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SPI Must Know
US Physics Must Know from Reynolds
| Question | Answer |
|---|---|
| Sound is | a longitudinal, mechanical wave |
| In a longitudinal wave | the particles move parallel to the direction of sound travel |
| One cycle of sound is made up of what? | one compression and one rarefaction |
| The four acoustic variables are: | pressure, density, temperature, and particle motion |
| Frequency is | the number of cycles per second |
| Units for Frequency | Hertz (Hz) |
| Frequency Range for Audible Sound | 20Hz to 20,000Hz |
| Frequency Range for Ultrasound | 20,000Hz to 20kHz |
| Clinical Imaging Frequency Range | 2MHz to 10MHz |
| Increasing Frequency | Decreases image depth because attenutaion increases |
| Increasing Frequency | Improves axial resolution because shorter pulses are created |
| Increasing Frequency | Improves lateral resolution because higher frequencies produce a narrower beam width |
| Increasing Frequency | Decreases period which decreases pulse duration leading to an improvement in axial resolution |
| Shorter SPL | Shorter PD |
| The Narrower the Beam | The better the lateral resolution |
| Shorter Pulses | Provide for better axial resolution |
| For Soft Tissue, Wavelength(mm)= | 1.54(mm/us)/frequency(MHz) |
| Increasing the Frequency | Decreases the wavelength which decreases SPL leading to an improvement in axial resolution |
| The Shorter the Pulse | The better the axial resolution |
| Average Speed of Sound in Soft Tissue | 1540m/s, 1.54mm/us |
| The Stiffer the Material | The higher the propagation speed |
| Propagation Speed for Air | 333m/s |
| Propagation Speed for Bone | 3500m/s |
| Amplitude | The strength of the outgoing signal or returning echo |
| Units for Amplitude | Decibels(dB) |
| An Increase in Transmit Gain | Increases amplitude, power, pressure, intensity, beam width, depth of penetration, brightness of the signals throughout the display, MI, and the risk of bioeffects. |
| Units for Power | Watts(W) or milliwatts(mW) |
| Intensity is | Power(W)/Area(cm^2) |
| There have been no proven biological effects for | unfocused sound beams below 100mW/cm^2 SPTA unfocused beams or 1W/cm^2 SPTA focused beams |
| Decibels(dB) are the units for: | amplitude, transmit gain, gain, attenuation, TGC, and dynamic range |
| An increase in intensity by 40dB represents | An increase in intensity by 10,000 times |
| For soft tissue, 3dB represents | a 50% loss in original intensity |
| Another term for stiffness is | Bulk Modulus |
| For perpendicular incidence | A reflection will occur at the boundary of two media if the acoustic impedances are different |
| For a soft-tissue interface | Over 99% of the sound energy is reflected with less than 1% of the sound energy transmitted |
| For a fat-muscle interface | over 99% of the sound energy is transmitted with only 1% reflected |
| Specular Reflectors | are large, smooth surfaced reflectors that require perpendicular incidence |
| Scatter Reflectors | are small, rough surfaced reflectors that are not angle dependent, but are frequency dependent |
| Increase Frequency | Increase Backscatter |
| The best example of a Rayleigh Scatterer is | the Red Blood Cell |
| Reflections from scatter reflectors | are weaker than reflections from specular reflectors |
| Harmonic imaging transmits | a fundamental frequency and processes the second harmonic (twice the fundamental frequency) |
| Two types of harmonic imaging | contrast harmonic imaging and tissue harmonic imaging |
| In contrast harmonics | multiples of the fundamental frequency are created by the reflector (e.g. contrast bubble) |
| Tissue harmonics | are created by the propagation of sound through tissue |
| Harmonic imaging may be improved by | pulse inversion or power modulation |
| Harmonic imaging improves | lateral resolution |
| Harmonic imaging decreases | side lobe/grating lobe artifacts |
| Harmonic imaging reduces | near field reverberation |
| Perpendicular incidence (90 degrees) | is also normal incidence |
| Perpendicular incidence predicts | the amount of reflection (intensity reflection coefficient (IRC)) at a boundary |
| Perpendicular incidence equation | [(Z2-Z1)/(Z2+Z1)]^2 x 100 |
| Oblique incidence | is present when the US beam strikes a boundary at an angle other than 90 degrees |
| The reflected angle always equals the | incidence angle |
| Refraction is | the change in the direction of sound travel and is associated with lateral displacement of structures on the display |
| Snell's Law (Law of Refraction) | predicts when refraction will occur and how much refraction will occur by predicting the transmitted angle |
| There will be no refraction if | there is perpendicular incidence or if there is oblique inceidence and the propagation speeds of the two media are equal |
| The primary cause of attenuation in soft tissue is | absorption (conversion of sound energy into heat) |
| The attenuation coefficient(dB/cm) for soft tissue is | 0.5 dB/cm/MHz (0.5 x frequency) |
| Total Attenuation(dB)= | 0.5 x frequency(MHz) x path length(cm) |
| The Piezoelectric Effect is | the conversion of electrical energy into sound energy (transmission) and sound energy into electrical energy (reception) |
| Increasing the Frequency | increases attenuation and decreases depth of penetration |
| The thinner the piezoelectric element and the higher the propagation speed of the element | the higher the frequency of the element |
| The Matching Layer | is placed on the front surface of the piezoelectric element, is 1/4 wavelength thick, and reduces the impedance difference between the piezoelectric element and the soft tissue |
| The Damping Block | is placed behind the piezoelectric element, reduces the ringing of the element, reduces SPL and PD which improves axial resolution |
| The damping block | increases bandwidth and reduces the QF |
| Modern US transducers | have wide bandwidths with low QF |
| The Near Field (Fresnel zone, near zone) | is the sound beam region located between the transducer face and the focal point |
| Beam width decreases where? | In the near field |
| The focal point | is the narrowest portion of the sound beam where intensity is the greatest and lateral resolution is the best |
| The Far Field (Fraunhofer zone, far zone) | is where the sound beam diverges, intensity decreases, and lateral resolution is poor |
| The near field length (NFL) can be increased by what? | By increasing the transducer diameter (best way) and/or by increasing the frequency |
| Focusing | decreases the beam width in the near field, improves lateral resolution but does not extend the near field and increases beam divergence in the far field |
| Internal focusing curves what? | The piezoelectric element |
| External focusing | places a curved lens on the piezoelectric element or transducer face |
| Electronic transmit focusing | curves the pulses going to the piezoelectric element |
| Electronic receive focusing | uses delay lines to focus the returning echoes |
| Only transmit focusing is controlled by? | The sonographer |
| Lateral Resolution is dependent upon what? | Beam width (the narrower the beam width, the better the lateral resolution) |
| Lateral resolution is improved by what? | Increasing the transducer diameter, frequency, harmonics, and/or by focusing |
| Axial resolution is dependent upon what? | The spatial pulse length and pulse duration (the shorter the pulse, the better the axial resolution) |
| Axial resolution is improved by what? | Increasing the frequency (sonographer) or by damping (manufacturer) |
| The linear sequenced array transducer | is a large electronic transducer which creates a linear display |
| The sector phased array transducer | is a small electronic transducer where all the piezoelectric elements are fired but with a brief time delay in order to beam steer and beam focus |
| The annular array transducer | utilizes piezoelectric elements arranged in a circle, is electronically fired and electronically focused, but is mechanically steered |
| The mechanical transducer | creates a cross sectional image by placing a piezoelectric element on a motor, is fixed focus where an acoustic standoff may be used to alter the placement of the focus |
| Beam scanning infers what? | That the sound beam is being swept through the tissue manually, mechanically, or electronically to create a cross sectional image |
| Beam steering is what? | Mechanically or electronically directing the sound beam toward a certain direction |
| Beam focusing is what? | Mechanically (internal/external focus) or electronically (transmit/receive) decreasing the sound beam width in the near field |
| The range equation uses what? | The propagation speed and the round trip time (divided by 2) to determine he distance to a reflector |
| For soft tissue, how long does it take for an US pulse to travel 1 cm round trip? | 13 us |
| Pulse Repetition Frequency (PRF) | is the number of pulses created per second |
| PRF is determined primarily by what? | Image depth since only one pulse can travel in the body at one time |
| Image depth and PRF are related how? | Inversly (increase depth, decrease PRF; decrease depth, increase PRF) |
| Duty Factor(DF) is | the percentage of time the machine spends producing a pulse; it is unitless |
| The pulser delivers the electrical impulse to what? | The piezoelectric element |
| The sonographer can control what with the transmit gain control? | The pulser |
| Amplification(gain, receiver gain, overall gain) does what? | Amplifies all returning signals regardless of depth, increases the strength of all the echoes throughout the display and is a safer gain to use as compared to transmit gain |
| Compnesation(TGC, DGC) compensates for what? | Attenuation |
| Compression reduces what? | The dynamic range |
| Rejection eliminates what? | Low amplitude signals and noise |
| Demodulation has two components: | rectification and smoothing |
| Dynamic Range (dB) is what? | The range of echo amplitudes that a particular component can process. |
| Dynamic range can be influenced by: | the sonographer (compression, log compression, dynamic range) |
| Decreasing Dynamic Range | Will increase contrast |
| Increasing Dynamic Range | Will increase the shades of gray displayed |
| A-mode displays returning echoes as what? | Spikes |
| B-mode displays returning echoes as what? | Bright dots |
| B-mode displays what? | The strength of the returning signals(amplitude) and the depth at which they are located. |
| M-mode is a B-mode operation displaying what? | Motion over time with excellent temporal resolution. |
| A frame is a collection of what? | Pulses where one pulse equals one scan line. |
| A frame contains how many scan lines? | 100-150 scan lines (pulses) |
| What is the standard frame rate? | 30 frames per second (30 Hz) |
| Temporal resolution is improved how? | By increasing the frame rate. |
| To improve temporal resolution (to increase frame rate) | decrease image depth, field of view, number of foci, and/or line density |
| What is the heart of the memory? | The scan converter |
| What is the digital scan converter? | A computer which uses the binary system where the number column is the multiple of 2 and the digits for the binary system are 0 and 1. |
| The digital scan converter is composed of what? | A matrix(s) or "checkerboards" with pixels able to hold a binary digit (0 or 1) |
| The more bits per pixel | the greater the number shades of gray |
| The number of shades of gray that a particular digital scan converter can display can be calculated how? | By the formula 2 to the power of x (e.g. a 4 bit scan converter can display 16 shades of gray) |
| Preprocessing occurs when? | Before memory |
| Postprocessing is what? | The assignment of gray shades to numbers leaving the memory. |
| If the image is frozen, what is being utilized? | Postprocessing (preprocessing does not allow for manipulation of a frozen image) |
| The display | is a television cathode ray tube (CRT) or computer monitor which writes in the raster scan format and is able to display up to 60 frames per sec (Hz) |
| Step 1 in the processing of echo information | pulser to transducer |
| Step 2 in the processing of echo information | reflection to transducer |
| Step 3 in the processing of echo information | receiver functions applied to the reflection (amplification, compensation, compression, demodulation, rejection) |
| Step 4 in the processing of echo information | analog to digital conversion (preprocessing) with storage of the binary number in the memory (scan converter) |
| Step 5 in the processing of echo information | the assignment of a shade of gray to the binary number leaving the memory |
| Step 6 in the processing of echo information | the display as a bright dot (B-mode) |
| The five resolutions are: | axial, lateral, temporal, contrast, and elevational |
| What is the Doppler principle? | The change in the frequency of sound due to motion of the sound source and/or the observer. |
| What is the Doppler shift? | The difference between the transmit frequency and the received frequency; is in the audible range; can be positive or negative |
| An increase in transmit frequency or RBC velocity will increase what? | The magnitude of the Doppler shift |
| To avoid aliasing, Doppler utilizes what? | Lower frequencies as compared to imaging. |
| The best way to perform a Doppler exam is what? | To be parallel (0 degrees) to blood flow |
| Fast Fourier Transform (FFT) is what? | The method used for spectral analysis in conventional (PW, CW) Doppler |
| What do the x, y, and z axis represent for the FFT? | The x axis represents time, the y axis represents velocity, and the z axis represents the amplitude (strength) of the returning signal |
| What is the Doppler wall filter? | The Doppler wall filter is a high pass filter which eliminates high amplitude (strong)(e.g. wall motion) and low velocity signals. |
| What is spectral broadening? | The vertical thickening of the Doppler spectrum, results in window fill-in and indicates turbulent flow. |
| CW Doppler requires how many piezoelectric elements? | Two |
| The primary advantage of CW Doppler is what? | It resolves high velocities without aliasing (no inherent sampling rate). |
| The primary disadvantage of CW Doppler is what? | Range ambiguity |
| PW Doppler requires how many piezoelectric elements? | One |
| The primary advantage of PW Doppler is what? | Range resolution (range discrimination) |
| The primary disadvantage of PW Doppler is what? | Aliasing due to the Nyquist limit (PRF/2) |
| What can you do to avoid aliasing? | Increase velocity scale, decrease image sampling depth, decrease transducer frequency, alter the baseline, increase Doppler intercept angle (e.g 0 degrees towards 90 degrees), utilize high PRF or CW |
| What is color flow Doppler? | A multigate PW Doppler technique which presents the mean velocity of blood flow. |
| Can aliasing occur in color flow Doppler? | Yes, because it is a PW technique. It may occur in normals because the PRF is lower as compared to imaging and appears as apparent flow reversal. |
| What is packet size? | The number of pulses sent down one line to create one scan line of color. |
| Increasing the packet size will improve what? | The estimation of the mean velocity, but it will decrease the frame rate. |
| What will increase the color Doppler frame rate? | Decreasing image depth, packet size, field of view, and/or line density. |
| What is autocorrelation? | The rapid technique of converting Doppler shift information into color. |
| What does a mosaic flow represent? | Turbulent flow |
| Green may be added to indicate what? | Variance (turbulent flow) |
| What is ghosting or flash artifact? | The placement of color on moving tissue structures. |
| What are reverberations? | Multiple echoes, equally spaced usually located close to the transducer. |
| Refraction may result in what? | Lateral displacement of structures (e.g side-by-side double image of a structure) or edge shadowing |
| What will result in a resolution artifact? | Structures being too close axially, laterally, or in the elevational plane to be distinguished. |
| What are side lobe and grating lobe artifacts? | Extraneous ultrasound energy emanating from the primary beam. |
| What are two types of reverberation artifact? | Ring down (originating from a gas bubble) and comet tail(originating from a highly reflective structure e.g bullet). |
| If the propagation speed of a material is less than 1540 m/s, then reflectors will be placed where? | Too far from the transducer. |
| If the propagation speed of a material if greater than 1540 m/s, then reflectors will be placed where? | Too close to the transducer. |
| Shadowing involves what? | A highly attenuating structure resulting in a low amplitude signal. |
| Enhancement involves what? | A low attenuating structure resulting in a high amplitude signal. |
| When will range ambiguity occur? | If depth of penetration(cm) x number of foci x lines per frame x frame rate is greater than 77,000. |
| What is the best way to practice ALARA? | To reduce the scanning time and transmit gain. |
| Bioeffects may be what? | Thermal, cavitation, or direct mechanical |
| What is stable cavitation? | The expansion and contraction of a gas bubble which may result in microstreaming. |
| What is transient cavitation? | The implosion of a gas bubble and may lead to a significant bioeffect. |
| What indicates the risk of cavitation? | The mechanical index (MI) |
| What indicates the risk of an unsafe rise in temperature? | The thermal index (TI) |
| A cracked ultrasound transducer can result in what? | An electrical shock to the patient. |
Created by:
jmontroy