SINGLE ELEMENT TRANSDUCERS: CONSTRUCTION AND OPERATION Flashcards
Transducer Design
First transducers employed a single-element circular ____ to both transmit and receive sound
Info obtained from scanning depends a large part on beam characteristics»_space;> influence transducer design
Design criteria
Proper frequency in ____ range
Capability of doing ____ wave operation
____ control
Uniform ____
Limited ____ extent
disk MHz pulsed directional intensity spatial
Transducer Design
What does an ultrasound transducer do?
____»_space; ultrasound ____»_space; ____ energy
____ energy»_space; ultrasound ____»_space; ____
So how does a transducer work?
electricity transducer mechanical mechanical transducer electricity
Piezoelectric Properties
Piezoelectric Principle
(Greek) ____ = to press,
____ = amber
“PRESSURE ELECTRICITY”
effect 1st described by
the Curies in 1880
piezo
elektron
Piezoelectric Properties
In certain materials, when sound is applied perpendicular to their surface, an ____ charge is created
-OR-
Some materials produce a ____ when deformed by applied pressure
electrical
voltage
Piezoelectric Properties
AND……………
Certain materials, when exposed to an electrical shock, will begin to ____ and transmit a sound wave
vibrate
Piezoelectric Properties
US probes contain elements that have this piezoelectric effect
These elements are called
Medical US ____ convert electricity to sound
transducers
Piezoelectric Properties
Transducers usually driven by 1 cycle of AC voltage for US imaging which produces a
____ – ____ cycles US pulse
Longer driving voltages (5-30 cycles) are used for ____ techniques
2
3
doppler
Piezoelectric Properties
MAN-MADE CERAMICS
____ ZIRCONATE TITANATE (PZT)
____ lead titanate
____ sulfate
polyvinylidine difloride (PVF2)
Those not naturally piezoelectric, are made so by placing the material in a strong electronic field while they are at a ____ temperature (poling)
lead
barium
lithium
high
Piezoelectric Materials
MAN-MADE CERAMICS
LEAD ZIRCONATE TITANATE (PZT)
Easy to ____
Effective at ____ wattage
____
shape
low
inexpensive
Piezoelectric Properties
If this critical temperature, the Curie point (Curie point for PZT of 3650 C = 6890 F) is subsequently exceeded, the element will lose its ____ properties
piezoelectric
Dipoles
There are regions of ____ and ____ charges on a molecule
Normally, randomly arranged and cannot migrate
positive
negative
Dipoles
Material is heated above Curie temperature (at which point the molecules are released and can move freely)
A pair of charged plates (one ____, one ____) is placed across the material
The negative ends point to the ____ plate, and the positive ends point to the ____ plate (opposites attract)
positive
negative
positive
negative
Dipoles
Material is then cooled ____ the Curie temperature with the electric plates still in place; the molecules then maintain their alignment
below
Dipoles
Material is then cooled ____ the Curie temperature with the electric plates still in place; the molecules then maintain their alignment
below
Dipoles
When the voltage is switched, the molecules twist the other way»_space; ____ the thickness
decreasing
Dipoles
Flipping back/forth cause expansion/contraction (about 10-6m, a few microns) of the crystal, which creates mechanical ____
When the expanding/____ crystal is placed on the body, sound waves are passed into the body
And vice versa
vibrations
contracting
CW Transducers
frequency of sound equals
frequency of driving voltage
(reasonably near operating frequency)
sound
voltage
CW Transducers
frequency of ____ equals
frequency of driving ____
(reasonably near operating frequency)
sound
voltage
Determinants of Frequency
PW Transducers
OPERATING FREQUENCY
a.k.a. ____ frequency is the transducers preferred frequency of operation
resonance
PW Transducers
Operating frequency is determined by:
propagating ____ of transducer material (4-6 mm/ms )
-and-
the ____ of the element (0.2-1mm)
speed
thickness
PW Transducers
f (MHz) = ct / 2 x thickness
Primarily:
the ____ of the element (0.2-1mm)
thickness
PW Transducers
THICKNESS CORRESPONDS TO ____ THE WAVELENGTH
WAVELENGTH DECREASES AS FREQUENCY ____
THINNER ELEMENTS ____ FREQUENCIES
half
increases
higher
Transducers Construction
____/Backing Material
____ Layer
____
____ Shield
Housing and ____
damping matching electrodes radiofrequency insulation
Damping/Backing Material
Immediately ____ the crystal
Composed of epoxy resin loaded with tungsten powder (medical US) or air (therapeutic US); acoustic ____ comparable to that of the crystal, absorbs sound energy
behind
impedance
Damping/Backing Material
PURPOSE:
TO DIMINISH THE RINGING OF THE CRYSTAL
Reduces number of ____ in each pulse
GOOD!
Reduces ____ duration & ____, improving ____
GOOD!
cycles
duration
SPL
resolution
Damping/Backing Material
PURPOSE:
TO DIMINISH THE RINGING OF THE CRYSTAL
Reduces the ____ and ____ (ability to detect weak echoes) of the transducer because it lowers the intensity (amplitude) of the output power
BAD!
efficiency
sensitivity
Damping/Backing Material
Max transfer of energy (crystal – backing material) impedances are identical
Diagnostic Imaging - Typically pulses of __-__ cycles are generated with damped diagnostic imaging US transducers
CW Doppler - Transducers intended for CW Doppler use are not damped (____ efficiency)
1
3
higher
Dynamic Damping
____ means to suppress ringing
____ w/ opposite polarity applied to crystal
Counteracts expansion and ____
electronic
voltage
contraction
Matching Layer
Acoustic impedance of crystal is ____ (30Mrayls) compared to tissue (1.9 Mrayls)
Resulting in an ___% reflection and a ___% transmission; the acoustic impedance mismatch creates a long pulse and reduces ____ of beam entering patient = BAD
To shorten pulse. & improve energy transfer, a material of ____ impedance is placed between crystal and patient = GOOD
large 81 19 intensity intermediate
Matching Layer
Located between the crystal and the skin; may be ____ or multiple
Transducer elements have ____ density and sound speed;
IMPEDANCE 20 TIMES THAT OF TISSUES
causing ≈ ___% of incident intensity to reflect
single
high
90
Matching Layer
Matching layer allows more of the US beam to be ____ into the patient
transmitted
Matching Layer
Usually of an acoustic impedance between that of the crystal and ____
Usually designed to be
(l of the center frequency)
May have multiple matching layers to maximize the effectiveness of ____-hertz transducers
ST
multi
Electrodes
Located on ____ sides of the crystal to create the changing polarity
Formed by plating a thin film of ____ or ____ on the crystal surface; electrodes are then fastened to each surface, and then attached to an electrical connector at the inactive end of the transducer
opposite
gold
silver
Radiofrequency Shield
Transducers are sensitive to ____ interference, which contributes to the noise level
____ noise levels prohibit the detection of weak echoes
Shield is composed of a hollow metallic cylinder placed around the crystal and ____ material; electronically grounded
electromagnetic
high
backing
Housing Insulator
An electrically insulated case (usually plastic), that houses the entire ____ assembly
Provides ____ and support
Insulation usually of ____ or cork
transducer
protection
rubber
Bandwidth
Pulse ____ and Bandwidth
____ of Bandwidth
____ Bandwidth
____ Layer Effect on Bandwidth
duration
calculation
fractional
matching
Bandwidth
Range of ____ contained within an US pulse
A parameter that describes the distribution of ____ components in a wave
Fractional bandwidth = bandwidth/____ frequency
frequencies
frequency
operating
Bandwidth
Typical fractional bandwidths for modern transducers, ____%-____% (5MHz operating frequency with a bandwidth from ____ – ____ MHz)
Fractional bandwidth =
bandwidth/operating frequency
7.5 - 2.5 / 5 = 1
50
100
2.5
7.5
Bandwidth
____-bandwidth transducers (those having fractional bandwidth of > 70%) the voltage excitation can be used selectively to operate the same transducer at more than one frequency
Allows for ____ imaging
wide
harmonic
Bandwidth
short pulse ____ (large) bandwidth
long pulse ____ (small) bandwidth
short SPL ____ (large) bandwidth
broad (large) bandwidth ____ Q factor
broad
narrow
broad
low
Q Factor
Describes the “____” of the frequency of an US wave
Q factor = ____ frequency/bandwidth
Q factor is ____
purity
operating
unitless
Q Factor
Damping material bandwidth, Q factor and amplitude/intensity
broad bandwidth ____ Q factor
narrow bandwidth ____ Q factor
low
high
Q Factor
In diagnostic PW ULTRASOUND,
____ Q transducers (2-3) are desirable
In CW ULTRASOUND,
____ Q transducers (> 700) are desirable
For short pulses, # of pulses Q-factor
(____-____ cycles per pulse)
low
high
2
3
Q Factor
Overall system bandwidth determined by ____ and instrument electronics
With wide-bandwidth transducers
(fractional bandwidth of ____%)
you may selectively operate the same transducer at more than one ____
transducer
70
frequency
—Long ring-down time
Longer Pulse = long PD
Narrow range of Frequencies = ____ Bandwidth
____ Q factor
Better ____
Used for Therapy and ____ Doppler
—Short ring-down time
Shorter Pulse = ____ PD
Wide range of Frequencies = ____ Bandwidth
____ Q-factor
Better ____
Used for ____-Echo Imaging
narrow high transmitters CW short broad low receivers pulse
Composite Piezoelectric Materials
Piezoelectric ceramics are often combined with a non-piezoelectric polymer to create materials called
____ HAVE ____ IMPEDANCE AND IMPROVED ____, ____ AND ____
composites lower bandwidth sensitivity resolution
Composite Piezoelectric Materials
Diced PZ material into an array of tiny pillars
Surrounded by epoxy –
____ dense, ____ impedance to 10-20 Mrayls
____ bandwidth
____ sensitivity to returning echoes
less
decreases
wider
higher
Under Development
____ Transducers
____
____ (via catheters)
cMUTs
____ micro-fabricated ultrasonic transducer
invasive
endocavitary
endovascular
capacitive
Under Development
Invasive Transducers
Advantages
Approaches allow transducer placement closer to ____ of interest
Avoids intervening tissues, ____
____ pathlength, ____ attenuation
Allows use of ____ frequencies resulting in better resolution
anatomy air reduces decreasing higher
Under Development
cMUTs
Capacitive microfabricated ultrasonic transducer
Elements are composed of ____ of microscopic silicon drums that include thin suspended membranes
thousands
Under Development
cMUTs
voltage applied to drum
creates an ____ force on the membrane
membrane ____ emitting a pulse of US
and
vise versa
electrostatic
vibrates
Under Development
cMUTs
Major Advantages
1. An impedance much ____ than ceramic elements
-Enables ____ bandwidth
-Fractional bandwidths > ____%
(fractional bandwidth = bandwidth/____ frequency)
lower
broader
100
operational
Under Development
cMUTs
Major Advantages
- ____-based technology = elements and electronics are combined in silicon
- Allows more electronic components to be
housed in ____ assembly - More signal processing accomplished
before sending ____ signals through
cable to instrument
semiconductor
probe
electric
Under Development
cMUTs
Major Advantages
- More efficient transducer
- ____ energy lost in damping because pulse
is coupled better to the tissue - May allow flexible transducers to be
made
- Conform to patient surface more effectively
and allow ____ area of acoustic coupling
less
broader
