EDELMAN INDEX / VOCAB Flashcards
ABSORPTION
~ occurs when U/S energy is converted into heat or reflected/refracted out of field
~ lower tissues return less U/S
~ directly related/proportional to frequency
~ causes high frequency waves to attenuate more than lower frequency waves
~ bone has highest absorption
~ high in muscle and skin, low in fluid-filled
~ calorimeter measures total power in a sound beam undergoing absorption
REGISTRATION ACCURACY
~ the ability of the system to place reflections in proper positions while imaging from different orientations
RANGE ACCURACY = VERTICAL DEPTH CALIBRATION
~ the system’s accuracy in placing reflectors at correct depths located parallel to the sound beam
~ differences and errors between scan and pin locations caused by system malfunction OR speed of sound not being 1540 m/s
ACOUSTIC IMPEDANCE /
CHARACTERISTIC IMPEDANCE
~ impedance is the acoustic resistance to sound traveling in a medium
~ multiply the density of a medium by the speed at which sound travels through it
~ calculated in Rayls, represented by Z
~ typical values = 1.25 to 1.75 Mrayls
ACOUSTIC VARIABLES
~ pressure = concentration of force in an area, density = concentration of mass in a volume, distance = measure of particle motion
~ pressure (Pa / pascals), density (kg / cm3), distance (cm / mm / feet / mile)
~ used to distinguish between sound waves and other types of waves
ACOUSTIC PARAMETERS
~ used to describe characteristics of a sound wave
~ 7 parameters = period, frequency, amplitude, power, intensity, wavelength, propagation speed
ACOUSTO - OPTICS
~ based on interaction of sound and light
~ Schlieren shadowing system allows view of the shape of a sound beam in a medium
~ an example is jet shock waves from the interaction of sound and light
AIUM CONCLUSIONS
~ no confirmed harmful bioeffects from exposure to diagnostic U/S have been reported
~ possible that bioeffects may be identified in the future
~ the benefits outweighs the risks
~ appropriate to use diagnostic
~ use to benefit patient, not entertain
IN VITRO BIOEFFECTS (AIUM)
~ in vitro bioeffects research is important
~ in vitro bioeffects are real even though they may not apply to the clinical setting
~ in vitro bioeffects research that claims direct clinical significance be viewed with caution
ALARA
~ As Low As Reasonably Achievable
~ when modifications to either output power/receiver gain can improve the quality, the first choice is one that will minimize the patient’s U/S exposure
~ image too dark, first increase receiver gain (does not increase patient exposure)
~ image too bright, first decrease output power
ALIASING
~ most common error with pulsed Doppler
~ occurs when sampling rate is too low in comparison to the measured blood velocities
~ false identity where very high velocities in one direction are incorrectly displayed as going opposite
~ flow above baseline indicates flow towards transducer
~ top of display is Nyquist limit, bottom is aliasing
~ display can happen in opposite direction too
ALIASING CONDITIONS
~ occurs ONLY with pulsed (PW) Doppler
~ occurs when Doppler sampling rate is too low compared to measured blood velocities
ALIASING LIMITS
~ Nyquist limit is the highest Doppler frequency or velocity that can be measured w/out the appearance of aliasing
~ aliasing appears when Doppler shift exceeds the Nyquist limit
~ Nyquist limit is 1/2 of the PRF
ALIASING SAMPLE VOLUME DEPTH
~ velocities are sampled many times/sec
~ sampling rate is the system’s PRF
~ sample volume too deep, PRF/Nyquist is low and velocity is sampled less (no measuring accuracy which creates aliasing)
~ sample volume shallow, PRF/Nyquist is high and velocity is sampled many times/sec (measures high velocities w/out aliasing)
ALIASING TRANSDUCERS
~ higher frequency transducers create more aliasing
~ smaller Doppler shifts from lower frequency transducers are less likely to exceed the Nyquist limit
~ higher frequency transducers create higher Doppler shifts, lower frequency transducers create lower Doppler shifts
ALIASING TECHNIQUES
~ ridding aliasing improves ability to measure the maximum velocity with Doppler
~ adjust scale to maximum
~ select new ultrasonic view with shallower sample volume
~ select a lower frequency transducer
~ use baseline shift
~ use continuous wave Doppler
ALIASING ADJUSTING SCALE TO MAX
~ sometimes PRF is not maximized (so Nyquist isn’t as well)
~ maximizing PRF raises Nyquist, less aliasing
~ higher PRF reduces sensitivity to low velocities
~ with very high velocities, aliasing artifact can occur even when scale is maximized!!!!!
ALIASING USE SHALLOWER SAMPLE VOLUME
~ shallower sample volume increases the PRF
~ can also adjust the PRF to raise it
~ use a new view with shallower depth
~ no disadvantages to this method!!!!
ALIASING USE LOWER FREQ TRANSDUCER
~ Doppler shift is directly related to freq of transducer (lower Doppler w/lower freq trans)
~ lower frequency sound reduces the height of Doppler spectrum
~ no significant disadvantage except lower freq sound produces lower quality image
ALIASING USE BASELINE SHIFT
~ bidirectional Doppler, flow towards trans above, flow away below
~ slides the display baseline down so that the entire velocity scale is devoted to 1 direction
~ visually appealing but aliasing can remain
~ baseline shift ineffective when Doppler shift so high signal completely wraps around itself
ALIASING USE CONTINUOUS WAVE DOPPLER
~ never appears with CW Doppler
~ range ambiguity limitation where exact location of moving blood cells cannot be determined (results from overlap blending of transmit and receive beams to form spectrum)
ALIASING FLOW REVERSAL
~ colors touching each other correspond with map (flow reversal indicated thru middle of map, aliasing indicated thru outside colors on map)
A MODE DISPLAY
~ appears as a series of upward spikes
~ x - axis represents reflector depth
~ y - axis represents strength/amplitude
AMPLIFICATION / RECEIVER GAIN
~ first function of the receiver
~ measured in db (typically 60 to 100)
~ required as electrical signals are too low to be displayed
~ each electronic signal returning made larger
~ ALL signals undergo an equal amount of amplification
~ patient exposure are not altered with amplification changes (unlike output power)
~ amplification alone cannot make an image uniform in brightness top to bottom
~ does not improve signal-to-noise ratio since they are amplified equally
AMPLITUDE
~ the “bigness” of the wave
~ measured mostly in pascals, but also g/cm3, distance (cm/inches) or decibels too!!!!
~ difference between maximum value and average/undisturbed value of an acoustic variable
~ difference between minimum value and average value of acoustic variable
~ measured from baseline up or down
~ typical U/S from 1 million pascals - 3 million
AMPLITUDE PEAK TO PEAK
~ the difference between maximum / minimum values of an acoustic variable
~ twice the value of the amplitude
AMPLITUDE VS. INTENSITY
~ power is related to amplitude SQUARED
~ intensity is related to amplitude SQUARED
ANALOG NUMBERS
~ can have an unlimited continuous range of values
~ real world numbers
ANALOG SCAN CONVERTER
~ first type of scan converter making gray scale imaging possible
~ funnel-shaped vacuum tube w/electron gun in smaller end
~ charged electrons contain image information and get shot out of electron gun
~ dielectric matrix / silicon wafer in larger end
~ electrons strike matrix where they are stored
~ dielectric matrix dots contain electron bucket (electrical storage element)
~ now obsolete due to limitaitons
ANALOG SCAN CONVERTER LIMITATIONS
~ image fade (stored charges on the silicon wafer dissipate over time)
~ image flicker (caused by switching between read and write modes)
~ instability (picture quality depends on many factors including length of use, room temperature, humidity)
~ deterioration (image degrades as the device ages)
ANALOG AND/TO DIGITAL IMAGE DATA
~ analog signals changed to digital form and stored in computer memory, converted back to analog again for display
~ transducer producers very low-voltage signals so weak they are susceptible to noise
~ digital information advantage is less noise
ANALOG TO DIGITAL PROCESS
1) analog to digital converter allows digital information to be in 0’s and 1’s
2) digital information is stored in scan converter’s computer memory (any processing of the reflected signals before storage is called pre processing)
3) digital image information is processed by computer (any processing after storage in scan converter is called post processing)
4) digital signals must be translated back to analog by digital to analog converter as digital cannot be displayed on analog devices
5) new analog signal is presented on analog video display
ANALOG TO DIGITAL EXAMPLE
~ computer mouse
~ motion is analog signals converted to digital to move cursor on screen
DIGITAL TO ANALOG EXAMPLE
~ iPod (0’s / 1’s represent song)
~ converted to analog for earphones
ANATOMY OF SOUND BEAM (TERMS)
~ focus ~ near zone ~ focal length/near zone length ~ far zone ~ focal zone
ANATOMY OF SOUND BEAM
Starting point beam width is exactly the same as the transducer diameter
ANATOMY OF SOUND BEAM (FOCUS)
~ focus/focal point is location where the beam is the narrowest
~ one half the width of the beam as it leaves transducer
ANATOMY OF SOUND BEAM (NEAR ZONE)
~ near field / Fresnel zone
~ beam gradually narrows to eventually one half the width of the active element
ANATOMY OF SOUND BEAM (FOCAL LENGTH)
~ Focal depth/near zone length
~ distance from transducer to focus
ANATOMY OF SOUND BEAM (FAR ZONE)~
~ far field / Fraunhofer zone
~ starts at focus and extends deeper
~ beam diverges / spreads out
ANATOMY OF A SOUND BEAM (FOCAL ZONE)
~ region around focus where the beam is relatively narrow
