SPI III: Test 4 - Pulse-Echo Instrumentation & Artifacts

M mode

motion mode; depth in vertical axis

M mode

• depth is vertical

• time is horizontal

M mode

combines A mode and B mode to show motion

• since it shows brightness, it can also be considered B mode

B mode

B scan or brightness mode (gray scale sonography)

• 2D or 3D

A mode

amplitude mode

• used in opthamology

A mode

• depth (time) is horizontal

• amplitude is vertical

slice thickness/partial volume artifact

beam width perpendicular to the scan plane is greater than 2 adjacent reflectors and displays as echoes within an anechoic structure

how to improve slice thickness artifact

using tissue harmonic imaging

speckle

granular appearance of images

what causes speckle?

interference of echoes from the distribution of scatterers in tissue

constructive speckle

echoes add together

destructive speckle

echoes completely or partially cancel out

reverberation

equally spaced reflections of diminishing amplitude with increased imaging depth

reverberation

reverberation

two or more strong reflectors cause multiple reflections

• comet tail

• twinkle (color)

ring down

type of reverberation that appears as multiple parallel lines or solid band behind a reflector

what causes ring down?

vibrations of air bubbles

ring down

mirror image

form of reverberation where there’s a duplication of a structure on the opposite side of a strong reflector

where is mirror image common?

the pleura and diaphragm

• also occurs in doppler

mirror image

refraction

change of direction of the sound beam from one medium to the next

refraction

displays structures laterally from their correct locations

• duplication

refraction

multipath artifact

displays structure deeper than it should be

multipath artifact

oblique incidence causes beam to reflect at an angle

multipath artifact

beam later reflects off another structure and echo takes longer to return (deeper)

multipath

focal banding

region of increased brightness at focal zone

what causes focal banding?

increased intensity of the beam

• intensity = power / area

how to fix focal banding

adjust TGCs

focal banding

grating lobes

additional weaker beams emitted from an array transducer

side lobes

beams from a single element in different directions than the beam

grating lobes and side lobes

strong reflector duplicates structures laterally creating an arc appearance

grating lobes and side lobes

propagation speed error

occurs when the speed of sound in soft tissue is faster or slower than the assumed 1.54 mm/μs

explain this image about propagation speed error

1. reflector is actually here

2. slower speeds place echoes deeper - takes longer to return

3. faster speeds place echoes closer - returns sooner

range ambiguity

all echoes are not received before the next pulse is emitted

range ambiguity

places structures much closer to the surface than they should be; also occurs in doppler

what causes range ambiguity?

PRF is too high

range ambiguity

shadowing

weakening of echoes distal to:

• strongly attenuating structure

• strongly reflecting structure

• from the edges of a refracting structure

shadowing

what causes edge shadowing?

refraction along the edge of a curved structure that decreases intensity of sound posterior to the curved edge

edge shadowing

enhancement

strengthening of echoes distal to a weakly attenuating structure

enhancement

increased brightness behind a weakly attenuating structure

enhancement

how to reduce shadowing and enhancement

spatial compounding and other speckle reduction techniques

aliasing

flow exceeds velocity range & nyquist limit (PRF/2)

aliasing

appearance of doppler information (spectral or color) on the wrong side of the baseline

aliasing

how to reduce or eliminate aliasing

• shift the baseline

• increase the PRF

• increase the doppler angle

• use a lower operating f

• use a CW device

flash artifact

sudden burst of color doppler that demonstrates an extension of color beyond the region of blood flow

what causes flash?

typically tissue or transducer motion

how to reduce flash

increasing wall filter

flash

damaged crystal

gain

determines amount of amplification of echoes

• ratio of output to input (dB)

time gain compensation (TGCs)

amplifies selectively based on arrival time (depth) and compensates for the effect of attenuation on an image

TGCs

allows for display of echoes from similar reflectors at different depths in a similar way

• uniform brightness

dynamic range

ratio of largest to smallest amplitude or power a system can handle (dB)

image display

brightness is proportional to the echo strength

frame

each individual image

frame rate

number of images entered into memory per second

image memory

image frames are stored here

cine loop

storing the last several frames acquired before freezing

freeze

holding and displaying one frame out of a sequence

persistence

reduces noise and smooths the image by frame averaging

persistence

higher levels are appropriate for slow-moving structures

small (narrow) dynamic range

few shades of grey, black to white, high contrast

high (broad) dynamic range

more shades of grey (good contrast resolution)

edge enhancement

sharpens boundaries to make them more detectable and measurements more precise

write zoom

increases number of pixels or scan lines

frequency compounding

echo frequency spectrum is divided into frequency bands by filters, processed separately and then recombined

frequency compounding

can be adjusted to emphasize penetration, resolution, and tissue texture

frequency compounding

commonly used to reduce noise and improve contrast resolution

write zoom

increases number of pixels or scan lines

read zoom

number of pixels or scan lines is the same as the original image (can look like a reduction in pixels)

harmonic imaging

improves image quality by sending pulses of some frequency into the body but then imaging echoes of frequency double that sent in

how does harmonics improve image quality?

• lateral resolution improvement

• grating lobes are eliminated

• superficial reverberation reduced or eliminated

panoramic imaging

expands the image beyond the normal limits of the field of view of the transducer

• new echoes are added to the image in the direction in which the scan plane is moving

spatial compounding

averaging of frames that view anatomy from different angles

• smoothes imaging surfaces and visualization of structures behind highly attenuating structures

how does spatial compounding improve image quality?

• reduction in speckle and clutter artifacts

• smooths imaging surfaces

• visualization of structures behind a highly attenuating structure

coded excitation

uses a series of pulses and gaps, rather than a single driving pulse

coded excitation

ensembles of pulses drive the transducer to generate a single scan line

coded excitation

this approach accomplishes functions such as multiple foci, separation of harmonic echo bandwidth from transmitted pulse bandwidth, increased penetration, reduction of speckle with improved contrast resolution, and gray-scale imaging of blood flow

assumptions of the ultrasound system?

• sound travels in a straight line

• echoes originate only from objects located on the beam axis

• the amplitude of the returning echoes is related directly to the reflecting or scattering properties of distant objects

• the distance to reflecting or scattering objects is proportional to the round-trip travel time

gray-scale maps

assignment of specific display brightness to numbers retrieved from the memory

contrast resolution

the ability of a gray-scale display to distinguish between echoes of slightly different intensities

contrast resolution

depends on the number of bits per pixel in the image memory

temporal resolution

ability of a display to distinguish closely spaced events in time and to present rapid moving structures correctly

temporal resolution

dependent on the frame rate

• if frame rate increases, this improves

• if PRF increases, FR increases

what is the acoustic power determined by?

primarly the amplitude of the ultrasound wave

what does the thickness of the element affect?

the frequency

• thicker element = lower frequency

• thinner element = higher frequency

what does changing to a higher frequency transducer result in? and how does a sonographer compensate?

results in improved image resolution, with the pitfall being a decrease in depth penetration; sonographer compensates by selecting the correct frequency for the area of the body being scanned