Chapters 3 Flashcards
near field
fresnel.
the area between the face of the transducer and the beam focus.
far field
fraunhofer.
the region past the focus.
focus
where the beam reaches its minimum diameter.
focal region
depth of field.
region over which the beam is most tightly focused.
Detail resolution
ability to distinguish between two objects in any of the three dimesnions: axial, lateral, and elevation.
Operating frequency
the center frequency of the transmit bandwidth.
Aperture
size of a transducer element (for single-element) or a group of elements (for an array).
Apodization
nonuniform driving of elements in an array to reduce grating lobes.
Array
a transducer assembly containing several piezoelectric elements.
Axial
in the direction of the transducer axis (sound travel direction).
Axial resolution
the minimum reflector separation along the sound path that is required to produce separate echoes ( to distinguish between two reflectors).
Beam
region containing continuous wave sound; region through which a sound pulse propagates.
CMUT
capacitive micromachined ultrasonic transducer that contains miniature elements that are comprised of two electrically conducting layers facing each other; one of which is fixed and the other which is a flexible membrane.
able to be integrated with electrocircuits
Composite
combination of a piezoelectric ceramic and a nonpiezoelectric polymer.
have a lower impedance
have a higher bandwidth, sensitivity, and resolution
Convex array
curved linear array.
Crystal
element.
Curie point
temperature at which an element material loses its piezoelectric properties/poling.
Damping
material attached to the rear face of a transducer element to reduce pulse duration; the process of pulse duration reduction.
Detail resolution
the ability to image fine detail and to distinguish closely spaced reflectors.
Disk
a thin, flat, circular object.
Dynamic aperture
aperture that increase with increasing focal length (to maintain constant focal width).
Dynamic focusing
continuously variable reception focusing that follows the increasing depth of the transmitted pulse as it travels.
Element
the piezoelectric component of a transducer assembly.
Elevational resolution
the detail resolution in the direction perpendicular to the scan plane. It is equal to the section thickness and is the source of section thickness artifact.
Far zone (far field):
the region of a sound beam in which the beam diameter increases as the distance from the transducer increases.
Focal length
distance from a focused transducer to the center of a focal region or to the location of the spatial peak intensity.
Focal region
region of minimum beam diameter and area.
Focal zone
length of the focal region.
Focus
: the concentration of the sound beam into a smaller beam area than would exist otherwise.
Grating lobes
additional weaker beams of sound traveling out in directions different from the primary beam as a result of the multielement structure of transducer arrays.
Lateral
perpendicular to the direction of sound travel.
Lateral resolution
minimum reflector separation perpendicular to the sound path that is required to produce separate echoes.
Lead zirconate titanate
a ceramic piezoelectric material.
Lens
a curved material that focuses a sound or light beam.
Linear
adjectival form of line.
Linear array
array made of rectangular elements arranged in as straight line.
Linear phased array
linear array operated by applying voltage pulses to all elements, but with small time differences (phasing) to direct ultrasound pulses out in various directions.
Linear sequenced array
linear array operated by applying voltage pulses to groups of elements sequentially.
Matching layer
material attached to the front face of a transducer element to reduce the reflections at the transducer surface.
1-3 layers are used
Natural focus
: the narrowing of a sound beam that occurs with an unfocused flat transducer element.
Near zone (near field):
the region of sound beam in which the beam diameter decreases as the distance from the transducer increases.
Operating frequency
preferred frequency of operation of a transducer.
Phased array
an array that steers and focuses the beam electronically (with short time delays).
Phased linear array
linear sequenced array with phased focusing added; linear sequenced array with phased steering of pulses to produce a parallelogram-shaped display.
Piezoelectricity
conversion of pressure to electric voltage.
Probe
transducer.
Resolution
the ability to distinguish echoes in terms of space, time, or strength (called detail, temporal, and contrast resolutions, respectively).
Resonance frequency
operating frequency.
Scanhead
transducer assembly.
Sector
a geometric figure bounded by two radii and the arc of the circle included between them.
Sensitivity
ability of an imaging system to detect weak echoes.
Side lobes
weaker beams of sound traveling out from a single element in directions different from those of the primary beam.
Sound beam
the region of a medium that contains virtually all of the sound produced by a transducer.
Source
an emitter of ultrasound; transducer.
Transducer
a device that converts energy from one form to another.
Transducer assembly
transducer element(s) with damping and matching materials assembled in a case.
Ultrasound transducer
a device that converts electric energy to ultrasound energy and vice versa.
Vector array
linear sequenced array that emits pulses from different starting points and (by phasing) in different directions.
footprint is small
presents sector display with a noncurved top
What are the names transducers can be called?
probes, scanheads, transducer assemblies
What’s a naturally occurring substance that is inherently piezoelectric?
Quartz
What is the Curie Temperature?
365 Celsius.
Materials are heated above this to infuse them with piezoelectric properties.
If the material is once again heated above this temperature, the material loses its piezoelectricity properties.
transducers should never be heat sterilized.
List the steps of sound production.
1) Voltages are applied to the crystal and deforms it.
2) The thickness of the crystal will increase or decrease depending on the applied voltage polarity.
3) This creates an alternating pressure that travels as a sound pulse.
4) Returning sound pressure waves deform the crystal and create a voltage across it.
5) This voltage is transmitted to the electrodes connected to the crystal.
6) the higher the amplitude of the echo, the higher the deformity of the crystal and the higher the voltage produced.
7) the voltage signals are amplified and shown on display as gray dots.
The frequency of the sound produces is equal to what?
to the frequency of the driving voltage
What’s another way to call the operating frequency?
The resonant frequency
Define resonant frequency?
Each crystal has a resonant frequency which is the frequency it’s most efficient at in converting electricity to sound energy and vice versa.
Maximum energy transfer between mechanical and electrical energy occurs when?
when the crystal thickness is 1/2 of the wavelength of the ultrasound.
The thinner the element, the ____.
the higher the resonant frequency.
Resonant frequency is influenced by what?
the propagation speed (because of wavelength = c/f)
What’s the most common c among modern crystals?
4-6 mm/microsecond
What percentage of the PRP is ultrasound emitted in pulsed ultrasound?
1%. aka the PD is 1%.
99% is listening time.
THe PRF is determined by what?
the voltage repetition frequency
Direct piezoelectric effect.
mechanical to electrical
Reverse piezoelectric effect.
electrical to mechanical
For the best imaging results we need the ____ pulse of sound possible and the ____ amplitude signals.
The shortest pulse of sound possible and the highest amplitude signals.
Ring down time.
the time it takes a crystal to stop ringing (vibrating) after the voltage pulse has been applied to it.
Transducer assembly includes:
1) Casing - usually plastic; waterproof and insulated.
2) element - thickness and shape depends on the transducer specifications. Discoid for annular and square or rectangular for phase arrays.
3) damping layer - behind the element.
4) matching layer - in front of the element.
5) filler material - vacant spaces are filled with epoxy
6) electrical circuitry - electrodes are attached to the elements.
Transducers meant for CW have?
longer driving voltages and are not damped.
Q Factor.
Quality factor. describes the purity of the vibration of the crystal - the frequency homogeneity of the beam. unitless Q factor = operating frequency/bandwidth or 1/ factional bandwidth or 1/ (bandwidth/operating frequency) is inversely proportional to bandwidth
Bandwidth.
the range of frequencies in a pulse.
refers only to the frequency that have an amplitude greater than 1/2 of the resonant frequency’s amplitude.
shorter pules = broader bandwidth
probes are labeled in MHz by its resonant frequency on its frequency bandwidth curve
For short pulses (1-3 cycles), the Q is?
is roughly = to the number of cycles.
High Q transducers
narrow bandwidth
long ring down time
better transmitter
good for doppler (because of higher amplitude)
Low Q transducers.
wide bandwidth
short ring down time
better receiver
good for 2D imaging (because it has better AR)
What’s the optimal length of an element?
1/2 of the wavelength
What’s the optimal length of the matching layer(s)?
1/4 of the wavelength
Near field length
transducer diameter^2/4 x wavelength
therefore, if the frequency is high, the NFL will be increase
If the aperture increases. the near zone length increases
Do natural unfocused elements have a focus point?
Yes.
Focused transducers can narrow the diameter.
What’s the most commonly used material for ultrasound?
Lead zirconate titanate (PZT)
Poling.
putting the crystal in high temperatures and a strong electrical field to align the positive and negative poles in a specific direction.
Operating frequency
determined by the c within it and the thickness of it.
a thinner crystal can expand and contract quicker.
operating frequency is proportional to 1/thickness
a higher c means the crystal can expand and contract faster, reducing the T
operation frequency is directly proportional to the c in the crystal
operation frequency (MHz) = c in crystal (mm/microsec)/ 2 x crystal thickness (mm)
for CW, operation frequency = frequency of transmit voltage
crystal impulse response
the response of a crystal to a single, short duration pulse
a short impulse response = fewer cycles in the pulse and thus improved AR
unfocused transducer
nothing has been added to affect its natural focus.
Matching layers have ____ impedance.
intermediate impedance.
Composite materials have a ___ impedance value than PZT
lower
Define resolution.
ability to resolve physical tissue characteristics in each of the 3 physical dimensions (axial, lateral, and elevation)
affects how well you can tell two structures apart and measurements.
P
Power
total energy being transmitted to a medium
units are watts (W)
power decreases as sound moves through the medium
determined by the source
we can affect it by altering the transmit gain
power = amplitude^2
I
Intensity is the amount of energy that is transferred to a particular area I = P (W)/area (cm^2) determined by the source we can affect by altering transmit gain I = (A)^2/area units are W/cm^2 or mW/cm^2
Harmonics
even and odd multiples of the fundamental frequency
compressions travel faster than rarefactions so the wave is nonsinusoidal.
nonsinusoidal waves have multiple frequencies of the fundamental frequency aka harmonics
bandwidth
the range of frequencies contained in a pulse
the range of usable frequencies a device can operate
pulses are not uniform, they have different amplitudes and frequencies
highest useable frequency - lowest usable frequency
e.g. 8-3= 5 MHz bandwidth
broader bandwidth
shorter pulses aka higher frequencies it containes
fractional bandwidth
bandwidth/operating frequency
describes the size of bandwidth in comparison to the operating frequncy
reciprocal to the Q factor
attenuation coefficient for soft tissue is?
0.5 dB/cm/MHz
total attenuation formula
attenuation coefficient (dB/cm/MHz) x path length (cm)
units for attenuation?
dB
why do we use TGC
to compensate for attenuation
bc like tissues have to appear like regardless of the depth
causes of attenuation?
absorption (most prevalent one in soft tissue)
reflection (most prevalent one in bone)
scattering (most prevalent one in air)
absorption
conversion of sound energy into heat
result of internal friction forces
significant in bioeffects
the amplitude of an echo is determined by what?
tissue properties
other ways to say perpendicular incidence?
normal, direct, orthogonal
an echo is created every time there’s an ___ difference.
impedance
specular reflectors
smooth interface
larger than the wavelength
create high amplitude reflections
highly angle dependent
non-specular reflectorss
scatterers
rough interface
similar or smaller than the wavelength
not angle dependent
reflections head off in all different directions
if frequency increases, scatter increases
rayleigh scatterer
reflects sound equally in all directions
e.g. RBC
impedance
is the resistance to sound propagation
determined by the stiffness, density of the tissue and the propagation velocity in the tissue
Z(rayls) = p(kg/m^3) x c (m/s)
the greater the impedance mismatch, the greater the amplitude of the echo
Intensity reflection coefficient
what percentage of the incident intensity gets reflected
IRC = the reflected intensity/ the incident intensity
Ir/Ii
what artifact can refraction cause
it can cause a structure to be misplace on the display
refraction
requires oblique incidence and different propagation speeds in the mediums
Huygen’s principle aka fresnel principle
every point along a wave front is a source for a wavelet
these wavelets interfere with each other
which results in a beam in an hour glass shape
phase
describes the relationships of waves in respect to time
in-phase
when the max and min amplitude occurs at the same time
the signal is amplified (constructive interference)
out-of-phase
when the max and min amplitude occurs at different times
this causes destructive interference
speckles represent what?
represent interference patterns, NOT the scatterers themselves
contrast agents
must be easy to administer, non-toxic, and small enough to pass through capillaries
increases echogenecity
most contain microbubbles of gas in a polymer or lipid shell
e.g. definity and optison
the range equation
used to calculate reflector distance for display on screen
distance to reflector (mm) = 0.5 x c (mm/microsec) x pulse round trip time (microsec)
for soft tissue: d = 0.77 x pulse round trip time
if the angle is lower in c2, the c in c2 is slower
repeat
T and wavelength are not related at all; they just have a common relative: f
repeat
HID
d
what determines the wavelength
the source and the medium
bc c and f = wavelength
start every exam with the ___ frequency.
highest
operating frequency for a continuous wave is equal to what?
the frequency of the transmit voltage
a short crystal impulse response means?
fewer cycles in the pulse and improved AR
natural focus of an unfocused transducer
determined by the operating frequency and the crystal diameter.
natural focus is the depth at which the beam reaches its narrowest beamwidth.
a larger diameter results in a deeper focus.
a higher frequency and damping material ___ AR.
improves
lateral resolution
determined by beamwidth
narrow beamwidth means better lateral resolution.
beamwidth varies with depth.
lenses, curved elements, electronic focusing, and mirrors are used to alter the LR
elevation resolution
=elevation beamwidth
varies with depth
best where beamwidth is narrowest
where does the highest intensity occur?
at the focus
How many dimensions do 2D transducers have?
3: axial, lateral, and elevation
Other names for lateral.
LATAS lateral azimuthal transverse angular side by side
Other names for axial.
LARRD longitudinal axial range radial depth
electronic steering?
is achieved by using small phase delays between pulses that drive the elements of phased array probes
define array.
a collection of crystals which can be used together to form a larger and more flexible transducer.
electronic focusing
uses phase delays like steering.
can also be changed by changing the number of active elements
an increase in the curved delay pattern = focus moves closer to the probe and vice versa
allows multiple foci
2D arrays
can be applied to all formats (sector, linear, etc)
controls elevation
can created 3D and 4D images
reduction in artifcats
can electronically steer and focus in both lateral and elevation
have at least three rows of elements
composite materials
wider bandwidth
makes harmonic imaging feasible
lower impedance of polymer provides a better match to tissue
other names for transducers.
probes
scanheads
transducer assemblies
what’s a naturally occuring substance that is inherently piezoelectric?
quartz
what’s the curie temp?
365 celsius
damping material
mixture of metal powder and plastic/epoxy
dampens ring down effect by absorbing vibrations
decreases SPL, amplitude, PD, Q factor, and sensitivity
improves axial resolution and bandwidth
generally 2-3 cycles per pulse
the transducer assembly includes:
casing element(s) damping layer matching layer filler material electrical circuitry
damping material
aka backing material mixture of metal powder and plastic/epoxy shortens the PD, SPL, and A increases axial resolution and bandwidth decreases Q factor and senstivity
are CW probes damped?
No
Q factor
Quality factor
describs the purity of the vibrations of the crystal aka the frequency homogenity of the beam
unitless
=operating frequency/ bandwidth
=1/fractional bandwidth aka bandwidth/operating f
for pulses 1-3 cycles long, Q is roughly the number of cycles
What does high Q mean?
narrow bandwidth
long ringdown time
better transmitter
good for doppler (because of higher A)
What does low Q mean?
wider bandwidth
short ring down time
better receiver
good for 2D imaging (because of better AR)
bandwidth
is the range of frequency in a pulse
refers only to the frequencies that have an A greater than 1/2 of the resonant frequency’s A
short pulses = broader bandwidth
probes are labeled in MHz by its resonant frequency on its frequency bandwidth curve
what is the most efficient thickness for the matching layer?
1/4 of the wavelength
What’s the optimal length of an element?
1/2 of the wavelength
Resonant/operating frequency the frequency the crystal is
most efficient in converting energy
Near field length formula
transducer diameter^2/ 4 x wavelength
so if frequency increases, the NFL increases
near field length
image resolution is better in the near field
if the aperture increases, the near zone length increases
beam diameter = lateral resolution
even natural unfocused elements have a focus point
what are the two types of real time imagers?
mechanical and electronic transducers
Types of mechanical sector scanners include? What’s one of their major disadvantages?
rotating: has a wheel that rotates
oscillating: has one crystal that moves about a pivot point
oscillating mirror: single stationary crystal; beam is directed by a moving mirror
disadvantage: fixed focal point
Electronic scanners (arrays)
have multiple rectangular elements
elements can be arranged in a straight or curved line
operated in 2 ways: sequencing or phasing
annular arrays
has ring-shaped crystals in concentric rings doesnt allow electronic steering field of view is sector phasing used for dynamic focusing focused beam is cone shaped provides focus in the elevation plane
elevation plane aka
Z plane
auto scanning
aka scanning, sweeeping, steering the beam performed automatically by electronics
scan lines are in rapid sequential order to create frames quickly
only possible with array probes
linear arrays (linear sequenced, linear, linear switched)
straight line of rectangular crystals
rectangular fov
image is composed of many parallel scan lines
voltages are pulsed to groups of elements sequentially
linear arrays (curved linear, convex, curved, convex sequenced)
operates identical to linear sequenced arrays
a variation of the linear array
has vurved line of rectangular crystals
scan lines fan out bc of the curved construction
wider fov than linear array
linear arrays in general
each element is wavelength wide
4-8 elements are activated at a time
each pulse of the group of elements results in 1 scan line
electronic focusing achieved by voltage delays
focused in z plane by using curved crystals or lesn
fov is as wide as the length of the physical array
phased arrays
aka electronic sector transducers
elements are 1/4 wavelength wide
operated by applying voltage to most or all elements with small time differences known as phasing
fov is usually sector format
can be focused in z plane if more than one layer of elements is present for phasing
dynamic focus an dmultpile foci
phasing can be applied to linear and curved linear arrays
when phasing is applied to linear, it’s called
linear phased array
aka phased array
phasing allows pulse to be shaped and steered
fov is usually pallelogram
vector array
combo array
uses phasing on each element group in a linear sequenced array to steer pulses
small probe footprint
side and grating lobes result in
noise and less sensitivity
how to reduce side and grating lobes
more elements = less side lobes
apodization: reducing voltage to elements on the side to make them less sensitive and therefore less sable to register side and grating lobes
reception steering
is sequential listening in an array probe
dynamic focus
continually changing reception focus
widening of aperture, increase focal length
like a camera focusing on someone moving closer and farther
dynamic aperture
aperture increases with increasing focal length in order to mantain focal width
detail resolution aka
spatial resolution
AR
reflector separation must be greater than half the SPL for the two reflectors to be resolved separately
sono systems proved 1mm AR
y-axis
LR
if the reflector separation is greatehr than the beamwidth, they will be resolved separately
focusing reduces beamwidth and improves LR
2mm LR is acceptable
how to improve AR
use the highest f possible
zoom
how to improve LR
use the highest f
place area of interest in near field or focal zone
line density
number of lines in the image
higher line density = higher image quality
frame rate
real time imaging requires FR of 30 frames per sec or greater
temporal resolution
ability of a display to distinguish cloesly spaced events in time
improves with higher FR
smaller size of fov improves temp resolution
improves by higher f, decreased depth, and decreased sector width
the deeper one images, the ___ it will take to receive echoes
longer
Line density (LD) x frame rate=
PRF
limited by the c and max depth imaged
focusing improves?
resolution
how can beams be focused?
curved elements, lens, and by phasing.
multiple foci
requires multiple pulses per scan line, each focus at a different depth
improves detail resolution
decreases temporal resolution
beamwidth is determined by
wavelength. focal length,, and aperture
image resolution has 3 aspects
detail, contrast, and temporal
virtual beam forming
feature of OP2
LR and elevation resolution is greatly improved
