Cerebral Blood Flow Measurement For Neurological Assessments: Functional Transcranial Doppler Ultrasound

Transcranial Doppler (TCD) is a special type of ultrasound developed to measure cerebral blood flow (CBF) and subsequently shown to be valuable in an increasingly larger number of clinical applications. Physically, TCD operates in the same way and is as safe as ultrasound imaging used during pregnancy. Changes in CBF in response to controlled stimuli have been shown to be valuable indicators of a variety of pathological conditions. In addition to measuring CBF, TCD can detect the passage of emboli in the bloodstream due to a stark difference in acoustic properties when emboli pass through the ultrasound beam. This unmistakable change in ultrasound signal echo between emboli and blood is due to density and acoustic impedance differences. The TCD signal is processed in real-time for visual display and measurement of cerebral blood flow velocities (CBFV). Two scanning techniques, the free-hand and continuous monitoring techniques, are the two primary methods for performing TCD. The free-hand technique is suitable for short-term monitoring (30 min or less) but is not effective for continuous monitoring due to operator fatigue and inconsistent transducer placement. The second method, continuous TCD monitoring, requires a fixation apparatus to hold the Doppler probe(s) in a static location at the scalp insonation site. Due to its low cost, real-time measurement capability, and easy portability, TCD has remained a valuable imaging tool when compared to more costly and stationary modalities. More recently, TCD has been used in conjunction with cerebral function tests (functional TCD, or fTCD) as a proxy for neuronal activity for indication in an ever-increasing number of brain studies. The future of TCD and fTCD as biomarkers for neurological assessment is exciting indeed.