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.     

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.     

Selective multivessel labeling approach for perfusion territory imaging in pseudo-continuous arterial spin labeling

Recently, a new method for perfusion territory imaging named superselective pseudo-continuous arterial spin labeling was introduced. The method uses additional time-varying gradients to create a circular labeling spot that can be adjusted in size and thus adapted to individual arteries. In this study, the additional gradients are adjusted in such a way that an elliptical labeling spot is formed, which can be applied to label the blood in multiple vessels simultaneously in conjunction with an increased labeling efficiency compared with the original superselective approach. When compared with other selective multivessel strategies, the proposed technique allows for an improved and flexible adaption of the labeling focus to different anatomical variations of the arteries in the neck so that a total of five perfusion territories from the data acquired in three measurements can be recalculated in a reduced scan time. These include not only the perfusion territories of the cerebrum but also the perfusion territories in the cerebellum fed by individual vertebral arteries.     

Dual vessel arterial spin labeling scheme for regional perfusion imaging.

Regional perfusion imaging (RPI) based on pulsed arterial spin labeling and angulated inversion slabs has been recently proposed. The technique allows mapping of individual brain perfusion territories of the major feeding arteries and could become a valuable clinical tool for evaluation of patients with cerebrovascular diseases. Here we propose a new labeling scheme for RPI where lateral and posterior circulations are labeled simultaneously. Two scans instead of three are sufficient to obtain the same perfusion territories as in the original approach, allowing for a 33% reduction in the total RPI protocol time. Moreover, the position of the inversion slabs with respect to vascular anatomy facilitates the planning and allows potentially better labeling efficiency. The new approach was tested on seven healthy volunteers and compared to the original labeling scheme. The results showed that the same perfusion territories and regional CBF values can be obtained.     

Superselective arterial spin labeling applied for flow territory mapping in various cerebrovascular diseases

In three example patients suffering from internal carotid artery occlusion, intracranial steno‐occlusive disease, and symptomatic arteriovenous malformation (AVM), a new method named superselective pseudo‐continuous arterial spin labeling (pCASL) was used in addition to clinical routine measurements. The capabilities of this method are demonstrated to gain important information in diagnosis, risk analysis, and treatment monitoring that are neither accessible by digital subtraction angiography nor by existing selective arterial spin labeling methods and thus to propose future applications in clinical routine. In all cases superselective pCASL enabled the assessment of tissue viability and of territorial brain perfusion at different levels starting from major brain feeding vessels to collateral circulation at the level of the Circle of Willis to even distal branching arteries. This made it possible to estimate the contribution of an extracranial‐intracranial bypass to the brain perfusion; to depict individual arteries to important functional brain areas; to identify en‐passant feeding vessels of an AVM and to track possible changes in their perfusion territories after intervention. J. Magn. Reson. Imaging 2013;38:496–503. © 2013 Wiley Periodicals, Inc.     

Brain perfusion territory imaging: methods and clinical applications of selective arterial spin-labeling MR imaging

The ability to visualize perfusion territories in the brain is important for many clinical applications. The aim of this overview is to highlight the possibilities of selective arterial spin-labeling (ASL) magnetic resonance (MR) imaging techniques in the assessment of the perfusion territories of the cerebral arteries. In the past decade, the optimization of selective ASL MR techniques to image the cerebral perfusion territories has resulted in numerous labeling approaches and an increasing number of clinical applications. In this article, the methods and clinical applications of selective ASL MR imaging are described and the importance of perfusion territory information in studying cerebral hemodynamic changes in patients with cerebrovascular disease is shown. In specific patient groups with cerebrovascular disease, such as acute stroke, large artery steno-occlusive disease, and arteriovenous malformation, selective ASL MR imaging provides valuable hemodynamic information when added to current MR protocols. As a noninvasive tool for perfusion territory measurements, selective ASL may contribute to a better understanding of the relation between the vasculature, perfusion, and brain function.     

Continuous artery-selective spin labeling (CASSL).

A new technique for selective spin labeling of individual arteries is presented. It is based on continuous arterial spin labeling (CASL) with an amplitude-modulated control experiment. Precessionary motion of the labeling gradient about the axis of the artery, combined with an appropriate frequency modulation of the labeling RF pulse, restricts the adiabatic inversion to the desired artery. In phantom studies, it was found that the level of selectivity could be controlled by the sequence parameters, and that the achievable labeling efficiency was at a level of approximately 80% compared to a regular, nonselective CASL experiment. In a volunteer study we acquired high-quality images of the perfusion territories of the internal carotid artery (ICA), the basilar artery (BA), the middle cerebral artery (MCA), and both anterior cerebral arteries (ACAs). The results show the method's flexibility for different geometries and flow velocities. Potential applications include perfusion territory imaging of smaller cerebral arteries, and selective angiography techniques.     

Theoretical analysis of the effect of imperfect slice profiles on tagging schemes for pulsed arterial spin labeling MRI.

Pulsed arterial spin labeling (ASL) techniques provide a noninvasive method of obtaining qualitative and quantitative perfusion images with MRI. ASL techniques employ inversion recovery and/or saturation recovery to induce perfusion weighting, and thus the performance of these techniques is dependent on the slice profiles of the inversion or saturation pulses. This article systematically examines through simulations the effects of slice profile imperfections on the perfusion signal for nine labeling schemes, including FAIR, FAIRER, and EST (UNFAIR). Each sequence is evaluated for quantitative accuracy, suppression of stationary signal, and magnitude of perfusion signal. Perfusion effects are modeled from a modified Bloch equation and experimentally determined slice profiles. The results show that FAIR, FAIRER, and EST have excellent tissue suppression. The magnitude of the perfusion signal is comparable for FAIR and FAIRER, with EST providing a slightly weaker signal. For quantitative measurements, all three methods underestimate the perfusion signal by more than 20%. Of the additional six ASL techniques examined, only one performed well in this model. This method, which combines inversion and saturation recovery, yields improved signal accuracy (<15% difference from the theoretical value) and tissue suppression similar to that of FAIR and its variants, but has only half the signal. Magn Reson Med 46:141-148, 2001.     

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.     

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

Continuous labeling by flow‐driven adiabatic inversion is advantageous for arterial spin labeling (ASL) perfusion studies, but details of the implementation, including inefficiency, magnetization transfer, and limited support for continuous‐mode operation on clinical scanners, have restricted the benefits of this approach. Here a new approach to continuous labeling that employs rapidly repeated gradient and radio frequency (RF) pulses to achieve continuous labeling with high efficiency is characterized. The theoretical underpinnings, numerical simulations, and in vivo implementation of this pulsed continuous ASL (PCASL) method are described. In vivo PCASL labeling efficiency of 96% relative to continuous labeling with comparable labeling parameters far exceeded the 33% duty cycle of the PCASL RF pulses. Imaging at 3T with body coil transmission was readily achieved. This technique should help to realize the benefits of continuous labeling in clinical imagers. Magn Reson Med 60:1488–1497, 2008. © 2008 Wiley‐Liss, Inc.     

Pseudo‐continuous transfer insensitive labeling technique

Transfer insensitive labeling technique (TILT) was previously applied to acquire multislice cerebral blood flow maps as a pulsed arterial spin labeling (PASL) method. The magnetization transfer effect with TILT is well controlled by using concatenated radiofrequency pulses. However, use of TILT has been limited by several challenges, including slice profile errors, sensitivity to arterial transit time and intrinsic low signal‐to‐noise ratio (SNR). In this work, we propose to address these challenges by making the original TILT method into a novel pseudo‐continuous arterial spin labeling approach, named pseudo‐continuous transfer insensitive labeling technique (pTILT). pTILT improves perfusion acquisitions by (i) realizing pseudo‐continuous tagging with nonadiabatic pulses, (ii) being sensitive to slow flows in addition to fast flows, and (iii) providing flexible labeling geometries. Perfusion maps during both resting state and functional tasks are successfully demonstrated in healthy volunteers with pTILT. A comparison with typical SNR values from other perfusion techniques shows that although pTILT provides less SNR than inversion‐based pseudo‐continuous ASL techniques, the modified sequence provides similar SNR to inversion‐based PASL techniques. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Magnetic resonance elastography of liver in healthy asians: Normal liver stiffness quantification and reproducibility assessment

Arterial spin labeling (ASL) methods allow for quantitative mapping of tissue perfusion in absolute units, without the use of contrast agents. In this technique, the magnetization of arterial blood water is labeled by magnetic inversion or saturation, and the delivery of labeled blood water to tissues is observed. In this review three classes of labeling methods for ASL are described and compared: continuous, pulsed, and velocity‐selective. The quantification of perfusion from ASL data is discussed, and methods for the extraction of new types of information using ASL and related techniques, such as mapping of vascular territories or venous oxygenation, are described. J. Magn. Reson. Imaging 2014;40:1–10 . © 2014 Wiley Periodicals, Inc .     

Arterial spin labeling in combination with a look-locker sampling strategy: inflow turbo-sampling EPI-FAIR (ITS-FAIR).

Arterial spin labeling (ASL) permits quantification of tissue perfusion without the use of MR contrast agents. With standard ASL techniques such as flow-sensitive alternating inversion recovery (FAIR) the signal from arterial blood is measured at a fixed inversion delay after magnetic labeling. As no image information is sampled during this delay, FAIR measurements are inefficient and time-consuming. In this work the FAIR preparation was combined with a Look-Locker acquisition to sample not one but a series of images after each labeling pulse. This new method allows monitoring of the temporal dynamics of blood inflow. To quantify perfusion, a theoretical model for the signal dynamics during the Look-Locker readout was developed and applied. Also, the imaging parameters of the new ITS-FAIR technique were optimized using an expression for the variance of the calculated perfusion. For the given scanner hardware the parameters were: temporal resolution 100 ms, 23 images, flip-angle 25.4 degrees. In a normal volunteer experiment with these parameters an average perfusion value of 48.2 +/- 12.1 ml/100 g/min was measured in the brain. With the ability to obtain ITS-FAIR time series with high temporal resolution arterial transit times in the range of -138 - 1054 ms were measured, where nonphysical negative values were found in voxels containing large vessels.     

Arterial spin labeling perfusion MRI in pediatric arterial ischemic stroke: Initial experiences

Purpose

To investigate the feasibility and utility of arterial spin labeling (ASL) perfusion MRI for characterizing alterations of cerebral blood flow (CBF) in pediatric patients with arterial ischemic stroke (AIS).

Materials and Methods

Ten children with AIS were studied within 4 to 125 hours following symptom onset, using a pulsed ASL (PASL) protocol attached to clinically indicated MR examinations. The interhemisphere perfusion deficit (IHPD) was measured in predetermined vascular territories and infarct regions of restricted diffusion, which were compared with the degree of arterial stenosis and volumes of ischemic infarcts.

Results

Interpretable CBF maps were obtained in all 10 patients, showing simple lesion in nine patients (five hypoperfusion, two hyperperfusion, and two normal perfusion) and complex lesions in one patient. Both acute and follow‐up infarct volumes were significantly larger in cases with hypoperfusion than in either hyper‐ or normal perfusion cases. The IHPD was found to correlate with the degree of stenosis, diffusion lesion, and follow‐up T₂ infarct volumes. Mismatch between perfusion and diffusion lesions was observed. Brain regions presenting delayed arterial transit effects were tentatively associated with positive outcome.

Conclusion

This study demonstrates the clinical utility of ASL in the neuroimaging diagnosis of pediatric AIS. J. Magn. Reson. Imaging 2009;29:282–290. © 2009 Wiley‐Liss, Inc.     

Evaluation of systematic quantification errors in velocity-selective arterial spin labeling of the brain.

Velocity-selective (VS) sequences potentially permit arterial spin labeling (ASL) perfusion imaging with labeling applied very close to the tissue. In this study the effects of cerebrospinal fluid (CSF) motion, radiofrequency (RF) field imperfections, and sequence timing parameters on the appearance and quantitative perfusion values obtained with VS-ASL were evaluated. Large artifacts related to CSF motion were observed with moderate velocity weighting, which were removed by inversion recovery preparation at the cost of increased imaging time. Imperfect refocusing and excitation pulses resulting from nonuniform RF fields produced systematic errors in the ASL subtraction images. A phase cycling scheme was introduced to eliminate these errors. Quantitative perfusion images were obtained with CSF suppression and phase cycling. Gray matter blood flow of 27.7 ml 100 g(-1) min(-1), approximately half the value reported in studies using spatially-selective ASL, was measured. Potential sources for this underestimation are discussed.     

Minimizing acquisition time of arterial spin labeling at 3T.

An improved arterial spin labeling (ASL) perfusion technique that combines pseudo-continuous labeling and a T2*-insensitive sequence (GRASE) with background suppression was used to acquire perfusion maps in normal volunteers and stroke patients. It is shown that perfusion measurements obtained in less than 1 min of scan time are reproducible, with a coefficient of variation of 7%. The perfusion maps generated from these data can be used to characterize the stroke lesion.     

Application of variable‐rate selective excitation pulses for spin labeling in perfusion MRI

Arterial spin labeling offers great potential in clinical applications for noninvasive measurement of cerebral blood flow. Arterial spin labeling tagging methods such as the flow sensitive alternating inversion recovery technique require efficient spatial inversion pulses with high inversion accuracy and sharp transition zones between inverted and noninverted magnetization, i.e., require a high performance inversion pulse. This work presents a comprehensive comparison of the advantages offered by a variable‐rate selective excitation variant of the hyperbolic secant pulse against the widely used conventional hyperbolic secant pulse and the frequency offset corrected inversion pulses. Pulses were compared using simulation and experimental measurement in phantoms before being used in a flow sensitive alternating inversion recovery‐arterial spin labeling perfusion measurement in normal volunteers. Both the hyperbolic secant and frequency offset corrected inversion pulses have small variations in inversion profiles that may lead to unwanted subtraction errors in arterial spin labeling at a level where the residual signal is comparable to the desired perfusion contrast. The variable‐rate selective excitation pulse is shown to have improved inversion efficiency indicating its potential in perfusion MRI. The variable‐rate selective excitation pulse variant also showed greatest tolerance to radiofrequency variation and off‐resonance conditions, making it a robust choice for in vivo arterial spin labeling measurement. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Ventilation-perfusion ratio of signal intensity in human lung using oxygen-enhanced and arterial spin labeling techniques.

This study investigates the distribution of ventilation-perfusion (V/Q) signal intensity (SI) ratios using oxygen-enhanced and arterial spin labeling (ASL) techniques in the lungs of 10 healthy volunteers. Ventilation and perfusion images were simultaneously acquired using the flow-sensitive alternating inversion recovery (FAIR) method as volunteers alternately inhaled room air and 100% oxygen. Images of the T(1) distribution were calculated for five volunteers for both selective (T(1f)) and nonselective (T(1)) inversion. The average T(1) was 1360 ms +/- 116 ms, and the average T(1f) was 1012 ms +/- 112 ms, yielding a difference that is statistically significant (P < 0.002). Excluding large pulmonary vessels, the average V/Q SI ratios were 0.355 +/- 0.073 for the left lung and 0.371 +/- 0.093 for the right lung, which are in agreement with the theoretical V/Q SI ratio. Plots of the V/Q SI ratio are similar to the logarithmic normal distribution obtained by multiple inert gas elimination techniques, with a range of ratios matching ventilation and perfusion. This MRI V/Q technique is completely noninvasive and does not involve ionized radiation. A limitation of this method is the nonsimultaneous acquisition of perfusion and ventilation data, with oxygen administered only for the ventilation data.     

血管编码动脉自旋标记MR脑灌注成像初步研究

目的 初步应用血管编码动脉自旋标记MR脑灌注成像技术选择件标记双侧颈内动脉及后循环的血流分布区.方法 使用伪连续动脉自旋标记成像方法 对7名健康志愿者和6例脑血管病患者的左、右颈内动脉及椎基底动脉编码进行头部横断而成像和图像后处理,得到来源于上述不同血管的脑血流量(CBF)的灌注分布图,计算7名志愿者的双侧大脑厌、白质及半脑的CBF.比较脑血管病患者的血流分布结果 与DSA图像的一致性及低灌注区域与液体衰减反转恢复(FLAIR)T2WI的高信号区域大小.结果 定量测量正常志愿者的半CBF为(32.6±4.3)ml·min-1·100 g-1,脑白质血流最(10.8±0.9)ml·min-1·100 g-1,脑灰质血流量(55.6±2.9)ml·min-1·100 g-1.脑血管病患者的脑血流分布异常、侧支循环血流分布与DSA对应良好;所有患者低灌注区域比FLAIR T2WI显示的高信号区域范围更大.结论 血管编码动脉自旋标记MR脑灌注成像可以无创地定性并定量不同血管来源的脑血供.     

Efficient visualization of vascular territories in the human brain by cycled arterial spin labeling MRI.

Intracranial vascular territories are usually visualized with the use of angiographic techniques. However, because it is difficult to visualize the distal vascular bed with any angiographic technique, it is also difficult to infer the parenchymal borders of vascular territories. Arterial spin labeling (ASL) MRI provides information on cerebral perfusion, and a regional ASL (regASL) approach offers the potential to visualize perfusion of the anterior and posterior circulation separately. Current techniques perform the labeling in each feeding artery as a separate experiment, which is very time-consuming. In this work a very time-efficient regASL technique is presented that acquires all vascular territories within the same experiment. Images with different combinations of labeled arteries are combined to delineate the vascular territory of interest. Five subjects were examined with a clinical 1.5T MR scanner. A sharp delineation of the middle cerebral artery (MCA) and posterior cerebral artery (PCA) territories with whole-brain coverage was achieved in all subjects. The resulting signal-to-noise ratios (SNRs) for conventional regASL and the proposed cycled regASL were 9.3+/-2.4 and 15.3+/-5.1, respectively. An optimized setup was achieved by combining the regional labeling scheme with an efficient readout technique, which yielded a total measurement time of 2 min for three vascular territories.