physics final exam review questions &
answers
define the piezoelectric effect - answer the property of certain crystals
to emit electricity when pressure is applied to them
occurs with reception
what is the most common material used for transducer crystals -
answer lead zirconate titanate (PZT)
what is another term for the near field of a sound beam - answer
fresenl zone
what is the purpose of transducer matching layers - answer - decreases
the impedance mismatch between crystal and skin
- helps avoid large reflection at the skin surface
what factors determine operating frequency for pulsed wave
transducers - answer - crystal thickness (inverse relationship)
- crystals vibrate in thickness mode (expand and contract)
- crystal prop speed (direct relationship)
- most crystals are cut to a thickness half of the desired frequency
wavelength
,what is the importance of the curie temperature to ultrasound imaging
- answer - it is the temperature between 300oC - 400oC at which
certain crystals will lose their piezoelectric properties
- this is why we don't autoclave our transducers
what are characteristics of the focus portion of a sound beam - answer -
diameter of the beam is the smallest at this portion
- area of greatest lateral resolution
- area of maximum sensitivity
- depth varies w/ frequency or focal depth adjustments
- width varies with additional focusing, crystal diameter/aperture
what ensures short spatial pulse lengths in a pulsed wave transducer -
answer - SPL depends on the wavelength x number of cycles
- shorter SPL comes from using higher frequencies (inversely related)
- wavelength and SPL are directly related
- backing/damping material shortens crystal ring time and this will
create short pulses too because fewer cycles will be produced
how are piezoelectric crystals affected by electrical stimulation and by
pressure stimulation - answer PRESSURE: property of certain crystals to
emit electricity when pressure is applied to them (piezoelectric effect)
, ELECTRICITY: property of certain crystals to expand or contract when
electricity is applied to them (reverse piezoelectric effect)
what determines the operating frequency of a continuous wave
transducer - answer - oscillator : responsible for crystal vibration
- voltages are constantly sent to crystal so ultrasound is always
transmitted (not pulsed)
- voltage frequency (f of oscillator = f of crystal vibration = f of TDR)
describe the elevational plane of a sound beam and list its alternative
names - answer - the plane on the outside of the beam also known as z-
axis
- the 3rd dimension on the screen (shows thickness of an organ)
- aka slice thickness, elevational, z-axis
- want the dimension as thin as possible because the image comes out
as 2D usually
describe how huygen's principle is related to a sound beam's hourglass
shape - answer - microparticles within crystal create secondary wavelets
that diffract/spread as they move away from TDR
- these wavelets interfere with each other, combining constructively to
determine hourglass shape of main soundbeam
answers
define the piezoelectric effect - answer the property of certain crystals
to emit electricity when pressure is applied to them
occurs with reception
what is the most common material used for transducer crystals -
answer lead zirconate titanate (PZT)
what is another term for the near field of a sound beam - answer
fresenl zone
what is the purpose of transducer matching layers - answer - decreases
the impedance mismatch between crystal and skin
- helps avoid large reflection at the skin surface
what factors determine operating frequency for pulsed wave
transducers - answer - crystal thickness (inverse relationship)
- crystals vibrate in thickness mode (expand and contract)
- crystal prop speed (direct relationship)
- most crystals are cut to a thickness half of the desired frequency
wavelength
,what is the importance of the curie temperature to ultrasound imaging
- answer - it is the temperature between 300oC - 400oC at which
certain crystals will lose their piezoelectric properties
- this is why we don't autoclave our transducers
what are characteristics of the focus portion of a sound beam - answer -
diameter of the beam is the smallest at this portion
- area of greatest lateral resolution
- area of maximum sensitivity
- depth varies w/ frequency or focal depth adjustments
- width varies with additional focusing, crystal diameter/aperture
what ensures short spatial pulse lengths in a pulsed wave transducer -
answer - SPL depends on the wavelength x number of cycles
- shorter SPL comes from using higher frequencies (inversely related)
- wavelength and SPL are directly related
- backing/damping material shortens crystal ring time and this will
create short pulses too because fewer cycles will be produced
how are piezoelectric crystals affected by electrical stimulation and by
pressure stimulation - answer PRESSURE: property of certain crystals to
emit electricity when pressure is applied to them (piezoelectric effect)
, ELECTRICITY: property of certain crystals to expand or contract when
electricity is applied to them (reverse piezoelectric effect)
what determines the operating frequency of a continuous wave
transducer - answer - oscillator : responsible for crystal vibration
- voltages are constantly sent to crystal so ultrasound is always
transmitted (not pulsed)
- voltage frequency (f of oscillator = f of crystal vibration = f of TDR)
describe the elevational plane of a sound beam and list its alternative
names - answer - the plane on the outside of the beam also known as z-
axis
- the 3rd dimension on the screen (shows thickness of an organ)
- aka slice thickness, elevational, z-axis
- want the dimension as thin as possible because the image comes out
as 2D usually
describe how huygen's principle is related to a sound beam's hourglass
shape - answer - microparticles within crystal create secondary wavelets
that diffract/spread as they move away from TDR
- these wavelets interfere with each other, combining constructively to
determine hourglass shape of main soundbeam
