Eur Radiol (2011) 21:875–881 Table 2 Prevalence of various types of blood supply to the deep brain structures ICA internal carotid artery; BA basilar artery; the deep brain structures were supplied by more than one artery in some patients. a indicates a significant difference compared to healthy control subjects. (p<0.03, two-sided Fishers’ exact test) Caudate nucleus Healthy controls Symptomatic patients Asymptomatic patients Lentiform nucleus Healthy controls Symptomatic patients Asymptomatic patients Thalamus Healthy controls Symptomatic patients Asymptomatic patients Ipsilateral ICA Contralateral ICA 18 (100%) 15 (100%) 19 (95%) 18 (100%) 15 (100%) 20 (100%) 15 (83%) 12 (80%) 15 (75%) – – – 0 (0%) 4 (27%)a 1 (5.0%) 1 (5.6%) 2 (13%) 2 (10%) 879 BA – – – 1 (5.6%) 1 (6.7%) 0 (0%) 16 (89%) 14 (93%) 16 (80%) In one of the five patients in whom the caudate nucleus was supplied by the contralateral ICA, anterior collateral flow was not detected. This may be explained by supply of part of the caudate nucleus by the recurrent artery of Heubner. This artery originates from the anterior cerebral artery close to its junction with the anterior communicating artery and although hard to detect on MR angiography, is present in almost all hemispheres examined in post-mortem studies [22, 23]. Blood flow from the contralateral ICA via the recurrent artery of Heubner to the caudate nucleus is therefore not necessarily accompanied by detectable ante- rior collateral flow via the circle of Willis. This illustrates the importance that perfusion territory selective ASL-MRI might in comparison with MRA. In both the healthy control subjects and patients with a symptomatic ICA stenosis our results show that the lenti- form nucleus can be supplied by either the ipsilateral ICA or the posterior circulation. A previous study that investi- gated the effect of anatomical features of the circle of Willis on the perfusion territory patterns in the deep brain structures found that the lentiform nucleus was supplied by the contralateral ICA in 6% of patients with cerebral ischemia without carotid artery stenosis [24]. In none of the patients in our study the lentiform nucleus received blood from the contralateral ICA. This apparent discrepancy can be explained by chance or by the fact that in the patients from the previous study, occlusions in the anterior and middle cerebral artery may have resulted in recruitment of (leptomeningeal) collateral blood flow to the lentiform nucleus via the contralateral carotid artery. In addition to the different patient populations, the perfusion territory selective ASL labeling techniques also differed. No differences were found between the patients with ICA stenosis and the healthy control subjects with respect to the supply routes to the thalamus. We did find a large inter-subject variation of the perfusion contributions to the Fig. 3 Perfusion territory image a of a patient with a right-sided symptomatic ICA stenosis and a right-sided unilateral fetal-type variant of the circle of Willis. Colors represent the perfusion territory of the right ICA (blue), left ICA (red) and the basilar arteries (green). The caudate nuclei and lentiform nuclei on both sides are supplied by their respective ipsilateral ICA. Furthermore, the thalamus on the left side is partially supplied by both its ipsilateral ICA and basilar artery, while the thalamus on the right side (symptomatic side) is completely supplied by its ipsilateral ICA. Circle of Willis on 2D phase-contrast MRA, phase encoded in the right-left direction b and anterior- posterior direction c. Blood flowing to the patient’s right and anterior direction is black and blood flowing to the patient’s left and posterior direction is white. Phase contrast MRAs show anterior to posterior flow through the right posterior communicating artery (besides white star; white in b and c). Maximum intensity projection d shows a right- sided fetal-type posterior communicating artery (besides white star), which explains why the right-sided thalamus is not supplied by the basilar artery