Final Study Guide

near zone length (NZL)

the distance from the transducer face to the location where the beam is the smallest in diameter

increase

an increase in diameter of element/aperture causes a ___________ in NZL

increase

an increase in frequency causes a ____________ in NZL

decrease

an increase in diameter causes a ______________ of beam divergence in far field

decrease

an increase in frequency causes a ____________ of beam divergence in the far field

phantom

• stimulates tissue properties of soft tissue, cystic, and solid structures

• attenuation is similar to soft tissue

resolution

small nylon fibers in phantoms are used to evaluate __________

hydrophone

a microphone that detects sound waves under water

electrical interference

strong electrical interference from other nearby electrical devices can create interference patterns

faulty elements

piezoelectric crystals may become damaged or breaks in wiring can occur; displayed as anechoic vertical lines

focal banding

• horizontal brightness at the level of the focus

• occurs because of the increased intensity in the focal zone

adjust the TGCs

to fix focal banding:

side lobes

caused by echoes from strong reflector outside central beam; single element

grating lobes

echoes from weaker beams outside the main beam from an array transducer; appears as duplicated structures laterally

harmonics

side lobes/grating lobes are reduced with:

slice thickness (partial volume filling)

beam is too thick in elevational plane; gives the appearance of debris in an echo-free structure

speckle

the granular appearance of images that is caused by interference of echoes from the distribution of scatterers in tissue

spatial compounding & persistence

speckle can be reduced by using:

mirror image

• occurs when beam encounters a large specular reflector that acts as a mirror

• appears as duplication of objects opposite of a highly reflective structure

reverberation

• occurs between two strong reflectors→sound waves bounce back and forth between structures

• appears as equally spaced reflectors on the image

harmonics

reverberation can be reduced with:

comet tail

• type of reverberation artifact

• small focal reflectors cause a narrow reverberation artifact

• useful in diagnosis of adenomyomatosis/cholesterolosis of the GB and colloid thyroid nodules

ring-down

• type of reverberation artifact

• associated with gas→helps diagnose gas conditions

refraction

• aka duplication or ghost artifact

• change in beam directions when beam encounters boundary between two materials with different propagation speed

• causes object to be projected laterally

sliding transducer laterally

refraction can be reduced by:

edge shadowing

• type of refraction artifact

• caused by a combination of reflection and refraction at the margin of a well-defined object

• appears as a shadow originating from the edge of a structure

spatial compounding

edge shadowing can be reduced with:

multipath

• beam reflects off the structure at an angle

• displays object deeper than it really is because beam does not travel in a straight line

propagation speed error

a structure of lower or higher propagation speed than 1540m/s; cannot be eliminated

range ambiguity

occurs when echoes from the preceding pulse arrive at a time when another pulse has been emitted

decrease output power/gain, switch to one focal zone, decrease PRF

to reduce range ambiguity:

shadowing

caused by a highly attenuating structure

dirty shadow

• occurs behind a strongly attenuating structure but the structure produces some echoes

• appears as hypoechoic shadow behind structure

enhancement

caused by a structure of low attenuation adjacent to a structure of normal attenuation

gain & TGCs and spatial compounding

enhancement can be reduced by:

aliasing

• most common Doppler artifact

• display of Doppler information in the wrong color or on the wrong side of the baseline

Doppler shift exceeds one-half of PRF (Nyquist limit)

the cause of aliasing is:

• shift baseline

• increase PRF

• increased Doppler angle

• use a lower operating frequency

• switch to CW

Ways to fix aliasing

mirror image

• duplication of Doppler shifted echoes on the opposite side of a strong reflector or the baseline

• causes: highly reflective surface, high Doppler gain, Doppler angles near 90 degrees

change transducer position, decrease Doppler gain

How to fix mirror image in Doppler

change transducer position

to fix shadowing:

clutter/wall thump

• type of noise artifact

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

wall filter

to fix clutter, increase:

flash (ghosting)

sudden burst of Doppler signal caused by tissue motion, transducer motion, talking/coughing, bowel

increase PRF, decrease gain, hold still/wait

how to fix flash

twinkle

• reverberation artifact

• similar to comet tail but with color

• useful in identifying kidney stones

axial resolution

• minimum reflector separation necessary to resolve reflectors parallel to sound beam

• does not change with depth

SPL

axial resolution is determined by:

SPL

axial resolution is improved by reducing:

lateral resolution

• minimum reflector separation necessary to resolve reflectors perpendicular to the beam

• varies with depth

beam width

lateral resolution is determined by:

focusing

lateral resolution is improved with:

temporal resolution

the ability to follow moving structures in temporal detail

directly

temporal resolution is _____________ related to FR

line density, lines per frame, depth & PRF, sector width, and number of foci

temporal resolution is dependent on:

contrast resolution

the ability of the gray scale display to distinguish subtle differences in echogenicity of adjacent tissues

contrast resolution

____________ is determined by # of pixels in an image and # of shades of gray that can be displayed in each pixel

system’s memory and dynamic range settings

contrast resolution is controlled by:

resolution

increased bandwidth improves _______________

quality factor

increased bandwidth reduces:

damping (backing) material

epoxy resin attached to back of element that absorbs vibrations and reduces # cycles/pulse

matching layer

has impedance value between crystal and tissue to improve sound transmission into body; minimizes reflection

coupling gel

eliminates air layer between transducer and skin

transducer arrays

• transducer assemblies with several elements

• each element is independently controlled

• required for real-time scanning

• use electronic means of sweeping, steering, and focusing beam→accomplished by sequencing and phasing

sequencing

voltage pulses are applied to small groups of elements in succession, time delay set between pulses

phasing

• voltage pulses are applied to elements in rapid succession

• allows for sweeping, steering, and focusing beam

beam steering

• sweeping the beam to produce automatic scanning

• accomplished with phasing

electronic (transmit) focusing

• longer time delays (greater curvature) create a more superficial focus

• shorter time delays (less curvature) moves focus deeper

aperture focusing (variable aperture)

• smaller groups (less elements) used for short focal lengths

• larger groups (more elements) used for foci located at increasing depths

aperture, focal length, and wavelength

____________ determine beam width at focus

aperture

to maintain the same beam width for increasing focal lengths, ______________ must also be increased

dynamic aperture

aperture changes as focal point is moved

dynamic (receive) focusing

• focusing occurs during echo reception

• improves lateral resolution

dead zone

the distance between the front surface of the transducer and the first identifiable echo

thermal index (TI)

the ratio of total acoustic power required to cause a rise in temperature of 1 degree C anywhere in the beam

attenuation (heat) & SPTA intensity

thermal index relates:

output power and exposure time

thermal index is directly related to:

frequency

TIS increases with an increase in:

focal diameter

TIB increases with:

2

the thermal index in adults should be less than:

0.7

the TI for fetuses should be less than:

mechanical index (MI)

• a measure of pressure amplitude that occurs in tissue

• indicator of cavitation

peak rarefactional pressure

mechanical index is directly related to:

1.9

the FDA limit says mechanical index should be less than or equal to:

0.3-0.7

the mechanical index for fetuses should be less than or equal to:

A-mode (amplitude mode)

was displayed on a graph with x-axis being depth and y-axis showing strength

B-mode (brightness mode)

• 2D images called B-scan

• displayed dots with brightness showing strength and location

spatial compounding

• aka compound imaging

• multiple frames are formed at varying angles which are averaged together to create one image

• uses phasing

• improves image quality by reducing speckle, reducing clutter & reverberation artifacts, reducing shadowing, and improving presentation of specular reflections

frame rate

spatial compounding does not affect:

persistence (frame averaging)

• goal is to reduce signal to noise ratio

• angle not changed between frames

• useful in color Doppler but movement of anatomy must be minimal

frequency compounding (frequency averaging/frequency fusion)

• averages 2 or more frames created at different frequencies to form an image

• improves texture because it combines multiple frequencies

• broad bandwidth transmitted, narrow bandwidths received

edge enhancement

• sharpens boundaries to make them more detectable and allow for more precise measurements

• preprocessing technique

harmonics

• transmit at a lower frequency and receive at higher frequency

• improve image quality and lateral resolution

• reduce grating lobe, reverberation, and clutter artifacts

depth

__________ is inversely related to PRF and FR

dynamic range

the ratio of maximum to minimum amplitude a system can handle

compression

• technique that decreases the dynamic range

• some is controlled by operator but some is not

• weak echoes can be lost

Doppler effect

change in frequency (and wavelength) caused by motion of a sound source, receiver, or reflector

Doppler shift

• the difference between the frequency of the received sound and the frequency of the emitted sound

• can determine direction (positive or negative) and velocity

• units are in kHz

• fD=fR-fT

Doppler equation

• we measure shift to calculate velocity

• fD= (2/c) x v x fT x cos0

wall filter

allows us to filter out unwanted motion from the vessel walls