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SON 111 MIdterm Rev

Physics Midterm Review

What improves sound transmission? Matching layer(s) reduce the reflection of sound at the transducer element surface.
How thick is the matching layer? 1/4 of a wavelength
What are the ways to focus a transducer? curved transducer element (internal focusing), curved reflector (mirror), add a lens, phased array (electronic focusing)
What does beam diameter (width) depend on? frequency (wavelength), aperature size, distance from the element
At one NZL, beam diameter = ? 1/2 transducer diameter (face)
At 2x NZL, beam diameter does what? increases in proportion to distance
What does a matching layer do? Stops total reflection at skin boundary. It improves sound transmission.
Describe matching layers. Impedence is btwn the transducer & skin. It is placed on the transducer face & multiple matching layers are used to accomodate the different frequencies coming out of the transducer.
How thick is the matching layer? 1/4 of a wavelength
What are the names for the near field? Near zone, fresnel zone
What is the near zone? A region that extends from the element out to a distance of one near zone length OR distance from the face of the transducer to the point where the beam converges.
Transducer element increases, NZL ? increases
Frequency increases, NZL ? increases
What are the other names for far field? far zone, fraunhofer zone
What is the far zone? Region that lies beyond a distance of one NZL where the beam diverges.
What happens to the far zone when the beam is focused? The more focused the beam, the more the far zone diverges.
What is the preferred frequency of operation? (3 names) Operating/Fundamental/Resonance frequency
What determines the resonance frequency? Propagation speed of transducer element (PZT crystal) 4-6 mm/us & thickness of the transducer element
What is damping material & what's another name for it? Backing material--mixture of metal powder & epoxy.
What does damping material do? It reduces the # of cycles in the pulse therefore limits SPL & improves resolution.
What is the drawback of damping material? decreased efficiency & sensitivity
What is the most important function of damping material? improves resolution (decreases PD & SPL)
What is quality factor? Tells us how pure our US frequency is going to be. It approx = # of cycles in the pulse.
How do we determine Q factor? Operating frequency/ bandwidth
How are Q Factor & bandwidth related? inversely
How do we improve Q factor? Damping material. Damping material increases BW therefore decreases Q factor giving us a purer frequency. Q factor of 1 is better than 2.
What is bandwidth? Range of frequencies in PW ultrasound. US pulses contain frequencies in addtion to the operating frequency. Range is usually 6 dB. The range is of frequencies that have 1/2 or greater the amplitude of the OF.
The shorter the pulse creates ? more frequencies, decrease in # of cycles--the wider the BW
Who discovered the piezoelectric effect & when? Pierre Curie 1880
Describe peizoelectric effect. Some materials produce a voltage when deformed by an applied pressure.
What is the Curie Point? The strong electric field that is 300 degrees Celcius that gives ceramics the piezoelectric effect.
What is PZT? lead zirconate titanate--used in modern transducers
How is the NZL determined? By the size & operating frequency of the element. Increases proportionately w/ increasing frequency & increases with diameter squared.
What is diffraction? Spreading out of the sound beam--from a small point source (transducer), sound goes out in all directions
What is an example of diffraction? Dropping a pebble in calm water--the transducer is not the pebble, all elements along the transducer face cause a wave of their own.
What is aperature? Size of the group of elements that produce one scan line or size of the transducer that is actually producing sound at that time.
Increase depth--aperature gets? bigger
Decrease depth--aperature gets? smaller--The machine controls the aperature size & we control it 2nd hand by controlling the depth.
What is Snell's Law? Law of Refraction that says the transmitted angle is greater than the incident angle if the propagation speed through medium 2 is greater than the propagation speed of medium 1.
What is refraction? Change in direction of sound due to propagation speed & oblique incidence. If PS are equal, no refraction occurs.
What is acoustic impedence? How tissue handles the soundwave.
What is the formula for AI? Z=pc---average soft tissue Z=1,630,000 rayls
What is oblique incidence? Occurs when direction of sound travel occurs btwn 2 media NOT perpendicular. Deals with propagation speed & angles.
What is incidence? How a sound wave strikes a boundary.
What is perpendicular incidence? Normal incidence--occurs when direction of sound travel is perpendicular to the boundary (90 degree angle).
What is diffused sound? Scattering--wavelength is large compared to the boundary---redirection of sound in many directions
What type of boundary does scattering occur? Rough surfaces & heterogeneous media (RBCs)
What is specular reflection? Wavelength is small compared to the boundary--boundary is smooth.
What does scattering do? Allows the tissue parenchyma to be seen as well as the tissue interface. ***Helps make imaging less dependent on scanning angle***
What is propagation speed? Speed with which a wave moves through a medium (m/s)
What determines propagation speed? Stiffness & density of the medium
Stiffness increases, PS? Density increases, PS? increases decreases
What do contrast agents do? Strengthen echogenicity of blood, strengthen echogenicity of perfused organs, & improve contrast btwn lesions & normal tissue.
What is a transducer? A device through which energy can flow. It converts one form of energy into another. Energy isn't lost just changed.
What does the crystal do when the piezoelectric hits it? Contracts & expands
What are harmonics? Fundamental frequency is the frequency of the transducer. Harmonics improve the quality of the sonographic images. Waveform is sawtooth.
What are the intensities that we know & use? SPTP--Spatial peak temporal peak---highest SATA--Spatial avg temporal avg---lowest SPTA--Spatial peak temporal ave---what we use
What are multiple focuses? Focusing at mutiple depths.
How does multiple focuses work? One pulse can only focus at one location. If there are 2 focuses, it will have 2x more scan lines than 1. Your echoes will have to go to 1 focus & come back & then go to the next focus & come back. Slows down frame rate & creates more scan lines.
What does focus do to the sound beam? Causes greater beam divergence in the far field. The longer the NZL, the less beam divergence in the far field.
What is transmit focus? Focuses at a certain point so it's already focused when it returns.
What is Dynamic focus? Receive Focus--"listening" focus--Set at a particular depth & echoes that are received from that particular depth are focused when they return. Continually changes as the tissues are scanned.
What are the 2 ways that transducers operate? Sequencing & phasing
What is applying voltage to groups of elements in succession? Sequencing
What is applying voltage to all elements at almost the same time? Phasing-- <1us difference
What are the ways that transducers are constructed? linear, convex, array
What is axial resolution? Minimum separation req'd along the sound path parallel to the sound beam.
What is lateral resolution? Minimum separation btwn 2 reflectors that are perpendicular to the sound beam that can produce 2 separate echoes.
The ability to resolve structures parallel to the sound beam. Axial resolution
The ability to resolve structures perpendicular to the sound beam. Lateral resolution
What improves axial resolution? #1--using a higher frequency transducer (reduces wavelength). 2--More damping materials (reduces # of cycles).
What is the formula for axial resolution? AR = 1/2 SPL
What improves lateral resolution? Focusing. The best AR & LR is obtained at the focus.
Which resolution can be controlled better? Axial resolution b/c it's easier to change transducers or frequency than it is to use more damping material
LR = ? beam width--smaller beam width--better focusing therefore better LR
What is the useful frequency range for resolution? 2-15 MHz---2 for penetration & 10 for best resolution
What is a group of parts that form a unit? Array
What is parallel scan lines prod by pulses origninating at different pts across the surface of the array but travel in the same verticl direction? linear
Pulses that travel out in different directions. Curved
Scan lines originate at diff pts & travel out in diff directions. Vector
Ring-shaped annular
What is focus by phasing? Provides electronic control of focusing.
What does increased curvature do to the focus? Moves the focus closer to the transducer.
What does decreased curvature do to the focus? Moves the focus deeper.
What is dynamic aperature? Not all elements are used to generate all pulses--smaller aperature for short focal depths & larger aperature for longer focal depths.
What are grating lobes? Intensities not contained within the sound beam that escape out of the side of the beam in multi-element transducers.
What is apodization? Also called Dynamic apodization--center of each element is hit with the electrical energy--reduces grating lobes.
Describe linear array. voltage hits groups of elements, fixed focus, produces rectangular image
What are the other names for linear array? Linear Switched Array & Linear Sequential Array
What is Curved Array? Voltage hits groups of elements, fixed focus, sector image w/ curved footprint
What is another name for Curved Array? Curvilinear Array
What is Phased Array? Steered & focused by phasing, sector image w/ pointed top
What is another name for Phased Array? Linear Phased Array
What is Phased Linear Array? Voltage hits groups of elements, focused by phasing in a curved pattern, rectangular image.
What is another name for Phased Linear Array? Phased Curved Array
What transducer is the parallelogram associated with? phased linear array
phasing applied to linear sequencing array. Voltage hits groups of elements, steered by phasing, linear, sector image with flat footprint. Vector Array
What is annular array & why is it a hybrid transducer? Round elements, focused by phasing but steered mechanically, sector image with rounded footprint. Uses a reflector that wobble back & forth to produce an image.
Created by: rachelf618