Figure 5:

Delayed arterial flow in chronic ICAD. Normal vasculature and perfusion are seen in the right hemisphere. The left M1 segment has severe chronic stenosis, with diminished signal in distal MCA branches on the Time-of-Flight MRA (A). The corresponding FLAIR image (B) is without findings to suggest a recent infarct, corroborated by a lack of clinical symptoms (note that DWI was not performed). On PCASL with PLD of 1800ms (C) apparent hypoperfusion is seen throughout the left MCA territory, along with arterial transit artifacts in the Sylvian M2 branches and watershed areas (arrow and arrowheads). On a second PCASL with PLD of 2500ms (D) the perfusion signal in the posterior temporal lobe and in the watershed areas normalizes (dotted circles) and the artifacts are markedly reduced, indicating that perfusion is maintained via delayed (collateral) flow. Macrovascular artifacts remain in the Sylvian fissure, consistent with an arrival time >2500ms in these branches, likely persistent ATA. Estimating CBF in the parenchyma fed by these vessels is possible with PLD >2500ms, although the reduction of signal with T1 relaxation will further decrease SNR. (C) and (D) are perfusion maps.