Chapter 2: Ultrasound Principles Flashcards
echoes on the image that were not caused by actual reflectors in the body
artifacts
ultrasound has the ability to cause changes to the tissue if proper settings are not used
bioeffects
principle of constantly transmitting a sound wave into the patient to obtain a spectral Doppler waveform
continuous wave
Tool for measuring blood flow quantitatively or qualitatively using pulsed-wave or continuous-wave techniques
Doppler
Principle of sending in a small group of sound waves and then waiting for that pulse to come back so that an image can be displayed. Also used for spectral and color Doppler.
pulsed wave
the part of the ultrasound machine that transmits and receives sound via an array of piezoelectric elements
transducer
pressure/mechanical waves
sound waves
a series of reflectors along one vertical line
scan line
all the scan lines displayed on the screen
frame
propagative of wave is parallel to the movement of molecules within the medium
longitudinal wave
simplest unit of a wave
cycle
can be measure by height, length, and other parameters
cycle
the number of cycles that occur in 1 second
frequency
the frequency of a pulsed wave transducer is determined primarily by:
thickness of piezoelectric element
other names for frequency
operating frequency
center frequency
resonating frequency
units for frequency
Hz
typical range of frequency in ultrasound
2-20 MHz
time taken for one cycle to occur in milliseconds
period
speed at which sound moves through a medium
propagation speed
What is propagation speed determined by?
only by the medium through which sound is traveling
What is the average propagation speed of soft tissue?
1540 m/s
length in (mm) of one cycle of sound, from the beginning of the wave to the end of the wave
wavelength
What is wavelength determined by?
propagation speed divided by the operating frequency
height of a cycle, from the baseline to the peak of the cycle
amplitude
Units of amplitude
pressure (pascals)
density (kg m^3)
particle motion (mm)
acoustic impedance unit
Rayls
property of the medium
acoustic impednace
What is acoustic impedance determined by?
the product of the density and propagation speed
With a difference in impedances between adjacent tissues,
no reflection is generated
The larger the differences in impedances, the _____ the return echo.
larger
pulse is sent into the body by the transducer, and machine waits for that pulse to return before transmitting the next pulse
Pulsed-wave ultrasound
number of pulses per second in Hz or kHz
pulse repetition frequency
inversely related to depth of reflector and unrelated to operating frequency
pulse repetition frequency
length of the pulse
spatial pulse length
equal to the wavelength multiplied by the number of cycles in a pulse
spatial pulse length
time taken for a pulse to occur, including dead time
pulse repetition period
measurement of only the transmission part of the pulse and does not include dead time
pulse duration
percentage of time machine is transmitting sound into patient
duty factor
equal to PD/PRP
duty factor
one element constantly receiving and one element constantly transmitting
continuous-wave ultrasound
DF of continuous wave
100%
used only for spectral Doppler
continuous wave
sound travels through tissue, some energy is lost
attenuation
may result from absorption of the beam
attenuation
conversion of sound to heat
absorption
lowest attenuation
water
highest attenuation
air
average attenuation rate through soft tissue
0.5 dB/cm/MHz
attenuation increases with ____.
frequency
larger than the wavelength of the transmitted beam and includes broad strucutres
specular reflectors
sound strikes perpendicularly, reflectors show up on ultrasound as a bright white line
specular reflector
sound strikes at an angle other than 90 degrees is not reflected back to the transducer and is not displayed
specular reflector
sound encounters a structure that is smaller than the transmitted beam’s wavelength
nonspecular reflector
scattering of a sound wave, beam spread out over many directions
nonspecular reflector
What reflector in not angle dependent?
nonspecular
Rayleigh scatterer
nonspecular reflector
occurs when reflector is very small compared with beam’s wavelength
Rayleigh scatterer
amount of scatter proportional to frequency to the fourth power
Rayleigh scatterer
the boundary of two tissues adjacent to one another
interface
occurs when two condition are met:
an angle of incidence perpendicular to interface
difference in acoustic impedance of two media
reflection
Amount of reflection is proportional to:
impedance mismatch
The farther apart two impedances are, the _____ the reflection
stronger
May occur if two different conditions met:
nonperpendicular, or oblique angle of incidence
difference in propagation speeds between two media
refraction
sound will be transmitted into the tissue at the same angle with no change in direction at the interface
perpendicular incidence
transmitted sound will change direction, assuming disparate propagation speeds
oblique incidence
if propagation speed of second medium greater than 1540 m/s, angle of the transmitted angle will be _____ than the incident angle.
greater
used by machine to determine travel time of pulse
range equation
measure time taken for a transmitted pulse of sound to return to the transducer and calculates the distance to the reflector
range equation
d- ct/z
range equation
sometimes called “curved” or “convex” array
curvilinear array transducer
commonly called linear array
linear sequential array
typically a small rectangular of square footprint transducer
phased array
curved near field with sloped sides
curvilinear array transducer
have ability to displayed at two image shapes
rectangular: rectangle with a flat top and bottom and straight sides
vector: slopes sides of rectangle to form a trapezoid; sometimes called virtual convex
linear array transducer
may have sector or vector image
both shaped like slices of pie
phased array transducers
piezoelectric elements are most commonly made of:
lead zirconate titanate
What is used on transducer to limit number of cycles in the pulse
damping material
improve transmission of sound into patient
damping material
source of electricity from a part of the machine
pulser
initial amount of power used to shock elements determines ______ or strength of sound wave.
amplitude
ALARA
as low as reasonably achievable
production of heat in tissue as sound travels
thermal bioeffects
creation of bubbles in the tissue
cavitation
Two indices used by manufacturers to provide information on risk of bioeffects at given technical settings
Mechanical index and thermal index
SPTA intensity no bioeffects unfocused transducer
100 mW/cm^2
SPTA intensity no bioeffects focused transducer
less than 1 W/cm^2
No bioeffects noted if temperature increase remains below:
1.5 degrees Celcius
resolution of reflectors that are parallel to the beam
axial resolution
shorter pulse = _____ axial resolution
better
The ____ the operating frequency, the better the axial resolution
higher
involves reflectors perpendicular to the beam
determined by the width of the beam
lateral resolution
Where is the best lateral resolution?
in region of focal zone
width of the beam
slice-thickness or elevational plane
frame rate; number of images produced per second
temporal resolution
influencers of temporal resolution
depth of image
width of image
number of focal zones
use of color Doppler
completed when a series of scan lines have been created
frame
The deeper the depth, the ____ the frame rate.
worse
The more focal zones used, the ____ the frame rate.
worse
the difference between the transmitted frequency of the ultrasound transducer and the returned frequency of the reflector
Doppler shift
Doppler shift: if a reflector is stationary
reflected frequency will be identical to transmitted frequency and Doppler shift will be zero
If a reflector is moving in a direction that is toward the transducer, then the reflected frequency will be greater than the transmitted frequency
Positive Doppler shift
reflector is moving away from the transducer, reflected frequency will be lower than the transmitted frequency
Negative Doppler shift
Doppler equation
V=c(Fd)/2f(cos0)
The most accurate Doppler shift
0 degrees
____ will provide the highest doppler shift
0 degrees
At ___, the Doppler shift is zero.
90 degrees
displays the signal information on a graph in which the frequency shift (converted to velocity) is displayed on the y-axis and time on the x-axis
spectral Doppler
wraparound of the spectral waveform causing positive shifts to be displayed as negative
aliasing
maximum frequency shift that can be sampled is equal to the ______ which is ____ of the PRF
Nyquist limit
1/2
elimination of aliasing
increase PRF or decrease Doppler shift
converts complex information into spectral waveform
Fast Fourier Transform
filling in of spectral window
spectral broadening
type of PW Doppler in which information regarding direction of flow and mean velocity is displayed as a color on top of the B-mode display
color Doppler
BART
blue away, red towards
created by sending multiple pulses of sound down the scan lines of a color gate to determine movement of the reflectors
color Doppler
processing algorithm used to process color Doppler
allows for identification of only mean velocity information, not peak systolic and end diastolic information obtained by spectral Doppler
autocorrelation
display of positive shift information as a negative shift
aliasing
also called
amplitude Doppler
color power angio
power Doppler
provides flow information that relies on the amplitude of the Doppler shift, but not the shift itself
power Doppler
What is amplitude determined by?
the number of red blood cells moving through the vessel, with higher amplitudes causing a stronger signal
movement artifact that obscures the image and makes it less or nondiagnostic
flash
adjusts brightness of all the dots on the screen equally
used when overall appearance of image is too bright or too dark
overall gain
adjusted when only part of the iamge is too bright or too dark
adjusts for attenuation in the far field
Time gain compensation
set number and location of focal zones
lateral resolution best
at or below area of interest
focal zone
determined by thickness of the element
frequency
controlled by PRF
should be set where area of interest is fully displayed
depth
produces images using sound energy returning from the patient that is double the operating frequency
produces images with better lateral resolution and fewer artifacts
tissue harmonic imaging
transducer sends beam into patient from different directions to minimize image artifacts and improve appearance of soft tissue
spatial compounding
permits measurement of higher velocities without aliasing
increasing scale/PRF
should be optimized so that spectral waveform should occupy about two-thirds of spectral window without touching the top or bottom of the display
PRF/scale
controls brightness of spectral waveform
spectral gain
permits sample depth selection
sample volume/range gate
used to stretch out a waveform so that individual parts of waveform can be more accurately measured
fast sweep speed
represents part of spectral waveform where velocity is zero
zero flow baseline
flips waveform so that negative shifts are displayed above the baseline and positive shifts below it
invert
The larger the gate, the _____ the frame rate
slower
The wider the gate, the ____ scan lines required
more
should be increased when a vessel not filling well with color, and scale is appropriately set
color gain
negative shifts displayed on top, positive shifts displayed below
invert
common aritfact consisting of linear repeating echoes
appears as a result of a strong specular reflector near the surface
reverberation
type of reverberation artifact
commonly caused by small bits of calcium or surgical clips
comet tail
occurs as the result of the attenuation of the sound
seen in presence of calcified plaque and overlying bone
shadowing
when sound travels through an area of decreased attenuation compared to the surrounding tissue, the tissue deep to the week attenuator appears brighter
enhancement
sound travels in the region of a strong specular reflector and causes a duplication of a reflector deep to the original structure
associated with array transducers
Grating lobes
caused by wall motion
produces a high-amplitude, low-frequency noise along baseline
eliminated with use of high-pass filters, also called a wall-filter
clutter
the principle of sending in a small group of sound waves and then waiting for that pulse to come back so an image can be displayed
pulsed-wave
The part of the ultrasound machine that transmits and receives sound via an array of piezoelectric elements
transducer
Tool for measuring blood flow quantitatively or qualitatively using pulsed-wave or continuous-wave techniques
Doppler
Echoes on the image not caused by actual reflectors in the body
artifacts
principle of constantly transmitting a sound wave into the patient to obtain a spectral-Doppler waveform
continuous-wave
The ability of ultrasound to cause changes to the tissue if proper settings are not used
bioeffects
What is the number of cycles that occur in 1 second?
frequency
What is the time taken for one cycle to occur?
period
What determines the propagation speed of sound?
the medium through which the sound is moving
What is the height of a cycle from baseline to the peak of the cycle?
amplitude
What is the number of pulses per second emitted by an ultrasound system?
pulse repetition frequency
What information is needed in order to determine spatial pulse length?
wavelength and the number of cycles per pulse
What is the percentrage of time the machine is transmitting sound in to the patient?
duty factor
What is the minimum number of piezoelectric elements necessary to perform continuous wave Doppler?
two
Which of the following has the highest attenuation?
a. water
b. muscle
c. bone
d. air
d
What type of reflection results when sound encounters structures that are smaller than the transmitted beam’s wavelength?
nonspecular
Which of the following must be present for reflection to occur?
a. acoustic impedance mismatch
b. difference in propagation speeds between two media
c. structures much smaller than the ultrasound beam’s wavelength
d. a change in the direction of the sound beam
a
What is a change in direction of the transmitted beam at an interface?
refraction
Assuming soft tissue, how long does it take an ultrasound pulse to reach a depth of 1 cm and return to the transducer?
13 milliseconds
Which transducer is most commonly used for peripheral and cerebrovascular examinations?
linear sequential array
Which transducer creates a “pie slice” shaped image?
phased array
Which of the following is added to a transducer to limit the number of cycles in a pulse?
a. damping material
b. matching layer
c. attenuation layer
d. lead zirconate titanate
a
After removing gel and fluids from a nonintracavitary transducer, what should the next step be in cleaning the transducer?
wipe down with a low-level transducer
What is the part of the ultrasound machine that provides the electricity that shocks the transducer?
pulser
What does the acronym ALARA stand for?
as low as reasonably achievable
What does the TI indicate?
risk of thermal bioeffects
Which term describes the resolution parallel to the beam?
a. temporal
b. axial
c. lateral
d. transverse
b
Where is the lateral resolution the best?
focal zone
A reflector moving toward a transducer would result in what type of Doppler shift?
positive shift
What angle results in the most accurate and highest Doppler shift?
0 degrees
On a spectral display, what is represented on the vertical axis?
velocity
What is complex processing technique that converts complex frequency shifts into a spectral waveform?
fast Fourier transform
Which of the following describes sending multiple pulses down one scan line to create a color Doppler image?
autocorrelation
What is a Doppler technique that provides flow information based on amplitude of the Doppler shift and not the shift itself?
power Doppler
Which control adjusts the overall brightness of the B-mode image?
gain
Which processing technique results in better lateral resolution and reduces reverberation artifact?
tissue harmonic imaging
Which control should be adjusted to permist the display of higher velocities in a spectral Doppler display?
PRF/scale
Which control should be adjusted if color is either not filling the vessel or is bleeding outside the vessel wall?
color gain
During an ultrasound evaluation of the aorta, a surgical clip is ecountered. What artifact would likely be present owing to this clip?
comet tail
What is an artifact caused by wall motion that can be released by using a wall filter?
clutter
Sound waves are ______ indicating that the movement of the molecules within the wave is parallel to propagation direction.
longitudinal
The typical frequency range used in medical diagnostic ultrasound is ____ MHz.
2-20
The average propagation speed in soft tissue that ultrasound machines assume is ____.
1,540 m/s
The property of the medium that is determined by the product of density and propagation speed that helps determine reflection of echoes is _____.
acoustic impedance
The parameter that primarily determines pulse repetition frequency and pulse repetition period is ______.
imaging depth
The loss of some energy in the sound beam as it travels through tissue is ______.
attenuation
The average rate of attenuation through soft tissue is _____.
0.5 dB/cm/MHz
The diaphragm is an example of a _____ reflector.
specular
A red blood cell is an example of a _____ scatterer.
Rayleigh
If the propagation speed in the transmitted medium is greater than the propagation speed in the incident medium, the angle of the transmitted beam will be _____ than the incident angle.
greater
The ultrasound machine uses the _____ equation to determine the travel time of an ultrasound pulse.
range
Modern transducers are ______, meaning they have the ability to use different frequencies that are present in the beam.
broadband
The _____ layer of a transducer is used to improve the transmission of sound into the patient.
matching
The piezoelectric elements within a transducer are usually made of _______.
lead zirconate titanate (PZT)
The measure of the amount of power in an ultrasound beam divided by the area of the beam is the _____.
intensity
A bioeffect of ultrasound that results in the creation of bubbles in the tissue is ______.
cavitation
No thermal bioeffects have been noted with an unfocused transducer with an SPTA intensity below ___ mW/com^2.
100
Lateral resolution is determined by the _____ of the beam.
width
Axial resolution is improved by increasing the _____ of the transducer.
frequency
The number of images produced per second is called the ______.
frame rate/temporal resolution
The ______ is the difference between the transmitted frequency of the ultrasound transducer and the returned frequency of the reflector when motion is present.
Doppler effect
A Doppler angle of _____ degrees results in no detectable shift.
90
A common artifact of PW spectral Doppler is _____ or wraparound of the spectral waveform causing positive shifts to be displayed as negative.
aliasing
The maximum frequency shift that can be sampled during PW Doppler is known as the ______/
Nyquist limit
Filling in of the spectral window because of a wide range of velocities at a given point in time is called _____.
spectral broadening
The process used in color Doppler to identify mean velocity and direction is called ______.
autocorrelation
Slider controls used to achieve uniform brightness across an image are known as _____.
TGCs
A processing technique that sends the beam into the patient from different directions to improve the appearance of soft tissue is known as ______.
spatial compounding
A Doppler control that allows the display of more or fewer spectral waveforms on the screen at one time is _____.
sweep speed
______ artifact occurs as a result of increased attenuation of sound and is often seen posterior to bone or calcified plaque.
comet tail
