Physics

Question 1

Doppler equation

Answer 1

Doppler shift = V cos theta 2Ft/c

Question 2

Five functions of US receiver

Answer 2

Amplification
Compensation
Compression
Demodulation
Rejection

Question 3

Amplification

Answer 3

enlargement of returning signal (“gain), makes returning signals brighter. No change or increase in energy.

Question 4

Compensation (aka time gain compensation)

Answer 4

makes echos from similar structures appear with similar brightness.

deeper structures = more attenuation = less bright
Compensation will selectively amplify signals coming from deeper structures

Question 5

Compression

Answer 5

reduces total range of signals
“compresses” the size of the difference b/t signals

More compression = less dynamic range

Question 6

Characteristics of low compression

Answer 6

Higher dynamic range
more shades of gray
fewer bright brights and dark darks

Question 7

What are the two main types of resolution?

Answer 7

Spatial & temporal

Spatial → discern two objects as separate.

Temporal → ability to locate moving structures

Question 8

Three types of spatial resolution and what determines each?

Answer 8

Axial → spatial pulse length
Longitudinal → beam width
Elevational → beam height or “slice thickness”

Question 9

Equation for axial resolution.

Relationship b/t axial resolution and SPL?

Answer 9

Axial resolution = SPL/2

Shorter pulse length = higher frequency = increased axial resolution

Question 10

What determines temporal resolution?

Answer 10

Pulse repetition frequency (PRF)

Temporal resolution increases with higher frame rate (increased PRF) and decreased depth

Question 11

Equations for Nyquist limit, PRF, and frame rate

Answer 11

Nyquist limit = PRF/2

PRF = Nyquist limit x2

PRF = 1/PRP

PRF = c/(depth)(2)

Frame rate = PRF/#lines

Question 12

Synonyms for lateral resolution?

Answer 12

LATA.

Lateral, angular, transverse, azimuthal

Question 13

What variables affect frame rate?

Answer 13

pulses per scan line
line density (# lines/image)
sector width
depth (listening time)

Question 14

How to optimize images?

Answer 14

Decrease depth
Narrow sector width
Focal point at ROI → narrowest beam width → highest lateral resolution

Question 15

What is TGC and LGC used for?

Answer 15

Adjusts gains in horizontal and vertical plane. Both used to compensate for attenuation.

LGC → compensates for enhancement artifact.

Question 16

What is compression?

Answer 16

Post-processing decrease in returning amplitudes without changing relative difference b/t them. Increased compression = decreased dynamic range.

Question 17

What is the relationship between compression and dynamic range? What are the features of low and high compression?

Answer 17

Inversely related.

Low compression → large dynamic range, less bright brights and dark darks, smoother, more shades of gray, less contrasted

More compression → less dynamic range, brighter brights/darks, more contrast, less shades of gray, harsher image

Question 18

Why are low frequency transducers used to image deeper structures?

Answer 18

Less attenuation than higher frequency probes, at expense of lower spatial resolution.