Eur Radiol (2011) 21:875–881 the anterior-posterior direction and one in the right-left direction (TR/TE, 9.4/5.9 ms; flip angle, 7.5º; FOV, 250× 187.5; single slice, 8 averages; slice thickness, 13 mm; velocity sensitivity, 40 cm/s; scan time, 20 s) [19]. Perfusion territory and collateral blood flow assessment The decision whether the caudate nucleus, lentiform nucleus and thalamus were supplied by the ipsilateral ICA, the contralateral ICA or the basilar artery was made by one author (NH), blinded to the MR angiography images, on the perfusion territory selective ASL-MR images. For this purpose the perfusion territories of the left ICA, right ICA and basilar artery were first identified using k-means clustering with Matlab (The MathWorks, Natick, Mass, version 7.5) [20]. Perfusion contributions were then scored by evaluating the head portion of the caudate nucleus, the entire lentiform nucleus, and entire thalamus. The ventricular system, anterior limb and posterior limb of the internal capsule, and external capsule were used to delineate the deep brain structures. The morphology of the circle of Willis and the direction of blood flow through the primary collaterals were determined separately, blinded to the perfusion territory selective ASL-MR images, with the time-of-flight and phase-contrast MR angiography images. Anterior collateral flow was defined as blood flow across the anterior communicating artery with retrograde flow in the precom- municating part of the anterior cerebral artery (A1 segment) ipsilateral to the stenosis. Posterior-to-anterior blood flow in the posterior communicating artery was considered to represent posterior collateral flow. Each circle of Willis was assessed for a missing or hypoplastic A1 segment and for the presence of a fetal-type posterior communicating artery, of which the diameter is larger than the precommunicating segment of the ipsilateral posterior cerebral artery. SPSS (SPSS Inc., Chicago, Illinois, USA version 15) was used for statistical analyses. The frequencies of supply by a specific artery were compared between the symptomatic patients, asymptomatic patients and healthy control subjects with a two-tailed Fisher’s exact test. A p-value< 0.05 was considered to indicate statistical significance. Values are expressed as mean ± standard deviation (SD). Results 877 various perfusion patterns of the caudate nucleus, lentiform nucleus and thalamus in the healthy control subjects, symptomatic patients and asymptomatic patients. In some patients the deep brain structures received blood from more than one brain-feeding artery. The caudate nucleus was predominantly supplied by the ipsilateral ICA in all healthy control subjects and symp- tomatic patients, and in 19 (95%) of the 20 asymptomatic patients. The caudate nucleus was supplied by the contra- lateral ICA in one patient (5%) with an asymptomatic ICA stenosis. In patients with a symptomatic ICA stenosis the caudate nucleus received additional perfusion from the contralateral ICA in 4 (27%) of the 15 patients, whereas this was not seen in healthy controls (p = 0.03). There was no difference in the supply of the caudate nucleus between patients with an asymptomatic and patients with a symp- tomatic ICA stenosis, and between the asymptomatic patients and the healthy control subjects. The lentiform nucleus was supplied by the ipsilateral ICA in all groups. In one of the 18 healthy control subjects and in one of the 15 patients with a symptomatic ICA stenosis, the lentiform nucleus received additional blood flow contribution from the basilar artery. The thalamus was predominantly supplied by the basilar artery. The ipsilateral and contralateral ICA supplied the thalamus with blood in respectively 17 and one of the 18 healthy control subjects, 12 and 3 of the 15 symptomatic patients, and 15 and 2 of the 20 asymptomatic patients. In 4 of the 5 (80%) subjects in whom the caudate nucleus received blood from the contralateral ICA, there was anterior collateral flow, compared to 1 of 48 subjects (2%) without a contributing contralateral ICA (p < 0.01). The collateral flow distribution in the circle of Willis did not affect the perfusion territories of the lentiform nucleus and thalamus between subjects with or without anterior or posterior collateral flow. Regarding the anterior collateral circulation, there were no subjects with a missing or hypoplastic A1 segment. Two asymptomatic patients and one symptomatic patient showed a unilateral fetal-type posterior communicating artery. In these three patients, the thalamus was supplied by the ipsilateral ICA. Figure 3 is an example of the perfusion territories and anatomical mor- phology of the circle of Willis in a symptomatic patient with a right-sided ICA stenosis and a right-sided unilateral fetal-type posterior communicating artery. The demographic and clinical characteristics of the partic- ipants are outlined in Table 1. Figure 2 shows the perfusion territories of the caudate nucleus, lentiform nucleus and thalamus, and the corresponding blood flow through the circle of Willis of a patient with a symptomatic right-sided ICA stenosis. Table 2 summarizes the prevalences of Discussion The current study shows that in patients with a symptomatic stenosis of the ICA, the caudate nucleus is more often supplied with blood from the contralateral ICA via collaterals than in healthy control subjects. Secondly, the contribution of