Transducers & the US Beam

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)