1. Principles of Magnetic Resonance Imaging Alessandro Sbrizzi UMC Utrecht

2. A recent history
1946 Felix Bloch and Edward Purcell independently discover the magnetic resonance phenomena (Nobel Prize in 1952)
1971 Raymond Damadian: nuclear magnetic relaxation times of tissues and tumors differed→Clinical Application
1973/1974 Paul C. Lauterbur and Peter Mansfield: spatial localization through Gradient Fields →Imaging (Nobel Prize in 2003)

3. MRI in 1980

4. The present About 100 million MRI scans per year (worldwide): Tumors
Multiple Sclerosis
Epilepsy
Neuro-degenerative diseases
Ischemic Stroke
Stenosis or aneurysms (MR Angiography)
Cardiac
Brain Functioning (fMRI)
MR guided surgery (MRI-Linac)
...

6. Brain vessels smaller than 1 mm are visualized

7. Basic physical principles I
Nuclei can be seen as small tiny rotating magnets
Represented by magnetic moment vector
In the presence of external magnetic field B0 aligned along the z-axis they precess around it at the Larmor frequency ω = γ |B0|.
Same effect as a spinning top (only much faster)
γ is a constant (gyromagnetic ratio)
Governing equation (by F. Bloch):
See also:

8. Basic physical principles II
The transverse component of μ is randomly distributed over a small volume V (net sum over V = 0)
Only the longitudinal component of μ is slightly different than 0, but it can not be measured.
To measure the magnetic moments, we need them to acquire a net transverse magnetization which differs from 0
See also:

9. Basic physical principles III
Idea: apply an additional external field, BRF in the xy plane which rotates μ around it.
Since μ is still precessing, consider a rotating reference frame with the same rotating rate ω = γ |B0|.
In the rotating frame, μrot is frozen, no longer precessing.
We apply BRF such that in this rotating frame it appears to be static too: BRF = (A cos ωt, A sin ωt, 0) thus Brot = (A,0,0).
Since ω is in the radio-frequency range, BRF is called a radio-frequency field.
This condition is called resonance.
See also:

10. Basic physical principles IV
Bloch equation in the rotating frame:
Effect in the rotating frame:
μrot precesses around the x axis as long as the RF field is ON.
Effect in the laboratory frame:
μ quickly precesses around B0 and slowly precesses around BRF
See also:

11. Basic physical principles V
Macroscopic view: Net effect on μ over a small volume V is described by the net magnetization vector M = (Mx , My , Mz):
When only static B0 is present: M is aligned along it (no transverse component).
When also RF field is on: M is tilted and acquires a transverse component which can be measured.
See also:

12. Basic physical principles VI
Once M has a transverse component, signal can be collected:
G is a third type of (time-dependent) magnetic field which is needed in order to spatially encode the signal from the spins (more on this later on…)
Since we are interested in the value of Mx+i My over the spatial coordinates (i.e. image), we need to solve this equation for Mx+i My .

13. Basic physical principles VII
The MRI scanner:
The main, static magnetic field B0 (to align the spins)
The Radio Frequency field BRF (to tilt the spins)
The Gradient field, G (spatial localization)