Model-free arterial spin labeling quantification approach for perfusion MRI

Asymmetrical cerebral perfusion demonstrated by noninvasive arterial spin-labeling perfusion imaging in a patient with corticobasal degeneration

This is to report arterial spin-labeling (ASL) regional cerebral blood flow (rCBF) mapping findings in a case of corticobasal degeneration (CBD). A 68-year-old man gradually developed limb-kinetic apraxia, myoclonus, and rigidity in the left hand and was diagnosed as having CBD. Magnetic resonance imaging and single photon emission computed tomography revealed atrophy and decreased blood flow, respectively, in the right hemisphere, findings that were compatible with CBD. In addition, ASL rCBF mapping demonstrated asymmetrical hypoperfusion areas in locations typical for CBD, proving its potential usefulness for routine clinical differential diagnosis.

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

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

[Arterial spin labeling: state of the art]

Arterial spin labeling (ASL) perfusion MR imaging is a technique by which water from circulating arterial blood is magnetically labeled and acts as a diffusible tracer allowing non-invasive measurement of cerebral blood flow. In this paper, the technique and current applications in neuroimaging will be reviewed.

CURRENT STATUS

First, the technical principles of ASL will be reviewed and both available techniques (continuous and pulsed ASL) explained. A review of the literature will demonstrate advances with the techniques of ASL and its clinical impact. Clinical research involves normal volunteers and patients with ischemic and tumoral pathologies.

CONCLUSION

Recent technical advances have improved the sensitivity of ASL perfusion MR imaging. The routine clinical use of ASL at 3.0 Tesla should increase over the next few years.

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

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

Assessment of vascular supply of hypervascular extra-axial brain tumors with 3T MR regional perfusion imaging

BACKGROUND AND PURPOSE

The vascular supply of extra-axial brain tumors provided by the external carotid artery has not been studied with RPI. The purpose of this work was to determine whether RPI assessment is feasible and provides information on the vascular supply of hypervascular extra-axial brain tumors.

MATERIALS AND METHODS

Conventional ASL and RPI studies were performed at 3T in 8 consecutive patients with meningioma. On the basis of MRA results, we performed RPI by placing a selective labeling slab over the external carotid artery. Five patients underwent DSA before surgery. Two neuroradiologists independently evaluated the overall image quality, the degree of tumor perfusion, and the extent of the tumor vascular territory on conventional ASL and RPI.

RESULTS

In overall quality of conventional ASL and RPI, no images interfered with interpretation. In comparisons of the vascular tumor territory identified by the conventional ASL and RPI techniques, the territories coincided in 3 cases, were partially different in 4, and completely different in 1. The interobserver agreement was very good (kappa = 0.82). In 5 patients who underwent DSA, the 4 patients in whom the dominant supply was the external carotid artery were scored as coincided or partially different. The 1 patient in whom the vascular supply was from the internal carotid artery was scored as completely different.

CONCLUSIONS

RPI with selective labeling of the external carotid artery is feasible and may provide information about the vascular supply of hypervascular extra-axial brain tumors.

Absolute quantification of cerebral blood flow: correlation between dynamic susceptibility contrast MRI and model-free arterial spin labeling

PURPOSE

To compare absolute cerebral blood flow (CBF) estimates obtained by model-free arterial spin labeling (ASL) and dynamic susceptibility contrast MRI (DSC-MRI), corrected for partial volume effects (PVEs).

METHODS

CBF was measured using DSC-MRI and model-free ASL (quantitative signal targeting with alternating radiofrequency labeling of arterial regions) at 3 T in 15 subjects with brain tumor, and the two modalities were compared with regard to CBF estimates in normal gray matter (GM) and DSC-to-ASL CBF ratios in selected tumor regions. The DSC-MRI CBF maps were calculated using a global arterial input function (AIF) from the sylvian-fissure region, but, in order to minimize PVEs, the AIF time integral was rescaled by a venous output function time integral obtained from the sagittal sinus.

RESULTS

In GM, the average DSC-MRI CBF estimate was 150+/-45 ml/(min 100 g) (mean+/-SD) while the corresponding ASL CBF was 44+/-10 ml/(min 100 g). The linear correlation between GM CBF estimates obtained by DSC-MRI and ASL was r=.89, and observed DSC-to-ASL CBF ratios differed by less than 3% between GM and tumor regions.

CONCLUSIONS

A satisfactory positive linear correlation between the CBF estimates obtained by model-free ASL and DSC-MRI was observed, and DSC-to-ASL CBF ratios showed no obvious tissue dependence.

The QUASAR reproducibility study, Part II: Results from a multi-center Arterial Spin Labeling test-retest study

Arterial Spin Labeling (ASL) is a method to measure perfusion using magnetically labeled blood water as an endogenous tracer. Being fully non-invasive, this technique is attractive for longitudinal studies of cerebral blood flow in healthy and diseased individuals, or as a surrogate marker of metabolism. So far, ASL has been restricted mostly to specialist centers due to a generally low SNR of the method and potential issues with user-dependent analysis needed to obtain quantitative measurement of cerebral blood flow (CBF). Here, we evaluated a particular implementation of ASL (called Quantitative STAR labeling of Arterial Regions or QUASAR), a method providing user independent quantification of CBF in a large test-retest study across sites from around the world, dubbed "The QUASAR reproducibility study". Altogether, 28 sites located in Asia, Europe and North America participated and a total of 284 healthy volunteers were scanned. Minimal operator dependence was assured by using an automatic planning tool and its accuracy and potential usefulness in multi-center trials was evaluated as well. Accurate repositioning between sessions was achieved with the automatic planning tool showing mean displacements of 1.87+/-0.95 mm and rotations of 1.56+/-0.66 degrees . Mean gray matter CBF was 47.4+/-7.5 [ml/100 g/min] with a between-subject standard variation SD(b)=5.5 [ml/100 g/min] and a within-subject standard deviation SD(w)=4.7 [ml/100 g/min]. The corresponding repeatability was 13.0 [ml/100 g/min] and was found to be within the range of previous studies.

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

PURPOSE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Simultaneous measurement of arterial transit time, arterial blood volume, and cerebral blood flow using arterial spin-labeling in patients with Alzheimer disease

BACKGROUND AND PURPOSE

Cerebral hemodynamics abnormality in Alzheimer disease (AD) is not fully understood. Our aim was to determine whether regional hypoperfusion due to AD is associated with abnormalities in regional arterial blood volume (rABV) and regional arterial transit time (rATT) as measured by quantitative arterial spin-labeling (ASL) with multiple-delay time sampling.

MATERIALS AND METHODS

Nineteen patients with AD (9 men and 10 women; mean age, 74.5 +/- 8.6 years) and 22 cognitively healthy control subjects (11 men and 11 women; mean age, 72.8 +/- 6.8 years) were studied by using a quantitative ASL method with multiple-delay time sampling. From the ASL data, maps of regional cerebral blood flow (rCBF), rABV, and rATT were generated. A region of hypoperfusion due to AD was determined by statistical parametric mapping (SPM) analysis. Mean rCBF, rABV, and rATT values within the hypoperfused region were compared between the AD and control groups.

RESULTS

Despite the significantly lower rCBF (P = .0004) in patients with AD (27.8 +/- 7.1 mL/100 g/min) in comparison with control subjects (36.7 +/- 6.3 mL/100 g/min), no significant difference in rATT was observed between the control (0.48 +/- 0.09 seconds) and AD (0.47 +/- 0.10 seconds) groups. Mean rABV was lower in the AD group (0.22 +/- 0.10%) than in the control group (0.27 +/- 0.12%), though the difference did not reach the level of statistical significance.

CONCLUSIONS

Our results revealed that regional hypoperfusion in AD is not associated with rATT prolongation, suggesting that the mechanism of hypoperfusion is distinct from that in cerebrovascular diseases.

Relation between cerebral perfusion territories and location of cerebral infarcts

BACKGROUND AND PURPOSE

The perfusion territories of the brain-feeding arteries are difficult to assess in vivo and therefore standard cerebral perfusion territory templates are often used to determine the relation between cerebral infarcts and the feeding vasculature. In the present study, we compared this infarct classification, using standard templates, with the individualized depiction of cerebral perfusion territories on MRI.

METHODS

The ethics committee of our institution approved the study protocol. A total of 159 patients (92 male, 67 female; mean age, 58.9 years) with first-time clinical symptoms of cerebral ischemia were included in the study. Diffusion-weighted imaging was used for depiction of the area of ischemia and the perfusion territories of the left internal carotid artery, right internal carotid artery, and vertebrobasilar arteries were visualized with territorial arterial spin labeling MRI. Infarct locations with respect to cerebral perfusion territories were evaluated with and without territorial arterial spin labeling MRI images.

RESULTS

In 92% of the patients, the territorial arterial spin labeling images were of diagnostic quality. One hundred thirty-six patients showed areas of ischemia on diffusion-weighted images. The additional information from the territorial arterial spin labeling images changed the classification in 11% of the cortical or border zone infarcts (6 of 56), whereas no territorial changes were observed in lacunar, periventricular, cerebellar, and brainstem infarcts.

CONCLUSIONS

The diagnostic information provided by perfusion territory imaging in patients with stroke is valuable for the classification of cortical and border zone infarcts, whereas no change of the textbook-based classification was observed for other infarct types.

Territorial Arterial Spin Labeling in the Assessment of Collateral Circulation

Background and Purpose— Collateral circulation plays a vital role in patients with steno-occlusive disease, in particular for predicting stroke outcome. Digital subtraction angiography (DSA) is the gold standard for the assessment of collateral circulation, despite its invasive nature. Recently, the development of a new class of arterial spin labeling (ASL) methods allowed independent measurement of territorial flow information without the need for contrast media injection. Here, we compared combined territorial ASL (TASL) and MR angiography (MRA) against DSA in the assessment of collateral circulation.

Methods— Eighteen patients presenting with extra- or intracranial arterial steno-occlusive disease were recruited. All DSA studies were performed using a biplane angiography unit. MR imaging consisted of time-of-flight MRA and TASL, performed at 3T. Collateral circulation on both modalities was evaluated in consensus in a double-blinded manner by 3 neuroradiologists.

Results— Good agreement was found between DSA and TASL in the assessment of collateral flow: Cramer coefficient, V=0.53 ( P <0.0001) and Contingency coefficient, C=0.67, with kappa=0.70 and kappa=0.72 in the assessment of flow and collaterals, respectively. TASL and DSA successfully evaluated 89% and 98% of the vessels, respectfully. Failure was linked to motion-related artifacts in TASL, and highly tortuous vessels in DSA. Generally, combined MRA-TASL was comparable to DSA in diagnostic quality.

Conclusions— TASL provided radiological information comparable to DSA on collateral flow, with the advantage that it could be performed during routine MRI studies. TASL may provide insight on collateral perfusion in patients who may not otherwise be candidates for DSA, and may potentially replace it.

Modeling the effects of dispersion and pulsatility of blood flow in pulsed arterial spin labeling

Arterial spin labeling (ASL) is a method of using MRI to image cerebral perfusion. For the measurement to be calibrated, a model is required describing the kinetics of the flow of the inverted blood from the labeling region to the imaging region. It is common to assume plug-flow, but alternatives such as a Gaussian distribution of arrival times have also been suggested. In this study a physiologically based model for dispersion is developed and compared to existing models when fit to experimental data. The model is based on the assumption of parabolic flow in the major arteries, and also allows inclusion of cardiac pulsatility. It was found that fitting using the proposed model leads to higher perfusion estimates, with the difference becoming more pronounced in regions where the dispersion is greater. This suggests that current models may underestimate perfusion in these areas. However, fitting using the proposed model also leads to high uncertainties in parameter estimates due to non-orthogonality of the parameters. Effects due to pulsatility are expected to be observable, but when no cardiac-gating is used the mean curve over several cardiac cycles is predicted to closely match the curve which assumes constant flow.

Measuring the effects of remifentanil on cerebral blood flow and arterial arrival time using 3D GRASE MRI with pulsed arterial spin labelling

Arterial spin labelling (ASL) has proved to be a promising magnetic resonance imaging (MRI) technique to measure brain perfusion. In this study, volumetric three-dimensional (3D) gradient and spin echo (GRASE) ASL was used to produce cerebral blood flow (CBF) and arterial arrival time (AAT) maps during rest and during an infusion of remifentanil. Gradient and spin echo ASL perfusion-weighted images were collected at multiple inflow times (500 to 2,500 ms in increments of 250 ms) to accurately fit an ASL perfusion model. Fit estimates were assessed using z-statistics, allowing voxels with a poor fit to be excluded from subsequent analyses. Nonparametric permutation testing showed voxels with a significant difference in CBF and AAT between conditions across a group of healthy participants (N=10). Administration of remifentanil produced an increase in end-tidal CO(2), an increase in CBF from 57+/-12.0 to 77+/-18.4 mL/100 g tissue per min and a reduction in AAT from 0.73+/-0.073 to 0.64+/-0.076 sec. Within grey matter, remifentanil produced a cerebrovascular response of 5.7+/-1.60 %CBF per mm Hg. Significant differences between physiologic conditions were observed in both CBF and AAT maps, indicating that 3D GRASE-ASL has the sensitivity to study changes in physiology at a voxel level.

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

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

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

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

The BOLD response and vascular reactivity during visual stimulation in the presence of hypoxic hypoxia

A disproportionate increase in cerebral blood flow (CBF) relative to the cerebral metabolic rate of oxygen (CMRO(2)), in response to neuronal activation, results in a decreased oxygen extraction fraction (OEF) and hence local 'hyperoxygenation'. The mismatch is the key 'physiological substrate' for blood oxygenation level dependent (BOLD) fMRI. The mismatch may reflect inefficient O(2) diffusion in the brain tissue, a factor requiring maintenance of a steep [O(2)] gradient between capillary bed and neural cell mitochondria. The aim of this study was to assess vascular responsiveness to reduced blood oxygen saturation, using both BOLD fMRI and the CBV-weighted vascular space occupancy (VASO)-dependent fMRI technique, during visual activation in hypoxic hypoxia. Our fMRI results show decreased amplitude and absence of initial sharp overshoot in the BOLD response, while VASO signal was not influenced by decreasing oxygen saturation down to 0.85. The results suggest that the OEF during visual activation may be different in hypoxia relative to normoxia, due to a more efficient oxygen extraction under compromised oxygen availability. The data also indicate that vascular reactivity to brain activation is not affected by mild hypoxia.

Clinical neuroimaging using arterial spin-labeled perfusion magnetic resonance imaging

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

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.

Arterial Spin Labeling: Benefits and Pitfalls of High Magnetic Field

Arterial spin labeling (ASL) techniques are MR imaging methods designed to measure the endogenous perfusion signal coming from arterial blood by manipulation of its magnetization. These methods are based on the subtraction of two consecutively acquired images: one acquired after preparation of the arterial blood magnetization upstream to the area of interest, and the second without any manipulation of its arterial magnetization. The subtraction of both images provides information on the perfusion of the tissue present in the slice of interest. Because ASL is a very low SNR technique, the shift from 1.5 T to 3.0 T should be regarded as a great way to increase signal-to-noise ratio (SNR). Furthermore, the concomitant increase in blood T(1) should improve the SNR of ASL further. Other effects related to poorer magnetic filed homogeneities and reduced T(2) relaxation times, however, will counterbalance both effects partially. In this article, the pros and cons of the use of ASL at high field are summarized, after a brief description of the major techniques used and their theoretical limitations. Finally, a summary of the few existing dedicated ASL perfusion techniques available are presented.