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

Quantification of rodent cerebral blood flow (CBF) in normal and high flow states using pulsed arterial spin labeling magnetic resonance imaging

PURPOSE

To implement a pulsed arterial spin labeling (ASL) technique in rats that accounts for cerebral blood flow (CBF) quantification errors due to arterial transit times (dt)-the time that tagged blood takes to reach the imaging slice-and outflow of the tag.

MATERIALS AND METHODS

Wistar rats were subjected to air or 5% CO(2), and flow-sensitive alternating inversion-recovery (FAIR) perfusion images were acquired. For CBF calculation, we applied the double-subtraction strategy (Buxton et al., Magn Reson Med 1998;40:383-396), in which data collected at two inversion times (TIs) are combined.

RESULTS

The ASL signal fell off more rapidly than expected from TI = one second onward, due to outflow effects. Inversion times for CBF calculation were therefore chosen to be larger than the longest transit times, but short enough to avoid systematic errors caused by outflow of tagged blood. Using our method, we observed a marked regional variability in CBF and dt, and a region dependent response to hypercapnia.

CONCLUSION

Even when flow is accelerated, CBF can be accurately determined using pulsed ASL, as long as dt and outflow of the tag are accounted for.

Simultaneous laser Doppler flowmetry and arterial spin labeling MRI for measurement of functional perfusion changes in the cortex

This study compares laser Doppler flowmetry (LDF) and arterial spin labeling (ASL) for the measurement of functional changes in cerebral blood flow (CBF). The two methods were applied concurrently in a paradigm of electrical whisker stimulation in the anaesthetised rat. Multi-channel LDF was used, with each channel corresponding to different fiber separation (and thus measurement depth). Continuous ASL was applied using separate imaging and labeling coils at 3 T. Careful experimental set up ensured that both techniques recorded from spatially concordant regions of the barrel cortex, where functional responses were maximal. Strong correlations were demonstrated between CBF changes measured by each LDF channel and ASL in terms of maximum response magnitude and response time-course within a 6-s-long temporal resolution imposed by ASL. Quantitatively, the measurements of the most superficial LDF channels agreed strongly with those of ASL, whereas the deeper LDF channels underestimated consistently the ASL measurement. It was thus confirmed that LDF quantifies CBF changes consistently at a superficial level, and for this case the two methods provided concordant measures of functional CBF changes, despite their essentially different physical principles and spatiotemporal characteristics.

Quantitative basal CBF and CBF fMRI of rhesus monkeys using three-coil continuous arterial spin labeling

A three-coil continuous arterial-spin-labeling technique with a separate neck labeling coil was implemented on a Siemens 3T Trio for quantitative cerebral blood flow (CBF) and CBF fMRI measurements in non-human primates (rhesus monkeys). The optimal labeling power was 2 W, labeling efficiency was 92+/-2%, and optimal post-labeling delay was 0.8 s. Gray matter (GM) and white matter (WM) were segmented based on T1 maps. Quantitative CBF were obtained in 3 min with 1.5-mm isotropic resolution. Whole-brain average DeltaS/S was 1.0-1.5%. GM CBF was 104+/-3 ml/100 g/min (n = 6, SD) and WM CBF was 45+/-6 ml/100 g/min in isoflurane-anesthetized rhesus monkeys, with the CBF GM/WM ratio of 2.3+/-0.2. Combined CBF and BOLD (blood-oxygenation-level-dependent) fMRI associated with hypercapnia and hyperoxia were made with 8-s temporal resolution. CBF fMRI responses to 5% CO2 were 59+/-10% (GM) and 37+/-4% (WM); BOLD fMRI responses were 2.0+/-0.4% (GM) and 1.2+/-0.4% (WM). CBF fMRI responses to 100% O2 were -9.4+/-2% (GM) and -3.9+/-2.6% (WM); BOLD responses were 2.4+/-0.7% (GM) and 0.8+/-0.2% (WM). The use of a separate neck coil for spin labeling significantly increased CBF signal-to-noise ratio and the use of small receive-only surface coil significantly increased signal-to-noise ratio and spatial resolution. This study sets the stage for quantitative perfusion imaging and CBF fMRI for neurological diseases in anesthetized and awake monkeys.

In vivo estimation of the flow-driven adiabatic inversion efficiency for continuous arterial spin labeling: a method using phase contrast magnetic resonance angiography

The accurate quantification of perfusion with arterial spin labeling (ASL) requires consideration of a number of factors, including the efficiency of the inversion and control pulses used for spin labeling. In this study the effects of spin velocity on continuous ASL efficiency when using the amplitude modulated control strategy were investigated using simulations of the Bloch equations. The inversion efficiency was determined in vivo by combining the simulations with phase-contrast velocity mapping data acquired at the level of the tagging plane. Using this novel method, an average inversion efficiency of 69% was calculated for a group of 28 subjects, in good agreement with experimental data reported previously. There was, however, a large range in inversion efficiency measured across the subject group (50-76%), indicating that the velocity dependence of the amplitude modulated control efficiency may introduce additional variability into the perfusion calculations if not properly taken into account.

Quantification of regional pulmonary blood flow using ASL-FAIRER

Pulsed arterial spin labeling (ASL) techniques have been theoretically and experimentally validated for cerebral blood flow (CBF) quantification. In this study ASL-FAIRER was used to measure regional pulmonary blood flow (rPBF) in seven healthy subjects. Two general ASL strategies were investigated: 1) a single-subtraction approach using one tag-control pair acquisition at an inversion time (TI) matched to the RR-interval, and 2) a multiple-subtraction approach using tag-control pairs acquired at various TIs. The mean rPBF averaged 1.70 +/- 0.38 ml/min/ml when measured with the multiple-subtraction approach, and was approximately 2% less when measured with the single-subtraction method (1.66 +/- 0.24 ml/min/ml). Assuming an average lung density of 0.33 g/ml, this translates into a regional perfusion of approximately 5.5 ml/g/min, which is comparable to other measures of pulmonary perfusion. As with other ASL applications, a key problem with quantitative interpretation of the results is the physical gap between the tagging region and imaged slice. Because of the high pulsatility of PBF, ASL acquisition and data analysis differ significantly between the lung and the brain. The advantages and drawbacks of the single- vs. multiple-subtraction approaches are considered within a theoretical framework tailored to PBF.

Velocity-selective arterial spin labeling

In pathologies in which slow or collateral flow conditions may exist, conventional arterial spin labeling (ASL) methods that apply magnetic tags based on the location of arterial spins may not provide robust measures of cerebral blood flow (CBF), as the transit delay for the delivery of blood to target tissues may far exceed the relaxation time of the tag. Here we describe current methods for ASL with velocity-selective (VS) tags (termed VSASL) that do not require spatial selectivity and can thus provide quantitative measures of CBF under slow and collateral flow conditions. The implementation of a robust multislice VSASL technique is described in detail, and data obtained with this technique are compared with those obtained with conventional pulsed ASL (PASL). The technical considerations described here include the design of VS pulses, background suppression, anisotropy with respect to velocity-encoding directions, and CBF quantitation issues.

Detrimental effects of BOLD signal in arterial spin labeling fMRI at high field strength

Arterial spin labeling (ASL) MRI is a useful technique for noninvasive measurement of cerebral blood flow (CBF) in humans. High field strength provides a unique advantage for ASL because of longer blood T(1) relaxation times, making this technique a promising quantitative approach for functional brain mapping. However, higher magnetic field also introduces new challenges. Here it is shown that the CBF response determined using ASL functional MRI (fMRI) at 3.0 T contains significant contamination from blood-oxygenation-level-dependent (BOLD) effects. Due to interleaved acquisitions of label and control images, difference in blood oxygenation status between these two scans can cause incomplete cancellation of the static signal upon image subtraction, resulting in a BOLD-related artifact in the estimated CBF hemodynamics. If not accounted for, such an effect can complicate the interpretation of the ASL results, e.g., causing a delayed onset and offset of the response, or inducing an artifactual poststimulus undershoot. The BOLD contribution also decreases the sensitivity of ASL-based fMRI. Correction methods are proposed to reduce the artifact, giving increased number of activated voxels (18+/-5%, P=0.006) and more accurate estimation of CBF temporal characteristics.

The chronic vascular and haemodynamic response after permanent bilateral common carotid occlusion in newborn and adult rats

Vascular growth and redistribution of flow can compensate for arterial occlusion and possibly reduce the effects of hypoperfusion. As yet there is limited information on the age-dependent nature of vasculature remodelling. In this study, we have monitored the vascular and morphologic changes using magnetic resonance imaging and histology in a chronic bilateral common carotid artery occlusion (BCCAO) model in both newborn and adult rats. Acutely, cerebral blood flow (CBF) decreased immediately after BCCAO, producing a state of oligemic hypoperfusion. At 6 months after BCCAO in both adult and neonatal rats, the CBF had normalised at control values. To investigate the underlying mechanism for the return of CBF to control values, intra- and extracerebral magnetic resonance angiograms (MRAs) were acquired. As expected, signal from the common carotid arteries was present in the sham-operated rats, but was absent in the BCCAO animals. India ink angiograms demonstrated more tortuous basilar arteries in the adult rats post-BCCAO and MRAs demonstrated more extracerebral midline collaterals in the neonatal rats post-BCCAO, indicating different modes of vascular adaptation dependent on the age at onset of the insult. Both groups had collateral vessels arising from the vertebral arteries, and BCCAO was also associated with increased diameter of basilar, posterior cerebral, posterior communicating, internal carotid, middle cerebral and anterior cerebral arteries. Our study suggests that the developing and mature animals exhibit different patterns of vascular remodelling and that the BCCAO hypoperfusion model will be useful for investigating age-dependent vascular events in response to vaso-occlusive disease.

Modulatory role of cyclooxygenase-2 in cerebrovascular coupling

To investigate the role of cyclooxygenase-2 (COX-2) in the cerebrovascular coupling, hemodynamic and neuronal responses to forepaw stimulation were measured in alpha-chloralose-anesthetized rats (N = 18) before and after intravenous administration of Meloxicam (MEL), a preferential COX-2 inhibitor, and following a bolus of prostaglandin E(2) (PGE(2)), a prominent vasodilatatory product of COX-2 catalyzed metabolism of arachidonic acid. The cerebral blood flow (CBF) and blood-oxygenation-level-dependent (BOLD) response was quantified using continuous arterial spin labeling magnetic resonance imaging. Neuronal activity was measured by recording somatosensory-evoked potentials (SEPs) via intracranial electrodes. Both MEL and PGE(2) had a significant effect on the activation-elicited CBF (P < 10(-6)) and BOLD (P < 10(-6)) responses, without affecting the baseline perfusion. Meloxicam decreased brain COX enzymatic activity by 57 +/- 14% and decreased the stimulation-induced CBF response to 32 +/- 2% and BOLD to 46 +/- 1% of their respective pre-drug amplitudes. In turn, PGE(2) bolus resulted in a partial recovery of functional hyperemia, with the CBF response recovering to 52 +/- 3% and the BOLD response to 56 +/- 2% of their values prior to MEL administration. There was no concomitant decrease in either amplitudes or latencies of SEP components. These findings suggest a modulatory role of COX-2 products in the cerebrovascular coupling and provide evidence for existence of a functional metabolic buffer.

Magnetic resonance imaging of Alzheimer's disease

A modern challenge for neuroimaging techniques is to contribute to the early diagnosis of neurodegenerative diseases, such as Alzheimer's disease (AD). Early diagnosis includes recognition of pre-demented conditions, such as mild cognitive impairment (MCI) or having a high risk of developing AD. The role of neuroimaging therefore extends beyond its traditional role of excluding other conditions such as neurosurgical lesions. In addition, early diagnosis would allow early treatment using currently available therapies or new therapies in the future. Structural imaging can detect and follow the time course of subtle brain atrophy as a surrogate marker for pathological processes. New MR techniques and image analysis software can detect subtle brain microstructural, perfusion or metabolic changes that provide new tools to study the pathological processes and detect pre-demented conditions. This review focuses on markers of macro- and microstructural, perfusion, diffusion and metabolic MR imaging and spectroscopy in AD.

Intravascular effect in velocity-selective arterial spin labeling: the choice of inflow time and cutoff velocity

Velocity-selective arterial spin labeling (VS-ASL) tags spins on a basis of flow velocity, instead of spatial distribution that has been commonly adopted in conventional ASL techniques. VS-ASL can potentially generate tags that are very close to the imaging plane and whereby avoid the error source of transit delay (deltat) variation independent of inflow time (TI). In practice, however, TI of VS-ASL should still be chosen with caution with respect to intravascular signal and cutoff velocity (V(c)). The presented study takes advantage of multiple TI and V(c) to systematically investigate the intravascular effect. Results demonstrate the presence of significant signal from large vessels in VS-ASL images for V(c) down to 4 cm/s. For perfusion measurement in human brain, low V(c) (<4 cm/s) is recommended. With V(c) = 2 cm/s, quantitative cerebral blood flow is 72.8 ml/100 ml/min, which is in agreement with the reported range using conventional ASL methods. In field strength of 3 T, numerical simulation shows that optimal signal-to-noise ratio efficiency can be achieved with TR/TI = 2092 ms/1664 ms for single slice and 4493 ms/1404 ms for slab imaging.

Personality factors correlate with regional cerebral perfusion

There is an increasing body of evidence pointing to a neurobiological basis of personality. The purpose of this study was to investigate the biological bases of the major dimensions of Eysenck's and Cloninger's models of personality using a noninvasive magnetic resonance perfusion imaging technique in 30 young, healthy subjects. An unbiased voxel-based analysis was used to identify regions where the regional perfusion demonstrated significant correlation with any of the personality dimensions. Highly significant positive correlations emerged between extraversion and perfusion in the basal ganglia, thalamus, inferior frontal gyrus and cerebellum and between novelty seeking and perfusion in the cerebellum, cuneus and thalamus. Strong negative correlations emerged between psychoticism and perfusion in the basal ganglia and thalamus and between harm avoidance and perfusion in the cerebellar vermis, cuneus and inferior frontal gyrus. These observations suggest that personality traits are strongly associated with resting cerebral perfusion in a variety of cortical and subcortical regions and provide further evidence for the hypothesized neurobiological basis of personality. These results may also have important implications for functional neuroimaging studies, which typically rely on the modulation of cerebral hemodynamics for detection of task-induced activation since personality effects may influence the intersubject variability for both task-related activity and resting cerebral perfusion. This technique also offers a novel approach for the exploration of the neurobiological correlates of human personality.

Neuroprotective effects of virally delivered HSPs in experimental stroke

Heat shock proteins (HSPs) are molecular chaperones with essential roles in modulating the proteolytic machinery and accelerating cell repair. Heat shock protein overexpression has been observed in vivo and in vitro under stresses including heat, nutrient deprivation and ischemia. Experiments in in vivo models of stroke indicate that transgenically overexpressed or virally delivered HSPs can enhance cell survival, but cannot always reduce lesion size. This study aims to assess the effects of virally delivered HSPs in a rat middle cerebral artery occlusion model of reversible focal cerebral ischemia using noninvasive magnetic resonance imaging. Attenuated herpes simplex virus carrying HSP27, HSP70, or a LacZ control was microinjected into the striatum 3 days before ischemia. Multislice T(2)-weighted images at 24 h after ischemia indicated that lesion volume was reduced by 44% in HSP27-treated animals compared with controls (P = 0.019). No significant differences were found between HSP70-treated and control animals (P = 0.88). Immunohistochemistry and Western blots revealed that HSP27 and HSP70 expression levels were equally high in injected hemispheres, but only the former had an effect on lesion size. This is the first evidence of the efficacy of gene therapy with any viral vector expressing HSP27 in an experimental model of stroke.

Quantification of cerebral perfusion using arterial spin labeling: two-compartment models

One of the advantages of arterial spin labeling (ASL) techniques over other techniques for measuring cerebral perfusion is that with ASL it is possible to achieve accurate quantification. This is particularly useful in the field of functional imaging, where accurate measurements of perfusion change can help untangle the complex physiological changes that occur following neuronal activation. However, the linearity of the perfusion estimate over a wide range of perfusion values may be more important than absolute values. For several years, single-compartment models have dominated the literature, and it has been assumed that the labeled water diffuses freely throughout the tissue voxel. However, recent work, as summarized in this review, has shown that this assumption is inaccurate and leads to an overestimation of perfusion at low perfusion rates, and an underestimation at high rates. The inclusion of restricted permeability of the capillary wall to water in a two-compartment model offers improved quantification.

Quantifying CBF with arterial spin labeling

The basic principles of measuring cerebral blood flow (CBF) using arterial spin labeling (ASL) are reviewed. The measurement is modeled by treating the ASL method as a magnetic resonance imaging (MRI) version of a microsphere study, rather than a diffusible tracer study. This approach, particularly when applied to pulsed ASL (PASL) experiments, clarifies that absolute calibration of CBF primarily depends on global properties of blood, rather than local tissue properties such as the water partition coefficient or relaxation time. However, transit delays from the tagging region to the image voxel are a potential problem in all standard ASL methods. The key to quantitative CBF measurements that compensate for this systematic error is to create a well-defined bolus of tagged blood and to ensure that all of the bolus has been delivered to an imaging voxel at the time of measurement. Two practical technical factors considered here are 1) producing a tagged bolus with a well-defined temporal width and 2) accounting for reduction in magnitude of the tagged magnetization due to relaxation. The ASL approach has the potential to provide a robust estimation of CBF, although the timing of water exchange into tissue and the effects of pulsatile flow require further investigation.

Quantifying CBF with pulsed ASL: technical and pulse sequence factors

We summarize here current methods for the quantification of CBF using pulsed arterial spin labeling (ASL) methods. Several technical issues related to CBF quantitation are described briefly, including transit delay, signal from larger arteries, radio frequency (RF) slice profiles, magnetization transfer, tagging efficiency, and tagging geometry. Many pulsed tagging schemes have been devised, which differ in the type of tag or control pulses, and which have various advantages and disadvantages for quantitation. Several other modifications are also available that can be implemented as modules in an ASL pulse sequence, such as varying the wash-in time to estimate the transit delay. Velocity-selective ASL (VS-ASL) uses a new type of pulse labeling in which inflowing arterial spins are tagged based on their velocity rather than their spatial location. In principle, this technique may allow ASL measurement of cerebral blood flow (CBF) that is insensitive to transit delays.

Pediatric Perfusion MR Imaging Using Arterial Spin Labeling

Cerebral blood flow (CBF, cerebral perfusion) mirrors cerebral metabolic demand and neuronal function, and therefore, is a vital parameter in the evaluation of pediatric brain injury and recovery. Until recently, measurement of CBF involved intravenous bolus injection of contrast agents or nuclear medicine methods that were technically difficult or ethically problematic in pediatrics. The development of arterial spin label (ASL) perfusion MR imaging as a noninvasive method for measuring CBF allows for the increased ability to measure this vital physiologic parameter in any age group. This article presents the technical aspects of performing ASL perfusion MR imaging in pediatrics, and discusses its current use in clinical studies and its potential for influencing important management strategies in specific disease entities.

Regional variation of cerebral blood flow and arterial transit time in the normal and hypoperfused rat brain measured using continuous arterial spin labeling MRI

Continuous arterial spin labeling (CASL) is a noninvasive magnetic resonance (MR) method for measuring cerebral perfusion. In its most widely used form, CASL incorporates a postlabeling delay to minimize the sensitivity of the technique to transit time effects, which otherwise corrupt cerebral blood flow (CBF) quantification. For this delay to work effectively, it must be longer than the longest transit time present in the system. In this work, CASL measurements were made in four coronal slices in the rat brain using a range of postlabeling delays. By doing this, direct estimation of both CBF and arterial transit time (delta(a)) was possible. These measurements were performed in the normal brain and during hypoperfusion induced by occlusion of the common carotid arteries. It was found that, in the normal rat brain, significant regional variation exists for both CBF and delta(a). Mean values of CBF and delta(a) in the selected gray matter regions of interest were 233 mL/100 g min and 266 ms, respectively, with the latter ranging from 100 to 500 ms. Therefore, use of a 500-ms postlabeling delay is suitable for any location in the normal rat brain. After common carotid artery occlusion, CBF decreased and delta(a) increased by regionally dependent amounts. In the sensory cortex, delta(a) increased to a mean value of 740 ms, significantly greater than 500 ms. These results highlight the importance of either (a) determining delta(a) as part of the CASL measurement or (b) knowing the approximate range of values delta(a) is likely to take for a given application, so that the parameters of the CASL sequence can be chosen appropriately.

Continuous arterial spin labeling perfusion measurements using single shot 3D GRASE at 3 T

Single shot 3D GRASE is less sensitive to field inhomogeneity and susceptibility effects than gradient echo based fast imaging sequences while preserving the acquisition speed. In this study, a continuous arterial spin labeling (CASL) pulse was added prior to the single shot 3D GRASE readout and quantitative perfusion measurements were carried out at 3 T, at rest and during functional activation. The sequence performance was evaluated by comparison with a CASL sequence with EPI readout. It is shown that perfusion measurements using CASL GRASE can be performed safely on humans at 3 T without exceeding the current RF power deposition limits. The maps of resting cerebral blood flow generated from the GRASE images are comparable to those obtained with the 2D EPI readout, albeit with better coverage in the orbitofrontal cortex. The sequence proved effective for functional imaging, yielding time series of images with improved temporal SNR with respect to EPI and group activation maps with increased significance levels. The method was further improved using parallel imaging techniques to provide increased spatial resolution and better separation of the gray-white matter cerebral blood flow maps.

Study design in fMRI: basic principles

There is a wide range of functional magnetic resonance imaging (fMRI) study designs available for the neuroscientist who wants to investigate cognition. In this manuscript we review some aspects of fMRI study design, including cognitive comparison strategies (factorial, parametric designs), and stimulus presentation possibilities (block, event-related, rapid event-related, mixed, and self-driven experiment designs) along with technical aspects, such as limitations of signal to noise ratio, spatial, and temporal resolution. We also discuss methods to deal with cases where scanning parameters become the limiting factor (parallel acquisitions, variable jittered designs, scanner acoustic noise strategies).

Quantification of cerebral arterial blood volume using arterial spin labeling with intravoxel incoherent motion-sensitive gradients

Quantification of cerebral arterial blood volume (CBVa) is important for understanding vascular regulation. To enable measurement of CBVa with diffusion-weighted (DW) arterial spin labeling (ASL), a theoretical framework was developed using the effects of intravoxel incoherent motion (IVIM). The pseudo-diffusion coefficient (D*) in the IVIM model was evaluated at 9.4 T in DW-ASL of rat brain under isoflurane anesthesia by variations of both post-labeling delay (w) and magnetization transfer ratio (MTR). D* and its volume fraction decreased at values of w>or=0.3 s, and the normalized apparent diffusion coefficient (ADC) increased with MTR, suggesting that D* is closely correlated with CBVa. Thus, the difference between ASL measurements with and without DW gradients is related to CBVa. The CBVa values measured by this approach were compared with values obtained using the modulation of tissue and vessel (MOTIVE) technique with ASL, which varies MT levels without changing spin labeling efficiency. CBVa values from both methods were highly correlated. The measured CBVa values were linearly correlated with cerebral blood flow (CBF) for a PaCO2 range of 25-50 mmHg; DeltaCBVa (ml/100 g)=0.007 (min-1)xDeltaCBF (ml/100 g/min). The DW-ASL approach is simple and easy to implement for human and animal CBVa studies.

Reducing contamination while closing the gap: BASSI RF pulses in PASL

Bandwidth-modulated selective saturation and inversion (BASSI) pulses are a class of frequency- and gradient-modulated radiofrequency (RF) pulses, derived from the hyperbolic secant pulse by temporal variation of the bandwidth parameter. These pulses afford optimal amplitude modulation, achieving uniform and highly selective profiles at any effective flip angle. In this paper, BASSI pulses are parameterized to obtain low RF energy pulsed arterial spin labeling (PASL) label pulses with minimal contamination of static spins outside the label region and highly selective PICORE/QUIPSS II saturation pulses allowing for small label gaps. They are compared to frequency offset corrected inversion (FOCI) label pulses and sinc saturation pulses in simulations and a phantom experiment. Drawing on the outstanding selectivity of bandwidth-modulated saturation pulses, a new noninvasive method to measure in vivo the contamination effects due to direct and indirect saturation of static spins by the label pulse is presented. In an in vivo study on four subjects, contamination effects in a QUIPSS II PASL implementation based on BASSI pulses are compared to those present in a state-of-the-art Q2TIPS sequence employing a FOCI label pulse. Residual contamination in the QUIPSS II/BASSI sequence is shown to be reduced by a factor of 3, compared to the Q2TIPS/FOCI sequence. In vivo human perfusion images obtained with a label gap of only 2 mm are presented.

Altered hemodynamics and regional cerebral blood flow in patients with hemodynamically significant stenoses

BACKGROUND AND PURPOSE

Blood oxygen level-dependent (BOLD) contrast largely depends on changes in cerebral blood flow (CBF). Because cerebrovascular disease may result in altered CBF, we assessed the temporal dynamics and magnitude of the BOLD response in patients with major arterial stenoses.

METHODS

Seven patients with hemodynamically significant stenoses affecting the anterior circulation (primarily left internal carotid and middle cerebral arteries) were compared with 7 neurologically healthy subjects. Continuous arterial spin-labeled perfusion MRI was used to measure resting CBF globally and within various vascular distributions. The BOLD response was acquired during a visually guided bilateral handball squeeze task while motor performance was recorded by a pressure transducer.

RESULTS

Baseline CBF was reduced in bilateral middle cerebral artery and left anterior cerebral artery territories in patients. A prolonged BOLD hemodynamic response was observed in patients in bilateral primary motor cortices but not visual cortex. Patients also exhibited a larger early negative BOLD response, or "initial dip," in left primary motor cortex. There were no differences in motor performance between groups, suggesting behavioral differences were not primarily responsible for the characteristics of the BOLD response.

CONCLUSIONS

An initial deoxygenation followed by a delayed hyperemic BOLD response was observed in patients, although resting flow values were not within an ischemic range. A simple visuomotor BOLD activation paradigm can reflect alterations in the hemodynamic response in patients with hemodynamically significant stenoses.

Continuous ASL (CASL) perfusion MRI with an array coil and parallel imaging at 3T

The purpose of this work was to assess the feasibility and efficacy of using an array coil and parallel imaging in continuous arterial spin labeling (CASL) perfusion MRI. An 8-channel receive-only array head coil was used in conjunction with a surrounding detunable volume transmit coil. The signal to noise ratio (SNR), temporal stability, cerebral blood flow (CBF), and perfusion image coverage were measured from steady state CASL scans using: a standard volume coil, array coil, and array coil with 2- and 3-fold accelerated parallel imaging. Compared to the standard volume coil, the array coil provided 3 times the average SNR increase and higher temporal stability for the perfusion weighted images, even with threefold acceleration. Although perfusion images of the array coil were affected by the inhomogeneous coil sensitivities, this effect was invisible in the quantitative CBF images, which showed highly reproducible perfusion values compared to the standard volume coil. The unfolding distortions of parallel imaging were suppressed in the perfusion images by pairwise subtraction, though they sharply degraded the raw EPI images. Moreover, parallel imaging provided the potential of acquiring more slices due to the shortened acquisition time and improved coverage in brain regions with high static field inhomogeneity. Such results highlight the potential utility of array coils and parallel imaging in ASL perfusion MRI.

Understanding and optimizing the amplitude modulated control for multiple-slice continuous arterial spin labeling

Multiple-slice perfusion imaging by continuous arterial spin labeling (CASL) is made possible by amplitude modulation (AM) of the labeling RF pulse, but perfusion sensitivity is reduced relative to the single-slice technique. A computer model of the Bloch equations for velocity driven adiabatic fast passage was developed to elucidate the compromised sensitivity to perfusion of the AM control technique for CASL. Calculations were performed over ranges of RF pulse amplitude, B1; magnetic field gradient, G; phase, phi, and frequency, f, of the modulation function; velocity, v, and relaxation times, T1 and T2, of blood. It was found that unless f>2piB1, phi determines the performance of the AM control; excessively high B1 or v reduces the efficiency of the AM control; and T1 relaxation dominates if f is too great. In vivo, in rat brain (n=5) at 2.35 T, the sensitivity of the AM technique to perfusion was 70% of the sensitivity of single-slice CASL.

Arterial spin labeling in small animals: Methods and applications to experimental cerebral ischemia

ASL enables noninvasive, quantitative monitoring of cerebral perfusion to be performed repeatedly over a period of hours. Thus, ASL is an attractive method for basic science studies of the time evolution and pathophysiology of diseases using animal models. In particular, ASL is valuable for basic science studies of evolving tissue status and viability in stroke using animal models of acute ischemia. This study describes both pulsed (PASL) and continuous ASL (CASL) studies of quantitative cerebral perfusion in rodent models of cerebral ischemia. Some technical factors pertinent to these studies are discussed, including a method for measuring arterial blood T(1) and double-echo PASL for measuring cerebral blood flow (CBF) and volume (CBV). Investigations of the CBF response to forebrain ischemia and reperfusion, and of regional variations in CBF and arterial transit time (ATT) are also discussed.

About being BOLD

The last decade has seen an unprecedented increase in the use of functional magnetic resonance imaging (fMRI) to understand the neural basis of cognition and behavior. Being non-invasive and relatively easy to use, most studies relied on changes in the blood oxygenation level dependent (BOLD) contrast as an indirect marker of variations in brain activity. However, the fact that BOLD fMRI is dependent on the blood flow response that follows neural activity and does not measure neural activity per se is seen as an inherent cause for concern while interpreting data from these studies. In order to characterize the BOLD signal correctly, it is imperative that we have a better understanding of neural events that lead to the BOLD response. A review of recent studies that addressed several aspects of BOLD fMRI including events at the level of the synapse, the nature of the neurovascular coupling, and some parameters of the BOLD signal is provided. This is intended to serve as background information for the interpretation of fMRI data in normal subjects and in patients with compromised neurovascular coupling. One of the aims is also to encourage researchers to interpret the results of functional imaging studies in light of the dynamic interactions between different brain regions, something that often is neglected.

Multislice continuous arterial spin-labeled perfusion MRI in patients with chronic occlusive cerebrovascular disease: a correlative study with CO2 PET validation

PURPOSE

To compare the use of multislice continuous arterial spin labeling (CASL) and CO(2) positron emission tomography (PET) to assess CBF in patients with chronic occlusive cerebrovascular disease for the validation of quantitative CASL perfusion in an altered hemodynamic state.

MATERIALS AND METHODS

Eleven patients with occlusive carotid artery disease were studied with CASL and conventional MRI. Cerebral blood flow (CBF) was also measured with O(15)-labeled CO(2) gas using a whole-body PET scanner. The average values within region of interests (ROIs) drawn on coregistered CASL and PET images were used for the linear regression analysis and to assess the effect of transit time on the quantification using CASL.

RESULT

In all patients there was a significant correlation between the CBF values from CASL and PET (r = 0.71 +/- 0.07, P < 0.001). The slope of regression lines varied widely among patients (0.54-1.77). Longer transit times were estimated in four of 11 patients in the hypoperfusion region.

CONCLUSION

Quantification of CBF using CASL is feasible and reasonable, even when employed in a routine clinical setting. However, the long transit time may lead to underestimation of the affected side in occlusive cerebrovascular disease.

Quantification of cerebral arterial blood volume and cerebral blood flow using MRI with modulation of tissue and vessel (MOTIVE) signals

Regional cerebral arterial blood volume (CBVa) and blood flow (CBF) can be quantitatively measured by modulation of tissue and vessel (MOTIVE) signals, enabling separation of tissue signal from blood. Tissue signal is selectively modulated using magnetization transfer (MT) effects. Blood signal is changed either by injection of a contrast agent or by arterial spin labeling (ASL). The measured blood volume represents CBVa because the contribution from venous blood was insignificant in our measurements. Both CBVa and CBF were quantified in isoflurane-anesthetized rats at 9.4T. CBVa obtained using a contrast agent was 1.1 +/- 0.5 and 1.3 +/- 0.6 ml/100 g tissue (N = 10) in the cortex and caudate putamen, respectively. The CBVa values determined from ASL data were 1.0 +/- 0.3 ml/100 g (N = 10) in both the cortex and the caudate putamen. The match between CBVa values determined by both methods validates the MOTIVE approach. In ASL measurements, the overestimation in calculated CBF values increased with MT saturation levels due to the decreasing contribution from tissue signals, which was confirmed by the elimination of blood with a contrast agent. Using the MOTIVE approach, accurate CBF values can also be obtained.

Single-shot 3D imaging techniques improve arterial spin labeling perfusion measurements

Arterial spin labeling (ASL) can be used to measure perfusion without the use of contrast agents. Due to the small volume fraction of blood vessels compared to tissue in the human brain (typ. 3-5%) ASL techniques have an intrinsically low signal-to-noise ratio (SNR). In this publication, evidence is presented that the SNR can be improved by using arterial spin labeling in combination with single-shot 3D readout techniques. Specifically, a single-shot 3D-GRASE sequence is presented, which yields a 2.8-fold increase in SNR compared to 2D EPI at the same nominal resolution. Up to 18 slices can be acquired in 2 min with an SNR of 10 or more for gray matter perfusion. A method is proposed to increase the reliability of perfusion quantification using QUIPSS II derivates by acquiring low-resolution maps of the bolus arrival time, which allows differentiation between lack of perfusion and delayed arrival of the labeled blood. For arterial spin labeling, single-shot 3D imaging techniques are optimal in terms of efficiency and might prove beneficial to improve reliability of perfusion quantitation in a clinical setup.

Towards quantification of blood-flow changes during cognitive task activation using perfusion-based fMRI

Multi-slice perfusion-based functional magnetic resonance imaging (p-fMRI) is demonstrated with a color-word Stroop task as an established cognitive paradigm. Continuous arterial spin labeling (CASL) of the blood in the left common carotid artery was applied for all repetitions of the functional run in a quasi-continuous fashion, i.e., it was interrupted only during image acquisition. For comparison, blood oxygen level dependent (BOLD) contrast was detected using conventional gradient-recalled echo (GE) echo planar imaging (EPI). Positive activations in BOLD imaging appeared in p-fMRI as negative signal changes corresponding to an enhanced transport of inverted water spins into the region of interest, i.e., increased cerebral blood flow (CBF). Regional differences between the localization of activations and the sensitivity of p-fMRI and BOLD-fMRI were observed as, for example, in the inferior frontal sulcus and in the intraparietal sulcus. Quantification of CBF changes during cognitive task activation was performed on a multi-subject basis and yielded CBF increases of the order of 20-30%.

Spatially-confined arterial spin-labeling with FAIR

PURPOSE

To investigate the effectiveness of slab-selective inversion in pulsed arterial spin labeling with body coil excitation as a means to reduce large vessel contamination of the perfusion signal.

MATERIALS AND METHODS

Studies were conducted by varying the tagging width in multislice flow-sensitive alternating inversion recovery (FAIR) in conjunction with body coil excitation on a Siemens Sonata whole-body 1.5-T scanner. The results of spatially-confined tagging were then compared with conventional nonselective tagging in the presence and absence of a bipolar gradient crusher pair in order to determine the effectiveness of suppressing vascular signal and to estimate the bolus width that reaches the capillary bed.

RESULTS

It is shown in five volunteers, ages 23-38 years, that depending on the average velocity of the arterial blood flow in the tagging region, a bolus of 6-8 cm in width reaches the capillary bed at a fixed inversion time TI of 1.4 seconds, while a bolus of 11.2-16.5 cm in width enters the imaging region. Further, noticeable velocity differences have been found among the participating subjects, with averages ranging from 10.1 to 13.9 cm/second.

CONCLUSION

The data suggest that it is advantageous to replace nonselective global tagging in FAIR perfusion imaging with body coil excitation by spatially-confined tagging to reduce undesired residual tagged blood in large vessels.

Neuroprotective effects of HSP70 overexpression after cerebral ischaemia--an MRI study

Heat shock proteins (HSPs) have been reported to increase cell survival in response to a wide range of cellular challenges. However, the role of HSP70 overexpression is still a matter of debate, with some reports showing protection and others not. In order to resolve these discrepancies and further investigate the action of these proteins in vivo, transgenic mice overexpressing HSP70 have been compared to wild-type mice in a middle cerebral artery occlusion model of permanent cerebral ischaemia. Previously, the effect of HSP70 was assessed histologically postmortem. In this report, magnetic resonance imaging (MRI) was used to assess the mice in vivo after the onset of stroke. The lesion volume, as measured at 24 h using T(2)-weighted MRI, was significantly smaller in HSP70 transgenic mice compared with wild-type mice. The smaller lesion size in HSP70 transgenic mice could not be attributed to differences in vascular anatomy or in cerebral blood flow during occlusion. Additionally, the apparent diffusion coefficient showed different spatial and temporal patterns between the groups, suggesting that the damage within the lesion may be less severe for HSP70 transgenic mice. Thus, we conclude that overexpression of HSP70 reduces the overall lesion size and may also limit the tissue damage within the lesion.

Grading of CNS neoplasms using continuous arterial spin labeled perfusion MR imaging at 3 Tesla

PURPOSE

To differentiate glioma grade based on blood flow measured using continuous arterial spin labeled (CASL) perfusion MRI, implemented at 3 Tesla for improved signal-to-noise ratio (SNR) and spin labeling effect.

MATERIALS AND METHODS

CASL perfusion images were obtained preoperatively in 26 patients with brain neoplasms (19 high-grade gliomas (HGGs; WHO grades 3 and 4) and seven low-grade gliomas (LGGs; WHO grades 1 and 2)). The mean and maximum tumor blood flow (TBF and TBFmax) were calculated in the neoplasm, including surrounding infiltrating tumor vs. edema. Measures normalized to global CBF (nTBF and nTBFmax) were also obtained.

RESULTS

Normalized measures of TBFmax provided the best distinction between HGG and LGG groups (Wilcoxon rank sum test, P = 0.01). Seventeen of 19 HGGs showed nTBFmax > 1.0, and 15 of 19 showed nTBFmax > 1.3. Four of seven LGGs showed nTBFmax < 1.0, and six of seven showed nTBFmax < 1.3. Absolute TBFmax also differed significantly between the HGG and LGG groups (P = 0.04). TBFmax in 11 of 17 HGGs was >50 mL/100 g/min (mean +/- SD = 94.9 +/- 71.7 mL/100 g/min). All but one LGGs showed TBFmax < or = 50 mL/100 g/min (mean +/- SD = 42.8 +/- 22.0 mL/100 g/min).

CONCLUSION

CASL perfusion MRI provides a quantitative, noninvasive alternative to dynamic susceptibility contrast perfusion MR methods for evaluating gliomas.

FAWSETS perfusion measurements in exercising skeletal muscle

Arterial spin labeling (ASL) techniques are now recognized as valid tools for providing accurate measurements of cerebral and cardiac perfusion. The labeling process used with most ASL techniques creates two problems, magnetization transfer (MT) effects and arterial transit time effects, that require compensation. The compensation process limits time resolution and hinders absolute quantification. MT effects are particularly problematic in skeletal muscle because they are large and change rapidly during exercise. The protocol presented here was developed specifically for quantification of perfusion in exercising skeletal muscle. The ASL technique that was implemented, FAWSETS, eliminates MT effects and arterial transit times. Localized, single-voxel perfusion measurements were acquired from rat hind limbs at rest, during ischemia and during three different levels of stimulated exercise. The results demonstrate sufficient sensitivity to determine the time constants for perfusion changes at onset of, and during recovery from, exercise and to distinguish the differences in the amplitude of the perfusion response to different levels of exercise. Additional measurements were conducted to demonstrate insensitivity to MT effects. The exercise protocol is easily adaptable to phosphorous magnetic resonance measurements, allowing the possibility to acquire local measurements of perfusion and metabolism from the same tissue in future experiments.

Functional magnetic resonance imaging reveals similar brain activity changes in two different animal models of schizophrenia

RATIONALE AND OBJECTIVES

In schizophrenia research, most of the functional imaging studies have been performed in psychotic patients, but little is known about brain areas involved in the expression of psychotic-like symptoms in animal models. The objective of this study was to visualize and compare brain activity abnormalities in a neurodevelopmental and a pharmacological animal model of schizophrenia.

METHODS

Blood perfusion of specific brain areas, taken as indirect measure of brain activity, was investigated in adult rats following either neonatal ventral hippocampal lesion or acute administration of phencyclidine. Quantitative perfusion magnetic resonance imaging was performed on five frontal brain slices using the continuous arterial spin labeling technique. The mean perfusion was calculated in several brain structures, which were identified on anatomical images.

RESULTS

Lesioned animals exhibiting deficits in prepulse inhibition of the startle reflex showed a significant blood perfusion increase in the nucleus accumbens, basolateral amygdala, ventral pallidum, entorhinal-piriform cortex, orbital prefrontal cortex, and in the bed nucleus of the stria terminalis, and a decrease of perfusion in the temporal cortex. Similar effects were seen following acute phencyclidine administration in naïve animals.

CONCLUSION

Our data point out specific cortical and subcortical brain areas involved in the development of psychotic-like symptoms in two different animal models of schizophrenia. The observed brain activity abnormalities are reminiscent of classical neuroimaging findings described in schizophrenic patients.

Four-phase single-capillary stepwise model for kinetics in arterial spin labeling MRI

An extended model for extracting measures of brain perfusion from pulsed arterial spin labeling (ASL) data while considering transit effects and restricted permeability of capillaries to blood water is proposed. We divided the time course of the signal difference between control and labeled images into four phases with respect to the arrival time of labeled blood water at the voxel of interest (t(A)), transit time through the arteries in the voxel (t(ex)), and duration of the bolus of labeled spins (tau). Dividing the labeled slab of blood water into many discrete segments, and adapting numerical integration methods allowed us to conveniently model restricted capillary-tissue exchange based on a modified distributed parameter model. We compared this four-phase single-capillary stepwise (FPSCS) model with models that treat water as a freely diffusible tracer, using both simulations and experimental ASL brain imaging data at 1.5T from eight healthy subjects (24-80 years old). The FPSCS model yielded less errors in the least-squares sense in fitting brain ASL data in comparison with freely diffusible tracer models of water (P = 0.055). These results imply that restricted permeability of capillaries to water should be considered when brain ASL data are analyzed.

Validation and advantages of FAWSETS perfusion measurements in skeletal muscle

This work discusses the strengths, limitations and validity of a novel arterial spin labeling technique when used specifically to measure perfusion in limb skeletal muscle. The technique, flow-driven arterial water stimulation with elimination of tissue signal (FAWSETS), offers several advantages over existing arterial spin labeling techniques. The primary goal of this study was to determine the perfusion signal response to changes in net hind limb flow that were independently verifiable. The range of perfusate flow was relevant to skeletal muscle during mild to moderate exercise. Localized, single voxel measurements were acquired from a 5 mm-thick slice in the isolated perfused rat hind limb at variable net flow rates. The results show that the perfusion signal is linearly proportional to net hind limb flow with a correlation coefficient of 0.974 (p = 0.0013). FAWSETS is especially well suited for studies of skeletal muscle perfusion, where it eliminates the need to compensate for magnetization transfer and arterial transit time effects. A conceptual discussion of the basic principles underlying these advantages is presented.

Effects of the apparent transverse relaxation time on cerebral blood flow measurements obtained by arterial spin labeling

Previous modeling studies have predicted that a significant fraction of the signal in arterial spin labeling (ASL) experiments originates from labeled water in the capillaries. Provided that the relaxation times in blood and tissue are similar, ASL data can still be analyzed with the conventional one-compartment Kety model. Such studies have primarily focused on T1 differences and have neglected any differences in transverse relaxation times (T2 and T2*). This is reasonable for studies at lower fields; however, it may not be valid at higher fields due to the stronger susceptibility effects of deoxygenated blood. In this study a tracer kinetic model was developed that includes T2* differences between capillary blood and tissue. The model predicts that a reduction in blood T2* at higher fields will attenuate the capillary contribution to the ASL signal. This in turn causes an underestimation of CBF when ASL data are analyzed with the one-compartment Kety model. We confirmed this prediction by comparing ASL data collected at 1.5 and 4 T, and at multiple gradient echoes (19, 32, 45, and 58 ms). A decrease in resting-state CBF with echo time (TE) was observed at 4 T, but not at 1.5 T. These results suggest that at higher fields AST data should be collected using gradient-echo techniques with short TEs, or with spin-echo techniques. Furthermore, the sensitivity of the CBF measurements to venous T2* may affect the interpretation of concurrent ASL/BOLD studies.

Simultaneous MRI acquisition of blood volume, blood flow, and blood oxygenation information during brain activation

Simultaneous acquisition of complementary functional hemodynamic indices reflecting different aspects of brain activity would be a valuable tool for functional brain-imaging studies offering enhanced detection power and improved data interpretation. As such, a new MRI technique is presented that is able to achieve concurrent acquisition of three hemodynamic images based primarily on the changes of cerebral blood volume, blood flow, and blood oxygenation, respectively, associated with brain activation. Specifically, an inversion recovery pulse sequence has been designed to measure VASO (vascular space occupancy), ASL (arterial spin labeling) perfusion, and BOLD (blood-oxygenation-level-dependent) signals in a single scan. The MR signal characteristics in this sequence were analyzed, and image parameters were optimized for the simultaneous acquisition of these functional images. The feasibility and efficacy of the new technique were assessed by brain activation experiments with visual stimulation paradigms. Experiments on healthy volunteers showed that this technique provided efficient image acquisition, and thus higher contrast-to-noise ratio per unit time, compared with conventional techniques collecting these functional images separately. In addition, it was demonstrated that the proposed technique was able to be utilized in event-related functional MRI experiments, with potential advantages of obtaining accurate transient information of the activation-induced hemodynamic responses.

CASL fMRI of subcortico-cortical perfusion changes during memory-guided finger sequences

Arterial spin labeling (ASL) perfusion functional magnetic resonance imaging (fMRI) is an attractive alternative to BOLD fMRI. Nevertheless, current ASL fMRI techniques are limited by several factors that hamper more routine applications in humans. One of these factors is restricted brain coverage so that whole-brain ASL fMRI studies have never been reported. The present study tested the ability of a multislice continuous ASL (CASL) fMRI approach using a small surface coil placed on the subject's neck to map changes in regional cerebral blood flow (rCBF) throughout the brain while healthy individuals (N = 15) performed memory-guided sequential finger movements at a mean rate of approximately 0.5 Hz. As predicted by results from a large number of studies, reliable task-related increases in flow were detected across subjects not only in primary and associative cortical areas but also in subcortical brain regions. When normalized to baseline, rCBF increased 31% in the hand representation area (HRA) of left primary motor cortex (M1), 13% in the left supplementary motor area proper (SMA), 10% in the left dorsolateral prefrontal cortex (DLPFC), 10-18% in the bilateral intraparietal sulci, 6% in the HRA of left putamen, 10% in the left thalamus, and 17% in the right anterior cerebellum. In addition to these increases, 6% and 4% decreases in rCBF were detected in the HRA of the right M1 and the bilateral posterior cingulate sulci, respectively. These results demonstrate that perfusion-based fMRI using CASL with a separate labeling coil can now be used to characterize task-related flow changes in most of the brain volume with adequate accuracy and sensitivity.

Arterial spin labeling blood flow magnetic resonance imaging for the characterization of metastatic renal cell carcinoma(1)

RATIONALE AND OBJECTIVE

This study sought to assess the feasibility of arterial spin labeling (ASL) blood flow (BF) magnetic resonance imaging (MRI) for the study of metastatic renal cell carcinoma (RCC) in the body, where the respiratory, cardiac, and peristaltic motions present challenges when applying ASL.

MATERIALS AND METHODS

ASL was performed using a background-suppressed single-section flow-alternating inversion recovery (FAIR) preparation and a single-shot fast spin-echo imaging sequence on a 3.0-T whole body imager. Tumor BF was evaluated for 26 patients with RCC metastatic to the liver, bone, lung, or lymph nodes before VEGF receptor inhibitor therapy. Two cases with tumor size change after treatment were also scanned 1 month after therapy. For validation, kidney cortex BF in five normal volunteers was measured with the same technique and compared with literature values.

RESULTS

ASL was successfully performed in all normal volunteers and in 20 of 26 patients. The six failures resulted from a systematic error, which can be avoided in future studies. For normal volunteers, measured kidney cortex BF was 275 +/- 14 mL/min/100 g, a value consistent with the literature. ASL determined tumor BF averaged across tumor volume and subjects was 194 mL/min/100 g (intersubject SD = 100), resulting in high perfusion signal and conspicuity of lesions. Bright signal was also seen in large vessels and occasionally in bowel. In the two cases studied 1 month after therapy, ASL perfusion changes were consistent with tumor size changes.

CONCLUSION

With background suppression, ASL MRI is a feasible method for quantifying BF in patients with renal cell carcinoma. This technique may be useful for evaluating tumor response to antiangiogenic agents.

Amplitude-modulated continuous arterial spin-labeling 3.0-T perfusion MR imaging with a single coil: feasibility study

Written informed consent was obtained prior to all human studies after the institutional review board approved the protocol. A continuous arterial spin-labeling technique with an amplitude-modulated control was implemented by using a single coil at 3.0 T. Adiabatic inversion efficiency at 3.0 T, comparable to that at 1.5 T, was achieved by reducing the amplitude of radiofrequency pulses and gradient strengths appropriately. The amplitude-modulated control provided a good match for the magnetization transfer effect of labeling pulses, allowing multisection perfusion magnetic resonance imaging of the whole brain. Comparison of multisection continuous and pulsed arterial spin-labeling methods at 3.0 T showed a 33% improvement in signal-to-noise ratio by using the former approach.

Continuous arterial spin labeling at the human common carotid artery: the influence of transit times

In evaluating the sensitivity of arterial spin labeling (CASL) and for quantification of perfusion, knowledge of the transit time from the labeling plane to the imaging slice is crucial. The purpose of the current study was to obtain estimates of transit times relevant under the specific experimental conditions of CASL in human subjects using a separate local labeling coil at the neck. Specifically, the post-label delay (PLD), i.e. the time between the end of the labeling period and the image acquisition, was varied either with or without additional application of crusher gradients to suppress intravascular signal contributions. The overall sensitivity change for varying the PLD between 1000 and 1700 ms was low. A tissue transit time from the neck to an axial supraventricular section through Broca's knee was obtained by fitting the PLD dependence to a two-compartment model. Averaging over subjects yielded 1930 +/- 110 ms for the tissue transit time, and 73 +/- 5 ml min(-1) 100 g(-1) for the cerebral blood flow. Small areas that exhibited a very high signal change upon labeling were indicative of regional variation in cerebral blood flow related to vascular anatomy.

Experience in implementing continuous arterial spin labeling on a commercial MR scanner

Continuous arterial spin labeling (CASL) is a technique for performing quantitative perfusion measurements without the need for exogenous contrast agent administration. This technique has seen limited use in the clinic due to problems of poor sensitivity and the potential for artifacts. In addition, CASL requires the application of long-duration radiofrequency pulses and the acquisition of a large number of images, which can cause difficulties when implemented on commercial MR scanners. This work details our experience in implementing CASL on a commercial MR scanner for the measurement of cerebral blood flow, including pitfalls regarding hardware, radiofrequency energy deposition, and practical application in human subjects. Results of studies to determine the optimal acquisition procedures are also presented.

Input parameter sensitivity analysis and comparison of quantification models for continuous arterial spin labeling

The regional cerebral blood flow (rCBF) values determined using continuous arterial spin labeling (CASL) are subject to several sources of variability, including natural physiologic variations, sensitivity to the input parameters, and the use of different quantification models. To date, a thorough analysis of the impact of input parameters and the choice of quantification model has not been performed. These sources of variability were investigated through computer simulations using bootstrap techniques on actual CASL data. Coefficients of variation for representative single voxels were 6.7% for gray matter and 29% for white matter, and for eight-voxel regions of interest they were 4.5% for gray matter and 23% for white matter. Comparison of nine CASL quantification models showed differences in gray matter rCBF values of up to 42%. An analysis of the sensitivity of the rCBF to input parameters for each of the nine quantification models demonstrated that accurate quantification of the inversion efficiency, tissue and arterial blood longitudinal relaxation times, and transit times were critical in calculating precise rCBF values. The large potential variations in rCBF and the effect of the choice of quantification model suggest that interpreting absolute rCBF values in CASL studies can be challenging and requires great care.

Improving the amplitude-modulated control experiment for multislice continuous arterial spin labeling

The use of an amplitude-modulated radiofrequency (RF) pulse for a control experiment is a proven method to control for off-resonance effects in multislice continuous arterial spin labeling (CASL) experiments. This method is also known as double adiabatic inversion. The adiabaticity factor of a single half-pulse, beta(1/2), and a new dimensionless parameter alpha, which is obtained from the labeling parameters and the flow velocity, are introduced. This makes it possible to distinguish three distinct cases: 1) With low alpha, a double inversion occurs. 2) With alpha > or = approximately 4, the efficiency with which the longitudinal magnetization is returned to the z-axis depends on the phase of the amplitude modulation at the time the spins cross the center of the labeling plane. 3) In the intermediate region, the efficiency shows undesirable fluctuations. In a Bloch equation simulation, three optimized parameter sets are determined. Near ideal performance should always be achieved by combinations of parameters for which beta(1/2) > or = approximately 2 and alpha approximately pi/beta(1/2). The efficiency increases were realized in a volunteer study, showing the practical application of the suggested optimization.