How was cerebral blood flow measured historically?

Today is the last topic of our cardiac physiology series "Measurement." I start of by looking at different techniques to measure non-invasive BP from auscultation (01:31) to oscillometry (07:03). Then I look at an invasive method being arterial lines and this is divided into how it works (11:15) and sources of error (17:54). The last LO for today covers different techniques to measure cardiac output (27:41). To finish as always, we look at the past SAQs (33:35). Thanks for listening! As always if you have any feedback let me know