Basic principles Flashcards
Amplitude
Refers to the maximum variation of particle displacement, particle density or acoustic pressure of a sound wave.
The units of measure are dependant on Pascals, the units for displacement (or distance) are expressed as mm, cm, or m and the unit for density is expressed as g/cm3
Wave length
Refers to the distance occupied by one complete cycle
when the amplitude is plotted against distance, that is, wavelength is the distance between two compression zones or between two rarefraction zones
Wvelength is expressed in units of distance such as m, cm, mm
Frame rate
Number of frames per second
Frequency
Number of cycles passing a given point per second (Hz)
Acoustic impedence
Resistance of a medium to transmission of sound waves
Attenuation
Reduction in signal strength as it transverses through a medium
Temporal resolution
Ability tpo accurately display position of moving structures at a particular instant in time
Period
Time taken for one complete wave cycle to occur
Propagation speed is determined by
Density of the medium and compressability of that medium
What assumption is violated to create a side lobe or grating lobe artifact?What causes side lobe and grating lobe artifact
the
assumption that the ultrasound waves are infinitely thin is
violated
Side and grating lobes are secondary beams around the central ultrasound beam and are produced by nonaxial vibrations of the piezoelectric elements
Side and grating lobe artifacts result in the blurring of the
edges of a displayed object (reduce lateral resolution)
Use of high frequencies increases attenuation. Yes or no?
Yes
-At shallower depths the frame rate is higher because the signal can return to the transducer faster.
-The least number of scan lines / smaller FOV & Decreasing the depth
Write zoom
Increases frame rate (Improves temporal resolution)
Improves spatial resolution and measurement accuracy
Are there an increased number of pixels in write zoom or read zoom? Which one improves resolution and what type of resolution does it improve?
Write zoom has an increased number of pixels per unit area
Therefore Write zoom improves spatial resolution
What is dynamic range?
The range between the strongest to weakest echoes
Colour hue/ brightness colour
Human eye can see more shades of colour and limited shades of grey. Therefore may improve visuals and perception of image
What is the reject control
Filter that sets the minimum echo level that will be displayed. Therefore it reduces electronic and acoustic noise. Doewnside is it can also remove low amplitude tissue echo.
What are imaging artefacts defined as?
False or misleading information introduced by the ultrasound machine.
Artefacts can be catergorised/ caused by
Interaction between the beam and tissue
Faulty or damaged instrumentation
Sonographer related vaiables e.g improper use of equipment
What are the five key assumptions that U.S machines use to produce 2D images?
- There is a constant rate of attenuation
- All echoes arise from the centre of a razor thin beam
- The propagation speed in soft tisue is 1540m/s
- The round trip time of a given echo is directly related to the depth of a reflector from the transducer
- The U.S beam travels in a straight line and reflects just once
Therefore… 2d imaging artefacts can be catergorised based on the violation of these assumptions, these four major catergories include
attenuation artefacts
Beam dimension artefacts
Depth of origin artefacts
Beam path artefacts
The u.s machine assumes there is a constant rate of attenuation. What is this rate?
1dB/CM/MHz
Acoustic enhancement artefact
Occurs when the beam passes through a structure that has low level of attenuation relative to the adjacent structures. As a result the structures beyond the low attenuating structure will appear brighter
Acoustic shadowing occurs due to attenuation. The weakening of the beam occurs when the beam…
-Encounters an interface where high acoustic mismatch exits
- Encounters a structure that has high absorption properties
- Diverges
-Is refracted
Name two attenuation artefacts
Acoustic shadowing
Acoustic enhancement
Name 3 Beam dimension artefacts
Beamwidth artefact
Depth of origin artifacts occur when what assumptions are violated?
propagation speed 1540 m/s ,
round trip time directly related to depth
U.S beam travelling in a straight line
Two types of depth of origin artefacts?
-Propagation speed artefacts-speed propagation means calculation is wrong and echo is therefore placed in the wrong area
-Range ambiguity artefacts-second pulse emitted before the first has arrived (high prf). thinks the 2nd echo is the 1st
Beam path artifacts occur when what assumptions are violated?
-the beam doesnt travel in a straight line and reflects more than once
-the propagation speed of 1540m/s in tissue is broken
common types of beam path artefacts?
-reverberation-acoustic mismatch between two close interfaces, perpendicular to the beam
-refraction -speeds different in different media create bending look
-mirror image-large variance in acoustic impedance
Why doesnt aliasing occur with CWD?
Aliasing does not occur with CWD because separate elements on the probe transmit while others simultaneously
“listen.” This eliminates the limitation set by the pulse repetition frequencies (i.e., pulses emitted per second).
what imaging is used to record velocities within the myocardium?
Tissue doppler imaging (TDI) / DTI)
PW picks up low amplitude and higher velocity, but TDI picks up?
DTI picks up high amplitude and low velocity
How to tell the difference between a PW and CW trace
PW has a spectral windowand a spectral width
