SAQs - Chapter 2
1. What factors determine the T1 and T2 relaxation time of a tissue?
• Inherent energy of the tissue.
• How well the molecular tumbling rate matches the Larmor frequency.
• How closely spaced the molecules are.
2. Why does fat have a short T1 and T2 relaxation time?
Because fat has a low inherent energy, a slow molecular tumbling rate and its molecules are packed together. This means energy exchange and spin-spin interactions are efficient and therefore T1 and T2 relaxation respectively occurs quickly.
3. Why does water have a long T1 and T2 decay time?
Because water has a high inherent energy, a fast molecular tumbling rate and its molecules are spaced far apart. This means energy exchange and spin-spin interactions are inefficient and therefore T1 and T2 relaxation respectively occurs slowly.
4. Define the term weighting.
Weighting means that parameters are selected to make one contrast mechanism dominate over the others.
5. What values of TR and TE are needed for PD weighting in a spin echo sequence and why?
The TR must be long so that neither fat nor water has had time to fully recover their longitudinal magnetization. The TE must also be short to minimize the T2 differences between the tissues.
6. List the main factors that make gradient echo sequences different from spin echo.
Variable flip angles, gradient rephasing, shorter TRs and scan times.
7. What parameter controls T2 decay and why?
The TE controls T2 decay as it determines how much dephasing is allowed to occur before the signal is read.
8. What types of contrast will the following produce?
(a) TR 250 ms, TE 5 ms, flip angle 120°.
(b) TR 50 ms, TE 15 ms, flip angle 35°.
(a) T1 weighting.
(b) T2* weighting.
9. What term is used to indicate gradient rephasing?
Rewinding.
10. What term is used to indicate gradient dephasing?
Spoiling.