Ch 6 Resolution

Question 1

What are the 3 aspects to imaging resolution?

Answer 1

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

Question 2

What is axial resolution?

Answer 2

-Type of detail resolution
-Min reflector separation required along the direction the sound travels to produce separate echoes

Question 3

Increasing frequency improves which type of resolution?

Answer 3

Axial resolution - b/c it creates smaller wavelengths

Question 4

What is the axial resolution formula?

Answer 4

AR (mm) = SPL (mm) / 2

Question 5

List 3 ways we can improve our axial resolution?

Answer 5

-Shorter SPL
-Dampening/backing material
-Increasing frequency

Question 6

What is lateral resolution?

Answer 6

-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)

Question 7

Lateral resolution is improved by what?

Answer 7

Focusing

Question 8

Where is the focal region with lateral resolution?

Answer 8

Where bean width is narrowest (best at or just before focus)

Question 9

Where does the beam diverge?

Answer 9

In the far field, past the focus (poor lateral resolution here)

Question 10

List 2 ways we can change the focus?

Answer 10

-Aperture (# of elements)
-Phasing (electronic delay profiles)

(we can change the width of the sound beam (aka our lateral resolution) by changing the focus)

Question 11

What happens if we put our focus far away from the ROI?

Answer 11

Can’t see finer details anymore

Question 12

What is elevational resolution?

Answer 12

-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

Question 13

Focus in the elevation plane is fixed in 1D array probes + is determined by what 2 things?

Answer 13

-The lens
-The size + shape of crystals

Question 14

What is the slice thickness/partial volume artifact?

Answer 14

-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)

Question 15

What can slice thickness artifacts lead to?

Answer 15

False diagnoses (ex. false debris/sludge in a cyst)

Question 16

How can we reduce slice thickness artifacts?

Answer 16

Reduce by using tissue harmonic imaging:

-Creates narrow sound beams in the lateral + elevation plane, therefore improves these types of resolution

Question 17

What is contrast resolution?

Answer 17

-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)

Question 18

How many shades of grey is there in u/s?

Answer 18

256

Question 19

List 4 ways contrast resolution can be changed by the sonographer?

Answer 19

-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)

Question 20

List 3 ways the contrast resolution can be changed simply by the path the sound beam takes?

Answer 20

-Acoustic impedance
-Refraction
-Attenuation

Question 21

What is temporal resolution?

Answer 21

-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!!

Question 22

What is FR?

Answer 22

The # of images that are produced per second

Question 23

What is FR dependent on?

Answer 23

-Depth
-FOV (# scan lines in a frame)

Question 24

Differentiate PRP + PRF?

Answer 24

PRP: ringing + listening, the time to build a single scan line

PRF: amount of scan lines or pulses per second

Question 25

List 3 ways temporal resolution can be improved by?

Answer 25

-Reduce depth
-Fewer scan lines
-Smaller sector width

Question 26

FR formula?

Answer 26

FR = 1 / FT

Question 27

How does multiple foci affect temporal resolution?

Answer 27

Reduces FR + temporal resolution

Question 28

How does multiple foci affect lateral resolution?

Answer 28

Improves it

Question 29

For each focus on each scan line, how many pulses are required?

Answer 29

1

1 focus = 1 pulse per scan line
Multiple foci = multiple pulses per scan line

Question 30

When would we use multiple foci?

Answer 30

Restricted to applications when temporal resolution is not as critical

(remember it improves lateral resolution, but degrades temporal resolution)

Question 31

Give an example of when multiple foci would be helpful + when it would not be helpful?

Answer 31

Helpful:
-static imaging (structures that don’t move)
-very high lateral resolution
-breast + liver imaging can use up to 8 foci

Non-helpful:
-fast moving structures
-pediatric or adult cardiac sonography