Functional Transcranial Doppler Ultrasound for Monitoring Cerebral Blood Flow

Summary

Functional transcranial Doppler ultrasound complements other functional imaging modalities, with its high temporal resolution measurement of stimulus-induced changes in cerebral blood flow within the basal cerebral arteries. This Methods paper gives step-by-step instructions for using functional transcranial Doppler ultrasound to perform a functional imaging experiment. 

Abstract

Functional transcranial Doppler ultrasound (fTCD) is the use of transcranial Doppler ultrasound (TCD) to study neural activation occurring during stimuli such as physical movement, activation of tactile sensors in the skin, and viewing images. Neural activation is inferred from an increase in the cerebral blood flow velocity (CBFV) supplying the region of the brain involved in processing sensory input. For example, viewing bright light causes increased neural activity in the occipital lobe of the cerebral cortex, leading to increased blood flow in the posterior cerebral artery, which supplies the occipital lobe. In fTCD, changes in CBFV are used to estimate changes in cerebral blood flow (CBF).

With its high temporal resolution measurement of blood flow velocities in the major cerebral arteries, fTCD complements other established functional imaging techniques. The goal of this Methods paper is to give step-by-step instructions for using fTCD to perform a functional imaging experiment. First, the basic steps for identifying the middle cerebral artery (MCA) and optimizing the signal will be described. Next, placement of a fixation device for holding the TCD probe in place during the experiment will be described. Finally, the breath-holding experiment, which is a specific example of a functional imaging experiment using fTCD, will be demonstrated.

Introduction

In neuroscience research, it is often desirable to monitor real-time brain activity noninvasively in a variety of environments. However, conventional functional neuroimaging modalities have limitations that impede the ability to capture localized and/or rapid activity changes. The true (non-jittered, non-retrospective) temporal resolution of functional magnetic resonance imaging (fMRI) is currently of the order of a few seconds¹, which may not capture transient hemodynamic changes linked to transient neural activation. In another example, although functional near-infrared spectroscopy (fNIRS) has high temporal resolution (milliseconds) and reas....

Protocol

All human subject research was performed in accordance with the Institutional Review Board of the University of Nebraska-Lincoln, and informed consent was obtained from all subjects.

1. Locating the MCA signal by freehand TCD

NOTE: “Freehand” TCD refers to operation of TCD with a handheld transducer to find a CBFV signal before beginning an fTCD experiment.

1. Setting TCD parameters
   1. Keep the power at a reasonably high value (e.g., 40.......

Discussion

Critical steps in the protocol include 1) finding the MCA, 2) placing the headband, and 3) performing the breath-holding maneuver.

Modifications may be necessary depending on the subjects in the study. For example, subjects with Alzheimer’s disease may have difficulty following instructions, necessitating the use of a capnograph to ensure compliance with breath-holding instructions¹⁵. Young children may have difficulty following i.......

Materials

Name	Company	Catalog Number	Comments
Aquasonic	Parker Laboratories, Inc., Fairfield, NJ, USA	01-50	Ultrasound Gel
Doppler Box X	DWL Compumedics Gmbh, Singen, Germany	Model "BoxX"	Transcranial Doppler with 2-MHz monitoring probes
Kimwipes	Kimberly-Clark Professional	34256	Delicate Task Wipers
Transeptic	Parker Laboratories, Inc., Fairfield, NJ, USA	09-25	Cleaning Spray