Pseudocontinuous arterial spin labeling with optimized tagging efficiency
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Abstract The adiabatic inversion of blood in pseudocontinuous arterial spin labeling (PCASL) is highly sensitive to off‐resonance effects and gradient imperfections and this sensitivity can lead to tagging efficiency loss and unpredictable variations in cerebral blood flow estimates. This efficiency loss is caused by a phase tracking error between the RF pulses and the flowing spins. This article introduces a new method, referred to as Optimized PCASL (OptPCASL), that minimizes the phase tracking error by applying an additional compensation RF phase term and in‐plane gradients to the PCASL pulse train. The optimal RF phase and gradient amplitudes are determined using a prescan procedure, which consists of a series of short scans interleaved with automated postprocessing routines integrated to the scanner console. The prescan procedure is shown to minimize the phase tracking error in a robust and time efficient manner. As an example of its application, the use of OptPCASL for the improved detection of functional activation in the visual cortex is demonstrated and temporal signal‐to‐noise ratio (SNR), image SNR, and baseline cerebral blood flow measures are compared to those acquired from conventional PCASL. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.Keywords:
Arterial spin labeling
Purpose To evaluate the impact of rejecting intermediate cerebral blood flow (CBF) images that are adversely affected by head motion during an arterial spin labeling (ASL) acquisition. Materials and Methods Eighty participants were recruited, representing a wide age range (14–90 years) and heterogeneous cerebrovascular health conditions including bipolar disorder, chronic stroke, and moderate to severe white matter hyperintensities of presumed vascular origin. Pseudocontinuous ASL and T 1 ‐weigthed anatomical images were acquired on a 3T scanner. ASL intermediate CBF images were included based on their contribution to the mean estimate, with the goal to maximize CBF detectability in gray matter (GM). Simulations were conducted to evaluate the performance of the proposed optimization procedure relative to other ASL postprocessing approaches. Clinical CBF images were also assessed visually by two experienced neuroradiologists. Results Optimized CBF images (CBF opt ) had significantly greater agreement with a synthetic ground truth CBF image and greater CBF detectability relative to the other ASL analysis methods ( P < 0.05). Moreover, empirical CBF opt images showed a significantly improved signal‐to‐noise ratio relative to CBF images obtained from other postprocessing approaches (mean: 12.6%; range 1% to 56%; P < 0.001), and this improvement was age‐dependent ( P = 0.03). Differences between CBF images from different analysis procedures were not perceptible by visual inspection, while there was a moderate agreement between the ratings (κ = 0.44, P < 0.001). Conclusion This study developed an automated head motion threshold‐free procedure to improve the detection of CBF in GM. The improvement in CBF image quality was larger when considering older participants. J. Magn. Reson. Imaging 2015;42:1377–1385.
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Regional cerebral blood flow (rCBF) abnormalities have been documented in adults with depression. Investigators used a new noninvasive method of measuring rCBF (arterial spin labeling) from functional magnetic resonance imaging in 25 medication-naive adolescents (age range, 13–17 years) with major depressive disorder (MDD) and 26 matched healthy controls. Arterial spin labeling provides an absolute estimate of rCBF derived from subtraction of tagged magnetically …
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Abstract 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. J. Magn. Reson. Imaging 2007;26:855–862. © 2007 Wiley‐Liss, Inc.
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Arterial transit time (ATT), a key parameter required to calculate absolute cerebral blood flow in arterial spin labeling (ASL), is subject to much uncertainty. In this study, ASL ATTs were estimated on a per-voxel basis using data measured by both ASL and positron emission tomography in the same subjects. The mean ATT increased by 260 +/- 20 (standard error of the mean) ms when the imaging slab shifted downwards by 54 mm, and increased from 630 +/- 30 to 1220 +/- 30 ms for the first slice, with an increase of 610 +/- 20 ms over a four-slice slab when the gap between the imaging and labeling slab increased from 20 to 74 mm. When the per-slice ATTs were employed in ASL cerebral blood flow quantification and the in-slice ATT variations ignored, regional cerebral blood flow could be significantly different from the positron emission tomography measures. ATT also decreased with focal activation by the same amount for both visual and motor tasks (approximately 80 ms). These results provide a quantitative relationship between ATT and the ASL imaging geometry and yield an assessment of the assumptions commonly used in ASL imaging. These findings should be considered in the interpretation of, and comparisons between, different ASL-based cerebral blood flow studies. The results also provide spatially specific ATT data that may aid in optimizing the ASL imaging parameters.
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Objective: To investigate cerebral blood flow(CBF) characteristics in patients with Alzheimer's disease by using a whole-brain 3D pseudocontinuous arterial spin-labeling technique.Methods: Sixteen AD patients and 16 age-matched cognitively normal control subjects were studied at 3.0T MR system.CBF maps were preprocessed by SPM8 software and compared by two-sample t test between the AD and control groups.Results: Patients with AD showed lower CBF values compared with that in control group throughout the brain(P0.001),most prominently in the following regions: frontal lobe(L),inferior orbitofrontal(R),parahippocampal(R),inferior parietal(R and L),anterior parietal(R),supermarginal(R),precunei(R),middle cingulated(R),postcentral(R) and middle temporal(R)(P=0.000).Regarding partial volume effects,corrected CBF in patients with AD decreased obviously in bilateral temporal gyris and parieto-occipital border zones(P0.001).Conlusion: 3D pseudo-continuous arterial spin labeling imaging technique can monitor brain function by measuring CBF changes in patients with AD.
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Alzheimer’s disease (AD) depicts dynamic changes in regional brain function from early stages of the disease. Arterial spin labeling- (ASL-) based MRI methods have been applied for detecting regional cerebral blood flow (rCBF) perfusion changes in patients with AD and mild cognitive impairment (MCI). Nevertheless, the results obtained from ASL studies in AD and MCI are still controversial, since rCBF maps may show both hypoperfusion or hyperperfusion areas in brain structures involved in different cognitive functions. The goal of this review is to provide the current state of the art regarding the role of ASL for detecting distinctive perfusion patterns in subjects with MCI and/or AD. The ability to obtain this information using a noninvasive and widely available modality such as ASL should greatly enhance the knowledge into the broad range of hemodynamically related changes taking place during the cognitive decline process in AD.
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PCASL was used to measure cerebral blood flow (CBF) in eighteen patients with high-grade gliomas (III and IV) during normal breathing followed by a breath-holding task (ten periods of 21s interleaved with periods of normal breathing) to assess baseline CBF and cerebrovascular reactivity (CVR). All patients completed the task successfully and CBF and CVR maps were generated. CBF ratio in contrast-enhanced tumor area was higher in grade IV than grade III gliomas, as expected. CVR in tumor areas was decreased compared to GM CVR values. More studies are needed to assess the CVR heterogeneity in tumoral tissue.
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Arterial spin labeling (ASL) magnetic resonance imaging uses arterial blood water as an endogenous tracer to measure cerebral blood flow (CBF). In this review, based on ASL studies in the resting state, we discuss state-of-the-art technical and data processing improvements in ASL, and ASL CBF changes in normal aging, mild cognitive impairment (MCI), Alzheimer's disease (AD), and other types of dementia. We propose that vascular and AD risk factors should be considered when evaluating CBF changes in aging, and that other validated biomarkers should be used as inclusion criteria or covariates when evaluating CBF changes in MCI and AD. With improvements in hardware and experimental design, ASL is proving to be an increasingly promising tool for exploring pathogenetic mechanisms, early detection, monitoring disease progression and pharmacological response, and differential diagnosis of AD.
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