Part 1: L4, MRI theory

Question 1

What is MRI scanning?

Answer 1

• Provides images of organs and tissue using H1NMR
• Ascertaining the concentrations of protons of different spins
• Diseases can be detected from differences in H1 NMR resonances between normal and abnormal tissue

Question 2

How does MRI work? (basis)

Answer 2

• Putting a proton into magnetic field (spin quantum number of 1/2 with associated magnetic moment)
• Proton magnetic moment can thus adopt two orientations, each with a different energy in the presence of an external magnetic field (antiparallel and parallel)
• In body tissues, more line up in parallel creating a small additional magnetization M in the direction of B0
• In MRI, interested in measuring relaxation from these moments (net value from bulk sample) AKA rate of decay

Question 3

Pulse length and flip angle

Answer 3

• Pulse length: Strength of pulse
• Flip angle: How far out of bulk magnetisation the pulse is pushed (90 degrees = no bulk magnetisation, 180 degrees - totally against)

Question 4

Why is the free induction decay useful to study?

Answer 4

• FID magnitude decays in an exponential manner with a time constant T2 (Decay due to spin-spin relaxation)
• Magnitude signal dependent on proton density and Mxy -> informative of proton environment

Question 5

Spin lattice relaxation:

Answer 5

T1 (longitudinal):

• Give pulse -> time taken to recover longitudinal orientation of M along z axis
• Positive enhancement
• T1 relaxation - z magnetisation increases (and xy decreases)
• ‘T1 time’ = time interval for 63% recovery

Question 6

Spin-spin relaxation:

Answer 6

T2 (Transverse):

• Observing loss of coherence in xy plane (or phasing of spins lost)
• Negative enhancement
• ‘T2 time’ = time interval for 37% loss of original transverse magnetization

Question 7

Distinguishing between tissues using MRI:

Answer 7

• Relaxation allows you to distinguish between textures (partly on basis of water content)
• e.g. Grey matter T1 = 950ms, T2 = 100ms
• e.g. White matter T1 = 600ms, T2 = 80ms

Question 8

Contrast mechanisms in MRI

Answer 8

• Difference of proton density (cannot be modified)
• Modification of T1 or T2 relaxation times
• Susceptibility effects (T2*)
• Resonance Frequency Shifting

Question 9

Common T1 and T2 agents:

Answer 9

• T1 is commonly a gadolinium complex (reduces longitudinal relaxation time and gives good positive contrast)
• T2 is commonly iron oxide (resulting in negative contrast)

Question 10

Mechanism for T1 contrast agents:

Answer 10

• Relaxation occurs when nuclear spin interacts with a magnetic field oscillating at or near the Larmor frequency for the excitation
• The most common sources of such fields are other nuclei - unpaired electrons and nuclear quadrupoles (imagine small magnets in the body facilitating this specific exchange)
• Dipole-dipole interactions occur where there are magnetic poles that interact with the nuclear magnetic moment (mu)

Question 11

Relaxation around Gd3+:

Answer 11

• Coordinated water molecules make a direct contribution to inner-sphere relaxation -> very fast relaxation
• Bulk solvent molecules experience the paramagnetic effect when they diffuse around the metal centre (outer-sphere relaxation); fairly shielded by inner sphere of water molecules

Question 12

Factors in water exchange:

Answer 12

• Ligands occupy 7/8 coordination sites of Gd3+ ions -> 1 or 2 water molecules can attach (9 total in both cases)
• A slow water exchange rate limits the relaxation enhancement
• The tuning of the steric environment in the vicinity of the Gd centre increase the dissociative water exchange rate as well as increase relaxivity

Question 13

What is larmor frequency?

Answer 13

• Processional path around magnetic field of the magnetic moment of the proton around
• Related to strength of magnetic field (B0)

Question 14

Why are gradients important in MRI (physical basis):

Answer 14

• On top of B0, add gradient field to excite protons in a slice of matched strength -> where rf and field match, signal occurs
• 3 gradients in x, y, z define a single spot (usually image in slices to generate a full image)
• Each slice has finite width delta-z relative to the range of frequencies delta-F

Question 15

Why is Gadolinium useful as a T1 agent?

Answer 15

• Gadolinium (III) has a very high magnetic moment due to its 7 unpaired electrons in the 4f orbital (paramagnetic)
• Paramagnetic
• One of the lanthanides

Question 16

Charge of lanthanides? How are f orbitals of lanthanides involved in bonding an relevance as a T1 agent?

Answer 16

• Generally 3+
• F orbitals not very involved in bonding -> very high magnetic moment due to unpaired electrons

Question 17

Types of water relaxation around Gd3+:

Answer 17

• Water exchange -> can be controlled (rate)
• Prototropic exchange (of second sphere): Occurs quite rapidly due to lewis acidity