Vessel-selective, non-contrast enhanced, time-resolved MR angiography with vessel-selective arterial spin labeling technique (CINEMA–SELECT) in intracranial arteries

Arterial Spin Labeling-Based MR Angiography for Cerebrovascular Diseases: Principles and Clinical Applications

Arterial spin labeling (ASL) is a noninvasive imaging technique that labels the proton spins in arterial blood and uses them as endogenous tracers. Brain perfusion imaging with ASL is becoming increasingly common in clinical practice, and clinical applications of ASL for intracranial magnetic resonance angiography (MRA) have also been demonstrated. Unlike computed tomography (CT) angiography and cerebral angiography, ASL-based MRA does not require contrast agents. ASL-based MRA overcomes most of the disadvantages of time-of-flight (TOF) MRA. Several schemes have been developed for ASL-based MRA; the most common method has been pulsed ASL, but more recently pseudo-continuous ASL, which provides a higher signal-to-noise ratio (SNR), has been used more frequently. New methods that have been developed include direct intracranial labeling methods such as velocity-selective ASL and acceleration-selective ASL. MRA using an extremely short echo time (eg, silent MRA) or ultrashort echo-time (TE) MRA can suppress metal susceptibility artifacts and is ideal for patients with a metallic device implanted in a cerebral vessel. Vessel-selective 4D ASL MRA can provide digital subtraction angiography (DSA)-like images. This review highlights the principles, clinical applications, and characteristics of various ASL-based MRA techniques. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY: Stage 2.

Development and practical evaluation of a saturation effect learning simulator for inflow magnetic resonance angiography

The quality of visualization in inflow magnetic resonance angiography (MRA) depends highly on the excitation state of the longitudinal magnetization obtained using specified imaging parameters. In addition, signal intensity changes controlled by the preparation pulse-such as inversion recovery (IR) and saturation recovery (SR)-can potentially be used as quantitative physiological values. Although having practitioners understand these relationships both qualitatively and quantitatively is important, handling clinical equipment in practical learning or experiments involves limited opportunities. The simulator corresponds to a three-dimensional spoiled gradient echo sequence and allows users to freely input multiple virtual excitation effects in space and time. The purpose of this study was to quantitatively evaluate the agreement between the measured MRAs obtained in flow phantom tests and virtual MRAs simulated under similar conditions. We imaged two vascular flow phantoms on a 3.0 T MR system using three-dimensional (3D) time-of-flight (TOF) MRA and 3D inversion recovery tissue signal suppression (IR-suppression) MRA protocols. We evaluated quantitative values for consistency between the measured and virtual MRAs images with matched spatial resolution. Then we assessed the coincidence by reformatting maximum-intensity projection images with 1 mm isotropic pixels, with it ranging from 89.6 to 92.0% and 89.1 to 92.9% for TOF MRA and IR-suppression MRA, respectively. These results may be useful as a reference index for the theoretical study of MRA images by practitioners, for complementary validation by phantom testing, or for the development of MRI-related simulators.

Pseudocontinuous Arterial Spin Labeling: Clinical Applications and Usefulness in Head and Neck Entities

Simple Summary

Conventional imaging methods, such as ultrasonography, computed tomography, and magnetic resonance imaging may be inadequate to accurately diagnose lesions of the head and neck because they vary widely. Recently, the arterial spin labeling technique, especially pseudocontinuous arterial spin labeling (pCASL) with the three-dimensional (3D) readout method, has been dramatically developed to improve diagnostic performance for lesion differentiation, which can show prominent blood flow characteristics. Here, we demonstrate the clinical usefulness of 3D pCASL for diagnosing various entities, including inflammatory lesions, hypervascular lesions, and neoplasms in the head and neck, for evaluating squamous cell carcinoma (SCC) treatment responses, and for predicting SCC prognosis.

Abstract

As functional magnetic resonance imaging, arterial spin labeling (ASL) techniques have been developed to provide quantitative tissue blood flow measurements, which can improve the performance of lesion diagnosis. ASL does not require contrast agents, thus, it can be applied to a variety of patients regardless of renal impairments and contrast agent allergic reactions. The clinical implementation of head and neck lesions is limited, although, in recent years, ASL has been increasingly utilized in brain lesions. Here, we review the development of the ASL techniques, including pseudocontinuous ASL (pCASL). We compare readout methods between three-dimensional (3D) turbo spin-echo and 2D echo planar pCASL for the clinical applications of pCASL to head and neck lesions. We demonstrate the clinical usefulness of 3D pCASL for diagnosing various entities, including inflammatory lesions, hypervascular lesions, and neoplasms; for evaluating squamous cell carcinoma (SCC) treatment responses, and for predicting SCC prognosis.

Optimization of 4D-MR angiography based on superselective pseudo-continuous arterial spin labeling combined with CENTRA-keyhole and view-sharing (4D-S-PACK) for vessel-selective visualization of the internal carotid artery and vertebrobasilar artery systems

PURPOSE

This study investigated the optimal labeling position and gradient moment for 4D-MR angiography based on superselective pseudo-continuous arterial spin labeling combined with CENTRA-keyhole and view-sharing (4D-S-PACK) for vessel-selective flow visualization of the internal carotid artery (ICA) and vertebrobasilar artery (VBA) systems.

METHODS

Seven healthy volunteers were scanned with a 3.0 T MR scanner. To visualize the ICA system, the labeling focus was placed in the right ICA at 55, 75 and 95 mm below the imaging slab. To visualize the VBA system, the labeling focus was placed in the basilar artery (BA), upper vertebral artery (VA upper), and lower vertebral artery (VA lower). Two sizes of labeling focus were created using gradient moments of 0.5 and 0.75 mT/m ms. The contrast-to-noise ratio (CNR) was measured in the middle cerebral artery (MCA) and posterior cerebral artery (PCA) branches.

RESULTS

CNRs increased as the distance between the center of the imaging slab and the labeling position decreased in all MCA segments. CNRs obtained with VA lower tended to be higher than those obtained with BA and VA upper in all PCA segments. Selective vessel visualization was achieved with the gradient moment of 0.75 mT/m ms for the ICA and VBA system.

CONCLUSION

The optimal 4D-S-PACK gradient moment was found to be 0.75 mT/m ms for the ICA and VBA systems. When visualizing the ICA system, the labeling position should be placed as close as possible to the imaging slab. When visualizing the VBA system, the labeling position should be placed at VA lower .

Intracranial 3D and 4D MR Angiography Using Arterial Spin Labeling: Technical Considerations

In the 1980’s some of the earliest studies of arterial spin labeling (ASL) MRI have demonstrated its ability to generate MR angiography (MRA) images. Thanks to many technical improvements, ASL has been successfully moving its position from the realm of research into the clinical area, albeit more known as perfusion imaging than as MRA. For MRA imaging, other techniques such as time-of-flight, phase contrast MRA and contrast-enhanced (CE) MRA are more popular choices for clinical applications. In the last decade, however, ASL-MRA has been experiencing a remarkable revival, especially because of its non-invasive nature, i.e. the fact that it does not rely on the use of contrast agent. Very importantly, there are additional benefits of using ASL for MRA. For example, its higher flexibility to achieve both high spatial and temporal resolution than CE dynamic MRA, and the capability of vessel specific visualization, in which the vascular tree arising from a selected artery can be exclusively visualized. In this article, the implementation and recent developments of ASL-based MRA are discussed; not only focusing on the basic sequences based upon pulsed ASL or pseudo-continuous ASL, but also including more recent labeling approaches, such as vessel-selective labeling, velocity-selective ASL, vessel-encoded ASL and time-encoded ASL. Although these ASL techniques have been already utilized in perfusion imaging and their usefulness has been suggested by many studies, some additional considerations should be made when employing them for MRA, since there is something more than the difference of the spatial resolution of the readout sequence. Moreover, extensive discussion is included on what readout sequence to use, especially by highlighting how to achieve high spatial resolution while keeping scan-time reasonable such that the ASL-MRA sequence can easily be included into a clinical examination.

The advantages of radial trajectories for vessel-selective dynamic angiography with arterial spin labeling

OBJECTIVES

To demonstrate the advantages of radial k-space trajectories over conventional Cartesian approaches for accelerating the acquisition of vessel-selective arterial spin labeling (ASL) dynamic angiograms, which are conventionally time consuming to acquire.

MATERIALS AND METHODS

Vessel-encoded pseudocontinuous ASL was combined with time-resolved balanced steady-state free precession (bSSFP) and spoiled gradient echo (SPGR) readouts to obtain dynamic vessel-selective angiograms arising from the four main brain-feeding arteries. Dynamic 2D protocols with acquisition times of one minute or less were achieved through radial undersampling or a Cartesian parallel imaging approach. For whole-brain dynamic 3D imaging, magnetic field inhomogeneity and the high acceleration factors required rule out the use of bSSFP and Cartesian trajectories, so the feasibility of acquiring 3D radial SPGR angiograms was tested.

RESULTS

The improved SNR efficiency of bSSFP over SPGR was confirmed for 2D dynamic imaging. Radial trajectories had considerable advantages over a Cartesian approach, including a factor of two improvements in the measured SNR (p < 0.00001, N = 6), improved distal vessel delineation and the lack of a need for calibration data. The 3D radial approach produced good quality angiograms with negligible artifacts despite the high acceleration factor (R = 13).

CONCLUSION

Radial trajectories outperform conventional Cartesian techniques for accelerated vessel-selective ASL dynamic angiography.

Recent Advances in Pediatric Brain, Spine, and Neuromuscular Magnetic Resonance Imaging Techniques

Magnetic resonance imaging (MRI) is a powerful radiologic tool with the ability to generate a variety of proton-based signal contrast from tissues. Owing to this immense flexibility in signal generation, new MRI techniques are constantly being developed, tested, and optimized for clinical utility. In addition, the safe and nonionizing nature of MRI makes it a suitable modality for imaging in children. In this review article, we summarize a few of the most popular advances in MRI techniques in recent years. In particular, we highlight how these new developments have affected brain, spine, and neuromuscular imaging and focus on their applications in pediatric patients. In the first part of the review, we discuss new approaches such as multiphase and multidelay arterial spin labeling for quantitative perfusion and angiography of the brain, amide proton transfer MRI of the brain, MRI of brachial plexus and lumbar plexus nerves (i.e., neurography), and T2 mapping and fat characterization in neuromuscular diseases. In the second part of the review, we focus on describing new data acquisition strategies in accelerated MRI aimed collectively at reducing the scan time, including simultaneous multislice imaging, compressed sensing, synthetic MRI, and magnetic resonance fingerprinting. In discussing the aforementioned, the review also summarizes the advantages and disadvantages of each method and their current state of commercial availability from MRI vendors.

Arterial spin labeling for the measurement of cerebral perfusion and angiography

Arterial spin labeling (ASL) is an MRI technique that was first proposed a quarter of a century ago. It offers the prospect of non-invasive quantitative measurement of cerebral perfusion, making it potentially very useful for research and clinical studies, particularly where multiple longitudinal measurements are required. However, it has suffered from a number of challenges, including a relatively low signal-to-noise ratio, and a confusing number of sequence variants, thus hindering its clinical uptake. Recently, however, there has been a consensus adoption of an accepted acquisition and analysis framework for ASL, and thus a better penetration onto clinical MRI scanners. Here, we review the basic concepts in ASL and describe the current state-of-the-art acquisition and analysis approaches, and the versatility of the method to perform both quantitative cerebral perfusion measurement, along with quantitative cerebral angiographic measurement.

Selective arterial spin labeling in conjunction with phase-contrast acquisition for the simultaneous visualization of morphology, flow direction, and velocity of individual arteries in the cerebrovascular system

PURPOSE

In various cerebrovascular diseases the visualization of individual arteries and knowledge about their hemodynamic properties, like flow velocity and direction, can become important for an accurate diagnosis. Magnetic resonance angiography methods are intended to acquire this information, but often a single acquisition is not sufficient to retrieve all of this desired information.

METHODS

Using selective arterial spin labeling (ASL) methods, a single artery of interest can be tagged and visualized, whereas quantitative information about hemodynamics can be retrieved using phase-contrast techniques that are often limited regarding their selectivity. In this study, a method that allows for velocity mapping of individual arteries by incorporating phase-contrast preparation into selective ASL angiography measurements is presented. Several postprocessing steps are required to generate velocity and directional-encoded maps of selected arteries from the data acquired in a single scan.

RESULTS

The method was successfully evaluated in healthy volunteers, and a first application in two selected patients is presented. In one patient, an aneurysm of the middle cerebral artery is investigated, and in the second patient it is used to visualize an arterio-venous malformation.

CONCLUSION

Selective ASL imaging in conjunction with phase-contrast acquisition allows for investigating hemodynamic properties of individual arteries. Magn Reson Med 78:1469-1475, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Evaluation of Intracranial Arteriovenous Malformations With Four-Dimensional Arterial-Spin Labeling-Based 3-T Magnetic Resonance Angiography

OBJECTIVE

We aimed to assess the usefulness of 3-T 4-dimensional (4D) arterial spin-labeling (ASL)-based magnetic resonance angiography (MRA) with color-coded time-of-arrival (TOA) maps for the evaluation of cerebral arteriovenous malformations (AVMs).

METHODS

Our study included 6 patients with cerebral AVMs. They underwent 4D-ASL MRA at 3T and digital subtraction angiography. A pseudocontinuous arterial spin labeling protocol with look-locker sampling was used for spin labeling. Two independent readers reviewed the 4D-ASL MRA images with color-coded TOA maps for the nidus size, arterial feeders, and venous drainage. Two other readers consensually reviewed the digital subtraction angiography images.

RESULTS

The cerebral AVMs were demonstrated on all 4D-ASL MRA images. In 5 high-flow AVMs, the color-coded TOA maps were especially useful for identifying the feeder/drainer. Intermodality agreement was excellent for the nidus size (κ = 1.0), very good for arterial feeders (κ = 0.88), and good for venous drainage (κ = 0.80).

CONCLUSIONS

The 4D-ASL 3-T MRA with color-coded TOA maps is useful for assessing the gross angiographic characteristics of intracranial AVMs.

Optimization of 4D vessel-selective arterial spin labeling angiography using balanced steady-state free precession and vessel-encoding

Vessel-selective dynamic angiograms provide a wealth of useful information about the anatomical and functional status of arteries, including information about collateral flow and blood supply to lesions. Conventional x-ray techniques are invasive and carry some risks to the patient, so non-invasive alternatives are desirable. Previously, non-contrast dynamic MRI angiograms based on arterial spin labeling (ASL) have been demonstrated using both spoiled gradient echo (SPGR) and balanced steady-state free precession (bSSFP) readout modules, but no direct comparison has been made, and bSSFP optimization over a long readout period has not been fully explored. In this study bSSFP and SPGR are theoretically and experimentally compared for dynamic ASL angiography. Unlike SPGR, bSSFP was found to have a very low ASL signal attenuation rate, even when a relatively large flip angle and short repetition time were used, leading to a threefold improvement in the measured signal-to-noise ratio (SNR) efficiency compared with SPGR. For vessel-selective applications, SNR efficiency can be further improved over single-artery labeling methods by using a vessel-encoded pseudo-continuous ASL (VEPCASL) approach. The combination of a VEPCASL preparation with a time-resolved bSSFP readout allowed the generation of four-dimensional (4D; time-resolved three-dimensional, 3D) vessel-selective cerebral angiograms in healthy volunteers with 59 ms temporal resolution. Good quality 4D angiograms were obtained in all subjects, providing comparable structural information to 3D time-of-flight images, as well as dynamic information and vessel selectivity, which was shown to be high. A rapid 1.5 min dynamic two-dimensional version of the sequence yielded similar image features and would be suitable for a busy clinical protocol. Preliminary experiments with bSSFP that included the extracranial vessels showed signal loss in regions of poor magnetic field homogeneity. However, for intracranial vessel-selective angiography, the proposed bSSFP VEPCASL sequence is highly SNR efficient and could provide useful information in a range of cerebrovascular diseases. © 2016 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.

Utility of noncontrast-enhanced time-resolved four-dimensional MR angiography with a vessel-selective technique for intracranial arteriovenous malformations

PURPOSE

To evaluate the utility of a vessel-selective four-dimensional (4D) magnetic resonance angiography (MRA) technique for the evaluation of intracranial arteriovenous malformations (AVMs).

MATERIALS AND METHODS

Twelve AVM patients were evaluated retrospectively. Time-of-flight (TOF) MRA, nonvessel-selective 4D-MRA (NS-4D-MRA), and vessel-selective 4D-MRA (VS-4D-MRA) were performed using a 3T MR unit in all patients, and used to identify feeding arteries and draining veins and measure nidus size. The diagnostic accuracy of the three techniques was compared using digital subtraction angiography (DSA). If a multifeeder was observed, the percentage of blood flow of each feeding artery to the entire nidus was evaluated and compared to the DSA findings using the "error value," defined as the degree of overestimation of the blood flow. All imaging findings were assessed by two neuroradiologists.

RESULTS

In both raters, the detectability of feeding arteries by VS-4D-MRA (12 and 11 patients) was significantly higher than those of TOF-MRA (7 and 6 patients) and NS-4D-MRA (8 and 7 patients) (P &lt; 0.016). The detectability of drainer veins by TOF-MRA (10 and 10 patients) was significantly higher than that of VS-4D-MRA (7 and 6 patients). In the percentage of the blood flow of each feed artery to the entire nidus, the DSA findings (error value; 27.1 ± 5.7) indicated overestimations of the blood flow compared to the VS-4D-MRA (error value; 7.1 ± 3.9) (P &lt; 0.001).

CONCLUSION

VS-4D-MRA was shown to be a useful technique for the evaluation of intracranial AVMs, especially for detecting feed arteries and estimating details of the nidus structure. J. MAGN. RESON. IMAGING 2016;44:834-845.

Arterial Spin Labeling Techniques 2009-2014

PURPOSE

Arterial spin labeling (ASL) techniques have been implemented across a diverse range of clinical and experimental applications. This review aims to evaluate the current feasibility of ASL in clinical neuroradiology based on recent improvements to ASL sequences and highlight areas for potential clinical applications.

METHODS AND MATERIALS

In December 2014, a literature search was conducted on PubMed Central, EMBASE, and Scopus using the search terms: "arterial spin labeling, neuroradiology," for studies published between 2009 and 2014 (inclusive). Of 483 studies matching the inclusion criteria, the number of studies using continuous, pseudocontinuous, pulsed, and velocity-selective ASL sequences was 42, 209, 226, and 3, respectively. Studies were classified based on several common clinical applications according to the type of ASL sequence used. Studies using pulsed ASL and pseudo-continuous ASL were grouped based on common sequences.

RESULTS

The number of clinical studies was 264. Numerous studies applied ASL to stroke management (43 studies), drug testing (21 studies), neurodegenerative diseases (40 studies), and psychiatric disorders (26 studies).

CONCLUSIONS

This review discusses several factors hindering the implementation of clinical ASL and ASL-related radiofrequency safety issues encountered in clinical practice. However, a limited number of search terms were used. Further development of robust sequences with multislice imaging capabilities and reduced radiofrequency energy deposition will hopefully improve the clinical acceptance of ASL.

Prospective motion correction and selective reacquisition using volumetric navigators for vessel-encoded arterial spin labeling dynamic angiography

PURPOSE

The aim of this study was to improve robustness to motion in a vessel-encoded angiography sequence used for patient scans. The sequence is particularly sensitive to motion between imaging segments, which causes ghosting and blurring that propagates to the final angiogram.

METHODS

Volumetric echo planar imaging (EPI) navigators acquired in 275 ms were inserted after the imaging readout in a vessel-encoded pseudo-continuous arterial spin labeling (VEPCASL) sequence. The effects of movement between segments on the images were tested with phantom experiments. Deliberate motion experiments with healthy volunteers were performed to compare prospective motion correction (PMC) with reacquisition versus no correction.

RESULTS

In scans without motion, the addition of the EPI navigator to the sequence did not affect the quality of the angiograms in comparison with the original sequence. PMC and reacquisition improved the visibility of vessels in the angiograms compared with the scans without correction. The reacquisition strategy was shown to be important for complete correction of imaging artifacts.

CONCLUSION

We have demonstrated an effective method to correct motion in vessel-encoded angiography. For reacquisition of 15 segments, the technique requires approximately 30 s of additional scanning (∼25%). Magn Reson Med 76:1420-1430, 2016. © 2015 International Society for Magnetic Resonance in Medicine.