%, P < .01, all values mean +/- SD). Accounting for the reduced T1 of hyperoxygenated blood, we found a smaller CBF decrease, which did not reach significance in any of the regions. Global CBF dropped from 50 +/- 10 mL per 100 g/minute to 47 +/- 10 mL per 100 g/minute following 100% oxygen inhalation, a decrease of 5 +/- 14% (P > .17). The root-mean-square variability of the initial and final room air CBF measurements was 7-8 mL per 100 g/minute.

CONCLUSIONS

The DeltaM signal intensity decreased significantly with oxygen inhalation; however, after accounting for changes in blood T1 with oxygen, CBF decreases were small. Such measurements support the use of hyperoxia as an MR imaging contrast agent and may be helpful to interpret hyperoxia-based stroke trials.

Hippocampal hyperperfusion in Alzheimer's disease

Many of the regions with the earliest atrophy in Alzheimer's Disease (AD) do not show prominent deficits on functional imaging studies of flow or metabolism. This paradox may provide unique insights into the pathophysiology of AD. We sought to examine the relationship between function and atrophy in AD using MRI blood flow and anatomic imaging. 22 subjects diagnosed with AD, mean Mini Mental State Exam (MMSE) score 22.2, and 16 healthy elderly controls were imaged with a volumetric arterial spin labeling blood flow MRI technique and an anatomical imaging method using the identical spatial resolution, image orientation, and spatial encoding strategy. Cerebral blood flow (CBF) and gray matter (GM) maps derived from the imaging were transformed to a standard anatomical space. GM and CBF maps were tested for significant differences between groups. Additionally, images were tested for regions with significant mismatch of the CBF and GM differences between groups. CBF was significantly lower in the bilateral precuneus, parietal association cortex and the left inferior temporal lobe but was non-significantly increased in the hippocampus and other medial temporal structures. After correction for GM loss, CBF was significantly elevated in the hippocampus and other medial temporal structures. The hippocampus and other regions affected early in AD are characterized by elevated atrophy-corrected perfusion per cm(3) of tissue. This suggests compensatory or pathological elevation of neural activity, inflammation, or elevated production of vasodilators.

Mapping resting-state functional connectivity using perfusion MRI

Resting-state, low-frequency (<0.08 Hz) fluctuations of blood oxygenation level-dependent (BOLD) magnetic resonance signal have been shown to exhibit high correlation among functionally connected regions. However, correlations of cerebral blood flow (CBF) fluctuations during the resting state have not been extensively studied. The main challenges of using arterial spin labeling perfusion magnetic resonance imaging to detect CBF fluctuations are low sensitivity, low temporal resolution, and contamination from BOLD. This work demonstrates CBF-based quantitative functional connectivity mapping by combining continuous arterial spin labeling (CASL) with a neck labeling coil and a multi-channel receiver coil to achieve high perfusion sensitivity. In order to reduce BOLD contamination, the CBF signal was extracted from the CASL signal time course by high frequency filtering. This processing strategy is compatible with sinc interpolation for reducing the timing mismatch between control and label images and has the flexibility of choosing an optimal filter cutoff frequency to minimize BOLD fluctuations. Most subjects studied showed high CBF correlation in bilateral sensorimotor areas with good suppression of BOLD contamination. Root-mean-square CBF fluctuation contributing to bilateral correlation was estimated to be 29+/-19% (N=13) of the baseline perfusion, while BOLD fluctuation was 0.26+/-0.14% of the mean intensity (at 3 T and 12.5 ms echo time).

Quantification of pain-induced changes in cerebral blood flow by perfusion MRI

The purpose of this study was to assess if the functional activation caused by painful stimuli could be detected with arterial spin labeling (ASL), which is a non-invasive magnetic resonance imaging (MRI) technique for measuring cerebral blood flow (CBF). Because ASL directly measures blood flow, it is well suited to pain conditions that are difficult to assess with current functional MRI, such as chronic pain. However, the use of ASL in neuroimaging has been hampered by its low sensitivity. Recent improvements in MRI technology, namely increased magnetic field strengths and phased array receiver coils, should enable ASL to measure the small changes in CBF associated with pain. In this study, healthy volunteers underwent two ASL imaging sessions, during which a painful thermal stimulus was applied to the left hand. The results demonstrated that the ASL technique measured changes in regional CBF in brain regions that have been previously identified with pain perception. These included bilateral CBF changes in the insula, secondary somatosensory, and cingulate cortices, as well as the supplementary motor area (SMA). Also observed were contralateral primary somatosensory and ipsilateral thalamic CBF changes. The average change in CBF for all regions of interest was 3.68ml/100g/min, ranging from 2.97ml/100g/min in ipsilateral thalamus to 4.91ml/100g/min in contralateral insula. The average resting global CBF was 54+/-9.7ml/100g/min, and there was no change in global CBF due to the noxious thermal stimulus.

Modeling and optimization of Look-Locker spin labeling for measuring perfusion and transit time changes in activation studies taking into account arterial blood volume

This work describes a new compartmental model with step-wise temporal analysis for a Look-Locker (LL)-flow-sensitive alternating inversion-recovery (FAIR) sequence, which combines the FAIR arterial spin labeling (ASL) scheme with a LL echo planar imaging (EPI) measurement, using a multireadout EPI sequence for simultaneous perfusion and T*(2) measurements. The new model highlights the importance of accounting for the transit time of blood through the arteriolar compartment, delta, in the quantification of perfusion. The signal expected is calculated in a step-wise manner to avoid discontinuities between different compartments. The optimal LL-FAIR pulse sequence timings for the measurement of perfusion with high signal-to-noise ratio (SNR), and high temporal resolution at 1.5, 3, and 7T are presented. LL-FAIR is shown to provide better SNR per unit time compared to standard FAIR. The sequence has been used experimentally for simultaneous monitoring of perfusion, transit time, and T*(2) changes in response to a visual stimulus in four subjects. It was found that perfusion increased by 83 +/- 4% on brain activation from a resting state value of 94 +/- 13 ml/100 g/min, while T*(2) increased by 3.5 +/- 0.5%.

Arterial Spin Labeling: a one-stop-shop for measurement of brain perfusion in the clinical settings

Arterial Spin Labeling (ASL) has opened a unique window into the human brain function and perfusion physiology. Altogether fast and of intrinsic high spatial resolution, ASL is a technique very appealing not only for the diagnosis of vascular diseases, but also in basic neuroscience for the follow-up of small perfusion changes occurring during brain activation. However, due to limited signal-to-noise ratio and complex flow kinetics, ASL is one of the more challenging disciplines within magnetic resonance imaging. In this paper, the theoretical background and main implementations of ASL are revisited. In particular, the different uses of ASL, the pitfalls and possibilities are described and illustrated using clinical cases.

Multivariate and univariate analysis of continuous arterial spin labeling perfusion MRI in Alzheimer's disease

Continuous arterial spin labeling (CASL) magnetic resonance imaging (MRI) was combined with multivariate analysis for detection of an Alzheimer's disease (AD)-related cerebral blood flow (CBF) covariance pattern. Whole-brain resting CBF maps were obtained using spin echo, echo planar imaging (SE-EPI) CASL in patients with mild AD (n=12, age=70.7+/-8.7 years, 7 males, modified Mini-Mental State Examination (mMMS)=38.7/57+/-11.1) and age-matched healthy controls (HC) (n=20; age=72.1+/-6.5 years, 8 males). A covariance pattern for which the mean expression was significantly higher (P<0.0005) in AD than in HC was identified containing posterior cingulate, superior temporal, parahippocampal, and fusiform gyri, as well as thalamus, insula, and hippocampus. The results from this analysis were supplemented with those from the more standard, region of interest (ROI) and voxelwise, univariate techniques. All ROIs (17/hemisphere) showed significant decrease in CBF in AD (P<0.001 for all ROIs, alphacorrected=0.05). The area under the ROC curve for discriminating AD versus HC was 0.97 and 0.94 for covariance pattern and gray matter ROI, respectively. Fewer areas of depressed CBF in AD were detected using voxelwise analysis (corrected, P<0.05). These areas were superior temporal, cingulate, middle temporal, fusiform gyri, as well as inferior parietal lobule and precuneus. When tested on extensive split-half analysis to map out the replicability of both multivariate and univariate approaches, the expression of the pattern from multivariate analysis was superior to that of the univariate.

The effect of labeling parameters on perfusion-based fMRI in nonhuman primates

The blood oxygenation level-dependent (BOLD) signal is the most commonly used modality of functional magnetic resonance imaging (fMRI) today. Although easy to implement, it is an ambiguous signal since it results from a combination of several hemodynamic factors. Functional cerebral blood flow changes, as measured by using arterial spin labeling (ASL), typically occur in the parenchyma and have been demonstrated to be more closely coupled to neural activation compared with BOLD. However, the intrinsically low signals from ASL techniques have hindered its widespread application to fMRI for basic research and even more so for clinical applications. Here, we report the first implementation of continuous ASL in the anaesthetized macaque at high magnetic field of 7 T. The technique was optimized to permit maximum signal-to-noise ratio of functional perfusion-based images at high spatial resolution. The effect of labeling parameters, such as label time and post-label delay (PLD), on functional cerebral blood flow (fCBF) in the visual cortex was evaluated. Functional cerebral blood flow maps did not change with increasing label time after 2,000 ms, indicating that a label time of 2,000 ms is sufficient for reliable mapping of fCBF. The percent changes obtained using fCBF were better localized to gray matter, than those obtained with BOLD. A short PLD of 200 ms revealed significantly higher fCBF changes at the cortical surface, indicating large-vessel contamination, than a long PLD of 800 ms. However, the effect of the PLD on fCBF was smaller than on baseline CBF. These results are of importance for high-resolution applications, and when accurate quantification is required for studies in monkeys as well as in humans.

Hyperemic flow heterogeneity within the calf, foot, and forearm measured with continuous arterial spin labeling MRI

Arterial spin labeling (ASL) is a noninvasive magnetic resonance imaging (MRI) technique for microvascular blood flow measurement. We used a continuous ASL scheme (CASL) to investigate the hyperemic flow difference between major muscle groups in human extremities. Twenty-four healthy subjects with no evidence of vascular disease were recruited. MRI was conducted on a 3.0 Tesla Siemens Trio whole body system with a transmit/receive knee coil. A nonmagnetic orthopedic tourniquet system was used to create a 5-min period of ischemia followed by a period of hyperemic flow (occlusion pressure = 250 mmHg). CASL imaging, lasting from 2 min before cuff inflation to 3 min after cuff deflation, was performed on the midcalf, midfoot, and midforearm in separate sessions from which blood flow was quantified with an effective temporal resolution of 16 s. When muscles in the same anatomic location were compared, hyperemic flow was found to be significantly higher in the compartments containing muscles known to have relatively higher slow-twitch type I fiber compositions, such as the soleus muscle in the calf and the extensors in the forearm. In the foot, the plantar flexors exhibited a slightly delayed hyperemic response relative to that of the dorsal compartment, but no between-group flow difference was observed. These results demonstrate that CASL is sensitive to flow heterogeneity between diverse muscle groups and that nonuniform hyperemic flow patterns following an ischemic paradigm correlate with relative fiber-type predominance.

Vessel-encoded arterial spin-labeling using pseudocontinuous tagging

A new signal-to-noise ratio (SNR) efficient method is introduced for the mapping of vascular territories based on pseudocontinuous arterial spin labeling (ASL). A pseudocontinuous tagging pulse train is modified using additional transverse gradient pulses and phase cycling to place some arteries in a tag condition, while others passing through the same tagging plane are in a control condition. This is combined with a Hadamard or similar encoding scheme such that all vessels of interest are fully inverted or relaxed for nearly all of the encoding cycles, providing optimal SNR. The relative tagging efficiency for each vessel is measured directly from the ASL data and is used in the decoding process to improve the separation of vascular territories. High SNR maps of left carotid, right carotid, and basilar territories are generated in 6 min of scan time.

Theoretical basis of hemodynamic MR imaging techniques to measure cerebral blood volume, cerebral blood flow, and permeability

Cerebrovascular hemodynamic assessment adds new information to standard anatomic MR imaging and improves patient care. This article reviews the theoretic underpinnings of several potentially quantitative MR imaging-based methods that shed light on the hemodynamic status of the brain, including cerebral blood flow (CBF), cerebral blood volume (CBV), and contrast agent permeability. Techniques addressed include dynamic susceptibility contrast (which most simply and accurately estimates CBV), arterial spin labeling (a powerful method to measure CBF), and contrast-enhanced methods to derive permeability parameters such as the transport constant Ktrans.

Atorvastatin therapy is associated with greater and faster cerebral hemodynamic response

Hypercholesterolemia in midlife increases the risk of subsequent cognitive decline, neurovascular disease, and Alzheimer's disease (AD), and statin use is associated with reduced prevalence of these outcomes. While statins improve vasoreactivity in peripheral arteries and large cerebral arteries, little is known about the effects of statins on cerebral hemodynamic responses and cognition in healthy asymptomatic adults. At the final visit of a 4-month randomized, controlled, double-blind study comparing atorvastatin 40 mg daily to placebo, 16 asymptomatic middle-aged adults (15 had useable data) underwent blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI), arterial spin labeling (ASL) quantitative cerebral blood flow (qCBF), dynamic susceptibility contrast (DSC) and structural imagings of the brain. Using a memory recognition task requiring discrimination of previously viewed (PV) and novel (NV) human faces, fMRI was used to elicit activation from brain regions known to be vulnerable to changes associated with AD. The BOLD signal amplitude (PV > NV) and latency to each stimulus were tested on a voxel basis between the atorvastatin (n=8) and placebo (n=7) groups. Persons randomized to atorvastatin not only showed significantly greater BOLD amplitude in the right angular gyrus, left superior parietal lobule, right middle temporal and superior sulcus than the placebo group, but also decreased hemodynamic response latencies in the right middle frontal gyrus, left precentral gyrus, left cuneus and right posterior middle frontal gyrus. However, neither the resting cerebral blood flow (CBF) measured with ASL nor the mean transit time (MTT) of cerebral perfusion calculated from DSC showed differences in these regions in either group. The drug related BOLD differences during memory recognition suggest that atorvastatin may have improved cerebral small vessel vasoreactivity, possibly through an effect on endothelial function. Furthermore, these results suggest that the effect of atorvastatin on the task-induced BOLD signal may not be a simple consequence of baseline flow change.

Quantitative accuracy of delayed hyperperfusion in MRI of transient ischemia in rats

A strong hyperperfusion was reported in transient ischemic tissue between 48 and 72 hours after middle cerebral artery occlusion (MCAO). Cerebral blood flow (CBF) estimated by continuous arterial spin labeling (CASL) with short delay after tagging was sensitive to cerebral blood volume (CBV) change. The delayed hyperperfusion may indicate a CBV increase after MCAO. For confirmation of the delayed hyperperfusion, we investigated a transit-time dependency in CASL at two days after MCAO. We also acquired CBF using the dynamic susceptibility contrast (DSC) at the same day. We have confirmed the CBF in transient ischemic tissue is quite higher (179.1+/-21.6 ml/100g/min) than normal tissue (121.0+/-6.9 ml/100g/min) with CASL using tagging delay of 0.4 sec. CBF estimated by DSC also show delayed hyperperfusion in transient ischemic tissue. These results confirm existence of physiological delayed hyperperfusion in transient ischemic area.

Magnetization transfer effects on the efficiency of flow-driven adiabatic fast passage inversion of arterial blood

Continuous arterial spin labeling experiments typically use flow-driven adiabatic fast passage inversion of the arterial blood water protons. In this article, we measure the effect of magnetization transfer in blood and how it affects the inversion label. We use modified Bloch equations to model flow-driven adiabatic inversion in the presence of magnetization transfer in blood flowing at velocities from 1 to 30 cm/s in order to explain our findings. Magnetization transfer results in a reduction of the inversion efficiency at the inversion plane of up to 3.65% in the range of velocities examined, as well as faster relaxation of the arterial label in continuous labeling experiments. The two effects combined can result in inversion efficiency reduction of up to 8.91% in the simulated range of velocities. These effects are strongly dependent on the velocity of the flowing blood, with 10 cm/s yielding the largest loss in efficiency due to magnetization transfer effects. Flowing blood phantom experiments confirmed faster relaxation of the inversion label than that predicted by T(1) decay alone.

Limbic activation to cigarette smoking cues independent of nicotine withdrawal: a perfusion fMRI study

Exposure to cigarette smoking cues can trigger physiological arousal and desire to smoke. The brain substrates of smoking cue-induced craving (CIC) are beginning to be elucidated; however, it has been difficult to study this state independent of the potential contributions of pharmacological withdrawal from nicotine. Pharmacological withdrawal itself may have substantial effects on brain activation to cues, either by obscuring or enhancing it, and as CIC is not reduced by nicotine replacement strategies, its neuro-anatomical substrates may differ. Thus, characterizing CIC is critical for developing effective interventions. This study used arterial spin-labeled (ASL) perfusion fMRI, and newly developed and highly appetitive, explicit smoking stimuli, to examine neural activity to cigarette CIC in an original experimental design that strongly minimizes contributions from pharmacological withdrawal. Twenty-one smokers (12 females) completed smoking and nonsmoking cue fMRI sessions. Craving self-reports were collected before and after each session. SPM2 software was employed to analyze data. Blood flow (perfusion) in a priori-selected regions was greater during exposure to smoking stimuli compared to nonsmoking stimuli (p<0.01; corrected) in ventral striatum, amygdala, orbitofrontal cortex, hippocampus, medial thalamus, and left insula. Perfusion positively correlated with intensity of cigarette CIC in both the dorsolateral prefrontal cortex (r2=0.54) and posterior cingulate (r2=0.53). This pattern of activation that includes the ventral striatum, a critical reward substrate, and the interconnected amygdala, cingulate and OFC, is consistent with decades of animal research on the neural correlates of conditioned drug reward.

An investigation of statistical power for continuous arterial spin labeling imaging at 1.5 T

Variance estimates can be used in conjunction with scientifically meaningful effect sizes to design experiments with type II error control. Here we present estimates of intra- and inter-subject variances for region of interest (ROI) from resting cerebral blood flow (CBF) maps obtained using whole brain, spin echo echoplanar (SE-EPI) continuous arterial spin labeling (CASL) imaging on 52 elderly subjects (age=70.5+/-7.9 years, 29 males). There was substantial intrasubject systematic variability in CBF of gray matter ROIs corresponding to a range of standard deviations=[39-168] (ml/(100 g min)). This variability was mainly due to two factors: (1) an expected inverse relationship between ROI volume and intrasubject variance and (2) an increased effective post-labeling delay for more superior slices acquired later in the sequence. For example, intrasubject variance in Brodmann area 4 (BA 4) was approximately 8 times larger than in hippocampus, despite their similar gray matter volumes. Estimated ROI-wise power was computed for various numbers of acquired CBF images, numbers of subjects, and CBF effect sizes for two experimental designs: independent sample t-test and paired t-test. The theoretical effects of pulse sequence and field strength on general applicability of these results are discussed.

Vascular space occupancy weighted imaging with control of residual blood signal and higher contrast-to-noise ratio

It has been recently proposed that the local cerebral blood volume change during brain activation can be measured by a series of images whose contrast is dependent on vascular space occupancy (VASO). VASO takes advantage of the inversion recovery sequence to acquire images when the longitudinal magnetization (Mz) of blood is relaxing through zero. The degree of blood suppression, however, is not always well controlled as a consequence of spatial variations in inversion efficiency and blood T1. Furthermore, while blood is eliminated, the Mz of other tissues is also small, which makes the contrast-to-noise ratio inherently low in VASO. In this paper, diffusion gradients were applied to demonstrate residual intravascular signal in the original VASO. An alternative VASO-weighted imaging was then proposed using a longer inversion time at which the Mz difference between blood and gray matter was optimized. A global saturation immediately after image acquisition was employed to eliminate the Mz disparity between inflowing blood and the residual in-plane blood from previous acquisition. Feasibility was evaluated by numerical simulation and functional experiments. In human visual cortex, the fractional VASO signal and cerebral blood volume changes were found to be -0.6% and 44%, respectively (voxel size = 3.4 x 3.4 x 5.0 mm3). As compared to the original VASO, the presented method provided a largely comparable activation map and hemodynamic curve but was not confounded by the existence of blood. Results also demonstrated its advantages of 1.6-fold higher CNR and insensitivity to variant tissue/blood T1 as well as inversion efficiency.

Arterial spin-labeling and MR spectroscopy in the differentiation of gliomas

BACKGROUND AND PURPOSE

Noninvasive grading of gliomas remains a challenge despite its important role in the prognosis and management of patients with intracranial neoplasms. In this study, we evaluated the ability of cerebral blood flow (CBF)-guided voxel-by-voxel analysis of multivoxel proton MR spectroscopic imaging ((1)H-MRSI) to differentiate low-grade from high-grade gliomas.

MATERIALS AND METHODS

A total of 35 patients with primary gliomas (22 high grade and 13 low grade) underwent continuous arterial spin-labeling perfusion-weighted imaging (PWI) and (1)H-MRSI. Different regions of the gliomas were categorized as "hypoperfused," "isoperfused," and "hyperperfused" on the basis of the average CBF obtained from contralateral healthy white matter. (1)H-MRSI indices were computed from these regions and compared between low- and high-grade gliomas. Using a similar approach, we applied a subgroup analysis to differentiate low- from high-grade oligodendrogliomas because they show different physiologic and genetic characteristics.

RESULTS

Cho(glioma (G)/white matter (WM)), Glx(G/WM), and Lip+Lac(G)/Cr(WM) were significantly higher in the "hyperperfused" regions of high-grade gliomas compared with low-grade gliomas. Cho(G/WM) and Lip+Lac(G)/Cr(WM) were also significantly higher in the "hyperperfused" regions of high-grade oligodendrogliomas. However, metabolite ratios from the "hypoperfused" or "isoperfused" regions did not exhibit any significant differences between high-grade and low-grade gliomas.

CONCLUSION

The results suggest that (1)H-MRSI indices from the "hyperperfused" regions of gliomas, on the basis of PWI, may be helpful in distinguishing high-grade from low-grade gliomas including oligodendrogliomas.

Noninvasive measurement of arterial cerebral blood volume using Look-Locker EPI and arterial spin labeling

This paper describes a method of noninvasively measuring regional arterial cerebral blood volume fractions (CBV(a)) in vivo using the combination of Look-Locker echo-planar imaging (LL-EPI) with arterial spin labeling (ASL). Using this technique the arterial inflow curve is rapidly sampled and the regional CBV(a) is measured, while tissue perfusion signals are suppressed. Two methods of spin labeling (LL-EPI flow-sensitive alternating inversion recovery (LL-EPI-FAIR) and LL-EPI signal targeting using alternating radiofrequency (LL-EPI-STAR)) are assessed and their advantages discussed. The application of vascular crushing to LL-EPI-FAIR is described and used to validate the insensitivity of the sequence to the perfusion difference signal. LL-EPI-STAR is used to assess changes in CBV(a) in response to a finger-tapping task. LL-EPI-STAR signal difference curves are shown to have a shortened vascular transit delay and increased peak signal change on activation. A 33 +/- 14% increase in CBV(a) on activation is found. CBV(a) is measured with a 6-s temporal resolution and the temporal response is compared with the BOLD signal change. CBV(a) is shown to increase more rapidly and return to baseline significantly faster than the BOLD signal change, which supports the suggestion that a change in CBV(a) is an input to the BOLD response.

Remifentanil-induced cerebral blood flow effects in normal humans: dose and ApoE genotype

BACKGROUND

Opioids have been linked to limbic system activation and, in animals, to neurotoxicity. Limbic system nonpharmacologic activation patterns have been linked to the Apolipoprotein E (ApoE) allelic distribution. We tested the hypothesis that, in the absence of surgery, small doses of remifentanil produce limbic system activation in humans which varies with dose and ApoE genotype.

METHODS

Twenty-seven ASA I-II volunteers received a remifentanil (Ultiva) infusion at four sequentially increasing doses: 0, 0.05, 0.1, and 0.2 microg x kg(-1) x min(-1) while receiving 100% oxygen. Cerebral blood flow (CBF) was measured at each dose globally and in the amygdala, cingulate, hippocampus, insula, and thalamus regions by pulsed arterial spin labeling magnetic resonance imaging. ApoE single nucleotide polymorphisms were determined in each subject.

RESULTS

Significant dose-related CBF increases, without correction for Paco(2), were detected in all areas. After normalizing for global CBF to correct for Paco(2) effects, the remifentanil-mediated increased CBF in the cingulate persisted, with decreased flow occurring in the hippocampus and amygdala. All these Paco(2)-corrected effects were reversed in the presence of the ApoE4 polymorphism.

CONCLUSION

Remifentanil at sedative doses produces both activating and depressing effects in various limbic system structures. The cingulate cortex seems to have the most susceptibility to remifentanil activation, and ApoE4 seems to produce relative activation of the hippocampus and amygdala.

Clinical neuroimaging using arterial spin-labeled perfusion magnetic resonance imaging

The two most common methods for measuring perfusion with MRI are based on dynamic susceptibility contrast (DSC) and arterial spin labeling (ASL). Although clinical experience to date is much more extensive with DSC perfusion MRI, ASL methods offer several advantages. The primary advantages are that completely noninvasive absolute cerebral blood flow (CBF) measurements are possible with relative insensitivity to permeability, and that multiple repeated measurements can be obtained to evaluate one or more interventions or to perform perfusion-based functional MRI. ASL perfusion and perfusion-based functional MRI methods have been applied in many clinical settings, including acute and chronic cerebrovascular disease, CNS neoplasms, epilepsy, aging and development, neurodegenerative disorders, and neuropsychiatric diseases. Recent technical advances have improved the sensitivity of ASL perfusion MRI, and increasing use is expected in the coming years. The present review focuses on ASL perfusion MRI and applications in clinical neuroimaging.

Arterial spin labeling for quantitative functional MRI

BOLD effect imaging is very effective for detection and localization of brain activity, and is the dominant functional imaging technique in cognitive psychology. Despite its efficiency to detect and localize active site, the technique does not lend itself easily to quantifiable measurements. A growingly popular alternative is the use of arterial spin labeling (ASL) to obtain perfusion maps as the indicator of cerebral activation. In this paper, the principles and challenges of arterial spin labeling are discussed and the development of a new fast, two-coil pseudo-continuous labeling scheme is presented. The new scheme permits collection of a multi-slice subtraction pair in less than three seconds, depending on the subject's arterial transit times. The theoretical basis of the technique, as well as a model for quantification of perfusion from the ASL data, are presented. Experimental data from functional imaging experiments were collected to demonstrate the technique and its characteristics.

Quantification of cerebral blood flow in nonhuman primates using arterial spin labeling and a two-compartment model

Noninvasive absolute quantification of cerebral blood flow (CBF) with high spatial resolution is still a challenging task. Arterial spin labeling (ASL) is a promising magnetic resonance imaging (MRI) method for accurate perfusion quantification. However, modeling of ASL data is far from being standardized and has not been investigated in great detail. In this study, two-compartment modeling of monkey ASL data in three physiological conditions (baseline, sensory activated and globally elevated CBF) is reported. Absolute perfusion and arterial transit times were derived for gray matter (GM) and white matter (WM) separately. The uncertainties of the model's result were determined by Monte Carlo simulations. The fitted CBF values for GM were 133 ml/min/100 ml at baseline condition, 165 ml/min/100 ml during visual stimulation and 234 ml/min/100 ml for globally elevated CBF after intravenous injection of acetazolamide. The ratio of GM to WM CBF was 2.5 at baseline and was found to decrease to 1.6 after application of acetazolamide. The corresponding arterial transit times decreased from 742 to 607 ms in GM and from 985 to 875 ms in WM. Monte Carlo simulations showed that absolute CBF values can be determined with an error of 11-15%, while the arterial transit time values have a coefficient of variation of 25-31%. With an alternative acquisition scheme, the precision of the arterial transit times can be improved significantly. The CBF values in the occipital lobe of the monkey brain quantified with ASL are higher than previously reported in positron emission tomography studies.

Reproducibility of continuous arterial spin labeling perfusion MRI after 7 weeks

BACKGROUND

Continuous arterial spin labeling (CASL) is a non-invasive technique for the measurement of cerebral blood flow (CBF). The aim of the present study was to examine the reproducibility of CASL measurements and its suitability to consistently detect differences between groups, regions, and resting states.

MATERIALS AND METHODS

Thirty-eight healthy subjects (19 female) were examined at 1.5 T on two measurement occasions that were seven weeks apart. Resting CBF was measured with eyes open and eyes closed.

RESULTS

In different regions of interest (ROIs) the repeatability estimates varied between 9 and 19 ml/100 g/min. There were no significant mean differences between occasions in all ROIs (P > 0.05). Greater CBF in the eyes-open than in the eyes-closed state was consistently present in the primary and secondary visual areas. Furthermore, CBF was consistently greater in the right than in the left hemisphere (P < 0.05) and differed between lobes and between arterial territories (P < 0.001). Finally, we consistently observed greater CBF in women than in men (P < 0.001).

CONCLUSION

This study demonstrates the suitability of CASL to consistently detect differences between groups, regions, and resting states even after seven weeks. This emphasizes its usefulness for longitudinal designs.

When perfusion meets diffusion: in vivo measurement of water permeability in human brain

Quantification of water permeability can improve the accuracy of perfusion measurements obtained with arterial spin labeling (ASL) methods, and may provide clinically relevant information regarding the functional status of the microvasculature. The amount of labeled water in the vascular and tissue compartments in an ASL experiment can be estimated based on their distinct diffusion characteristics, and in turn, water permeability determined from the relative vascular and tissue contributions. In the present study, a hybrid magnetic resonance imaging technique was introduced by marrying a continuous ASL method with a twice-refocused spin-echo diffusion sequence. Series of diffusion-weighted ASL signals were acquired with systematically varied b values. The signals were modeled with fast and slow decaying components that were associated with the vascular and tissue compartments, respectively. The relative amount of labeled water in the tissue compartment increased from 61% to 74% and to 86% when the postlabeling delay time was increased from 0.8 to 1.2 and to 1.5 secs. With a b value of 50 secs/mm2, the capillary contribution (fast component) of the ASL signal could be effectively minimized. Using the single-pass approximation model, the water permeability of gray matter in the human brain was estimated based on the derived relative water fractions in the tissue and microvasculature. The potential for in vivo magnetic resonance mapping of water permeability was showed using two diffusion weighted ASL measurements with b=0 and 50 secs/mm2 in both healthy subjects and a case of brain tumor.

Integrity of the Cerebral Blood-Flow Response to Hyperoxia After Cardiopulmonary Bypass

OBJECTIVE

In this study, the hypothesis that cardiopulmonary bypass (CPB) alters the cerebral blood flow (CBF) vasoconstrictive response to hyperoxia was tested.

DESIGN

A prospective, observational study was conducted.

SETTING

The study was conducted at a single university hospital.

PARTICIPANTS

Subjects were patients who presented for cardiac surgery with CPB.

INTERVENTIONS

CBF was measured before and after CPB in 12 subjects while breathing 21% O(2) and 100% O(2). CBF was measured by using continuous arterial spin labeling (CASL) perfusion magnetic resonance imaging. Arterial pO(2) (mmHg), pCO(2) (mmHg), hemoglobin (Hgb), and oxygen content (CaO(2)) were also measured.

MEASUREMENTS AND MAIN RESULTS

Mean age of the 12 subjects was 63 +/- 16 years. Hgb decreased from 12.0 (+/-2.4) g/dL to 9.2 (+/-2.9) g/dL postoperatively (p = 0.008). CBF increased by 39%, from 37.2 (+/-10.8) mL/100 g/min to 49.2 (+/-14.3)mL/100 g/min postoperatively (p = 0.01). In response to the hyperoxic challenge CBF decreased by 8.0 (+/-7.1) mL/100 g/min (21%) preoperatively and by 9.4 (+/-6.4) mL/100 g/min (19%) postoperatively (p = 0.58). By using multiple regression, the contribution of CPB to the hyperoxic CBF response (DeltaCBF) was evaluated, while controlling for other potentially important covariates known to influence CBF, including age, baseline CBF on 21% O(2), and changes in arterial pO(2), pCO(2), and CaO(2). CPB state was not found to be a significant covariate in controlling the CBF response to hyperoxia.

CONCLUSIONS

CPB does not impair the CBF response to hyperoxia.

Feasibility of velocity selective arterial spin labeling in functional MRI

Arterial spin labeling (ASL) magnetically inverts or saturates the spins in arterial blood and uses them as endogenous tracers. Conventionally, the tagging band is upstream or nonselective to the target slices. In the brain, ASL-based functional magnetic resonance imaging (fMRI) has been shown to detect activation better localized in gray matter than blood oxygenation level dependent contrast. More recently, velocity selective-ASL (VS-ASL) was proposed to tag spins according to their flow velocity. One desirable characteristic of VS-ASL is its capability to generate tags sufficiently close to the target slices and thereby circumvent the complication of non-zero transit delay. In this study, we investigate the feasibility of VS-ASL in fMRI by comparing it with a conventional ASL method (PICORE). The results from the visual cortex of healthy volunteers show that VS-ASL and PICORE have comparable spatial specificity in detecting the flow change induced by neuronal activity. Velocity selective-arterial spin labeling can further distinguish the contribution from different flow directions but spurious elevation of fractional signal change may occur when the VS tagging is applied off the direction of blood supply. The flow reaches the vicinity of perfusion at a cutoff velocity (Vc) of 2 cm/sec whereas the activation exclusively detected by Vc=4 cm/sec implies the arteriolar response to the neuronal activity and a respondent vessel diameter up to 240 microm. Velocity selective imaging can remove intravascular signal from the vessels where the flow velocity is above Vc.

Functional uncoupling of hemodynamic from neuronal response by inhibition of neuronal nitric oxide synthase

The cerebrovascular coupling under neuronal nitric oxide synthase (nNOS) inhibition was investigated in alpha-chloralose anesthetized rats. Cerebral blood flow (CBF), cerebral blood volume (CBV), and blood oxygenation level dependent (BOLD) responses to electrical stimulation of the forepaw were measured before and after an intraperitoneal bolus of 7-nitroindazole (7-NI), an in vivo inhibitor of the neuronal isoform of nitric oxide synthase. Neuronal activity was measured by recording somatosensory-evoked potentials (SEPs) via intracranial electrodes. 7-Nitroindazole produced a significant attenuation of the activation-elicited CBF (P<10(-6)), CBV (P<10(-6)), and BOLD responses (P<10(-6)), without affecting the baseline perfusion level. The average DeltaCBF was nulled, while DeltaBOLD and DeltaCBV decreased to approximately 30% of their respective amplitudes before 7-NI administration. The average SEP amplitude decreased (P<10(-5)) to approximately 60% of its pretreatment value. These data describe a pharmacologically induced uncoupling between neuronal and hemodynamic responses to functional activation, and provide further support for the critical role of neuronally produced NO in the cerebrovascular coupling.

Magnetic resonance approaches to brain aging and Alzheimer disease-associated neuropathology

The noninvasive, nonradioactive, quantitative nature of magnetic resonance techniques has propelled them to the forefront of neuroscience and neuropsychiatric research. In particular, recent advances have confirmed their enormous potential in patients with Alzheimer disease (AD). Structural and functional magnetic resonance (MR) imaging have demonstrated significant correlation with clinical outcomes and underlying pathology and are used increasingly in the AD clinic. This review will highlight the role of high-resolution structural MR imaging and functional magnetic resonance imaging in the identification of atrophic and hemodynamic changes in AD and their potential as diagnostic biomarkers and surrogates of therapeutic response. Advanced MR techniques based on diffusion, perfusion, and neurochemical abnormalities in the aging brain will be presented briefly. These newer techniques continue to expand our understanding of neuropathology in the aging brain and are likely to play an important clinical role in the future.

Functional imaging with Turbo-CASL: Transit time and multislice imaging considerations

The optimal use of turbo continuous arterial spin labeling (Turbo-CASL) for functional imaging in the presence of activation-induced transit time (TT) changes was investigated. Functional imaging of a bilateral finger-tapping task showed improved sensitivity for Turbo-CASL as compared to traditional CASL techniques for four of six subjects when scanned at an appropriate repetition time (TR). Both experimental and simulation results suggest that for optimal functional sensitivity with Turbo-CASL, the pulse TR should be set to a value that is 100-200 ms less than the resting-state TT. Simulations were also run to demonstrate the differences in TT sensitivity of different slices within a multislice acquisition, and the signal loss that is expected as the number of slices is increased. Despite the lower baseline ASL signal provided by the Turbo-CASL acquisition, one can achieve equal or improved functional sensitivity due in part to the signal enhancement that accompanies the decrease in TT upon activation. Turbo-CASL is thus a promising technique for functional ASL at higher temporal resolution.

Age dependence of cerebral perfusion assessed by magnetic resonance continuous arterial spin labeling

PURPOSE

To study the normal dependence of cerebral perfusion changes on age, to measure values of perfusion early in life, and to create a reference dataset.

MATERIALS AND METHODS

Perfusion maps were collected from a total of 44 healthy subjects (from four to 78 years old) using the arterial spin labeling (ASL) technique. The population was retrospectively divided into three age groups: children, teenagers, and adults. For each group, mean values of cerebral blood flow (CBF) were calculated in gray matter (GM) and white matter (WM). Results were compared across the three different age groups.

RESULTS

CBF values decreased with age (97+/-5 mL/100 g/minute in GM and 26+/-1 mL/100 g/minute in WM for the children, GM 79+/-3 mL/100 g/minute and WM 22+/-1 mL/100 g/minute for the teenagers, and GM 58+/-4 mL/100 g/minute, WM 20+/-1 mL/100 g/minute for the adults). The quantitative results suggest a rapid drop, rather than a gradual decrease, in cerebral perfusion between children and adult subjects, especially in the GM. This step in CBF occurs during adolescence, at approximately the 16th year of age.

CONCLUSION

ASL is a practical and quantitative technique suitable for perfusion measurement in children as well as adults. Perfusion measurements with ASL appear sensitive to neurophysiological changes occurring during brain maturation.

The effects of flow dispersion and cardiac pulsation in arterial spin labeling

The blood in the carotid arteries exhibits time-varying flow velocity as a function of cardiac phases. Despite this flow velocity variation, most current methods set forth for the analysis of arterial spin labeling (ASL) data have assumed that the tagged blood is delivered from the tagging region to the imaging region via simple plug flow, i.e., a single transit delay (deltat). In this study, we used a pulse oximeter to synchronize image acquisition at systole and diastole separately. The deltat dispersion was modeled with a Gaussian distribution and the effect of cardiac pulsation upon the ASL signal was evaluated on five healthy volunteers. ASL signals were collected at a series of inflow times (TI) using PICORE QUIPSS II: TR/TE/TI1 = 2400/3.2/700 ms, TI = {300, 500, 700, 900, 1100, 1300, 1500} ms, matrix size = 64 x 64, repetition = 100. Transit delay was found significantly shorter in systolic tag than diastolic tag (paired student's t-test, p < 0.001; mean difference across subjects = 54 ms). When the tag was applied in late systole, the ASL signal arrived in the target brain slice earlier, and was higher by 16% with TI = 700 ms. Intervoxel dispersion (-350 ms) dominated over intravoxel dispersion (< 200 ms). The disparity of ASL signals found between systolic and diastolic tags indicated that ASL imaging was sensitive to cardiac pulsations. We conclude that both flow dispersion and fluctuations in the ASL signal due to cardiac pulsations are significant.