Week 3 transducer manufacturing and beam formation Flashcards
Eqn for Quality factor
Q = Res freq/bandwidth = f0/(f2-f1)
for US, want low Q factor (ie wide bandwidth)
incr damping = incr bandwidth
Range eqn - how does the US machine determine structure depth
OR,
remember the go return time trick
V = d/t
V is 1540 m/s, t is time there and back, solve for d
structure depth = d/2
OR,
the go return time in soft tissue for a structure that is 1 cm deep is 13 μs
What is the purpose of the transducer housing/insulator
increase the signal/noise ratio
what is the purpose of the transducer damping material / backing material
limits # cycles, so creates short pulse length
stops crystal from vibrating too long
What is the relationship btw crystal thickness and freq
thick crystal = low resonance freq
thin crystal = high freq
How thick should you cut your crystal to get a desired freq?
What is the c for PZT?
thickness = λ/2
(cut crystal to half the wavelength of the desired freq)
λ = c/f
c (PZT) = 4mm/μs
What is the purpose of the transducer matching layer?
How thick should the matching layer be?
Matching layer has acoustic impedance btw crystal and skin, therefore reduces reflection at skin
Thickness = λ/4,
but transducer emits spectrum of freq, so use multiple matching layers to avoid narrowing bandwidth (also to create step downs in z?).
Huygen’s principle
each section of crystal produces its own wavelet;
wavelets interfere with each other;
forms complex wave that travels in 1 direction
Explain aperture
Aperture is crystal diameter.
Crystal is actually 250 crystals in a row. Can change diam by adjusting “2D width” and turning crystals on/off.
Beamwidth
- what happens when you incr freq?
- how do you calculate beamwidth at focal pt?
incr freq = decr. beamwidth
beamwidth at focal pt = F*λ /D
F is Focal distance and D is crystal diam.
Eqn for Near Field Length
NFL = D^2/(4λ)
D is crystal diam
Near field has good lateral res (bc signals are closer together than far field where they diverge).
To improve lateral res, incr NFL (incr D or incr f)
Note: incr f = decr penetration
Eqn for far field divergence
Sinθ = 1.2λ/D
θ is divergence angle
D is crystal diam
To improve far field lateral res, decr divergence ange (incr D or incr f)
Intensity profile of beam in near field versus far field
Near field - complex, near focal pt it is highest in the middle
*** higher intensity than far field
Far field - more uniform bell shaped intensity but weaker
What are side lobes and why are they problematic
- signal coming off side of beam, generally quite weak
- can cause artifact or noise
What are the different terms for axial res
LARRD
Longitudinal
Axial
Range
Radial
Depth
What are the different terms for lateral res
LATA
Lateral
Angular
Transverse
Azimuthal
Axial res formula
How to get better axial res (for res, better is a lower number)?
Axial res = SPL/2 = nλ/2
SPL is spatial pulse length = length of wave
incr freq = better axial res
What is lateral res?
How to get better lateral res?
lateral res = beam width
incr freq = decr beam width = better lateral res. (b/c lines are closer together)
What is the difference btw and external focus probe and an internal focus probe
external focus: lens or mirror, not used today
internal focus: curved crystal, used for transducer with f<5 MHz (can only shape thick enough crystal)
Explain electronic focusing
This is the type of focusing we use
There is a time delay behind each crystal. The outer crystal elements are excited first and the middle ones are excited last, so the beam converges at a focal point. When you turn the focus knob this changes the time delays and changes the focal point.
What is elevational res?
Can it be adjusted?
elevational res = beam thickness
Determined by height of the crystals and cannot be adjusted
Another name for near field
Fresnel zone
Another name for far field
Fraunhofer zone
