Fig. 2.

Basic principle of ASL with dual postlabeling delay (PLD).

(A) The effect of arterial transit time (ATT). ASL is highly sensitive to the arrival time of the labeled blood in the tissue, which is defined as ATT. For example, in patients with steno-occlusive cerebrovascular disease, labeled blood that travels via stenotic proximal arteries or collateral pathways exhibits prolonged ATT. Thus, ASL measurements that employ a conventional single PLD of 1.5 s (ASL with a PLD of 1.5 s) lead to underestimation of the CBF, and the slowly streaming CBF could be assessed on ASL with a PLD of 2.5 s (ASL with a PLD of 2.5 s). (B) Arterial transit artifact (ATA). Apparent hyperperfusion is demonstrated in territories where intravascular magnetically labeled protons are stagnant, a condition termed “ATA”. ATAs appear in nearly the same configuration on both PLD settings. (C) Effect of increased vascular permeability. Extravasation of labeled protons into the tissue due to increased vascular permeability or opening of the blood brain barrier also appears in nearly the same configuration on the dual PLD setting.

Two types of periictal ASL findings with dual PLD.

(D) Fast flow type is characterized by markedly increased signals of the epileptically activated cortex on ASL with a PLD of 1.5 s, which is markedly decreased or mostly washed out on ASL with a PLD of 2.5 s. Signals of the surrounding or contralateral cortices are decreased on ASL with a PLD of 1.5 s, probably due to the steal phenomenon, which is mostly improved on ASL with a PLD of 2.5 s. There is a tight topographical relationship between the epileptogenic lesion and the epileptically activated cortex. (E) The gradual flow type is characterized by increased signals of the epileptically activated cortex on ASL with a PLD of 1.5 s, which is further increased on ASL with a PLD of 2.5 s.