Quantitative MRI mOOCB0 Mapping · Exercise 5-1
Dual-echo B0 mapping: a single-voxel example
In this exercise, we will cover:
- How to compute B0 for a voxel of pure water
- How the fat signal affects the B0 measurement
- How to mitigate the effects of fat on the B0 measurement
Exercise 5-1Double-echo B0 mapping in a single voxel
Question 1a
You are scanning on a 3 T scanner, and your sequence has a minimum TE of 2 ms. Compute the difference in phase between that measurement and a measurement at 4 ms, for two cases:
Question 1b
What is the measured ΔB0 for each case, in µT?
Question 2a
Same question, on a 1.5 T scanner.
Question 2b
What is the measured ΔB0 for each case, in µT?
Question 1a
Your voxel now holds 50 % fat (−3.4 ppm from water). At 3 T, compute the difference in the signal’s phase between 2 ms and 4 ms:
Question 1b
What is the measured ΔB0 for each voxel, in µT?
Question 2a
Same question, on a 1.5 T scanner.
Question 2b
What is the measured ΔB0 for each voxel, in µT?
Question 1
Your voxel holds 30 % fat, at 3 T, with a freq. offset of 0 ppm. After TE = 0, at what TE are water and fat first back in phase (TE1)? And when are they next in phase (TE2)?
Question 2
With those two echoes, compute the difference in the signal’s phase between TE1 and TE2:
Question 3
What is the measured ΔB0 for each voxel, in µT?
Exercise 5-1 · Summary
Summary
- Two echoes give ΔB0 from the change in phase between them: ΔB0 = Δφ / (γ ΔTE), with Δφ in radians and ΔTE in seconds.
- Fat precesses 3.4 ppm slower than water (−434 Hz at 3 T), so in a voxel with fat the measured phase is that of the sum of the two, and the B0 measured is biased.
- Echoes taken when water and fat are in phase (every 2.30 ms at 3 T) remove that bias.