physics CH 8 transducers

Transducer

any device that converts one form of energy into another.

two functions that an ultrasound trx performs?

transmission- converts electrical energy into sound energy

reception- reflected sound pulse is converted into electricity.

Piezoelectric effect

the property of a material to create a voltage when it is mechanically deformed or when pressure is applied.

piezoelectric materials

materials that convert sound into electricity and viceversa

some naturally occuring are quartz and tourmaline.

man made are lead zirconate titanate (PZT)

why can’t naturally piezoelectric materials be used in a trx?

the impedances are too high.

PZT or active element

the piezoelectric crystal itself. it is ½ wavelength think. impedance is 20 times greater than the skin.

Matching Layer

positioned in front of the PZT at the transducer face. it matches the impedances to improve transmission. it is ¼ wavelength thick.

also called intermediate layer, middle layer, middle value

two things the matching layer does:

increases efficiency of sound energy transfer b/w the element and the medium. (creates a smoother energy transfer)

protects the active element.

what percentage of the beam will be refelcted back without a matching layer?

80%

Continuous wave trx have ___ matching layer

1

Backing material (damping element)

reduces “ringing” of the PZT and has the same impedance as the PZT

shortens the PD and length

crystal damping reduces axial resolution

increases bandwidth

reduces sensitivity of the PZT, CW does not have this and is more efficient

what increases the efficiency of sound transfer between the trx PZT and the skin?

the matching layer and gel

decreasing order of impedance:

PZT > matching layer > gel > skin

Three results related to the use of backing material:

decreased sensitivity

wide bandwidth

low quality factor

Bandwidth

the range of frequencies in the pulse. the difference between the highest and lowest frequencies- Frequency Range

Long duration events

narrow bandwidth

Short duration events

wide bandwidth

imaging probes produce pulses that are

wide bandwidth or broadband.

CW doppler

does not use backing material and produces a narrow bandwidth pulse.

Quality factor

unitless number that is inversely related to bandwidth

formula for quality factor

QF = main frequency / bandwidth

wide bandwidth probes have a ____ Q factor

low

narrow bandwidth probes have a ___ Q-factor

high

a shorter, dampened pulse has a

lower Q- factor

a longer, undampened pulse has a

higher Q- factor

Imaging transducers (PW)

pulses with short duration and length (low PD and low SPL)

uses backing material to limit ringing

reduced sensitivity

wide bandwidth/ broadband

lower Q- factor (related to # of cycles in a pulse)

improved axial resolution

Non-imaging transducers (CW)

creates CW

No backing material

increased sensitivity

narrow bandwidth

higher Q- factor

cannot create an image (has no depth resolution)

polarization

exposing a pzt material to a strong electrical field while it is being heated to a high temperature

Curie Point (curie temp)

the temperature at which the pzt is polarized

350 degrees C

680 degrees F