Magnetic Resonance in Medicine 56:1140–1144 (2006) Dual Vessel Arterial Spin Labeling Scheme for Regional Perfusion Imaging Ivan Zimine, 1 Esben Thade Petersen, 1 and Xavier Golay 1,2, * Regional perfusion imaging (RPI) based on pulsed arterial spin labeling and angulated inversion slabs has been recently pro- posed. The technique allows mapping of individual brain perfu- sion territories of the major feeding arteries and could become a valuable clinical tool for evaluation of patients with cerebrovas- cular diseases. Here we propose a new labeling scheme for RPI where lateral and posterior circulations are labeled simultane- ously. 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 vol- unteers and compared to the original labeling scheme. The results showed that the same perfusion territories and regional CBF values can be obtained. Magn Reson Med 56:1140–1144, 2006. © 2006 Wiley-Liss, Inc. Key words: regional perfusion imaging; arterial spin labeling; flow territories Imaging of individual perfusion territories of the brain’s major feeding arteries could be extremely useful for the evaluation of patients with various vascular disorders. For instance, it may help to identify the location and extent of brain regions at risk of possible ischemic damage. Another important clinical application is the assessment of collat- eral circulation in patients considered for vascular inter- ventions such as EC–IC bypass surgery and follow-up after the procedure (1). Currently, there are no clinically estab- lished techniques allowing direct visualization of different perfusion territories on the underlying anatomy. Several research groups have recently reported on the potential application of arterial spin labeling (ASL) for providing this information. ASL allows noninvasive measurement of brain perfusion using magnetically labeled blood as an endogenous contrast medium (2,3). In most ASL experi- ments, the labeling (inversion or saturation of blood magne- tization) is applied to all the supplying arteries and results in whole-brain perfusion maps. In order to get perfusion territories, individual arteries must be labeled separately. Several strategies for selective labeling have been proposed for both continuous and pulsed ASL. These include the 1 Department of Neuroradiology, National Neuroscience Institute, Singapore. 2 Singapore Bioimaging Consortium, A*STAR, Singapore. Contract grant sponsor: National Medical Research Council; Contract grant numbers: NMRC/0919/2004 and NMRC/CPG/009/2004; Contract grant spon- sor: SingHealth; Contract grant number: NHGA-RPR/04012. *Correspondence to: Xavier Golay, Department of Neuroradiology, National Neuroscience Institute, 11 Jalan Tan Tock Seng, Singapore 308433. E-mail: xavier_golay@sbic.a-star.edu.sg Received 18 April 2006; revised 21 June 2006; accepted 12 July 2006. DOI 10.1002/mrm.21049 Published online 19 September 2006 in Wiley InterScience (www.interscience. wiley.com). use of local surface coils (4), 2D pencil-beam inversion pulses (5), oblique (6) or oscillating (7) labeling planes, and hemisphere-selective (8) or angulated inversion slabs (9). While all of these approaches have their merits and draw- backs, particularly with respect to spatial selectivity of the label, only the regional perfusion imaging (RPI) method described by Hendrikse et al. (9) has been applied success- fully both in patients (10) and in a relatively large number of healthy subjects (11). Very recently, a selective FAIR tech- nique was used to assess hemispheric cerebral blood flow in patients undergoing carotid endarterectomy (12). In the original RPI implementation, three ASL scans with inversion slabs positioned over the left internal carotid artery (LICA), the right internal carotid artery (RICA), and the posterior circulation (POST, including both vertebral and basilar arteries) are used to image respective perfusion territories of these arteries. As noted by the authors, one limitation of this approach is the difficulty in achieving selective labeling, particularly of the posterior circulation, in about 30% of the subjects because of tortuous vascu- lar anatomy in the neck. Here we propose a new labeling scheme for RPI, where lateral and posterior circulations are labeled simultaneously. Acquiring two sets of data, label- ing left ICA and POST in one and right ICA and POST in another allows one to obtain the same perfusion territories information as the one provided by the original labeling scheme. In addition to scan time reduction, the dual ves- sel labeling scheme helps with the above issue of selective labeling because the positioning of the inversion slabs with respect to vascular anatomy is different. MATERIALS AND METHODS Labeling Scheme The original and the new labeling schemes for RPI are shown schematically in Fig. 1. In general, selection of the ICAs using the original labeling scheme is relatively easy and can be done in the majority of subjects. The difficulty of proper selection of the posterior circulation without con- comitant labeling of the ICAs arises when one or both of the vertebral arteries are near or cross the plane joining left and right ICAs. With the dual vessel labeling approach, posterior circulation is always included in the inversion slab, and one only needs to ensure that one of the ICAs is excluded. Perfusion data obtained using dual vessel labeling will contain combined perfusion territories. Assuming that the posterior circulation is labeled identically in both ASL scans, the territories can be recalculated in the following manner, POST = (LP + RP −|LP − RP|)/2, LICA = LP − POST, RICA = RP − POST, [1] © 2006 Wiley-Liss, Inc. 1140