2 Theory and Practice of MRA-Guided Transcranial Doppler Sonography Francisco L. Colino and Gordon Binsted Department of Human Kinetics, Faculty of Health and Social Development, The University of British Columbia, Kelowna, BC, Canada 1. Introduction Despite consisting of 2 – 3% of total body mass, the brain accounts for ~20% of the body’s oxygen consumption and therefore must receive significant blood supply to maintain homeostasis (Ainslie & Duffin, 2009). This profound dependency on blood supply belies the brain’s apparent ability to regulate blood supply so tightly. Hence, the brain’s ability to regulate its blood supply has been, and still is, the subject of considerable research (e.g., Kety & Schmidt, 1948; Panerai et al., 2009; Willie & Smith, 2011; Willie et al., 2011; also see Ainslie & Duffin, 2009 for a recent review). However, the dense cortical bone of the skull does not lend itself easily to normal ultrasound techniques. The most common tool for assessing cerebral blood flow regulation is transcranial Doppler sonography (TCD) as it operates at lower frequencies (usually 1 – 2 MHz), relative to conventional ultrasound frequencies (≥ 5 MHz), to penetrate the skull. Traditionally, researchers rely solely on an M-mode display 1 for information regarding depth, blood flow velocity, pulsatility index, etc. (Aaslid et al., 1986). Based on historical indicators they infer which vessel is insonated; for example, the user can perform simple stimulus-response tests to confirm that the identity of the vessel. A reduction in blood flow velocity following the vibration/compression of the external carotid would confirm middle cerebral artery (MCA) insonation. In a similar fashion, flow variation accompanying the opening/closing of the eyes is a simple confirmatory test for the posterior cerebral artery (PCA). Despite these well-established procedures to determine vessel identity, there are several notable problems with the current approach. First, TCD measures blood flow velocity, which is a relative measure; absolute flow cannot be estimated without knowing the vessel diameter. Specifically, according to Poiseuille’s Law, flow (Q) is determined by the vessel length (L), pressure difference (P), viscosity () and, most notably, vessel radius (). 4 8 pa P Q Lh  (1) 1 M-mode stands for ‘motion mode’. This modality returns echoes over time for one line of the B-mode image. Thus, movement of structures positioned in that line can be visualized in a time-varying fashion. Often M-mode and B-mode are displayed together on the ultrasound monitor.