Imaging technique, where a radiotracer is injected (which was essentially radioactively labeled water) into the participant. This radiotracer would then circulate through the body's vascular system, ultimately diffusing freely into brain tissue along with the blood. The radioactive tracer would begin to decay almost immediately, emitting tiny positively charged particles (called positrons) which could then be detected using specialized equipment. As the radiotracer travels with the blood, the amount of radioactvity detected reflects blood flow. That is to say, areas which received a lot of blood should also have received a lot of radioactively labeled water and consequently shown higher levels of radioactive positron emissions. Thus, in for each region of brain tissue, uptake of the radiotracer would be proportional to blood flow.
see PET Scan.
Yamaguchi S. [Clinical Applications of Positron Emission Tomography for Neurosurgery(4)Applications of Positron Emission Tomography for Assessing Brain Tumors]. No Shinkei Geka. 2017 Nov;45(11):1015-1024. doi: 10.11477/mf.1436203638. Japanese. PubMed PMID: 29172209. 1).