Brain Perfusion Territory Imaging: Methods and Clinical Applications of Selective Arterial Spin-labeling MR Imaging

Arterial Spin Labeling (ASL) fMRI: advantages, theoretical constrains, and experimental challenges in neurosciences

Cerebral blood flow (CBF) is a well-established correlate of brain function and therefore an essential parameter for studying the brain at both normal and diseased states. Arterial spin labeling (ASL) is a noninvasive fMRI technique that uses arterial water as an endogenous tracer to measure CBF. ASL provides reliable absolute quantification of CBF with higher spatial and temporal resolution than other techniques. And yet, the routine application of ASL has been somewhat limited. In this review, we start by highlighting theoretical complexities and technical challenges of ASL fMRI for basic and clinical research. While underscoring the main advantages of ASL versus other techniques such as BOLD, we also expound on inherent challenges and confounds in ASL perfusion imaging. In closing, we expound on several exciting developments in the field that we believe will make ASL reach its full potential in neuroscience research.

Robustness and reproducibility of flow territories defined by planning-free vessel-encoded pseudocontinuous arterial spin-labeling

Flow-territory mapping by MR imaging ASL noninvasively provides a unique insight into the distribution of cerebral perfusion. The introduction of planning-free vessel-encoded pCASL made flow-territory mapping feasible for clinical use, though lack of individual planning could impede reproducibility of this technique. We assessed the reproducibility of planning-free vessel-encoded pCASL in patients and controls. Results indicated that planning-free vessel-encoded pCASL is a reproducible method that could assist in clinical decision-making.

Dynamic alteration of regional cerebral blood flow during carotid compression and proof of reversibility

Background

It is difficult to non-invasively visualize changes in regional cerebral blood flow caused by manual compression of the carotid artery.

Purpose

To visualize dynamic changes in regional cerebral blood flow during and after manual compression of the carotid artery.

Material and Methods

Two healthy volunteers were recruited. Anatomic features and flow directions in the circle of Willis were evaluated with time-of-flight magnetic resonance angiography (MRA) and two-dimensional phase-contrast (2DPC) MRA, respectively. Regional cerebral blood flow was visualized with territorial arterial spin-labeling magnetic resonance imaging (TASL-MRI). TASL-MRI and 2DPC-MRA were performed in three states: at rest, during manual compression of the right carotid artery, and after decompression. In one volunteer, time-space labeling inversion pulse (Time-SLIP) MRA was performed to confirm collateral flow.

Results

During manual carotid compression, in one volunteer, the right thalamus changed to be fed only by the vertebrobasilar system, and the right basal ganglia changed to be fed by the left internal carotid artery. In the other volunteer, the right basal ganglia changed to be fed by the vertebrobasilar system. 2DPC-MRA showed that the flow direction changed in the right A1 segment of the anterior cerebral artery and the right posterior communicating artery. Perfusion patterns and flow directions recovered after decompression. Time-SLIP MRA showed pial vessels and dural collateral circulation when the right carotid artery was manually compressed.

Conclusion

Use of TASL-MRI and 2DPC-MRA was successful for non-invasive visualization of the dynamic changes in regional cerebral blood flow during and after manual carotid compression.

Vascular Pathology as a Potential Therapeutic Target in SCI

Acute traumatic spinal cord injury (SCI) is characterized by a progressive secondary degeneration which exacerbates the loss of penumbral tissue and neurological function. Here, we first provide an overview of the known pathophysiological mechanisms involving injured microvasculature and molecular regulators that contribute to the loss and dysfunction of existing and new blood vessels. We also highlight the differences between traumatic and ischemic injuries which may yield clues as to the more devastating nature of traumatic injuries, possibly involving toxicity associated with hemorrhage. We also discuss known species differences with implications for choosing models, their relevance and utility to translate new treatments towards the clinic. Throughout this review, we highlight the potential opportunities and proof-of-concept experimental studies for targeting therapies to endothelial cell-specific responses. Lastly, we comment on the need for vascular mechanisms to be included in drug development and non-invasive diagnostics such as serum and cerebrospinal fluid biomarkers and imaging of spinal cord pathology.

Arterial spin labeling measurements of cerebral perfusion territories in experimental ischemic stroke

Collateral circulation, defined as the supplementary vascular network that maintains cerebral blood flow (CBF) when the main vessels fail, constitutes one important defense mechanism of the brain against ischemic stroke. In the present study, continuous arterial spin labeling (CASL) was used to quantify CBF and obtain perfusion territory maps of the major cerebral arteries in spontaneously hypertensive rats (SHRs) and their normotensive Wistar-Kyoto (WKY) controls. Results show that both WKY and SHR have complementary, yet significantly asymmetric perfusion territories. Right or left dominances were observed in territories of the anterior (ACA), middle and posterior cerebral arteries, and the thalamic artery. Magnetic resonance angiography showed that some of the asymmetries were correlated with variations of the ACA. The leptomeningeal circulation perfusing the outer layers of the cortex was observed as well. Significant and permanent changes in perfusion territories were obtained after temporary occlusion of the right middle cerebral artery in both SHR and WKY, regardless of their particular dominance. However, animals with right dominance presented a larger volume change of the left perfusion territory (23 ± 9%) than animals with left dominance (7 ± 5%, P < 0.002). The data suggest that animals with contralesional dominance primarily safeguard local CBF values with small changes in contralesional perfusion territory, while animals with ipsilesional dominance show a reversal of dominance and a substantial increase in contralesional perfusion territory. These findings show the usefulness of CASL to probe the collateral circulation.

New insights in perinatal arterial ischemic stroke by assessing brain perfusion

Perinatal arterial ischemic stroke (AIS) is an important cause of long-term morbidity in children. Thus, there is an urgent need to better understand the mechanisms of stroke in newborns in order to develop effective treatment and prevention strategies. The purpose of this study was to assess brain perfusion within the first month of life in newborns with AIS. In this study, magnetic resonance imaging (MRI) and perfusion imaging by arterial spin labeling (ASL) were used to assess brain perfusion in four term newborns with AIS. One patient had a stroke within the territory of the right middle cerebral artery (MCA); the other three patients had a stroke within the territory of the left MCA. None of them displayed any hemorrhagic component. All four patients demonstrated abnormal brain perfusion in the stroke area. Cerebral blood flow (CBF) within the stroke area was increased in patient # 1. In all other three patients, CBF was decreased within the stroke center and increased in the periphery of the stroke area. These results show the feasibility of the ASL sequence in newborns with AIS and support its addition to the current MRI protocol used in these newborns as it provides useful information on brain hemodynamics. Its value for identifying salvageable tissue in newborns needs to be further assessed, as well as its potential role in stroke follow-up and for tissue-specific treatment screening.

Perspectives on optimizing trial design and endpoints in peripheral arterial disease: a case for imaging-based surrogates as endpoints of functional efficacy

Surrogate endpoints are important for validation of mechanism, early proof of concept, and the rational design of clinical trials for regulatory approval of drugs. The recent failure of several drugs in peripheral arterial disease (PAD) and in atherosclerosis highlights the importance of understanding drug effect and is a clarion call for better endpoints. This review focuses on aspects relating to the current state of surrogate endpoints in PAD and reviews emerging endpoints using imaging approaches that may have the potential of improving study design in PAD.

Distribution of cerebral blood flow in the caudate nucleus, lentiform nucleus and thalamus in patients with carotid artery stenosis

OBJECTIVE

To investigate the influence of internal carotid artery (ICA) stenosis on the distribution of blood flow to the caudate nucleus, lentiform nucleus, and thalamus.

METHODS

We studied 18 healthy control subjects, 20 patients with a unilateral asymptomatic ICA stenosis, and 15 patients with a recently symptomatic unilateral ICA stenosis. The contribution of the ICAs and the basilar artery to the perfusion of the deep brain structures was assessed by perfusion territory selective arterial spin labeling (ASL) MRI. Differences were tested with a two-tailed Fishers' exact test.

RESULTS

The caudate nucleus was predominantly supplied with blood by the ipsilateral ICA in all groups. In 4 of the 15 (27%) the symptomatic patients, the caudate nucleus partially received blood from the contralateral ICA, compared to none of the 18 healthy control subjects (p = 0.03). The lentiform nucleus and the thalamus were predominantly supplied with blood by the ipsilateral ICA and basilar artery respectively in all groups.

CONCLUSION

In patients with a symptomatic ICA stenosis, the caudate nucleus may be supplied with blood by the contralateral ICA more often than in healthy controls.

Modified pulsed continuous arterial spin labeling for labeling of a single artery

Imaging the contribution of different arterial vessels to the blood supply of the brain can potentially guide the treatment of vascular disease and other disorders. Previously available only with catheter angiography, vessel-selective labeling of arteries has now been demonstrated with pulsed and continuous arterial spin labeling methods. Pulsed continuous labeling, which permits continuous labeling on standard scanner radiofrequency hardware, has been used to encode the contribution of different vessels to the blood supply of the brain. Vessel encoding requires a longer scan and a more complex reconstruction algorithm and may be more sensitive to fluctuations in flow, however. Here a method is presented for single-artery selective labeling, in which a disk around the targeted vessel is labeled. Based on pulsed continuous labeling, this method is achieved by rotating the directions of added in-plane gradients. Numerical simulations of the simplest strategy show good efficiency but poor suppression of labeling at large distances from the target vessel. Amplitude modulation of the rotating in-plane gradients results in better suppression of distant vessels. In vivo results demonstrate highly selective labeling of individual vessels and a rapid falloff of the labeling with distance from the center of the labeling disk, in agreement with the simulations.

Whole animal imaging

Translational research plays a vital role in understanding the underlying pathophysiology of human diseases, and hence development of new diagnostic and therapeutic options for their management. After creating an animal disease model, pathophysiologic changes and effects of a therapeutic intervention on them are often evaluated on the animals using immunohistologic or imaging techniques. In contrast to the immunohistologic techniques, the imaging techniques are noninvasive and hence can be used to investigate the whole animal, oftentimes in a single exam which provides opportunities to perform longitudinal studies and dynamic imaging of the same subject, and hence minimizes the experimental variability, requirement for the number of animals, and the time to perform a given experiment. Whole animal imaging can be performed by a number of techniques including x-ray computed tomography, magnetic resonance imaging, ultrasound imaging, positron emission tomography, single photon emission computed tomography, fluorescence imaging, and bioluminescence imaging, among others. Individual imaging techniques provide different kinds of information regarding the structure, metabolism, and physiology of the animal. Each technique has its own strengths and weaknesses, and none serves every purpose of image acquisition from all regions of an animal. In this review, a broad overview of basic principles, available contrast mechanisms, applications, challenges, and future prospects of many imaging techniques employed for whole animal imaging is provided. Our main goal is to briefly describe the current state of art to researchers and advanced students with a strong background in the field of animal research.

Functional brain imaging in the clinical assessment of consciousness

Recent findings suggest that functional brain imaging might be used to identify consciousness in patients diagnosed with persistent vegetative state and minimally conscious state. Michael Rafii and James Brewer discuss the potential for fMRI's wider implementation in clinical practice, and associated caveats.

Arterial spin labeling in neuroimaging

Arterial spin labeling (ASL) is a magnetic resonance imaging technique for measuring tissue perfusion using a freely diffusible intrinsic tracer. As compared with other perfusion techniques, ASL offers several advantages and is now available for routine clinical practice in many institutions. Its noninvasive nature and ability to quantitatively measure tissue perfusion make ASL ideal for research and clinical studies. Recent technical advances have increased its sensitivity and also extended its potential applications. This review focuses on some basic knowledge of ASL perfusion, emerging techniques and clinical applications in neuroimaging.

Comparison of selective arterial spin labeling using 1D and 2D tagging RF pulses

Generic arterial spin labeling (ASL) techniques label all brain feeding arteries. In this work, we used two different selective ASL (SASL) methods to show the perfusion of one single artery. A slice selective inversion of an area including the desired vessel was compared to a multidimensional RF pulse with Gaussian profile to label only the artery of interest. Perfusion images with a resolution of 2 x 2 x 5 mm(3) are shown that were acquired after tagging only the internal carotid artery of healthy volunteers. In addition, both techniques were applied to a patient with an extra-intracranial bypass to illustrate its perfusion territory. These perfusion images are consistent with a standard angiography. SASL imaging with a resolution of 2 x 2 x 5 mm(3) is possible in a total scan time of 5 min. The presented MR techniques may in part replace the assessment of revascularization success by conventional angiography.

Continuous flow Overhauser dynamic nuclear polarization of water in the fringe field of a clinical magnetic resonance imaging system for authentic image contrast

We describe and demonstrate a system to generate hyperpolarized water in the 0.35 T fringe field of a clinical 1.5 T whole-body magnetic resonance imaging (MRI) magnet. Once generated, the hyperpolarized water is quickly and continuously transferred from the 0.35 T fringe to the 1.5 T center field of the same magnet for image acquisition using standard MRI equipment. The hyperpolarization is based on Overhauser dynamic nuclear polarization (DNP), which effectively and quickly transfers the higher spin polarization of free radicals to nuclear spins at ambient temperatures. We visualize the dispersion of hyperpolarized water as it flows through water-saturated systems by utilizing an observed -15-fold DNP signal enhancement with respect to the unenhanced (1)H MRI signal of water at 1.5 T. The experimental DNP apparatus presented here is readily portable and can be brought to and used with any conventional unshielded MRI system. A new method of immobilizing radicals to gel beads via polyelectrolyte linker arms is described, which led to superior flow Overhauser DNP performance compared to previously presented gels. We discuss the general applicability of Overhauser DNP of water and aqueous solutions in the fringe field of commercially available magnets with central fields up to 4.7 T.

Improved arterial spin labeling after myocardial infarction in mice using cardiac and respiratory gated look-locker imaging with fuzzy C-means clustering

Experimental myocardial infarction (MI) in mice is an important disease model, in part due to the ability to study genetic manipulations. MRI has been used to assess cardiac structural and functional changes after MI in mice, but changes in myocardial perfusion after acute MI have not previously been examined. Arterial spin labeling noninvasively measures perfusion but is sensitive to respiratory motion and heart rate variability and is difficult to apply after acute MI in mice. To account for these factors, a cardiorespiratory-gated arterial spin labeling sequence using a fuzzy C-means algorithm to retrospectively reconstruct images was developed. Using this method, myocardial perfusion was measured in remote and infarcted regions at 1, 7, 14, and 28 days post-MI. Baseline perfusion was 4.9 +/- 0.5 mL/g min and 1 day post-MI decreased to 0.9 +/- 0.8 mL/g min in infarcted myocardium (P < 0.05 versus baseline) while remaining at 5.2 +/- 0.8 mL/g min in remote myocardium. During the subsequent 28 days, perfusion in the remote zone remained unchanged, while a partial recovery of perfusion in the infarct zone was seen. This technique, when applied to genetically engineered mice, will allow for the investigation of the roles of specific genes in myocardial perfusion during infarct healing.

Review: Functional Neuroimaging Studies of Early Upper Limb Recovery After Stroke: A Systematic Review of the Literature

Background. Understanding mechanisms of recovery may result in new treatment strategies to improve motor outcome after stroke. Imaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) allow changes in brain activity after stroke recovery to be identified. Objective. To systematically review serial imaging studies on recovery within 6 months poststroke, assess their methodological quality, and identify trends in the association between task-related brain activation patterns and functional upper limb recovery. Methods. A literature search was performed using Medline, PICARTA, and EMBASE databases. Studies were appraised using binary weighted methodological criteria for internal, statistical, and external validity. Results. Twenty-two of the 869 identified studies met the inclusion criteria. Studies showed methodological weaknesses with respect to controlling for task performance, selecting appropriate outcome measures, and adequate presentation and execution of statistical analysis. After stroke, motor task performance shows unilateral overactivation of motor and nonmotor areas, a posterior shift in activity in the primary motor cortex, and bilateral recruitment of associated motor and nonmotor areas. Concomitant with neurological recovery, overactivation appears to diminish longitudinally, but not in all patients. Conclusion . Despite methodological shortcomings and heterogeneity, trends can be discerned. However, statistically sound associations with recovery are not consistent. The challenges in future research will be, controlling for confounding factors, finding outcomes that specifically measure dexterity of the paretic limb, to control for the extent of white matter damage and changes in perfusion in order to establish the longitudinal construct validity of fMRI and PET with regard to upper limb recovery after stroke.

Use of magnetic resonance imaging to predict outcome after stroke: a review of experimental and clinical evidence

Despite promising results in preclinical stroke research, translation of experimental data into clinical therapy has been difficult. One reason is the heterogeneity of the disease with outcomes ranging from complete recovery to continued decline. A successful treatment in one situation may be ineffective, or even harmful, in another. To overcome this, treatment must be tailored according to the individual based on identification of the risk of damage and estimation of potential recovery. Neuroimaging, particularly magnetic resonance imaging (MRI), could be the tool for a rapid comprehensive assessment in acute stroke with the potential to guide treatment decisions for a better clinical outcome. This review describes current MRI techniques used to characterize stroke in a preclinical research setting, as well as in the clinic. Furthermore, we will discuss current developments and the future potential of neuroimaging for stroke outcome prediction.

Perfusion in rat brain at 7 T with arterial spin labeling using FAIR-TrueFISP and QUIPSS

Measurement of perfusion in longitudinal studies allows for the assessment of tissue integrity and the detection of subtle pathologies. In this work, the feasibility of measuring brain perfusion in rats with high spatial resolution using arterial spin labeling is reported. A flow-sensitive alternating recovery sequence, coupled with a balanced gradient fast imaging with steady-state precession readout section was used to minimize ghosting and geometric distortions, while achieving high signal-to-noise ratio. The quantitative imaging of perfusion using a single subtraction method was implemented to address the effects of variable transit delays between the labeling of spins and their arrival at the imaging slice. Studies in six rats at 7 T showed good perfusion contrast with minimal geometric distortion. The measured blood flow values of 152.5+/-6.3 ml/100 g per minute in gray matter and 72.3+/-14.0 ml/100 g per minute in white matter are in good agreement with previously reported values based on autoradiography, considered to be the gold standard.

[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.

Depressive disorders: focally altered cerebral perfusion measured with arterial spin-labeling MR imaging

PURPOSE

To assess focal cerebral perfusion in patients with refractory depressive disorder (RDD), patients with nonrefractory depressive disorder (NDD), and healthy control subjects by using arterial spin-labeling (ASL) magnetic resonance (MR) imaging.

MATERIALS AND METHODS

This study was approved by the local ethical committee, and written informed consent was obtained from all participants. Twenty-four patients with RDD, 37 patients with NDD, and 42 healthy control subjects were included. From February 2006 to July 2007, all participants were imaged with a 3-T MR system. ASL and echo-planar images were subtracted and averaged to give perfusion-weighted images. Voxel-based analysis was performed. Region-of-interest analysis was applied to the bilateral hippocampi, thalami, and lentiform nuclei.

RESULTS

Patients with NDD showed reduced perfusion in the left prefrontal cortex versus control subjects and increased perfusion mainly in the limbic-striatal areas (P < .05). In contrast, patients with RDD had decreased perfusion predominantly in the bilateral frontal and bilateral thalamic regions (P < .05). Compared with patients with RDD, patients with NDD showed higher perfusion mainly in the limbic-striatal areas (P < .05). In region-of-interest analysis, the NDD group showed higher regional cerebral blood flow than both RDD and control groups in the left hippocampus (P = .045), right hippocampus (P = .001), and right lentiform nucleus (P = .049).

CONCLUSION

This study revealed alterations of regional perfusion in the brains of patients with RDD that differed from those in patients with NDD. These results are consistent with the concept that RDD is associated with decreased activity of the bilateral prefrontal areas; and NDD, with decreased activity of left frontal areas in conjunction with overactivity of the bilateral limbic system.

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.

Advances in magnetic resonance neuroimaging

Interest in advanced neuroimaging is growing and is certain to continue; new and faster sequences, better image quality, higher magnetic fields, and improved models of diffusion, perfusion, and functional connectivity are in constant development. The purpose of this article is to highlight recent advances in neuroimaging from two aspects: (1) those advances directly benefited by increases in field strength (increased T1, signal-to-noise ratio, magnetic susceptibility-sensitivity, and chemical shift) and how the increased signal-to-noise ratio can be used to trade off for other advantages and (2) those advances made in response to attempts to try to reduce the inherent artifacts encountered at higher field strengths (eg, reducing specific radiofrequency absorption in tissue and magnetic susceptibility).

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.