Ultrasound Imaging Artifacts and Scattering Description

10 Questions

What should you do to demonstrate turbulent flow in the presence of stenosis?

Maintain the scale and avoid excessive color gain.

Why is it recommended to keep the PW gate at approximately 1/3 the vessel size during Doppler sample gate imaging?

To avoid sampling all shifts in the vessel that can fill in the waveform.

How does a stenosis affect blood flow proximal to it?

Decrease in diameter upstream leads to higher diastolic pressure.

Which law can be applied to understand how a stenosis affects blood flow?

Pouseuille’s Law

What happens if the Doppler gate is too large for the vessel during imaging?

All shifts in the vessel are sampled, filling in the waveform.

How does velocity change at the stenosis according to the Bernoulli Effect?

It increases as the area decreases.

What impact does high Doppler gain have on the appearance of a waveform?

'Filling' the waveform with all shifts.

'Spectral broadening' in a Doppler waveform occurs due to...

'Filling' of shifts when gate is too large.

'High resistance waveform' proximal to a stenosis is characterized by...

'Increase in resistance'.

At what point along a vessel would you expect to find 'elevated PSV and EDV' according to the text?

Proximal to stenosis

Study Notes

Ultrasound Basics

Reflection: angle of incidence does not matter, results in unclear boundaries and shadowy appearance
Scattering: occurs at small interfaces, size equal to one wavelength, creates softer reflections and appearance
Rayleigh's scattering: when interface is smaller than one wavelength, scattered reflections are equal in all directions

Refraction

Occurs when sound beam changes direction or bends between mediums
Requires oblique incidence and different propagation speeds
Snell's Law: relates incident angle and refracted angle when mediums have different propagation speeds and oblique incidence
Critical angle: when US beam is extremely oblique, no transmission of sound

Pulsed Ultrasound

Generates pulses of 2-3 cycles only
Pulse echo principle: sends pulse, waits for echo, and uses listening time to know location of reflection
PRF (pulse repetition frequency): depends on depth, inversely related to depth
PRP (pulse repetition period): time between beginning of one pulse to beginning of the next pulse, includes listening time

Shadowing and Enhancement

Shadowing: helpful tool to identify calcified structures, increases with frequency
Posterior enhancement: caused by lack of attenuation, produces brighter echoes posterior to fluid-filled objects
Edge shadowing: dropout or shadowing at lateral edges of a round structure, caused by refraction

Artifacts

Double image: caused by refraction, produces two images side by side
Beam width artifact: loss of lateral resolution along the beam width, echoes appear as horizontal lines
Ways to fix artifacts: change angle, spatial compounding

Blood Flow and Vessel Dynamics

Bernoulli Effect: describes relationship between pressure and velocity at a change in vessel radius or diameter
Pressure and velocity are inversely related
Types of blood flow: laminar, parabolic, plug, and turbulent
Laminar flow: organized, concentric streamlines, spectral window in waveform
Parabolic flow: highest velocities in center of vessel, lowest next to wall
Plug flow: all layers move at the same velocity
Turbulent flow: abnormal, disorganized flow, often seen distal to stenosis

Doppler Ultrasound

Doppler sample gate: should be kept at approximately 1/3 the vessel size
Gate too large: spectral broadening, waveform appears disorganized
Gate too small: waveform appears normal
Doppler gain: too high, color "bleeds" out of vessel, underestimate disease
Doppler gain: too low, vessel appears not filled in, overestimate disease

Stenosis and Blood Flow

Proximal to stenosis: decrease in diameter, increase in resistance, high resistance waveform
At the stenosis: elevated PSV and EDV, velocity increases to maintain volume flow, lowest pressure
Distal to stenosis: turbulent flow, abnormal flow patterns