Unit 1
Principle of Piezoelectricity
principle that states that some materials, when deformed by an applied pressure, produce a voltage
Piezoelectricity
ultrasound transducers operate according to the Principle of _________________
voltages; pulses; voltages
transducers convert electric ____________ into ultrasound ________ and convert returning echoes back into ___________
Piezoelectric elements
aka crystal, active element, or transducer element
thickness; polarity
when a voltage is applied, the ___________ of the element increases or decreases depending on the _____________ of the voltage
natural
the type of Piezoelectric element that quartz, tourmaline, and Rochelle salt are
synthetic
type of Piezoelectric element that includes lead zirconate titanate (PZT)
easily available and low cost
most common material used
365 degrees C
What is the temperature at which material loses magnetic properties?
Curie point
365 degrees C is also known as the _________________
the crystals are cooled below Curie point and removed from alignment with the circuit
For an element to become Piezoelectric, the material is heated to Curie point, dipoles are polarized with strong electric current, and then __________________
parallel
after cooling the crystal below the Curie point, the aligned dipoles remain fixed ______________ to each other
destroys
heating to Curie point a second time ________________ piezoelectric properties
transmit
reverse piezoelectric effect is _____________ mode
pressure, voltage
during transmit mode, a piezoelectric element produces ______________ waves when deformed by applied ____________
voltage; pressure
during receive mode, a piezoelectric element produces ______________ when deformed by applied ____________
operating frequency
natural frequency of operation for the element
propagation speed; crystal thickness
operating frequency is determined by ___________ and ________________
higher
thinner elements operate at ____________ frequencies
Huygens’
ultrasound waves follow __________________ principle
sound beam
combination of all sound arising from different point-like sources on transducer crystal face
three
the sound beam is _____________ dimensional
axial plane
along the direction of sound travel (parallel)
lateral plane
perpendicular to the direction of sound travel
elevational plane
thickness of the sound beam
side lobes
additional beams that travel out in directions not included in main beam
source of artifact
single
side lobes occur with ______________ element transducers
distance
the transducer produces a sound beam with a width that varies according to the _____________ from the transducer face
intensity
_________________ is not uniform throughout the beam because the area varies
smaller
the ____________ the beam point of interest in the tissue, the better the resolution
near zone
aka Fresnel zone, near field
region extending from the transducer to the minimum beam width
beam width decreases with increasing distance from the transducer
decreases
for the near zone, beam width ______________ with increasing distance from the transducer
far zone
aka Fraunhofer zone, far field
the region that lies beyond the min beam width (natural focus)
beam width increases with increasing distance from the transducer
increases
for the far zone, beam width ________________ with increasing distance from the transducer
focal point
where the beam is the smallest and maximum intensity
resolution best at this point
focal point
Where is the resolution the best?
focal zone
where the beam is focused on each side of the focal point
area of max sensitivity, intensity, and best lateral resolution
as the diameter of the beam decreases, the intensity of the beam increases
increases
as the diameter of the beam decreases, the intensity of the beam _____________
near zone length
the distance from the transducer face to the location where the beam is the smallest in diameter
near zone
the __________________ is where additional focusing can be added
longer
the _______________ the near zone, the better
true
t/f: it is impossible to focus in the far zone
far field divergence
beam diameter increases after natural focus
increase
as frequency and crystal diameter increase, the Fresnel zone and near zone length ______________
one half
at a distance of one near zone length the diameter of the beam is equal to ____________ the crystal diameter
focusing the beam
contributes to:
better resolution (narrower beam)
stronger beam (decreased area)
near zone
focusing can only be achieved in ________________
mechanically
single element transducers can only be focused ______________
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
M-mode (motion mode)
combines A-mode and B-mode to show motion of tissue on a graph with the x-axis as time and the y-axis as depth
transducer
device that converts one form of energy to another
bandwidth
the range of frequencies
broad
shorter pulses = ______________ bandwidth (imaging)
narrow
longer pulses = _____________ bandwidth (Doppler)
transducer
broad BW allows imaging at multiple frequencies within BW range without changing ________________
damping material
epoxy resin attached to back of element that absorbs vibrations and reduces #cycles/pulse
decrease
_______________ in PD and SPL results in improved axial resolution for imaging
increased
_____________ BW with more damping decreases QF along with efficiency and sensitivity
diagnostic imaging
_________________ transducers are highly damped to generate 2-3 cycles/pulse
pulsed-wave Doppler
_______________ transducers are less damped to generate 5-30 cycles/pulse for better sensitivity
continuous wave Doppler
_______________ transducers are not damped (air) to reflect all energy into patient
broadens
damping ________________ BW
quality factor
determines sensitivity→ability to detect weak echoes
low
highly damped, broad BW transducers have a _________ Q factor with less sensitivity and efficiency
high
little damped, narrow BW transducers have a __________ Q factor with more sensitivity and efficiency
voltage excitation
___________________ can be used selectively to operate the same transducer at more than one frequency
matching layer
material placed on transducer face
has impedance value between crystal and tissue to improve sound transmission into body
minimizes reflection
reflection
matching layer minimizes ________________
coupling gel
eliminates air layer between transducer and skin
eliminate/diminish the strong reflection caused by air present between transducer and the skin (soft tissue)
improves transmission of sound into and out of the body
curved
transducer used for abdomen, pelvis, and OB
5-1 MHz
curved transducers use a frequency of ____________
sector
transducer used for adult echo and abdomen (Abd Doppler)
5-1 MHz
sector transducers use a frequency of _____________
linear (12-5 MHz)
transducer used for vascular, thyroid, scrotum, and MSK
12-5 MHz
when a linear transducer is being used to image vascular, thyroid, scrotum, and MSK, the frequency is _____________
linear (20-10 MHz)
transducer used for breast and MSK
20-10 MHz
when a linear transducer is being used for breast and MSK exams, the frequency is _____________
transducer arrays
transducer assemblies with several elements, each independently controlled
required for real-time scanning
use electronic means of sweeping, steering, and focusing the beam
involves the summation of waves (Huygens’ principle)
sequencing; phasing
transducer arrays use electronic means of sweeping, steering, and focusing the beam; this is accomplished by___________ and ____________
sequencing
voltage pulses are applied to small groups of elements in succession
time delays between pulses
duration is set by depth
allows for fast acquisition of images and frame rates which allow for real-time scanning
phasing
voltage pulses are applied to elements in rapid succession
very small time delays between pulses
allows for steering the beam direction
beam steering
sweeping the beam to produce automatic scanning
accomplished with phasing
focusing
_______________ occurs in all 3 planes
superficial
with electronic (transmit) focusing, longer time delays create a more ______________ focus
deeper
with electronic (transmit) focusing, shorter time delays create a ______________ focus
short
in aperture focusing, smaller groups (less elements) are used for ____________ focal lengths
increasing
in aperture focusing, larger groups (more elements) are used for foci located at ______________ depths
width
aperture, focal length, and wavelength determine the beam _______________ at the focus
increased
to maintain the same beam width at the focus for increasing focal lengths, the aperture must be ____________
dynamic aperture
aperture changes as focal point is moved
multiple
multiple pulses per scan line can generate ___________ foci
improves
generating multiple foci _____________ resolution
time
multiple pulses per scan line takes more _______________
dynamic (receive) focusing
focusing occurs during echo reception
done by the US machine
uses time delays of the electrical signals during echo reception
delay patterns change continuously
improves lateral resolution
linear array
straight line of rectangular elements, each about a wavelength wide
provides a rectangular image composed of many vertical, parallel scan lines
parallelogram
linear arrays can be steered to the right or left to produce a _______________ in Doppler
convex array
curved line of elements
pulses travel out in different directions from different points across the curved surface
produces sector scan
vector array
parallelogram-shaped display
phased array
compact line of elements, each about ¼ wavelength wide
voltage pulses applied to entire group of elements with varying time delays, resulting in sweeping of beam
produces pie-shaped image with scan lines radiating from common point (sector scan)