Unit 3_Transducers and the Beam

what is a transducer?

something that converts one form of energy to another

what do US transducers do?

convert electric energy into US (and vice versa)

Principle of Piezoelectricity states:

some materials, when deformed by an applied pressure, produce a voltage

the electric voltages applied to transducer are converted to _____________.

US echoes incident on the transducer and produce _____________.

ultrasound; electric voltages.

what do US transducers operate according to?

the Piezoelectric Principle

most common Piezoelectric material used?

PZT- lead zirconate titanate

• Easily available and low cost

other names for Piezoelectric elements?

• crystal

• active element

• transducer element

when a voltage is applied, what happens to the thickness of the element?

the thickness of the element increases or decreases depending on the polarity of the voltage

examples of natural Piezoelectric elements?

quartz, tourmaline, Rochelle salt

examples of synthetic (not naturally) Piezoelectric elements

lead zirconate titanate (PZT)

what is the Curie Point (number and definition)?

365 degrees Celcius

temperature at which material loses magnetic properties (aka dipoles can move freely)

explain process of how elements become Piezoelectric:

• Heat material to Curie point (365°C)

• Polarize dipoles with strong electric current/ field (alignment circuit).

• Cool crystal below Curie point and remove from the alignment circuit.

• The now aligned dipoles remain fixed parallel to each other and the material can exhibit its piezoelectric properties. 

what occurs if an element is reheated to Curie Point?

Heating to Curie point again destroys piezoelectric properties

single element transducers take the form of _________.

disks

linear array transducers contain

numerous elements that have a rectangular shape

what describes a probe, scan head, or transducer assembly?

element with casing and damping and matching material

what are CMUTs?

• capacitive micromachined ultrasonic transducers

• newer transducers that contain miniature elements with two electrically conducting layers facing each other

differences between CMUTs and PZT elements?

CMUTs have several advantages over PZT elements:

• broader BW

• improved detail resolution

operating frequency definition?

natural frequency of operation for the element

what is operating frequency determined by?

• Propagation speed of the element material

• Thickness of the transducer element

equation for operating frequency?

ƒ_o =  cPZT / 2 x cth

thickness and frequency relationship?

• Thickness & frequency are inversely related

  • thinner elements operate at higher frequencies

crystal thickness equation?

crystal thickness= wavelength / 2

other names for operating frequency?

• resonant frequency

• center frequency

typical diagnostic US elements are how thick? propagation speeds?

0.2 mm thick

4-6 mm/ µs

what is bandwidth (BW)?

range of frequencies a transducer generates in addition to the operating frequency

what bandwidths are produced with short pulses?

• broad BW

• IMAGING

what bandwidths are produced with longer pulses?

• narrow BW

• DOPPLER

what does a broad/wide BW allow for?

• imaging at multiple frequencies within BW range without changing transducers

• harmonic imaging!!

what is damping (backing) material?

mixture of a plastic or epoxy resin attached to the rear face of the transducer element

what does damping/ backing material do?

• absorbs vibrations

• reduces # of cycles/ pulse

  • broadens BW!

• improves resolution (axial?)

  • reduces SPL

relationship between damping and PD?

inversely!

• increased damping = decreased PD

• this is because damping decreases cycles/ pulse and PD= T * cycles/ pulse

relationship between damping and SPL?

inversely!

• increased damping = decreased SPL

• this is because damping decreases cycles/ pulse and SPL= wavelength * cycles/ pulse

relationship between BW and QF?

inversely!

• increased BW with more damping= decreased QF, sensitivity, and efficiency

what transducers are not damped?

CW because pulses are not used

how does damping material shorten pulses?

by allowing US energy to pass into it

CW transducers do not have ______________, which allows for….

backing material; all US energy to be reflected back into the patient

Con of Damping Material

• reduces amplitude —>because not as much constructive interference

  • decreases efficiency and sensitivity

how are diagnostic imaging transducers damped?

HIGHLY damped

• 2-3 cycles/pulse

how are PW Doppler transducers damped?

LESS damped

• 5-30

what does QF do?

determines sensitivity

• ability to detect weak echoes

lightly damped transducers have what kind of BW?

• narrow BW

• high QF, more sensitivity and efficiency

what is the matching layer?

material places on transducer face (front)

• typically two layers are used but in some cases two or three can be used

what does the matching layer do?

• improves sound transmission across element-tissue boundary

  • by reducing reflection

• impedance matching is intermediate between transducer element and tissue

  • reduces reflection, improves transmission

matching layer equation?

wavelength / 4

what would occur without the matching layer?

majority of emitted intensity reflected at the skin boundary

what occurs due to air between the transducer and skin surface?

reflection of virtually all sound and prevention of any penetration into the sound tissue

what does a coupling medium do?

aka gel

• eliminates air layer between transducer and skin

• facilitates passage of sound into and out of tissue

what occurs to a pulse as it travels away from the transducer? what causes this?

width of pulse changes

• superposition of waves

  • constructive interference

  • destructive interference

ultrasound waves follow what principle?

Huygen’s Principle

what is Huygen’s principle?

sound beam is a combination of all sound arising from different point like sources on a transducer crystal face

what determines lateral resolution?

width in scan plane

what is not uniform throughout the beam?

intensity (not uniform in space or time)

• Intensity= power/ area

• therefore, as beam gets smaller, intensity gets larger because the area is smaller.

what are side lobes?

additional beams that travel out in some directions NOT included in the beam

other names for Near Zone?

Fresnel Zone or Near field

what is the near zone?

region extending from the transducer to the minimum beam width

in the near zone, beam width decreases as….

distance from the transducer increases

what is Near Zone Length?

distance between transducer and minimum width of beam

what is the NZL determined by?

size and operating frequency of the element or group of elements

• for equation relationship with frequency, note that frequency and wavelength are inversely related

if aperture size increases, NZL ___________

NZL increases

if frequency increases, NZL ___________

increases

NZL equation?

NZL=  D²/4λ or D²f /4c

what is the focal zone

where the beam is focused on each side of the focal point

where is the best resolution?

focal zone, due to narrowest beam width

focal zone equation?

beam diameter at natural focus= D/2

other names for far zone?

far field or Fraunhofer zone

what is the far zone?

region after natural focus

what is divergence of the beam?

beam width increases with increasing distance from the transducer

far field divergence equation?

sin⁡θ=1.22λ/D 

relationship between far field divergence and diameter

inversely related

• increase in Diameter

• decrease in beam divergence

relationship between far field divergence and frequency?

inversely related

• increase in frequency

• decrease in beam divergence

what does focusing improve?

lateral resolution

• ONLY IN THE NEAR ZONE

beam width is decreased in …

the near zone and focal region

beam width is increased/ widened in…

the far zone

what is focal length?

• same thing as near zone length

• the distance from the transducer to the center of the focal region (distance from a focused transducer to Spatial Peak Intensity)

sound may be focused by:

• curved transducer elements

• using a lens

• phased arrays

define frame

a complete scan of the US beam

what does real time sonography do?

presents images (frames) in a rapid sequential format

electronic scanning is performed with…

arrays

automatic scanning of the sound beam is performed how??

electronically

automatic scanning provides a means for…

sweeping the sound beam through the tissues rapidly and repeatedly

how are arrays operated?

in two ways:

• sequencing

• phasing

how are elements arranged?

in a straight (linear) or curved (convex) line

what is sequencing?

sweeping of the beam to allow for real-time scanning

what is phasing?

beam steering and focusing

what is linear sequenced array?

• aka sequencing (which is sweeping the area)

• straight line of rectangular elements

how is linear sequenced array operated?

by applying voltage pulses to GROUPS of elements in succession

what does linear sequenced array do to image quality?

increases line density= improved image quality

another name for convex sequenced array?

curved array

what is convex sequenced array?

curved line of elements that created a sector image

difference between convex sequenced array and linear sequenced array?

operate similarly except pulses travel out in different directions from different points across the curved surface

what is phased array also known as?

linear phased array

how is phased array operated?

by applying voltage pulses to most or all of the elements (not groups) but with small time differences (delays) between them

• resulting sound pulse is sent out in a specific path direction

what is beam steering?

phasing is applied to arrays to steer the beam by sending out several pulses from each group with different phasing

what is electronic focusing?

phased array can also focus the beam

a greater curvature would place the focus…

closer to the transducer

less curvature moves the focus….

deeper

what does aperture focusing do?

varies the number of crystal activated to focus the beam

what groups of aperture focusing are used for short focal lengths?

smaller groups

what is the dimension perpendicular to the scanning plane known as?

slice/ section thickness dimension