Doppler ultrasonography principles

Basic, Basic Sciences

Fast facts

Medical U/S detects at 1-20MHz, whereas the human ear detects sound at 20-20,000 Hz

The speed that sound waves can move through a medium depends on the density and resistance of the medium

B-mode: aka brightness mode aka 2D mode is the DEFAULT mode to give a linear array of scan images when machine is turned on.

What determines Resistance to flow? Radius, viscosity and length

Poisseille’s Law:  Flow= pi x pressure x radius4/ (8 x viscosity x Length)

Doppler frequency changes in sound waves off a moving object. 

U/S images of flow- a series of pulses gets transmitted to detect blood movement

• Echoes from a stationary object(aka tissue)- same from pulse to pulse 

• Echoes from movies scatterers exhibits slight differences in time for significant to be returned to receiver

Doppler Frequency: differences in echo expressed as a phase shift

Doppler Sonogram: Doppler frequency which is processed and produce a color flow display

What does Doppler signal depend upon?

1. Blood velocity: increase in velocity will increase Doppler frequency

2. U/S frequency increases lead to increases in Doppler frequency

3. Choice of frequency: better sensitivity to flow or better penetration

4. Angle of U/S probe: increased frequency as Doppler U/S beam becomes more aligned with flow direction

Flow Imaging Modes 

1. Color - gives limited information over a large area and is used to identify vessels; red means blood moves towards probe and blue is blood moving away from probe

2. Spectral- looks at flow in just one site to allow calculation of velocity and indices

These are used in complement to each other

Color Flow Imaging

Puts color flow Doppler shifts onto a superimposed B mode image

Factors affecting:

A. Power: transmitted into tissue

B. Gain- sensitivity to flow signals

C. Frequency: trade off of penetration for sensitivity and resolution

D. Scale(aka pulse repetition)- low pulse repetitions frequency looks at low velocities and high pulse repetition frequency

Blood flow calculations

1. Velocity- can be calculated with the Doppler equation if the beam angle is known but can be prone to error

2. Absolute flow-fraught with errors

3. Wave form analysis-look at shape and spectrum to figure out proximal(PAD) and distal disease processes (uterine or fetal changes)

84%

Answered correctly

2017

Year asked

80%

Answered correctly

2016

Year asked

Author
Dalia Saha, MD