Image Quality

Question 1

What causes a reverberation artifact?

Answer 1

Highly reflective material (metal, calcification, air pocket / partial liquid)

Question 2

What is lateral (horizontal) resolution?

Answer 2

ability to distinguish between two adjacent objects separated by narrow space

Depends on: aperture size

res = (depth * wavelength) / aperture

Question 3

What is axial resolution?

Answer 3

depth perception

to resolve distance: d < 1/2 SPL (single pulse length)

depends on: Q-value, freq and velocity of sound

Question 4

What is elevation resolution?

Answer 4

slice thickness

depends on: length of transducer element

Question 5

How do the Doppler angle and Doppler shift frequency contribute to aliasing?

Answer 5

Aliasing occurs when Doppler shift frequencies above the Nyquist limit are misinterpreted as lower than their true value, wrapping around and appearing to flow in the opposite direction. This is due to insufficient sampling rate, as the Nyquist limit is defined as ½ of the Doppler pulse repetition frequency (PRF). As the equation below suggests, a small Doppler angle (θ) creates a larger Doppler shift

∆f = PRF/2 = (2 * f_0 * v * cosθ) / c

Question 6

What are two ways to reduce the effects of aliasing?

Answer 6

1. raise Nyquist limit
2. adjust Doppler angle closer to 90 degrees (reduce Doppler shift)