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SON 111 MIdterm Rev
Physics Midterm Review
| Question | Answer |
|---|---|
| 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
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