This protocol forms the basis for a functional TCD experiment because almost all functional TCD experiments require placement of a fixation device to record a stable signal over an extended period of time. The main advantage of functional TCD is its high temporal resolution measurement of changes in cerebral blood flow. Finding the middle cerebral artery using TCD takes practice.
It is crucial to hold the transducer steady and move very slowly. The fine motor control needed for making small adjustments in transducer position and direction takes time to develop. Practice on as many volunteers as you can find.
Visual demonstration is important for two reasons. First, it is helpful to see exactly where to place the transducer. Second, a crucial part of learning functional TCD is learning the sounds associated with the different arteries.
Begin by setting the parameters for a transcranial Doppler ultrasound or TCD. Keep the power at a reasonably high value during the initial search for the middle cerebral artery or MCA. Once the MCA signal is located, reduce the power as much as possible while still maintaining a good signal.
Set the sample volume to eight to 12 millimeters during the initial search for the MCA signal. If the signal is difficult to find, increase the gate size to increase the intensity of the signal. Set the gain at a medium level, with the goal of keeping background noise at a minimum.
Set the high pass filter cutoff between 50 and 150 hertz. If the subject is an adult, set the depth to 50 millimeters, which is the average midpoint depth for the M1 segment of the MCA. Apply enough ultrasound gel to cover the surface of the transducer.
Alert the subject that the gel may feel cold, then place the transducer on the temporal window. After placing the transducer on the scalp, search for the MCA signal, which will generally be located slightly anterior and rostral from the location of the initial transducer placement. If the TCD spectral signal is not immediately obvious, adjust the angle of the transducer while keeping it in the same location relative to the scalp.
Slowly angle the probe from rostral to caudal and posterior to anterior. If a signal is still absent, check the color M mode display for flow in the MCA, which is indicated by red color. Increment or decrement the signal depth in five millimeter steps and continue searching.
If flow was visible in M mode but not in the Doppler spectra, increase or decrease the depth until the flow signal is visible on the Doppler spectra. If a satisfactory signal is still not obtained, move the transducer to a nearby slightly more anterior position on the scalp, and repeat the search. When an optimal MCA signal is obtained, note the depth and maximum velocity.
Using a washable makeup pen, place a mark on the scalp where the optimal signal was found. Next, search for the bifurcation. Increase the depth until the signal from the bifurcation of the internal carotid artery into the middle cerebral artery and anterior cerebral artery is noted, typically at a depth of 51 to 65 millimeters.
Search for the optimum bifurcation spectral signal, striving for the highest velocity spectral signal possible. When an optimal bifurcation signal is obtained, note the depth of the bifurcation. Adjust the fixation device to the subject's approximate head size.
Alert the subject before placing the headset on his or her head. After placing the headset, adjust the fixation device's fit. And ask the subject if the device is too tight.
Loosen the mechanism of the fixation device so that the transducer can move freely. Apply enough ultrasound gel to the transducer to cover the face of the transducer. Adjust the fixation device so that the transducer is located over the top of the previously made mark.
Search for the optimal MCA spectral signal, striving for the highest velocity spectral signal possible. When the optimal MCA signal is found, tighten the mechanism of the fixation device to lock the transducer in place. Note the depth and all other settings.
Decrease the power as much as possible while still maintaining a spectral envelope that traces the maximum velocity accurately. Begin recording on the TCD software. Instruct the subject to breathe normally for three minutes to achieve a good baseline recording and allow cerebral blood flow velocity to stabilize from any previous experiments or stimuli.
Countdown slowly from three. On the count of one, ask the subject to begin breath holding after a normal inspiration. Place a marker in the TCD recording to signify the start of breath holding.
Have the subject hold their breath for 30 seconds or until they're no longer comfortable holding their breath. When the subject inhales, place a marker in the TCD recording to signify the end of breath holding. Continue monitoring cerebral blood flow velocity using TCD for at least 30 seconds following the end of breath holding to ensure that the velocity returns to baseline values.
Doppler spectra and color M modes from the midpoint of the M1 segment of the middle cerebral artery or MCA are shown here. The spectra were taken at the same position on the scalp, but at different angles. It's important to note that a very small change in angle can greatly improve Doppler signal strength.
A simple Doppler spectrum and M mode from the bifurcation of the ICA into the ACA and MCA is shown here. The overlapping red and blue shaded regions and the M mode image denote the MCA and ACA respectively. Doppler spectra and M mode images we're taken at different time points in the breath hold maneuver.
The mean blood flow velocity at baseline was 56 centimeters per second, increasing to 70 centimeters per second by the end of breath holding, and undershooting to 47 centimeters per second after breath holding ended. The entire breath holding experiment is shown here. The envelope shown by the white line increases gradually during breath holding remains elevated for approximately 15 seconds after breath holding ends, undershoots for approximately 20 seconds, and then recovers to baseline values.
Several examples of bilateral TCD spectra and M mode suitable for bilateral FTCD are shown here. It is important to remember that finding the MCA signal involves very controlled, fine motor movements. The only way to become proficient at finding the MCA is to practice on as many different people as possible.
This technique paved the way for researchers to measure brain activity in environments that were previously inaccessible. For example, unlike within MRI, subjects wearing TCD fixation devices can move freely, so brain lateralization during active tasks can be studied.
