Arterial spin labeling: its time is now

Abstract

It has been more than two decades since arterial spin labeling (ASL) perfusion MRI was introduced [1,2], yet its impact on research and clinical studies is just starting to be broadly appreciated. ASL has gone from an infancy where it was practiced in small animals by just a few experts, to a childhood of initial experience in humans and in more laboratories, and finally through an awkward adolescence where emerging capabilities were masked by imperfect coordination of methods, analysis, and distribution. As evidenced by the richness and variety of studies in this special edition of MAGMA, ASL has become a mature technology where widespread development and testing, exploration of clinical and research applications, and movement toward consensus on approaches are occurring today.While ASL methods are now advanced enough to provide routine, quantitative sequences for brain ASL and competent methods for some organs outside the brain, remaining limitations and the ongoing spirit of innovation in the community continue to drive new developments in ASL methods. Perhaps, nowhere is this more evident than in the wide assortment of labeling methods (and associated acronyms) that continue to be introduced and improved. In this issue alone, we see a paper on an innovative approach to mixing continuous and pulsed ASL [3], an exploration of the challenges of pseudo-continuous labeling in humans at 7 Tesla [4] and an investigation into unintended effects of pulses to shape the labeling bolus [5]. The exciting and exploding area of vessel-selective labeling to map perfusion territories is represented here by the New Concept article on random vessel encoding [6].A key issue restricting the reliability of ASL studies is the effect of motion and other instabilities on noise and artifacts in ASL. The successful approach of background suppression [7,8] is emphasized in the article of Maleki et al. [9], where suppression is optimized and used to image blood flow to the eye. Since background suppression does reduce ASL signal by a significant factor, the community remains uncertain whether or how much background suppression to use. For example, neither the reproducibility study of renal blood flow [10] nor the study of pulmonary perfusion in cystic fibrosis [11] employs substantial background suppression. Integrally related to the choice of background suppression is the choice of acquisition sequence. Traditionally, single-shot imaging with echoplanar imaging was used to minimize motion sensitivity, but the improving speed of other approaches and the reduced motion sensitivity provided by background suppression have encouraged the use of RARE, balanced SSFP [12], and even 3D spiral [8,13] or GRASE [14] sequences for acquisition. A key question in the choice and design of sequences is how many different images are required for quantification. Multiple images are required when T1 and especially transit time measurements are needed.The use of ASL in broad clinical populations has stimulated interest in better understanding arterial tran...

Cerebrovascular reactivity measured with arterial spin labeling and blood oxygen level dependent techniques

“…Ten averages were acquired, resulting in scan duration of 1.5 min. The PC slice was placed 2mm distal to the left carotid artery bifurcation and oriented perpendicular to the internal carotid and vertebral arteries, allowing simultaneous assessment of the four feeding vessels [36]. The slice was positioned based on a time-of-flight MRA sequence (TR/TE: 17 msec/4.12 msec, FA: 18°, voxel size: 1 × 1 × 2.5 mm 3 , 40 slices, one saturation band placed above the imaging slab, acquisition time: 2.9 min).…”

Perfusion Magnetic Resonance Imaging: A Comprehensive Update on Principles and Techniques

“…To minimize these static tissue signals, background suppression techniques were developed (95, 96). Since background suppression does reduce ASL signals, there is still debate as to whether or how much background suppression to use (97). Furthermore, the labeling pulse can cause substantial magnetization transfer (MT) effects in the static tissue of the image slices, particularly with CASL, due to the long labeling.…”

Cerebral Blood Flow is Diminished in Asymptomatic Middle-Aged Adults with Maternal History of Alzheimer's Disease

“…A limited number of studies have suggested that ASL CBF may be sensitive to neurometabolic alterations among persons with risk factors for AD including individuals with mild cognitive impairment (MCI;Johnson et al 2005;Xu et al 2007;Chao et al 2009;Dai et al 2009;Bangen et al 2012) and cognitively normal adults who have a FH of AD and/or carry the APOE4 allele (Fleisher et al 2009;Filippini et al 2011); with the latter set of studies yielding inconsistent findings. However, only recently has the ASL methodolgy become more fully developed and readily available across major scanner vendors (Alsop 2012), suggesting the need for a renewed evaluation of the utility of ASL CBF within the context of AD, from its preclinical stages to frank dementia. Therefore, in this cross-sectional study, we used a whole-brain CBF sequence with established fidelity (Gevers et al 2011) to evaluate resting CBF in a large sample (n = 327) of wellcharacterized adults spanning the AD spectrum from at-risk cognitively healthy middle-aged individuals to patients with probable AD.…”