Transcranial Doppler in the Era of Personalized Medicine: An Important Tool for the Assessment of Cerebrovascular Function.

Background: Transcranial Doppler (TCD) and transcranial color-coded duplex sonography (TCCS) have been referred to as the "brain stethoscope" as they provide real-time monitoring of both hemodynamic parameters and the structural image of the vessel and neighboring tissues. Its use has made important contributions to the field, with the ability to monitor important variables such as cerebral blood velocity to study cerebrovascular function. Although the use of B-mode with color-coded imaging allows identification of the vessel of interest in a more user-friendly approach compared to non-imaging TCD, TCD has unique functions that are complementary to TCCS studies. The aim of this review was to introduce these unique functions of TCD and discuss scenarios where the use of TCD would be justified without the combined use of TCCS.

Summary: Due to its portability and noninvasive nature, TCD is a reliable option for monitoring cerebrovascular conditions; with the ability to perform long periods of monitoring via a dedicated head frame or band. This provides an opportunity to monitor cerebrovascular function which could be named TCD functional monitoring (TCD fm). Importantly, TCD can be used to evaluate the main mechanisms involved in cerebral blood flow regulation such as cerebral autoregulation (CA) and neurovascular coupling (NVC). CA refers to the ability of the brain to maintain CBF despite changes in systemic arterial blood pressure. NVC evaluates the CBF response to local cerebral metabolism and neural activation. Both mechanisms are important in physiological and pathological conditions. Understanding and monitoring cerebrovascular regulation could develop new insights to design personalized treatments which may lead to better prognosis and outcome for patients. Moreover, TCD fm may be implemented for microembolism detection during long periods of recording, enhancing the ability to detect embolic activity. All these unique applications of TCD strengthen the importance of the method. Furthermore, several functions of TCCS could be accomplished using TCD, which is less expensive when compared to modern TCCS devices, and therefore is a reasonable option for low-income countries.

Key messages: TCD has utilities that are not covered by TCCS and considering the economic impact of broadening its use in specific areas, such as those with resource limitations, it is a reasonable option for the standard method for evaluation of cerebrovascular function. Further research is needed to combine these two modalities to provide a solution to the current limitations of using TCD and TCCS in isolation.