MRI

Question 1

Explain how the Net Magnetic Vector (NMV) is ‘flipped’ from the longitudinal to the transverse plane. (8 marks)

Answer 1

• NMV is the sum of individual magnetic fields generated by hydrogen protons.
• NMV precesses around B₀ at the Larmor frequency (ω = γB₀).
• this is proprtional to BO
• Energy transfer must occur at the same frequency for resonance.
• RF pulse is applied at the Larmor frequency to excite hydrogen nuclei.
• RF pulse provides energy, flipping NMV from the longitudinal (B₀) to the transverse (XY) plane.
• Once in the transverse plane, the NMV induces a signal in the receiver coil.

Question 2

Explain the parameter ‘TR’ in MRI. (3 marks)

Answer 2

Definition of TR 1 mark
TR (Repetition Time) is the time interval between successive RF pulses.

Influence on Image Contrast 1 mark
Short TR → T1-weighted images, Long TR → T2-weighted or PD-weighted images.

Impact on Signal and Scanning Time 1 mark
Short TR reduces scan time but limits T2 relaxation effects; long TR improves SNR but increases scan time.

Question 3

.During an MRI scan when the RF pulse is switched off, the protons will release the energy gained and ‘relax.’There are 2 types of relaxation mechanisms, spin-spin and spin-lattice relaxation. Describe each relaxation process to explain how image contrast is generated in MRI. (10 marks)

Answer 3

• Definition of Relaxation 1 mark
  Process by which protons return to equilibrium, releasing energy.

Spin-Lattice (T1) Relaxation 1 mark
Energy is released to surrounding tissues (lattice).

T1 Recovery Mechanism 1 mark
Longitudinal magnetisation recovers as protons realign with B₀.

T1 Time Significance 1 mark Different tissues have unique T1 times, affecting signal intensity.

Spin-Spin (T2) Relaxation 1 mark
Protons lose phase coherence due to spin-spin interactions.

T2 Decay Mechanism 1 mark Transverse magnetisation decreases as protons dephase.

T2 Time Significance 1 mark Different tissues have unique T2 times, affecting signal loss.

Impact of TR on Contrast 1 mark Short TR → enhances T1 contrast; long TR → reduces T1 effects.

Impact of TE on Contrast 1 mark Short TE → limits T2 effects; long TE → enhances T2 contrast.

Clinical Application 1 mark T1-weighted → anatomy imaging; T2-weighted → pathology detection

Question 4

Explain the terms T1 recovery and T2 decay in MRI. (8 marks)

Answer 4

Definition of Relaxation 1 mark Process by which excited protons return to equilibrium after RF pulse removal.

T1 Recovery 1 mark Refers to NMV regaining longitudinal magnetisation.

T1 Time Definition 1 mark Time taken for 63% of longitudinal magnetisation to recover.

T1 Contrast Dependence 1 mark T1-weighted images depend on TR; short TR → enhanced T1 contrast.

T2 Decay 1 mark Refers to the dephasing of transverse magnetisation.

T2 Time Definition 1 mark Time taken for 37% of transverse magnetisation to remain.

T2 Contrast Dependence 1 mark T2-weighted images depend on TE; long TE → enhanced T2 contrast.

Clinical Importance 1 mark Different tissue types exhibit unique T1 and T2 relaxation times, allowing for contrast differentiation in MRI.