Doppler Physics and Instrumentation

Doppler shifts

sonographic instruments measure:

velocity

sonographic instruments calculate:

200-15,000 Hz

Doppler shift range

Doppler shift frequency

the difference of frequency of the received sound and the frequency of transmitted sound

duplex imaging

• combination of 2D gray-scale imaging and Doppler information (color Doppler, power Doppler, spectral Doppler)

• allows for placing sample volume in a specific vessel

color Doppler

presents real-time blood flow or tissue motion information along with gray-scale anatomic imaging

gray-scale

non-Doppler shifted echoes create ___________________

signal processer

Doppler shifted echoes are detected in the ________________

autocorrelation

the signal processor uses a mathematical technique called ______________

autocorrelation

determines the mean, variance, and sign of the Doppler shift signal at each location along the scan line

variance

measures dispersion or the spread around the mean

100-400

____________ Doppler samples (locations) per scan line

pulses

multiple _________________ are involved in all images with autocorrelation because they are required in the autocorrelation process

3-32

___________ pulses = 1 scan line with autocorrelation

1

____________ pulse = 1 scan line with gray-scale

frame rate

a bigger color box lowers the _______________ because it takes time to generate more scan lines

qualitative

the color map provides _________________ information

hue

color perceived by viewer

luminance

brightness

saturation

the amount of hue in a mix with white

Nyquist limit

upper limit of Doppler shift that can be properly displayed by pulsed-wave

PRF

if Doppler shift exceeds ½ the _____________, aliasing occurs

color aliasing

improper color progression—>red to yellow to green to blue

flow reversal

if there is _________________, red and blue will be separated by black

• demonstrates blood flow (character of flow—>laminar, turbulent, speed)

• determines direction of flow

• demonstrates nonvascular motion

advantages of color Doppler:

• angle dependence

• lower FR

• lack of detailed spectral info—>cannot quantify velocity

color Doppler limitations:

• decrease output power

• decrease color gain

to fix color bleeding:

• decrease color gain

• increase color reject

to fix color noise:

ensemble length/packet size

• number of pulses used for each color scan line

• if high—>improved accuracy, improved detection of slow flow, decreased FR

priority

• selects gray-scale echo strength

• lower strength echoes displayed as color

low

__________ PRF allows for display of slow flow (small fD)

high

___________ PRF is for fast flow or eliminating aliasing

noise

smoothing/persistence reduces ______________

color reject

• threshold below which weak echoes are eliminated

• reduces noise

wall filter

• rejects frequencies below a threshold

• too low—wall motion appears in color

• too high—slow blood flow signals will be removed

flash

• sudden burst of color beyond region of blood flow

• improper display of color caused by movement of transducer or tissue (cardiac motion, artery pulsatility, respiration, talking, peristalsis)

decrease color gain, increasing wall filter, increasing PRF, decreasing persistence, reducing box size

to reduce flash:

mirror image

• additional color display at wrong location

• occurs when vessel lies proximal to a highly reflective interface

decrease output power, decrease color gain, change angle of insonation

to reduce mirror image artifact:

power Doppler

presents 2D Doppler information by color encoding the amplitude of the Doppler shifts

concentration/number

in power Doppler, the power of the Doppler shift is determined by the _________________ of the moving scatterers

• not angle dependent

• no aliasing

• improved sensitivity (slow flow in small vessels)

advantages of power Doppler:

• no directional information

• no flow speed or character information

disadvantages of power Doppler:

spectral Doppler

• presents Doppler shift frequencies in order

• determines their sign

• provides quantitative data for evaluating Doppler shift

phase quad detection

• a quadrate phase detector separates forward and reverse Doppler shift voltages

• splits received frequencies into 2 channels (positive shift and negative shift)

• occurs within the signal processor

phase quad detection

___________________ is also known as Doppler shift detector

fast fourier transform

• converts Doppler shift information into a visual spectrum

• breaks down complex signals into individual frequencies which display as vertical lines

• occurs within the signal processor

fast fourier transform

______________ is also known as the spectrum analyzer

continuous wave

• detects the Doppler shift within the region of overlap between the beams of transmitting and receiving transducer elements

• no depth selectivity

pulse wave

• emits ultrasound pulses and receives echoes using a single element transducer or array

• ability to select information from a particular depth (range gating, sample volume)

CW transducers

• dual element transducer assemblies (one for transmitting and one for receiving)

• not damped

• the region over which Doppler info can be acquired (Doppler sample volume) is the region of overlap between the transmitting and receiving beams

oscillator

• part of the beam former in CW Doppler instrument

• produces a continuously alternating voltage that drives the transducer element

RE amplifier, quad phase detector, and audio frequency amplifier

the signal processor in a CW Doppler instrument includes:

PW transducers

• damped using a single element transducer or array

• pulses are longer than those used in imaging

Doppler cursor

• line superimposed on the B-mode images that shows the path of the sound beam

• used in both CW and PW

range gating

enables depth selectivity and a small Doppler sample volume

sampling system

PW is a ___________________

beam width

with sample volume, width is equal to the _______________ at the sample volume

length of the pulse

with sample volume, length is limited by the __________________

narrow

a small sample size leads to a ____________ range of fDs

broad

a large sample size leads to a ____________ range of fDs

spectral broadening

• vertical thickening of the spectral trace (filling in of spectral window)

• produced by disturbed or turbulent flow

• can also be demonstrated with a large sample size in a vessel with laminar flow

inversely

the cosine of the angle and the Doppler angle are ______________ related

flow

the angle should be set parallel with ________________

Doppler gain

• amplifies received Doppler signal

• affects both spectral display (brightness) and volume

• should be set high enough to display low flow, but not so high that it produces noise

Doppler output power

• adjusts intensity of the transmitted beam

• increased transmitted intensity increases sensitivity

• should be balanced with Doppler gain

• moderate setting is ideal

PRF

• aka velocity scale

• adjusts max velocity displayed on spectrum

• low scale for low velocity flow and high scale for high velocity flow to avoid aliasing

fast

__________ sweep speed is good for detailed waveform analysis

slow

_____________ sweep speed is good for inclusion of multiple waveforms, venous response to limb compressions, demonstrate respiratory phasicity

wall filter

• aka high-pass filter

• removes data below selected frequency

• too low—>may demonstrate artifacts

• too high—>may eliminate low velocity flow

clutter/thump

• high-intensity, low-frequency Doppler shifts

• caused by heart, vessel wall, or cardiac valve motion

cross-talk

• a type of mirror-image artifact

• duplication of flow on the opposite side of the baseline

• occurs when the quadrate phase detector is saturated and there is leakage into the opposite direction channel

• important to determine if flow is real or artifact

decreasing Doppler gain

cross-talk can be eliminated by: