Pulse Ultrasound & Imaging

Question 1

How many cycles are there in a typical ultrasound pulse?

Answer 1

3

Therefore pulse duration is typically 3/f

Question 2

Describe the relationship between frequency and axial resolution

Answer 2

As frequency increases, the pulse duration decreases, allowing increased maximum pulse repetition frequency
This improves axial resolution

axial resolution = c x t / 2

t = pulse duration

Question 3

How does the ultrasound machine determine the depth of an object

Answer 3

d = (c x t) / 2

round path = 2d
propagation speed assumed at 1540m/s

Question 4

What artifact is related to pulse repetition frequency maximum?

Answer 4

Range ambiguity

A second pulse is transmitted before all echoes from the previous pulse are detected
These echoes are interpreted by the machine as having originated in the second pulse

Question 5

Calculate the timing of the last detectable echo

Answer 5

tp = 2P/c

P = depth of maximum penetration

Question 6

Show the relationship between PRF max and timing of the last detectable echo
How does the ultrasound machine use this information

Answer 6

1/tp = PRF

Given that tp = 2P/c
PRF = c/2P

The machine estimates the depth of penetration then calculates the PRF max using that depth
The machine then tries to use the maximum PRF at all times

Question 7

Show the relationship between frame rate and PRF

Then show the relationship between FR max, depth of penetration and propagation speed

Answer 7

PRF = FR x N

N = number of pulses required to form an image

FR max = c/2 x P x N

FR max x P x N = c/2

Given c is a physical constant, to increase any of the left side, we must decrease the others