Advanced Pulse Sequences Flashcards
Where is the main intensity information stored in k-space?
At the k-space origin.
How is contrast determined in k-space?
Contrast is determined by the magnitude of the transverse magnetisation when the k-space origin is sampled.
How are scan parameters chosen to maximise the contrast in the image?
Spin-Echo: Arrange it so that the k-space origin is aligned with the SE peak. i.e. that the echo time is twice the length of time between the 90 and 180-degree pulses.
Gradient Echo: The echo time is set as the time to reach the centre of k-space.
Why are turbo spin echoes possible?
T2 is longer than the time required to collect data, so there is plenty of transverse magnetisation left at the end of the k-space line, allowing us to sample multiple lines per excitation.
What are possible problems with TSE, and how can they be resolved?
Each 180-degree pulse delivers power into the tissue creating potential SAR issues.
Reducing the refocusing angle reduces the SAR.
How is the SAR related to the flip angle?
SAR is proportional to (Flip Angle)^2 or (B_1)^2.
How does the point spread function affect the signal collected from a TSE?
As the signal decays as we collect more lines, the T2 data that should have 1 value suddenly has multiple values.
We can multiply the k-space data by the PSF to correct for this but it alters the effective resolution of the image.
What is required for an Echo-Planar Imaging sequence to sample all of k-space in one FID?
Must collect all data within T2* (about 100ms), so need strong gradients to keep the same resolution.
What are the advantages of Gradient and Spin Echo Imaging (GRASE)?
Can reduce SAR compared to TSE by collecting multiple lines of k-space using gradient reversals like in EPI.
In a GRASE sequence, what are the uses of the different echoes?
Gradient echoes take less time, making the scan more time efficient. Spin Echoes are mainly used to refocus T2* decay into T2 decay.
What are the properties of a Fast Imaging with Steady-Stae Precession (FISP) sequence?
- Sequence in balanced: total gradient moment is zero,
- Magnetisation at end of cycle is refocussed and partially flipped back to z-axis (+ or -) establishing a steady state of magnetisation.
What are FISP sequences commonly used for?
It has a complex relationship between TR, TE, T1, T2, flip angle and contrast. The short TR and TE requires means that it is useful in cardiac imaging.
What are the advantages and disadvantages of spiral imaging?
+ Starts at k=0: short effective TE.
- Gradient modulation is complex: need to resample before FT.
How can the required gradient power be reduced in EPI?
Interleaving the k-space acquisitions. Full k-space sampled across multiple acquisitions.
