Functional Transcranial Doppler Ultrasound

“Hey Hannah – do you think there is much more to discover in ultrasound technology?” Hannah looked at me and…

It was about 30 years ago when I asked this question to a fellow graduate student while crossing the Duke quad. I was in the middle of a daunting doctoral program in ultrasound engineering—vanilla delay-and-sum beamforming had been the norm for many years, and we all knew speckle was a fundamental physics limitation. So what more could be done?

Perhaps it was the Carolina heat, or maybe I was addled from late-night calculations… but my, how short-sighted and naïve I was! Since that time, a host of new technologies and discoveries have proliferated within the ultrasound landscape, with many already making their way into the clinic. And of course, ultrasound has never lost its inherent competitive benefits of safety, mobility, and affordability. The future continues to look bright for our favorite imaging modality.

Today, I’d like to tell you about one of the more ‘niche’ ultrasound applications – functional transcranial Doppler ultrasound (fTCD). fTCD is an extension of transcranial Doppler ultrasound (TCD). Simply put, TCD is pulsed-wave Doppler of the basal cerebral arteries. Recently, a great clinical introduction to transcranial Doppler (TCD) was given on the Sonography Lounge.

The “functional” aspect of fTCD refers to monitoring changes in cerebral perfusion during neural activation by a functional task. These tasks could include motor, sensory, or cognitive stimuli. The fTCD response is based on neurovascular coupling – essentially, the link between neural activity and cerebral blood flow. Neurovascular coupling is something we don’t completely understand, but certainly something we can observe. One of the simplest (and most famous) examples is the increase in posterior cerebral artery blood flow velocity in response to a perceived visual change.

fTCD serves as a natural complement to other perfusion imaging modalities such as fMRI, PET, and fNIRS. The high temporal resolution (~100 Hz), anatomical target (deep branches off the circle of Willis), amenity to motion (robust during movement tasks), and safety couple well with the spatial and temporal extent and limits of these other modalities. Interestingly, because of these advantages, fTCD is being used in psychology and neuroscience research.

What medical information can fTCD results give us? Clinically, the change in cerebral blood flow might indicate hemispheric lateralization, help monitor intracranial pressure, show potential for stroke recovery due to somatosensory activation, and even predict preclinical Alzheimer’s disease – to name only a few! A wide range of clinical applications makes this easy-to-learn technique a tool with powerful potential.

By the way, how did Hannah (not her real name) answer my question? She just looked at me and laughed!

Greg Bashford, PhD, PE, is a Professor and Biomedical Engineer at the University of Nebraska.

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