Chapter 3 Transducers - quick Notes
thickness = c/(2*fo)
In pulsed mode what determines the operating frequency?
fo = c/(2*thickness)
Pd = Period * n
(D2 * f) / 6
Axial R = 1/2 SPL
Lateral R = Beamwidth
1/2 element thickness
SPL = nc/f
λ = c/f
fractional bandwidth = bandwidth/operating frequency
Q factor = operating frequency/bandwidth
bandwidth = max frequency - min frequency
man made piezoelectric material
improved bandwidth, sensitivity, resolution
How many cycle pulses does one cycle of alternating current produce?
How many cycles of alternating current are needed to produce CW?
5 - 30
Operating frequency equals
the freqency of the sound produced
What determines te operating frequency
thickness and speed of material
Thickness of the element equal
Typical thickness and speed of element
1 - 2 mm thick
4 - 6 mm/μs speed
What effect does thinner element have on frequency?
Higher frequencies which create better detail resolution
What is the importance of wide-bandwidth transdcers?
the transducer is driven at 1 - 3 selectable frequencies.
higher frequencies create better resolution
but if the depth is not suffient a low frequency can be chosen
make harmonics possible - low frequency goes in higher frequency returns
Pulse Repetition Voltage
The number of voltage pulses sent to the transducer each second
Pulse duration formula
Pd = Period * n
What is damping material made from?
epoxy and plastic
WHat does damping reduce?
WHat does damping increase
WHat is the reason for using backing material?
improve sound transmission across the element/tissue boundary
transducer elements are a solid with high impedance
impedance 20 times higher than skin, with out it 80% of intensity would be reflected at the skin.
How many layers of backing are used?
multiple layers are best
What is the purpose of Coupling medium?
eliminate the air between the transducer and skin
Why does little Francus like Ducks?
(D2 * f) / 6
What happens to near zone length with increasing frequency or aperture?
decrease of size with increasing depth
increase of size with increasing depth
What happens during focusing?
the end of the near zone is moved closer to the transducer and beam is narrowed.
What is the Focal length
depth of field
How is focusing complished?
made up of many elements
an element has been sliced
Define linear and describe the signifgance of linear transducers.
Linear transducers produce images that are rectangle
applying voltage to groups of elements in succession
Think of taking a group of photos in a line to be spliced togething in PS for panaramic.
Shoot slide shoot slide shoot slide shoot
Explain Linear squenced Array
element in a straight line
squenced - pulses originating at different points but all traveling in the same vertical direction
Define convex and describe the signifgance of convex transducers.
curved line (bowed outward)
Convex transducers produce sector-like images
Sequencing - pulses originating at different points and traveling out in different directions.
voltage is sent to most or all elements with small time differences so that the resulting pulse is sent out in a specific direction
How do you determine the direction of the steered beam?
voltage applied from left to right will go right
Explain electronic focus.
accomplished with a curved pattern of phasing.
Greater curvature places the focus closer to the transducer
Explain multi foci
multiple pulses per scan line each focused at a different depth
What are the advantages and disadvantages of multi foci
improves detail resolution
degrades temporal resolution
to maintain the same beam width at increasing focal lengths
larger groups are used for deeper focal lengths
Explain section thicnkess focus
With multiple rows of element phasing can be applied to focus the slice thickness
What is a grating lobe?
additional beams that go off in a direction out of the scan plane.
Array transducers only
*side lobes are produced from single element transducers
How are grating lobes reduced?
lowering amplitude of outer elements
dynamic - changed on the fly
Explain the signifigance of Vector Arrays
Phasing is applied to each element group during sequencing to steer
pulses in various directions.
Produced a vector shape image
How is a parallelogram image produced?
when phasing is applied to linear sequenced arrays
each pulse travels in the same direction but not straight down.
Explain dynamic reception focus
reception focus depth changed continually as the transmitted pulse travels through tissue and the echoes arrive from deeper and deeper locations
What is dynamic aperture?
aperture increases to maintain a constant focal width with changing focus
List the transducers
Linear Array - phased and sequenced
Convex Array - phased and sequenced
Phased Array - Phased
Vector Array - Phased and sequenced (linear - small)
What are the three aspects of Resolution?
What is Axial resolution?
resolution along the direction of the beam.
What determines Axial Resolution?
*# of pulses
Axial Resolution formula
What is the trade off for improving Axial / Lateral resolution by raising the frequency?
Depth is lost
What is Lateral resolution?
resolution across the beam
What determines Lateral Resolution?
How can Lateral Resolution be improved?
focusing - decreasing beam width
What is the useful U/S range of Frequency
2 MHz to 15 Mhz
Lower than 2 MHz and resolution sucks
Higher than 15 MHz and Depth is insufficient
What is the relationship between Axial Resolution and # of pulses?
What is the relationship between Axial / Lateral Resolution and Frequency?
The numeral value of Resolution goes down when frequency goes up but the numberal value is best at low numbers. Therefore making this a direct relationship.