Borderzone sign. These ASL subtraction images are from an 85 year-old man with dense left hemiparesis, acquired using PCASL with a labeling time of 1500 ms and a PLD of 1500 ms. Only the proximal portions of the arterial tree are present, indicating that the PLD was not long enough for the labeled spins to have reached the tissue, and that the ATT was prolonged bilaterally in this elderly patient. While longer PLD should improve the visualization of parenchymal CBF, it is not uncommon to see such a finding, known as the borderzone sign, in elderly patients with extremely delayed arrival times. 101×85mm (300 × 300 DPI)