Ch 6 Resolution Flashcards
What are the 3 aspects to imaging resolution?
Detail:
-ability to distinguish structures in each of the 3 dimensions of the u/s beam
-includes axial, lateral + elevational (slick thickness)
Contrast:
-ability to distinguish structures based on variations of brightness
Temporal:
-ability to distinguish closely spaced events in time
-improves with increased FR
What is axial resolution?
-Type of detail resolution
-Min reflector separation required along the direction the sound travels to produce separate echoes
Increasing frequency improves which type of resolution?
Axial resolution - b/c it creates smaller wavelengths
What is the axial resolution formula?
AR (mm) = SPL (mm) / 2
List 3 ways we can improve our axial resolution?
-Shorter SPL
-Dampening/backing material
-Increasing frequency
What is lateral resolution?
-Type of detail resolution
-Width of sound beam
-Min reflector separation in the direction perpendicular to the beam direction that can produce 2 separate echoes
(lateral resolution = beam width)
Lateral resolution is improved by what?
Focusing
Where is the focal region with lateral resolution?
Where bean width is narrowest (best at or just before focus)
Where does the beam diverge?
In the far field, past the focus (poor lateral resolution here)
List 2 ways we can change the focus?
-Aperture (# of elements)
-Phasing (electronic delay profiles)
(we can change the width of the sound beam (aka our lateral resolution) by changing the focus)
What happens if we put our focus far away from the ROI?
Can’t see finer details anymore
What is elevational resolution?
-Type of detail resolution
-Aka slice thickness plane
-Is known as the third dimension in the u/s beam (the one we can’t see in 2D imaging)
-Contributes to slice thickness/partial volume artifacts
Focus in the elevation plane is fixed in 1D array probes + is determined by what 2 things?
-The lens
-The size + shape of crystals
What is the slice thickness/partial volume artifact?
-Occurs when the slice thickness is larger than the size of the structure
-Results in structures being in front or behind the desired imaging plane
-Creates a filling in of what should be an anechoic structure (think GB)
What can slice thickness artifacts lead to?
False diagnoses (ex. false debris/sludge in a cyst)
How can we reduce slice thickness artifacts?
Reduce by using tissue harmonic imaging:
-Creates narrow sound beams in the lateral + elevation plane, therefore improves these types of resolution
What is contrast resolution?
-Ability to distinguish structures based on variations of brightness
-The strength of individual signals determines the level of brightness
(ex. specular reflectors are hyperechoic, whereas rayleigh scatters are anechoic)
How many shades of grey is there in u/s?
256
List 4 ways contrast resolution can be changed by the sonographer?
-Overall gains
-TGCs
-Dynamic range (ratio b/w the brightest + darkest parts of an image, it changes the # of shades of grey)
-Compression (the process of decreasing the differences b/w the smallest + largest echo-voltage amplitudes)
List 3 ways the contrast resolution can be changed simply by the path the sound beam takes?
-Acoustic impedance
-Refraction
-Attenuation
What is temporal resolution?
-Ability to accurately pinpoint an object’s location at a specific moment in time
-Ability to accurately image when the scan plane is constantly changing due to motion of structures, relative to the time required to produce an image (ex. cardiac valves)
-Heavily reliant on FR!!
What is FR?
The # of images that are produced per second
What is FR dependent on?
-Depth
-FOV (# scan lines in a frame)
Differentiate PRP + PRF?
PRP: ringing + listening, the time to build a single scan line
PRF: amount of scan lines or pulses per second
