MR Toolkit - Image Contrast

Questions to be answered in groups of three with the use of any material you might want to use. You have 30 mins to submit the answers, as a team you have to agree on the response given.

Image Contrast Tutorial

1. Single Slice GRE Sequence

Consider the properties of two different tissue types:

	Tissue 1	Tissue 2
T1 (ms)	1300	1300
T2 (ms)	60	100

When adjusting sequence parameters, which of the three options below would yield the greatest contrast between both tissues?

A. TR = 1000ms; TE = 77ms and α=90°
B. TR = 1000ms; TE = 160ms and α=90°
C. TR = 1000ms; TE = 0ms and α=45°

2. Steady State Signal

The signal of a single excitation at time TE is given by \(M_{0}sin(\alpha)e^{- \frac{\text{TE}}{T2}}\). If you repeat the sequence until it reaches steady state, what of the following expressions better describes your signal intensity?

A. \(M_{0}sin(\alpha)e^{- \frac{\text{TE}}{T2}}(1 - e^{- TR/T1})\)
B. \(M_{0}sin(\alpha)e^{- \frac{\text{TE}}{T2}}\frac{1 - e^{- TR/T1}}{1 - {cos(\alpha)e}^{- TR/T1}}\)
C. \(M_{0}sin(\alpha)e^{- \frac{\text{TE}}{T2}}\)

3. Signal intensity, T1 relaxation and flip angles.

Grey matter has a T1=1330ms (@ 3T). When imaging with a TR=2000ms (standard fMRI TR), the flip angle that generates the maximum signal for that tissue is 77°.

Signal Intensity and T1 relaxation times.

If grey matter tissue had a longer T1 (for example when imaging @ 7T), the optimum flip angle for the same TR would be:

A. Bigger than 77°
B. 77°
C. Smaller than 77°

Signal intensity and Flip Angles.

If the sequence were faster (for example with multiband 6), the optimal flip angle for the same tissue would be:

A. Bigger than 77°
B. 77°
C. Smaller than 77°

Can you explain qualitatively why we need to adapt flip angles?

2D vs. 3D encoding

If two tissues have the same T₂ but different T₁ values, of 700 and 1100ms, respectively (WM and GM at 3T), then the optimum contrast when using a GRE sequence with TR=600ms is achieved with a 90° flip angle.

Acquisition Time

When 3D encoding a volume of 200x200x200 pixels, how long does the acquisition last?

A. 400 minutes.
B. 80.000 minutes.
C. 2 minutes.

Flip Angles

To accelerate the previous acquisition the TR could be reduced to 6ms, in which case the optimum flip angle would be:

A. The same.
B. Reduced.
C. Increased.

Contrast

When comparing the acquisitions with TR=600 and TR=6ms , the signal difference between tissues with T1=700ms and T1=1100ms will be 15.5 and 1.7, respectively. Which of the acquisitions has the maximum contrast in a given amount of time? (note that the SNR for a given number of averages, N_av, increases \(\sqrt{N_{\text{av}}}\)).

A. 6ms.
B. 600ms.
C. They have the same contrast.

Spin History

Consider the case of a subject undergoing a functional study using a 2D EPI sequence (with TR=2500ms). During image acquisition, he moves along the z direction ~50% of the slice thickness. This movement happens halfway through the volume excitation. Motion correction can be used to reduce these nuisances…

A. Depending on the slice ordering (sequential – 1,2,3,4,5…- or interleaved – 1,3,5… 2,4,… -). You will get different artifacts depending on the direction of motion. Biggest signal variations are expected with interleaved acquisitions, but they can be easily modelled if the motion is known.
B. Depending on the slice ordering (sequential – 1,2,3,4,5…- or interleaved – 1,3,5… 2,4,… -). You will get different artifacts depending on the direction of motion. Biggest signal variations are expected with sequential slice ordering and will only affect one volume.
C. Once you perform motion correction of your data, you are good!.
D. It’s complicated.