A fast analysis method for non-invasive imaging of blood flow in individual cerebral arteries using vessel-encoded arterial spin labelling angiography

Magnetic Resonance in Med

2014

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Purpose: Vessel-encoded pseudocontinuous arterial spin labeling allows the acquisition of vessel-selective angiograms and vascular territory perfusion maps. The technique generates a periodic variation in inversion efficiency across space that can be manipulated to encode specific combinations of vessels. Currently, the choice of these encodings is limited to scenarios with few vessels and may not optimize the signalto-noise ratio (SNR). Here we present an automated, rapid method for calculating a minimal number of SNR optimal encodings for any number and arrangement of vessels. Theory and Methods: The proposed optimized encoding scheme (OES) is a Fourier-based method that finds SNR optimized encodings to best match the ideal encodings for a set of vessels. For nine or fewer vessels, the calculation takes less than 3 s. Results: In simulations, the OES method produces encodings for a range of vessel geometries that, on average, have an SNR efficiency 37% greater than that for random encoding. When labeling vessels in the neck in healthy subjects, the OES encodings result in images with higher SNR than other encoding methods. Conclusion:The OES results in a minimal number of encodings with a higher SNR efficiency than other encoding methods, regardless of the number or geometry of the vessels.

Section: Resultsmentioning

confidence: 99%

“…A Bayesian, maximum a posteriori (MAP) method was used to separate out vessel‐specific information . This method considers subsets of the encoding matrix, hence it can deal with poorly conditioned encoding matrices without significant loss of SNR.…”

Section: Methodsmentioning

confidence: 99%

An Optimized Encoding Scheme for Planning Vessel‐Encoded Pseudocontinuous Arterial Spin Labeling

Berry

Jezzard

Magnetic Resonance in Med

2014

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“…In order to separate the blood signals arising from each feeding artery from the vessel‐encoded data, a fast maximum a posteriori Bayesian version of the general framework for vessel‐encoded analysis was applied to the magnitude images. This method is SNR efficient and compensates for rigid body motion between the vessel localization scan and the vessel‐encoded acquisitions.…”

Section: Methodsmentioning

confidence: 99%

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

Schmitt

et al. 2016

NMR Biomed.

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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.

“…All VEPCASL images were analyzed using a Bayesian maximum a posteriori (MAP) method [38] to separate out vessel-specific information. In order to quantitatively compare the 2D accelerated Cartesian and radial approaches the SNR in the vessel-selective angiograms for each subject was calculated.…”

Section: Image Analysismentioning

confidence: 99%

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

Berry

Jezzard

2019

Preprint

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Object: 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 timeresolved balanced steady-state free precession (bSSFP) and spoiled gradient echo (SPGR) readouts to obtain dynamic vessel-selective angiograms arising from the four main brainfeeding arteries. Dynamic 2D protocols with acquisition times of one minute or less were achieved through radial undersampling or a Cartesian parallel imaging approach. For wholebrain 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 improvement 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.