876 either symptomatic or asymptomatic ICA stenosis, and to compare this distribution with that of healthy control subjects. Materials and methods The study was approved by the institutional ethical review board and written informed consent was obtained from all participants. Subjects Fifty-three subjects were included prospectively in the study. Thirty-five were functionally independent patients with a unilateral ICA stenosis of ≥50% and 18 were healthy control subjects. Of the patients, 20 had an asymptomatic ICA stenosis (12 males and 8 females; mean age ± SD, 69 ± 7 years) and 15 a symptomatic stenosis (9 male, 6 female; 71 ± 7 years). The symptom- atic patients had experienced a transient ischemic attack (TIA), non-disabling ischemic stroke, or amaurosis fugax on the side of the stenosis in the 3 months before inclusion and were seen at our hospital for diagnosis and treatment. The patients with an asymptomatic ICA stenosis were recruited from a hospital-based vascular screening study involving subjects with risk factors for atherosclerosis or clinically manifest atherosclerotic ves- sel disease [12]. Stenoses of the ICAs were measured with color Doppler-assisted Duplex ultrasonography and peak systolic velocity thresholds based on literature were used for the diagnosis of less than 50%, 50% to 70% or 70% to 99% stenoses [13]. The control group consisted of 18 healthy age and sex-matched volunteers (11 male, 7 female; 67 ± 7 years). All control subjects were recruited through local media advertisements and did not have a history of neurological disease or vascular pathology on MRI or MR angiography (MRA) of the brain. Subjects with diabetes mellitus or severe renal or liver dysfunction were excluded. Magnetic resonance imaging Each subject underwent an MRI investigation on a clinical 3 Tesla MRI system (Achieva, Philips Medical Systems, The Netherlands) equipped with an eight-element phased- array head coil and locally developed software for ASL- MR imaging. To depict the perfusion territories of the ICAs and the basilar artery, territory specific perfusion images were acquired with a pseudo-continuous selective ASL perfusion sequence, according to a previously published protocol [14, 15]. To position the imaging section, a T1-weighted spin-echo sequence was first obtained in the Eur Radiol (2011) 21:875–881 sagittal plane. Seventeen 7 mm slices with no slice gap were subsequently aligned parallel to the orbito-meatal angle, as shown in Fig. 1a. Labeling was performed by employing a train of 18°, 0.5 ms, Hanning-shaped radio- frequency pulses at an interpulse pause of 0.5 ms, for a duration of 1,650 ms, in combination with a balanced gradient scheme [16, 17]. The control situation, without labeling of arterial blood, was achieved by adding 180° to the phase of all even RF pulses. Perfusion territory selective ASL-MRI was accomplished through manipulating the spatial labeling efficiency by applying additional gradients between the labeling pulses in sets of 5 dynamics as demonstrated in Fig. 1b. Imaging was performed with single-shot echo planar imaging in combination with parallel imaging (SENSE factor 2.5) and background suppression. The background suppression consisted of a saturation pulse immediately before labeling and inversion pulses at 1,680 and 2,830 ms after the saturation pulse, 1,525 ms after the labeling stopped [18]. The following parameters were used: repetition time (TR), 4,000 ms; echo time (TE), 14 ms; field of view (FOV), 240 × 140 mm; matrix size, 89 × 79; 75 dynamics; scan time, 5½ min. A three-dimensional time-of-flight MR angiography was made with subsequent maximum intensity projection recon- struction using the following parameters: TR/TE, 30/6.9 ms; flip angle 20°; 2 averages; FOV, 100 × 100 mm; matrix, 256 × 256; 50 slices; slice thickness, 1.2 mm with 0.6 mm overlap; scan time, 3 min. Collateral blood flow direction was determined according to a previously published imaging protocol with 2 consecutive 2-dimensional phase-contrast MRI measurements, out of which one was phase-encoded in Fig. 1 Sagittal T1 weighted image a illustrating the imaging volume aligned parallel to the orbito-meatal angle and the labeling plane of the regional-perfusion selective ASL imaging. The labeling efficiency (b, white represents no label, blue represents full label) was spatially manipulated within the labeling plane in sets of five dynamics, in which: (1) no labeling applied (control), (2) non-selective labeling applied (globally perfusion weighted), (3) labeling varied in right-left (RL) direction (distance of 50 mm between full label and control situation), (4) labeling varied in anterior-posterior (AP1) direction (distance of 18 mm between full label and control situation), and (5) labeling varied in anterior-posterior direction (AP2, similar to AP1, but shifted 9 mm in posterior direction compared to the previous dynamic)