Chapter 18 Ultrasound II Flashcards
lines of sight
Each US pulse provides info for a single line of sight
Images are built up by generating a large number of lines of sight that are sequentially directed to cover the ROI in a patient
pulse repetition freqency
number of separate pulses (i.e. lines of sight) sent out every second
product of frame rate and lines per image
common pulse repetition frequency
4000 pulses/s
image frame rates
~ 30 frames/s
line density
of lines per image/ FOV
what does increasing line density do?
improves lateral resolution
how can line density be increased?
reduce frame rate
reduce FOV (but limits region of patient that can be seen)
increase pulse repetition frequency
what does reducing frame rate do?
will increase line density: improve lateral resolution but reduce temporal resolution
what does increasing pulse repetition frequency do?
increase line density which improves lateral resaolution, but also reduces listening intervals and thus decreases imaging depth
duration of each pulse
~ 1 us
listening interval
interval between pulses
transducer acts as a receiver
listening interval for 4 kHz pulse repetition frequency
250 us
increasing the frequency is a reduced listening interval fpr echo detection and vice versa
how is depth of interface producing the echo dtermined?
by the time interval between the emitted pulse and the returning echo
for v=1540 m/s a return time of 13 us is a depth of 1 cm (return trip of 2 cm)
return time of 26 us is depth of 2 cm
etc
different echo listening times and penetration depths for 4 kHz, 6 kHz, and 8 kHz PRF
4 kHz- 250 us- 20 cm
6 kHz - 167 us- 13 cm
8 kHz- 125 us- 10 cm
what would uncorrected echo data do?
show distant echoes as being much weaker than superficial echoes due to attenuation
how to US scanners compensate for increased attenuation with depth?
increase signal gain as the echo return time increases
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depth gain compensation
time gain compensation
time varied gain
swept gain
all mean the same thing
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what does the intensity of returning echoes along a line provide info about
differences in acoustic impedances between tissues
A mode imaging
depth on horizontal axis
echo intensity on vertical axis
ophthalmology uses A mode imaging
T-M mode imaging
time-motion
time on horizontal axis and depth on vertical axis
-displays time- dependent motion, valuable for studying rapid movement (cardiac)
B-mode
echo intensity is displayed as brightness value (B) along each line of sight
used for M -mode and 2D gray scale imaging
what do scan converters do?
compute 2D images from echo data from distinct beam directions which are subsequently displayed on a monitor
US image data and storage
512x512 matrix
1 byte per pixel
-each frame contains 0.25 MB of info
what is used for abdo imaging
convex arrays at ~ 4 MHz
what is used for superficial imaging
linear arrays at ~ 10 MHz
what is used for gyne and pelvic imaging
endo-array prove
transrectal
what is used for ped US
smaller footprint transducer
> 7 MHz
what is used for transcranial imaging?
- through acoustic windows through the skull such as temples or eyes
- 2 MHz
- phased arrays
1.5 D arrays
- lots of transducers in scan plane, small number in slice thickness direction
- focusing the small number transducer elements can be used to reduce the slice thickness and improve elevational resolution
- comparable lateral and elevational resolution
2D arrays
- can do volume averaging
- instead of sound waves being sent straight down and reflected back, they are sent at different angles
- returning echoes are processed and yield a 3D volume image
4D fetal US
3D picture in real time
don’t have the lag associated with computer constructed image
spatial compounding imaging
- multibeam imaging
- combines multiple lines of sight to form a single composite image
- echoes from the different directions are averaged together into a single composite image
- reduces angle-dependent artifacts and clutter, improves contrast and margin definition
- corners receive a subset of views compared to the center, which reduces image quality
- used for breast, peripheral blood vessels, musculoskeletal injuries
extended FOV US
uses static B-mode to permit large subject area to be viwed on single static image
- as images are acquired, they are stitched together
- results in single slice image covering the whole area of interest
-useful when you need to see a large patient area and there is limited motion
harmonic imaging
- requires broadband transducers
- receives signals at twice the transmit frequency
- reduces artifacts and clutter
- good for patients with thick and complicated body wall structure
- first harmonic (twice the frequency) is used- higher harmonics have too much attenuation
- high frequencies arise from non-linear interactions of US with tissues
cardiac frequencies for harmonic imaging
transmit at 1.5 to 2 MHz, receive at twice that
US contrast agents
- vascular and perfusion imaging
- encapsulated microbubbles
- micorbubbles produce harmonic frequencies
pulse inversion harmonic imaging
- uses 2 pulses, standard and phase reversed
- 2 pulses cancel out for normal tissue but not for microbubbles (contrast agents)
-in echocardiography, harmonic imaging has reduced imaging artifacts due to reverberations
reverberation
large number of reflected waves, which can be perceived as continuous sound
what is doppler effect?
changes in frequency resulting from a moving sound source
-objects moving toward detector reflect sound at higher frequencies and vice versa
what is the shift in frequency in doppler effect proportional to?
cos(theta)
theta is angle between US beam and moving object
max is for 0 or 180 degrees
at 90 degrees there is no doppler shift
also proprtional to reflector velocity
does doppler measure reflector velocity?
no, just change in frequency
frequency shift for moving blood for 2 MHz transducer
260 Hz for 10 cm/s
780 Hz for 30 cm/s
2600 Hz for 100 cm/s
frequency shift for moving blood for 5 MHz transducer
650 Hz for 10 cm/s
2000 Hz for 30 cm/s
6500 Hz for 100 cm/s
blunt flow
- in blood vessels
- constant flow across most of the cross-sectional area and reduced flow near the vessel walls
what happens to velocity of blood as a blood vessel narrows?
velocities increase
when can turbulent flow occur in blood vessel
when vessel is disrupted by plaque and stenoses
how is doppler US used to evaluate blood flow in vessels?
based on backscatter of blood cells
what does pulsed doppler provide?
- frequency shift
- also depth information
PRF values in doppler US
8 kHz
duplex scanning
combines real time imaging with doppler detection
B-mode images vs doppler
-Bmode images give info on stationary reflectors
Doppler shifts give info on flow present
spectral analysis
frequency shift as a function of time
clinical conditions have distinct spectral waveforms
brightness value
intensity at a given frequency shift at any moment in time
what PRF must be used to avoid aliasing in doppler spectral analysis?
2X highest doppler frequency shift
can also avoid aliasing by adjusting spectral baseline
color doppler
2d display of moving blood with frequency shifts encoded as colors
-flow info is provided by taking the average value of a number of samples obtained from each pixel
-gives info on direction and magnitude of flow over a selected ROI
-red = towards transucer
blue = away from transducer
-turbulent flow is green or yellow
where is color doppler info displayed?
on top of B-mode image
twinkle artifacts
in color doppler
occur behind a strong attenuator and show fuclutating reds and blues in tissues which have no flow
flash artifact
in color doppler
fill color box with sudden burst of color due to sudden movement of patient or transducer
power doppler
- uses same info as color doppler
- where color doppler would see 45 negative and 45 positive shifts as net 0 (no movement), power doppler would see it as 90 frequency shifts (i.e. magnitude)
- power doppler color doesn’t vary with direction of flow
- power doppler doesn;t have aliasing artifacts
- power doppler is more sensitive than color doppler and is used to detect slow blood flow
axial resolution
ability to seperate 2 objects lying along the axis of the beam
- determined by pulse length
- indepedent of depth
-~ 1/2 of pulse length, which is the US wavelength in tissue
how to improve axial resolution?
increase transducer frequency to reduce pulse length
axial resolution at 1.5 and 15 MHz
1 mm
0.1 mm
what two quality items are always trade-offs in US imaging?
spatial resolution and imaging depth
lateral resolution
ability to resolve 2 separate adjacent oibjects
what determines lateral resolution in US?
US beam width
narrow beam = better resolution
increasing number of lines per frame also improves lateral resolution
lateral resolution compared to axial resolution
lateral is 4X worse than axial
-lateral usually becomes worse at greater distance from transducer
elevational resolution
plane perpendicular to image plane
- aka slice thickness
- depends on height of transducer elements
- can be improves using 1.5D arrays
assumptions in US (that can lead to artifacts)
echo depth is proportional to echo time
sound travels in straight lines
attenuation is uniform
echoes reflected from side lobes and grating lobes
create artifacts
mirror image artifact
sound is relfected off of a large interface, causing parts of the image to be in the wrong location
speed displacement artifacts
caused by variability of speed of sound in different tissues
reverberation echoes
multiple reflections occuring from 2 adjacent surfaces
comet artifacts
occur when gap between reverberation echoes becomes very small, and reflections form one comet structure
ring-down artifacts
show a streak like appearance from misture of gas (air) and fluids
acoustic shadowing
reduced echo intensity behind a highly attenuating or reflecting object, creating a shadow
acoustic enhancement
increased echo intensity behind a minimally attenuating object
when can shadowing and enhancement be helpful?
provide clues to identify anatomical features like cysts and gallstones
tissue mimicking phantoms
used to QA US
identify axial, lateral, elevational resolution
uniform phantoms can identify faulty transducers, nonuniformities, extraneous sources of noise
-can help differentiate issues with image formation vs image display
ACR accreditation program
for US and breast US
- QA results must be documented
- most common issues relate to display on maladjusted monitors
-includes tests on uniformity, physical and mechanical inspection, geometric accuracy, US display, diagnostic display
spatial peak intensity
highest in beam
temporal average intensity
averaged over pulse and listening time
intensities in B mode, M mode, Doppler
B-mode: 10 mW/cm2
M-mode: 4 times higher
doppler: 50 times higher
what is cavitation
creation and collapse of microscopic bubbles
-can be caused by US
mechanical index
- estimates change of inducing cavitation
- gas-containing structures and contrast agents are more susceptible to effects of acoustic cavitation
thermal index
predicts rise in tissue temperature
-TI of 1 increases tissue temperature by 1 degree celcius
how many lines of sight make up an US image?
~ 100
transcucer frequency for breast imaging
10 MHz
offers good axial resolution and can penetrate a typical compressed breast thickness (6 cm)
where is extended FOV imaging most likely used?
MSK
is temporal resolution independent of pulse length?
yes
-affected by changes in PRF, line density, and FOV
US intensity refers to?
spatial peak intensity and temporal average intensity
