1. Magnetic Resonance Imaging
Basic Principles
V.G.Wimalasena Principal
School of Radiography

2. Introduction Modern 3 Tesla MRI unit Patient Couch RF Coil (for head)
Bore of the magnet
RF Coil (for head)
Main magnet body
Patient Couch

3. What is MRI?
Magnetic resonance imaging (MRI), or nuclear magnetic resonance imaging (NMRI), is primarily a Medical Imaging technique most commonly used in radiology to visualize the structure and function of the body.
It provides detailed images of the body in any plane.

4. MRI Vs CT
MRI provides much greater contrast between the different soft tissues of the body than CT does, making it especially useful in neurological (brain), musculoskeletal, cardiovascular, and oncological (cancer) imaging.
Unlike CT, it uses no ionizing radiation, but uses a powerful magnetic field to align the nuclear magnetization of (usually) hydrogen atoms in water in the body.

5. Uses RF fields
Radiofrequency fields are used to systematically alter the alignment of the nuclear magnetization of Hydrogen atoms, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner.
This signal can be manipulated by additional magnetic fields to build up enough information to construct an image of the body.

6. History
MRI is a relatively new technology, which has been in use for little more than 30 years (compared with over 110 years for X-ray radiography).
The first MR Image was published in 1973 and the first study performed on a human took place on July 3, 1977.
Magnetic resonance imaging was developed from knowledge gained in the study of nuclear magnetic resonance

7. Brief lay explanation of MRI physics
The body is mainly composed of water molecules which each contain two hydrogen nuclei or protons.
When a person goes inside the powerful magnetic field of the scanner these protons align with the direction of the field.

8. A second radiofrequency electromagnetic field is then briefly turned on causing the protons to absorb some of its energy.
When this field is turned off the protons release this energy at a radiofrequency which can be detected by the scanner.

9. The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up.
These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan.

10. Diseased tissue, such as tumors, can be detected because the protons in different tissues return to their equilibrium state at different rates.
By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue.

11. Use of contrast agents
Contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumours or inflammation.
Contrast agents may also be directly injected into a joint, in the case of arthrograms, MR images of joints.

12. Safety precaution
Unlike CT scanning MRI uses no ionizing radiation and is generally a very safe procedure.
But Patients with some metal implants, cochlear implants, and cardiac pacemakers are prevented from having an MRI scan due to effects of the strong magnetic field and powerful radiofrequency pulses.

13. Uses of MRI MRI is used to image every part of the body,
But is particularly useful in
neurological conditions,
disorders of the muscles and joints,
for evaluating tumors and
showing abnormalities in the heart and blood vessels.

14. System components Gradient amplifiers RF transmitter Operator consol
Magnet power supply
Gradient amplifiers
Shim power supply
RF transmitter
Operator consol
Magnet coils
Shim coils
Host computer
Gradient coils
RF coils
Magnet bore
Image processor
Image disk
RF receiver
Digitizer

15. Explaining Basic principles
This is an Integration of Two ways of explaining. i. e
Classically
Via quantum physics
It describes
Properties of atoms
Their interaction with magnetic fields

16. Atomic structure Central nucleus & orbiting electrons Nucleus
Nucleons
(Protons & neutrons)
Atomic number
Mass number
Electrically stable

17. Motion within the atom There are three types of motion within an atom
Electrons spinning on their own axis
Electrons orbiting the nucleus
The nucleus spinning about its own axis

18. The principles of MRI rely on the spinning motion of specific nuclei present in biological tissues
These are called (MR active nuclei)

19. MR active nuclei ?
MR active nuclei are Characterized by their tendency to align their axis of rotation to an applied magnetic field
Due to the laws of electromagnetic induction, nuclei that have a net charge and are spinning acquire a magnetic moment and are able to align with an external magnetic field

20. MR active nuclei continued..
Important Examples
Hydrogen 1
Carbon 13
Nitrogen 15
Oxygen 17
Fluorine 19
Sodium 23
Phosphorus 31
The nuclei with odd mass numbers undergoes this interaction
The result of this interaction is angular momentum or spin
Although neutrons have no net charge heir subatomic particles are not evenly arranged over the surface of the neutron and this imbalance enables the nucleus in which the neutron is situated to be MR active as long as the mass number is odd.

21. The magnetic moment alignment
The alignment of the magnetic moment is measured as the total of the nuclear magnetic moments and is expressed as a vector sum
The strength of the total magnetic moment is specific to every nucleus and determines the sensitivity to magnetic resonance

22. The hydrogen nucleus
The hydrogen nucleus is the MR active nucleus used in clinical MRI
Very abundant in the body
Solitary proton gives a relatively large magnetic moment

23. The hydrogen nucleus as a magnet
The nucleus contains one positively charged proton that spins
The spin of the proton induces a magnetic field around it and acts as a small magnet N N S S

24. The magnetic vector
The magnetic moment of each nucleus has vector properties.
i.e. it has size and direction and is denoted by an arrow
size
direction

25. Alignment of the magnetic moments
In the absence of an applied magnetic field the magnetic moments are randomly oriented
When placed in a strong external magnetic field the magnetic moments of the hydrogen nuclei align with this magnetic field , parallel or anti-parallel (as shown in next slide)

26. Alignment of the magnetic moments
Parallel
Anti-parallel
Random alignment in the absence of external magnetic field
Alignment
External magnetic field

27. The state of alignment
Quantum physics describes that the hydrogen nuclei only possesses two energy states or populations – low & high
Low energy nuclei align their magnetic moments parallel to the external magnetic field
High energy nuclei align their magnetic moments anti-parallel to the external magnetic field

28. Energy levels & field strength
Low energy population
Energy difference depends on field strength
high energy population

29. Energy levels & alignments
The energy level and the number of nuclei aligned in each direction is determined by the strength of the external magnetic field and the thermal energy level of the nuclei
Low thermal energy nuclei do not have enough energy to oppose the field and align parallel
High thermal energy nuclei have sufficient energy to oppose and may align anti-parallel

30. Alignment & field strength
Thermal energy depends on the body temperature
The main deciding factor to increase the number of parallel alignments is the high field strength of the external magnetic field
At thermal equilibrium the parallel population is higher than the anti-parallel population
Therefore there is a net magnetic moment parallel to the external magnetic field

31. The net magnetization vectorB0
Net Magnetization Vector (NMV)

32. Summary
The magnetic moment (of hydrogen in this case) is called the Net Magnetization Vector (NMV)
The static external magnetic field is called B0
The interaction of the NMV with B0 is the basis of MRI
The unit of B0 is Tesla or Gauss.
1 Tesla (T) = Gauss (G)

33. Summary continued…
When a patient is placed in the bore of the magnet the hydrogen nuclei within the patient align parallel and anti-parallel to B0.
A small excess of hydrogen nuclei line up parallel to B0 and constitute the NMV of the patient.
The energy difference between the two populations increases as B0 increases.
The magnitude of NMV is larger at high field strengths(B0 )

34. Precession
Each hydrogen nucleus that makes up the NMV is spinning on its own axis
The influence of B0 produce an additional spin or wobble
This path is called the precessional path and the speed at which the NMV wobbles around B0 is called the precessional frequency
Precessional path B0
Magnetic moment of the nucleus
Precession
Hydrogen nucleus

35. Precession continued….
Two populations;
High energy nuclei – spin down
Low energy nuclei – spin up
Their magnetic moments precess on a circular path around B0 as shown
Spin up nuclei B0
Precession
Spin down nuclei

36. The Larmor equation
The value of the precessional frequency is governed by the Larmor equation i.e
The precessional frequency (ω0) = Magnetic field strength(B0) x Gyro-magnetic ratio(γ)
ω 0 = B0 x γ
Gyro-magnetic ratio is a constant for a specific MR active nucleus and is expressed as the precssional frequency at 1.0 tesla. The unit is MHz / T

37. Precessional frequencies of Hydrogenγ B0 ω
1.5 T
63.86 MHz
42.57 Mhz/T
1.0 T
42.57 MHz
0.5 T
21.28 MHz

38. Resonance
Resonance is a phenomenon that occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation.
At resonance the object can absorb energy from the external source
Therefore Exchange of energy between two systems at a specific frequency is called resonance.

39. Nuclear Resonance
When a nucleus is exposed to an external perturbation that has an oscillation similar to its own natural frequency, the nucleus gains energy from the external force.
The nucleus gains energy and resonates if the energy is delivered at exactly its precessional frequency.

40. RF signal & Nuclear magnetic Resonance
Energy at the precessional frequency of hydrogen at all field strengths in clinical MRI corresponds to the radio frequency (RF) band of the electromagnetic spectrum
For resonance of hydrogen to occur, an RF pulse of energy at exactly the Larmor frequency of the hydrogen NMV must be applied
Other MR active nuclei that have aligned with B0 do not resonate because their precessional frequencies are different to that of hydrogen

41. Excitation & RF frequency
The application of an RF pulse that causes resonance to occur is termed excitation.
The absorption of energy causes an increase in the number of spin down hydrogen nuclei populations as some of the spin up nuclei gain energy via resonance and become high energy nuclei (next slide)
The energy difference corresponds to the energy required to produce resonance via excitation
As the field strength increases, the energy difference between the two populations also increases so that more energy (higher frequencies) are required to produce resonance.

42. Energy transfer during excitation
Low energy population
Some nuclei gain energy to join the high energy population
High energy population

43. The results of resonance
The first result is the NMV moves out of alignment away from B0
The angle to which the NMV moves out of alignment is called the flip angle
The magnitude of the flip angle depends upon the amplitude and duration of RF pulse
Usually the flip angle is 900 (see next slide). The transverse NMV rotates at the Larmor frequency

44. The flip angle & Transverse plane
B0 is now termed the longitudinal plane
The plane at 900 to B0 is termed the transverse plane
Longitudinal plane
Longitudinal plane B0
NMV
Flip angle
Flip angle 900
NMV
Transverse plane
Transverse plane

45. In phase / out of phase
The second result of resonance is that the magnetic moments within the transverse NMV move into phase with each other
Phase is the position of each magnetic moment on the precessional path around B0
Magnetic moments that are in phase are in the same place on the precessional path around B0 at any given time
MM that are out of phase are not in the same place on the precessional path

46. Phase of magnetic moments around the precessional path
Out of phase
In phase

47. Summary
For resonance of hydrogen to occur, RF at exactly the Larmor frequency of hydrogen must be applied
The result of resonance is an NMV in the transverse plane that is in phase
This NMV precesses in the transverse plane at the Larmor frequency

48. Formation of MR signal after removal of RF pulse
The MR signal
Formation of MR signal after removal of RF pulse

49. The MR signal
As a result of resonance the NMV is precessing in phase in the transverse plane.
According to Faraday’s laws of induction,
When a receiver coil (a conductive loop) is placed in the area of moving magnetic field a voltage is induced in it.
This Signal is produced when coherent (in phase) magnetization cut across the coil.

50. MR signal continued….
Therefore the moving NMV produces magnetic field fluctuations inside the coil
As the NMV precesses at the Larmor frequency in the transverse plane a voltage is induced in the coil.
This voltage constitutes the MR signal
Why would you expect the MR signal to be alternating?

51. The frequency of the MR signal is the same as the Larmor frequency
The magnitude of the MR signal depends on the amount of magnetization present in the transverse plane

52. Generation of the MR signal in the receiver coilB0
Precession of NMV
Receiver coil
NMV

53. Relaxation & The free induction decay signal
Switching off RF pulse
Relaxation
Recovery & decay
FID

54. Relaxation
When the RF pulse is turned off the NMV is again influenced by B0 , and, it tries to realign with it.
To do that it must lose the energy given to it by the RF pulse.
The process by which the NMV loses energy is called relaxation

55. Recovery & Decay As relaxation occurs the NMV returns to align with B0
When this happens,
The amount of magnetization in the longitudinal plane gradually increases – this is called recovery
The amount of magnetization in the transverse plane gradually decreases – this is called decay

56. The free induction decay signal
As the magnitude of transverse magnetization decreases so does the voltage induced in the receiver coil.
The induction of this reduced signal is called the free induction decay (FID) signal

57. Result of relaxation During relaxation
The NMV gives up absorbed energy and returns to B0
The magnetic moments of the NMV lose the transverse magnetization due to dephasing
Looking down on to transverse plane
In phase
Dephasing
Out of phase

58. T1 Recovery & T2 Decay Relaxation results in
recovery of magnetization in the longitudinal plane
and
decay of magnetization in the transverse plane.
The recovery of longitudinal magnetization is caused by a process called T1 recovery
The decay of transverse magnetization is caused by a process called T2 decay

59. T1 Recovery
T1 recovery is caused by the nuclei giving up their energy to the surrounding environment or lattice and it is often termed spin - lattice relaxation
The rate of recovery is an exponential process with a recovery time constant called T1

60. Recovery time constant -T1
T1 is the time it takes 63% of the longitudinal magnetization to recover in the tissue
100%
63%
Signal intensity T1
Time

61. T2 decay
This is caused by nuclei exchanging energy with neighbouring nuclei.
The energy exchange is caused by the magnetic fields of each nucleus interacting with its neighbour.
It is often termed spin-spin relaxation results in a decay or loss of transverse magnetization
The rate of decay is also an exponential process so that the T2 relaxation time is its time constant of decay

62. Time constant of decay – T2
100%
T2 is the time it takes 63% of the transverse magnetization to be lost
Signal intensity
37% T2
Time

63. Dephasing of the FID
A signal or voltage is only induced in the receiver coil if there is magnetization in the transverse plane that is in phase
Dephasing (T2)
Signal (FID)

64. Pulse timing parameters
The magnitude and timing of the RF pulses form the basis of MRI and are discussed in Next lesson