1. Chap.12 (3) Medical imaging
systems: MRI
Known as Nuclear Magnetic Resonance NMR
Science or black magic?
Source: Courtesy of Warner Bros

2. Principles of MRI
Tesla, one T = Gauss (Magnetic field of earth is 0.5 Gauss) -> 1 T is times stronger than earth

3. MRI
Source: Biomed resources

4. MRI
Source: MT Scott Diagnostic imaging

5. A brief recipe of MRI Put the subject into a strong magnetic field
Pass radiowaves through the subject
Turn of the radiowaves
Recieve radiowaves coming back from the subject
Convert the measured RF-data to an image

6. Elements contributing to a MRI
The quantitative properties of the nuclear spin
The radiofrequency (RF) exitation properties
Relaxationproperties of the tissue
Magnetic field strength and gradients
Thte timing of the gradients, RF-pulses and signal detection

7. Prerequisites for depicted nucleus
A nucleus that is to be pictured must have both:
Spin
Charge
Nucleus with even protonnumbers cannot be used because the spin will cancel each other

8. Single-proton
A single proton has a charge on the surface which is sufficient to form a small current-loop and generates a magnetic momentum µ
The proton has also a mass that creates an angle-moment J due to the spin

9. Hydrogenatoms
The hydrogenatom is the only large element in the body able to be depicted with MRI. (C, O and N have all even numbers in the proton number).
Hydrogen is everywhere in the body, primarily combined to water
= All MRI are in fact a picture of hydrogen

10. Angle momentum
J = m=mvr m v r J

11. Magnetic momentum A I
The magnetic momentum vector µ=IA

12. Precession og relaxation
Resonance – thea absorption of energy – occurs when radio frequency energy is applied
At the Larmor frequency and causes particles to change state and become excited
Spin-lattice decay (T1) and spin-spin decay (T2)
Faradays law of induction: If a mmagnetic field is moved through a conductor, a current
Will be produced in the conductor

13. Vector direction
The magnetic momentum and the angle momentum vector is aligned to the spin- axis.
µ=γJ
Where γ is the gyromagnetic ratio, constant for a given nucleus

14. Proton interaction with magnetism
Loaded particles spinning is constructing their own little magnetic field.
- Will line up in the same direction as an external magnetic field
Spinning particles with a mass have an angle momentum
The angle momentum works as a gyroscope and counteracts changes of the spin direction

15. Ref:

16. Larmour frequency The energy difference between
the two alignment states depends on the nucleus
 E = 2 z Bo
 Eh 
/2
known as Larmor frequency
/2= MHz / Tesla for proton
Ref: James Voyvodic

17. Resonance frequencies of common nuclei
Note: Resonance at 1.5T = Larmor frequency X 1.5
Ref: James Voyvodic

18. Electromagnetic Radiation Energy
X-Ray, CT
MRI

19. Magnetization Sum of all contributions from each nucleus
Large magnetic fields create a big magnetization M
Temperature dependency
To be able to measure the magnetization, we will have to disturb it
The quantity of energy supplied (durability for the RF-pulse at the resonance frequency) will decide how far the nuclei will be pushed away from B

20. Radiofrequency field
RF fields are used to manipulate the magnetization for a specific atom in a specific position
The hydrogen nucleus is tuned to a certain RF- frequecy
Eksternal RF-waves can be sent into the subject in order to disturb the hydrogen nucleus
Disturbed hydrogen nuclei will generate RF- signals with the same frequency – which can later be detected

21. To record an MRI signal
Needs a receive coil tuned in to the same RF-requency as the excitasjonscoil
Measure net magnetization
The signal oscillates at the resonansfrequency when the net magnetization vector rotates in the room
Signalamplitude will be weakened when the netto magnetization returns to the B-direction

23. MRI scanner

24. Relationship between parallell / antiparallell protones :
Important MRI equations
Larmorequation: ω=γB
Relationship between parallell / antiparallell protones :
Nn/Ne = ehν/kT =1+410-6 represents net magnetization at room temperature and 1 Tesla

25. T1 recording

26. T2 recording

27. MR images
T1 and T2
contrast

28. 3D picture construction
ω = γB

29. T1, T2 and proton-density

30. Vertical main field
Source: Oulun Yliopisto

31. Extremity
MRI

32. Interventional MRI

34. Adv/disadv MRI Adv: No harmful radiation Soft tissue imaging
High resolution images of T1 or T2 preferences
Disadv:
Expensive, large installation with superconducting magnets++
Very strong magnetic field
Claustrophobic
Not for frozen tissue