Unraveling the Mysteries of Doppler Ultrasound: A Comprehensive Yet Rough Guide

Functional Ultrasound Imaging is poised to transform the realm of Brain-Computer Interfaces, setting new benchmarks with its unparalleled spatial and temporal resolution, extraordinary sensitivity, and compact form factor.

Propagation of task-related brain activity in behaving primates w/ Functional Ultrasound Imaging

As a Brain-Computer Interface researcher on a mission to build the tools necessary to read and write to our neural code, one thought that often clouds my mind is which modality will drastically alter the trajectory of the space.

With significant advances in Ultrafast Ultrasound Imaging,Functional Ultrasound Imaging, also referred to as fUS, has been picking up storm in BCIs.

Functional Ultrasound is a non-invasive technique that employs ultrasound waves to detect changes in cerebral blood flow [refer to this article to understand how CBF is correlated to brain activity]. By quantifying these alterations, fUS can produce real-time images of brain activity, enabling a better comprehension of the intricate workings of the brain.

In this super rough memo, we will investigate the Fundamental Principles of Functional Ultrasound, specifically Doppler and how it can be leveraged to enrich our understanding of the brain.

Doppler Effect:

Analogous to that of a siren. The sound of a siren sounds as it approaches, passes and then drives away. The waveform compressed as the ambulance comes close and then stretches as the ambulance departs.

Sounds of Siren Passing = Doppler Effect

Doppler Ultrasound uses the same idea to determine the speed and direction of blood inside the arteries.

• The transducer is held at an angle to the blood vessel and the altered frequency of the echo returning to the transducer from the blood flow tells the computer the bloods speed and direction
• ▹ Displayed as different colours on the final image to show changes in or absence of blood flow
• ▹ Blood towards probe: + Pos. Doppler Shift
• ▹ Blood away from probe: -Neg. Doppler Shift
• The Reflector is almost always blood cells but could be tissue

Elastography:

• Used to differentiate tumours from healthy tissue based on the tissues relative stiffness
• Healthy tissue/ benign tumour: Compressible
• ▹ Malignant Tumours: Firm

Compression is now done by newer Elastography systems to send out high-pressure pulses that compress target tissues by a predefined amount of force and display the resulting compression levels in various coded colours.

Principles of Doppler Ultrasound

Doppler Effect in Ultrasound

• Blood Towards Probe: Pos. Doppler Shift
• Blood Away From Probe: Neg. Doppler Shift

Things that affect Doppler Shift:

• Probe Frequency (# of Waves that Pass a Fixed Place in a Given Time) -> Higher Freq Probe (like Linear) will generate greater doppler shift. However, the selection of probe is balance between sensitivity to flow (for high frequency) and penetration (for low frequency)
• Velocity of Blood -> Higher velocity equates to a greater doppler shift.
• Angle of Insonation -> Helps determine which views we want to interrogate (a valve or blood vessel), Always optimize angle of insonation to be parallel to direction of blood flow (meaning that the blood flow should be flowing directly towards or away the probe)

Higher Frequency Obtained if:

1. Velocity is Increased
2. Beam Aligned Parallel to Flow Direction
3. Higher Frequency is Used (Probe Frequency)

How to Optimize Angle of Insonation:

1. Choose Certain Views -> Allowing for optimal orientation of our probe relative to the area of interest
2. Employ Probe Manipulation -> Use of techniques like fanning or rocking the probe as we are in a given view to try to optimize our alignment
3. Use Angle Correct Function -> Correct for suboptimal alignment (generally does not work for under 60 degrees)

Direction of Doppler Shift:

The Bernoulli Equation:

• The change in measure across a given distance is called a pressure gradient.
• The pressure gradient results in a net force that is directed from high to low pressure.
• The force is called pressure gradient force

Spectral Doppler:

• Graphical representation to plot velocity of blood on the y axis over time on x axis
• Blood flow towards the probe is positive deflection above baseline
• Blood flow away from the probe is negative deflection below baseline

Mapping Spectral Doppler:

Pulsed Wave:

• Sends pulses of Ultrasound waves to a specific point, then listens for reflection (must pause first to listen)

Continuous Wave:

• Continuously sends Ultrasound waves and continuously listens for reflected signals

Pulsed Wave: Pulse Repetition Frequency (PRF)

• Pulsed wave must send and then wait to receive before emitting another pulse
• PRF is a cycle of emitting a wave into tissue and then receiving the returning echo [Huge Limitation]

• PRF is Determined by:
• ▹ Ultrasound Transducers
• ▹ Target Depth -> Deeper Structure = Longer to Listen

Nyquist Limit:

• The “upper limit” of the velocity that can be detected with PW is called the Nyquist Limit
• The Nyquist Limit is half of the PRF -> Waveforms must be sampled at least twice per wavelength to be reliably interpreted.
• Key Concept -> PW is subject to a nyquist limit
• If PRF is too low then aliasing will occur (signals will look similar) because the doppler frequency shift is greater than half the PRF which is the nyquist limit (meaning if the nyquist limit is exceeded, then signals will become indistinguishable)
• The Nyquist limit be adjusted for various applications
• ▹ Slower: Moving blood cells/ objects turn PRF/ Nyquist down
• ▹ Faster: Moving blood cells/ objects turn PRF/ Nyquist up

in contrast with CW:

• CW = continuously sending and receiving signals
• No designated “waiting period” -> NoPRF = No Nyquist Limit
• Can change your scale to measure much higher velocities

Pulsed Wave VS Continuous Wave:

Colour Flow Doppler

• NOT a form of Spectral Doppler
• ▹ Borrows spatial precision from PW model
• Uses short pulses from multiple locations to build a colour map
• Subject to a Nyquist Limit [aliasing is possible]
• ▹ Same as PW BUT colour map instead of spectral tracing

• We can make use of aliasing in order to identify areas of very high velocity blood flow as it is represented by a mix of yellow, blue, red colours
• Due to the machines senselessly plotting a flow as it is unable to resolve direction as it exceeds the nyquist limit

Optimizing Colour Doppler

• Nyquist/PRF can be adjusted for various applications -> presets do automatically [High Nyquist Limit will miss areas of low blood flow]
• Colour gain: can be manually adjusted -> increase until you see spontaneous colour speckling and then dial back
• Direction of blood flow to be parallel to the angle of insonation of the probe
• Colour box: As small as you can make it while capturing relevant structures
• Colour Signal will be suboptimal on:
• ▹ Poor quality images
• ▹ Very deep structures
• ▹ Large angle of insonation -> not very close to being parallel to blood flow

TCD — Non Invasive Way to Monitor for Intracranial Pathology

• 2 Methods:

1. Colour Doppler initially to locate blood vessels
2. Pulsed wave doppler to measure velocities

Thanks for reading! Connect on LinkedIn or feel free to email at mikaelhaji@gmail.com.