SPI 2 Flashcards
Beam width
As sound travels the width of the beam changes.
Start- same size as transducer diameter (aperture)
Gets progressively narrower until it reaches smallest diameter
Then diverges
Focus (focal point)
Location where beam reaches its minimum diameter.
Focal depth (focal length, near zone length)
The DISTANCE from the transducer face to the focus.
Near zone (Fresnel zone)
The REGION or zone in between the transducer and the focus
Sound beams converge here
Far zone (Fraunhofer zone)
The REGION or zone deeper than the focus, beyond the near field.
Sound beam diverges here.
Focal zone
Region surrounding the focus where the beam is “sort of narrow”
Picture is relatively good here.
Beam Diameter at transducer
Same as transducer diameter
Beam Diameter at end of near zone
1/2 of transducer diameter
Beam Diameter at 2 near zone lengths
Same as transducer diameter
Beam Diameter deeper than 2 near zone lengths
Greater than transducer diameter
Focal depth is determined by
Transducer diameter (aperture) Frequency
Shallow focus (focal depth)
Small diameter, low frequency
Deep focus (focal depth)
Large diameter, high frequency
Have lower intensity at focus than shallow
Sound beam divergence
Describes the spread of the sound beam in the deep far zone
Sound beam divergence is determined by
Transducer diameter
Frequency of the ultrasound
Less divergence
Narrower beam in far field, large diameter crystal, high frequency, improved lateral resolution in field
More divergence
Wider beam in far field, smaller diameter crystal, low frequency, degraded lateral resolution in far field.
Diffraction
V-shaped wave (Huygen’s wavelet)
Produced by tiny source with a size near the wavelength of the sound.
Waves diverge in this shape as they propogate.
Huygen’s principle
Hourglass shape of sound beam.
Result of the constructive and destructive interference of many sound wavelets emitted from numerous sound sources.
Resolution
The ability to image accurately
Axial resolution
Ability to distinguish two structures that are close to each other front to back, parallel to, or along the beam’s main axis
LARRD
Longitudinal Axial Range Radial Depth
Axial resolution is measured in what units?
Units of distance (mm, cm)
Shorter pulses provide what?
Better axial resolution
Can sonographer change axial resolution ?
No
“Short pulse” means?
Short spatial pulse length or a short pulse duration
Equation for axial res.
Axial res. (mm)= SPL (mm)/2
Axial res. Inproves with
Less ringing
Higher frequency
Axial res. is best using
Highest frequency and fewest number of cycles per pulse
Think of this with numerical questions
Lateral resolution
Minimum distance that two structures are separated by side to side, or perpendicular to the sound beam that produces two distant echoes
LATA
Lateral
Angular
Transverse
Azimuthal
Units of lateral res.
All units of length (mm)
A smaller number creates more accurate image
Lateral res. is equal to
Beam diameter
Beam width variation
Lateral res is best at
The focus because it is the narrowest
Lateral res. is not as good as
Axial resolution
Axial and lateral res. In high frequency pulsed US
Improved axial res. entire image
Inproved lateral res. In the far, far field (at depths greater than twice the focal depth)
Focusing alters the beam in what three ways?
Narrower “waist” in the US beam
shallower focus
smaller focal zone
Focusing is effective mainly where?
the near field and the focal zone
types of focus
fixed focus
adjustable
fixed focus
conventional or mechanical
-Lens external focus
-curved PZT crystal- internal focusing
poorest lateral resolution since not adjustable
adjustable focus
phased array-by electronics better lateral res.
electric focusing- adjustable
multi-focusing
single crystal transducers are what type of focus?
always fixed focus
2-D imaging is also called
B-scan or B-mode
Mechanical transducer characteristics
Image shape- sector steering- mechanical focusing- fixed (conventional) number of crystals and shape- 1, disc crystal defect- image loss
Linear switched transducer characteristics
image shape- rectangular steering- none focusing- fixed number and crystal shape- approx. 200, rectangular crystal defect- vertical line dropout
Linear phased array transducer characteristics
Image shape- sector
steering- electronic
focusing- electronic
number and shape of crystals- approx 200, rectangular
crystal defect- poor steering and focusing
Annular phased transducer characteristics
image shape- sector
steering- mechanical
focusing- electronic
number and shape of crystals- approx. 5, ring
Convex sequential transducer characteristics
image shape- blunted sector steering- none focusing- fixed number and shape of crystals- approx. 200, rectangular crystal defect- vertical dropout
Convex phased transducer characteristics
image shape- blunted sector
steering- electronic
focusing- electronic
number and shape of crystals- approx. 200, rectangular
crystal defect- poor steering and focusing
Vector transducer characteristics
image shape- flat top sector (trapezoid)
steering- electronic
focusing-electronic
number and shape of crystals- approx 200, rectangular
crystal defect- poor steering and focusing
contrast resolution
visualizing a variety of gray shades in an image
poor contrast resolution has
few gray shades
good contrast resolution has
many gray shades
spatial resolution
visualizing general detail in an image
spatial resolution is affected by
axial resolution, lateral resolution, gaps between scan lines
poor spatial resolution
limited detail
good spatial resolution
fine detail
temporal resolution
ability to accurately locate moving structures at any particular instant in time
temporal resolution is determined by
frame rate only
frame rate units
hertz Hz
frame rate is determined by what two factors?
imaging depth # of pulses per image
frame rate is limited by what two factors?
speed of sound in medium
imaging depth
fundamental limitation of temporal resolution
speed.
High temporal resolution
high frame rate shallow imaging fewer pulses per image single focusing narrow sector low line density associated with better movie but lower quality image
Low temporal resolution
low frame rate
deep imaging
more pulses per image
multi-focusing (improves lateral res)
wide sector
high line density (improves spatial res.)
associated with poor quality movie but higher quality image
if imaging depth doubles what happens to the frame rate?
it will be halved
Master synchronizer
communicates with all individual components of US system
organizes and times their functions
Transducer
converts electrical into acoustic energy during transmission
Pulser
controls the electrical signals sent to active elements; receives timing signal from synchronizer
Pulser determines what?
PRP PRF and pulse amplitude
creates firing pattern for phase array systems (beam former)
Pulser modes
Continuous wave Pulsed wave (single crystal) pulsed wave (arrays)
continuous wave
constant electrical signal
electrical frequency= sound’s frequency
Pulse waves (single crystal)
short duration electrical “spike”
one spike per pulse
Pulse waves (arrays)
many elements fired for each pulse
one electrical “spike” per fire elements
Receiver
processes electronic signal produced by transducer during reception and produces picture on the screen
Display
examples include monitor, audio speakers, paper record
Storage
media is permanently archived to this; ex: computer memory, hard drives
Transducer output
Determined by excitation voltage from Pulser
PZT crystal vibrates with magnitude related to pulser voltage
when transducer output changes, strength of every pulse transmitted to body changes
signal to noise ratio
signal- meaningful portion of data
noise- inaccurate portion of data
High signal to noise ratio
meaningful part of data is stronger than inaccurate portion
Low signal to noise ratio
inaccurate portion of data is stronger than meaningful part
increasing transducer output improves what?
signal to noise ratio
Receiver functions
amplification, compensation, compression demodulation, rejection (hint alphabetical order)
Amplification
aka receiver gain
adjustable
all signals are treated the same
Compensation
Time gain compensation TGC, depth compensation DGC
adjustable
signals treated differently based on reflector depth
creates uniform brightness from top to bottom of image
higher frequency more TGC
lower frequency less TGC
Compression
adjustable, decreases dynamic range, changes gray scale map
allows us to see all gray shades
dB add or subtract
Demodulation
not adjustable, changes form of signals
Reject
aka suppression or threshold
adjustable
only weak signals manipulated, strong signals not affected.
TGC curve
near gain delay slope knee far gain
Contrast agents
“micro-bubbles”
Injected into the circulation (IV) these agents create strong reflections that actually “light up” blood chambers or vessels.
Requirements for contrast agents
safe
strong reflector of US
long persistence
metabolically inert
adjustments to output power or receiver gain alter what?
brightness of the entire image
Output Power
affects brightness by adjusting strength of sound waves sent to the body from the transducer
affects patient exposure
When an image is too bright due to high output power what happens?
lateral and longitudinal resolutions degrade
Receiver gain
affects brightness by changing amplification of electronic signals after returning to the receiver
does not affect patient exposure
ALARA (As Low As Reasonably Achievable)
when an image is too bright or too dark, changes in output power and receiver gain can enhance the image. As first option always choose the option that will minimize patient exposure.
(decrease output power to decrease patient exposure if image is too bright)
look at pg. 101-104
Harmonics (harmonic frequency) imaging creates US scans from sound reflections at
twice the transmitted frequency
Harmonics are created where?
In the tissues (not the transducer or receiver)
Fundamental frequency is equal to?
the transducer frequency
What is non-linear behavior?
the small difference between speeds of high and low pressure (compressions and rarefactions of sound)
What does non-linear behavior do?
distorts the sound waves and creates harmonics
(think uneven) look at pages 105 and 106
Bistable properties
black or white on or off high contrast narrow dynamic range poor contrast resolution
Gray scale properties
many shades of gray multiple levels low contrast wide dynamic range good contrast resolution
Brightness
related to the brilliance of the image
how “lit up” is the image?
Contrast
determines the range of brilliancies that are displayed.
(are the whites white? are the blacks black?)
bistable images are high contrast
Analog VS. Digital
Analog- real world
Digital- computer world
Scan converters do what?
change the data format from penetrations (spokes) to horizontal lines of a display. Can be manipulated between the storing and displaying of the data
(ex. black on white can be changed to white on black)
Computer memory is called?
RAM
Random Access Memory
What is a Digital Scan Converter?
A microprocessor digitizes images. Converts image into numbers which are stored in memory
numbers can be processed and re-translated for display
What is a pixel?
the smallest element of a digital picture
What is pixel density?
more pixels per inch, the more detail in the image (spatial or detail resolution)
What is a bit?
smallest amount of digital storage
Bistable
having a value of either zero or one
group of bits is assigned to each pixel to store the gray scale color assigned to that pixel
The more bits you have per pixel-
the more shades of gray and the better the contrast resolution
To calculate the # of shades of gray -
multiply the number 2 by itself the same # of bits (2^n)
