Studied by 1 person
What kind of waves are ultrasound waves?
mechanical and longitudinal
Rarefactions are regions of ________ and ________.
low pressure and density
Compressions are regions of _______ and ________.
high pressure and density
Speed of sound (propagation speed) is determined by what?
the medium only; stiffness and density
Stiffness is also called what?
bulk modulus
If you increase stiffness what happens to propagation speed?
it increases
Sound cannot travel through a ________. It needs a medium
vacuum
What is the average speed of sound in soft tissue?
1,540 m/s or 1.54 mm/us
What is frequency determined by?
the sound source
What is frequency and what two things determine it?
the number of cycles per second; propagation speed of the crystal and thickness of the element (c/wavelength)
Audible frequency:
Ultrasound:
Infrasound:
20Hz-20,000Hz
above 20,000Hz (20kHz)
below 20Hz
Constructive interference
waves are in phase; stronger; reinforcement
Deconstructive interference
waves are out of phase; weaker
What is period?
the time for one cycle to occur (1/F)
What is wavelength?
the length of one cycle (c/F)
What is pulse duration?
the time that the pulse is on (# of cycles in a pulse x period)
What is duty factor?
the fraction or percentage of time that the pulse is on (pulse duration/PRP x 100)
Attenuation
the weakening of sound as it travels through the body through reflection, scattering and absorption
Attenuation is highest in ___ and lowest in _____.
air; fluid
What will increasing the frequency of the TRX do? (attenuation)
reduce penetration and increase attenuation
What is the average frequency TRX for an adult and a child?
3.5 MHz for an adult; 5 MHz for a child
Half value layer thickness (HVLT)
level at which 3dB attenuation has occurred
3dB =
1/2
What is absorption?
when energy is converted into heat
Reflection
specular reflectors are large and smooth like the kidney capsule
Rayleigh scatterers
smaller than the wavelength (RBCs) - frequency to the 4th
Attenuation coefficient in soft tissue
0.5 dB per MHz
What compensates for attenuation?
TGC
3 operator controls
intensity, power, and frequency
What does increasing power result in?
it increases exposure to patient, increases penetration, image brightness, and voltages applied to the crystal
What would decrease intensity?
increasing beam area
Power increases =
intensity increases
What is amplitude?
the maximum cyclic change in a quantity
What are the acoustic variables?
pressure, density, temperature, and motion (vibration)
What is the intensity associated with bioeffects?
SPTA
Intensities highest to lowest
SPTP, SATP, SPTA, SATA (TP is always highest, TA is always lowest)
Impedance
density x propagation speed (rayls)
What must there be in order for reflection to occur?
a change in impedances
Intensity transmission coefficient (ITC)
the percentage of sound transmitted at a boundary
Intensity reflection coefficient (IRC)
the percentage of sound that is reflected
Diffraction
the interaction associated with divergence of a sound beam after passing through a small aperture
Snells law
refers to the angle of sound transmission at an interface with oblique incidence and different propagation speeds
Refraction:
shows lateral misrepresentation
Acoustic enhancement
the hyperechoic region posterior to a low attenuating structure (fluid)
If the speed of sound is slower than the average in soft tissue, what will happen?
the instrument with place the structure further away than it actually is
How will multipath artifacts displace structures?
axially
What do grating lobes do?
put an extra echo on the image
What do slice thickness artifacts do?
fill in anechoic spaces
What produces comet tail artifact?
metal (surgical clips)
What produces ring down artifact?
gas
Shadowing
decrease in amplitude behind a highly attenuating structure
What does lowering the frequency of the transducer do? (spatial resolution)
decreases spatial resolution
Curving the element, or lens, will do what?
improve lateral resolution? (focusing)
Temporal resolution decreases with what 3 things?
an increase in scan lines, multifocus zones, and an increase in sector width
Which is superior, axial or lateral resolution?
axial
Where is the most accurate measurement taken?
along the beam path
How do you find the reflector depth if GRT is given?
divide by 13
What increases NZL?
increasing the frequency or the transducer diameter
FZ
the narrowest part of the beam; max intensity here
The far field is also called__________.
fraunhofer
The near zone is also called_________.
fresnel
Wide bandwidth transducers have what?
shorter pulses and better axial resolution
Array transducers have what?
multiple elements
Phasing
electronic time delays to steer and/or focus the beam
Annular arrays
electronically focused but mechanically steered
Linear phased array
best for cardiac and intercostal scanning
What is axial resolution improved by?
damping, increasing frequency, wider bandwidth, shorter pulse length
huygen’s principle
constructive and destructive interference; v shaped wavelets; hourglass shaped beam
Linear sequenced
rectangle shape
Linear phase
sector shape
Dynamic apodization (subdicing)
reduces grating lobes
What is lateral resolution improved by?
focusing; beam diameter for an unfocused beam is 1/2 the transducer diameter
What does the backing layer do?
improves axial resolution by reducing ringing and shortening the SPL; also reduces sensitivity
Formula for axial resolution
SPL/2
What does the matching layer do?
reduces acoustic impedance mismatch between element and tissue; gel acts as a matching layer; 1/4 a wavelength thick
Dynamic receive focus
uses delay lines after the echoes are detected (focusing during reception)
Q factor
characterizes the frequency bandwidth of the transmitted ultrasound wave; F/BW
What does multifocusing achieve? (frame rate)
reduces frame rate
Contrast resolution
the ability to distinguish between adjacent structures that produce echoes of similar amplitude
What will increasing line density do?
improve spatial resolution and decrease frame rate
Piezoelectric effect
acoustic to electric
F number
refers to the focusing of the sound beam
PZT (lead zirconate titanate)
material used for TRX
Shades of gray (bits per pixel) describes what?
contrast resolution
How many lines are in a TV frame?
525
Gain control
determines the amount of amplification that occurs in the receiver (increasing gain is not associated with increasing risk of bioeffects)
Output power control affects what?
the pulse component
TGC
equalizes the differences in echo amplitudes received at different depths; compensates for attenuation
Read magnification
post processing control; increases pixel size so some resolution is lost in the image
write magnification
preprocessing control; does not lose resolution
PRF
number of pulses per second
Compression
receiver function that decreases the difference between the smallest and largest receiver signal amplitude
Beam former
responsible for apodization, beam steering, and focus aperture
Order of the instrument:
pulser, beam former, receiver, memory, display
Dynamic range
the ratio of the largest to smallest signal a system can handle
Rectification
converts negative voltages to positive
Demodulation
converts from radio frequency to video form
D to A converter
numbers to proportional values of brightness
A to D
analog to digital form
Scan converter
where images are stored
Note 
Flashcard (93)