DETAILED DISCUSSION ON MRI ... MACHINE PHYSICS

1. MRI PHYSICS
(PART I)
BY
DR. ARIFKHAN S

2. MRI – MAGNETIC RESONANCE IMAGING
•
•
•
•
ATOM : Nucleus + electrons
Nucleus : Neutrons+ PROTONS
PROTONS spin and creates magnetic field.
The protons - being little magnets - align themselves in the
external magnetic field like
a compass needle in the magnetic field of the earth.
• May align parallel or anti-parallel

3. LARMOR EQUATION
•

4. MAIN MAGNET FIELD BO
• Purpose is to align H protons in H2O (little magnets)
[Main magnet and some of its lines of force]
[Little magnets lining up with external lines of force]

5. A SINGLE PROTON
There is electric charge
on the surface of the
proton, thus creating a
small current loop and
generating magnetic
moment .
J
+
+
+
The proton also
has mass which
generates an
angular
momentum
J when it is
spinning.
Thus proton “magnet” differs from a magnetic bar in that it
also possesses angular momentum caused by spinning.

6. VECTOR & COORDINATE SYSTEM

7. Protons in a Magnetic Field
Bo
Parallel
(low energy)
Anti-Parallel
(high energy)
Spinning protons in a magnetic field will assume two states.
If the temperature is 0o K, all spins will occupy the lower energy state.

8. • Naturally the preferred state of alignment is the one that
needs less energy
• So more protons are on the lower energy level, parallel to
the external magnetic field
• Proton’s are not stationary. Precession movement is seen
precession frequency : precess per second
Depends upon the strength of the magnetic field
The stronger the magnetic field, the faster the
precession rate
and the higher the precession frequency.
Magnetic field : B0

9. NMR: NUCLEAR MAGNETIC RESONANCE
• Resonance here refer to the change in energy states of the NUCLEI in response to A RF wave
of specific Radio Frequency.
• Resonance can also occur in an external magnetic field.
• Criteria:
•
Must have ODD number of protons or ODD number of neutrons.
• Examples:
•
1H, 13C, 19F, 23N,
•
40.08, 11.27 and 17.25 MHz/T
and 31P with gyromagnetic ratio of 42.58, 10.71,
• The main Resonating nuclei in Human body is H.

10. Net Magnetization
Bo
M
M
c
Bo
T

11. Energy Difference Between States
/2 = 42.57 MHz / Tesla for proton in hydrogen atom
Knowing the energy difference allows us to use
electromagnetic waves with appropriate energy
level to irradiate the spin system so that some spins
at lower energy level can absorb right amount of
energy to “flip” to higher energy level.

12. Basic Quantum Mechanics Theory of MR
Spin System Before Irradiation
Bo
Lower Energy
Higher Energy

13. RESONANCE AND RF PULSING
2 things happen to Resonating protons when RF pulse is applied
1- Energy Absorption
Increase number of High energy Spin Up nuclei and these may align anti parallel
2- Phase Coherence
NMV precesses in transverse plane at Larmor Frequency

14. • High Energy state but Not in phase
High energy state but in phase : COHERENCE

15. Basic Quantum Mechanics Theory of MR
The Effect of Irradiation to the Spin
System
Lower
Higher

17. Basic Quantum Mechanics Theory of MR
Spin System After Irradiation

18. THIS ENERGY IS JUST HANDED OVER TO THEIR SURROUNDINGS,
THE SO CALLED
LATTICE. AND THIS IS WHY THIS PROCESS IS NOT ONLY CALLED
LONGITUDINAL RELAXATION, BUT
ALSO SPIN-LATTICE-RELAXATION. THIS RELAXATION PRODUCES

19. • T1 relaxation Curve : after switching off RF change in longitudinal
MAGNETIZATION

20. • This time constant is the transversal relaxation time T2 , OR
• spin-spin-relaxation TIME

22. IMAGING TRICKS
• B0 MAGNETIC FIELD IS NOT UNIFORM.
• INTERNAL MAGNETIC FIELD OF PROTONS: TISSUE DIFFERENCE
• RF PULSING : DURATION AND INTERVAL

23. TISSUE FEATURES
• LIQUIDS HAVE LONG T1 AND LONG T2
When the lattice consists of pure liquid/water, it is difficult
for the protons to get rid of their energy,
• FATS HAVE SHORT T1 AND T2
The carbon bonds at the ends of the fatty acids have frequencies
near the Larmor frequency, thus resulting in effective energy
transfer.

24. MRI : THE MACHINE

25. EQUIPMENT
Gradient Coil
4T magnet
RF Coil
gradient coil
(inside)
Magnet
RF Coil

26. MAIN COMPONENTS OF A SCANNER
• Static Magnetic Field Coils
• Gradient Magnetic Field Coils
• Magnetic shim coils
• Radiofrequency Coil
• Subsystem control computer
• Data transfer and storage computers
• Physiological monitoring, stimulus display, and behavioral
recording hardware

27. MRI SCANNER COMPONENTS

28. STATIC MAGNETIC FIELD COILS
• Earths magnetic field is
0.3-0.7G (G: Gauss)
• 1 Tesla is 10000 G
• Magnets used for imaging mostly between .5 to 1.5 Tesla
• PERMANENT AND ELECTROMAGNET (SUPERCONDUCTING )
• The field should be homogenous. Inhomogeniety reduces the signal to noise
ratio ; i.e affects the image quality.

29. PERMANENT MAGNETS &
ELECTROMAGNETS
• Permanent magnet : is always magnetic and does not use any energy for work.
but is thermally instable, field strength is limited, weight (a .3T magnet will be around 100 Tons!!!!)
• Electromagnet:
1. Resistive Magnet: An electrical current is passed through a loop of wire and generates a magnetic field
2. Superconducting magnets: widely used in MR machines. Here the current carrying conductor is kept at
very low temperatures called SUPERCONDUCTING TEMPERATURE (4 Kelvin or -269 Celsius).
At this temperature the conducting material looses its resistance for electricity. And so produces constan
magnetic field s.
Cryogens are Helium and Nitrogen.
• Advantages of superconducting magnets are high magnetic field strength and excellent magnetic
field homogeneity

30. VOLUME COILS
• These completely surround the part of the body that is to be imaged.
• it is the transmitter for all types of examinations. It also receives the signal
when larger parts
of the body are imaged
• E.g The helmet-type head coil acts as receiver coil, the body coil transmitting
the RF pulses while imaging the head

31. SHIM COILS
• Shimming: the process by which an in-homogenous magnetic filed
is converted to a homogenous magnetic filed by electrical as well
as mechanical adjustments.
• Shim Coils are such devices used for shimming of the magnetic
field (i.e the STATIC MAGNETIC FIELDS)

32. IMAGING GRADIENT MAGNETS
• Used to vary magnetic field in known manner
• Each point has slightly different rate of precession & Larmor
Frequency.
• Variety of signal released by Protons returning to z-plane can used
to determine the composition of exact location of each point.
Gradient Function:
• Slice selection
• Frequency encoding
• Phase encoding

33. GRADIENT FUNCTIONS
• Slice selection : We can selectively excite nuclei in one slice of tissue by
incorporating a third magnetic field: the “gradient” magnetic field.
• The gradient magnetic field produces a linear change in the total
magnetic field.
• Here, “gradient” means “change in field strength as a function of
location in the MRI bore”.
• Since the gradient field changes in strength as a function of position,
we use the term “gradient amplitude” to describe the field

34. PHASE AND FREQUENCY ENCODING
Consider an MRI image composed of 9 voxels
(3 x 3 matrix)
All voxels have the same precessional frequency and are all “in phase” after
the slice select gradient and RF pulse

35. • When the Y “phase encode gradient” is on, spins on the top row
have relatively higher precessional frequency and advanced phase.
Spins on the bottom row have reduced precessional frequency and
retarded phase

36. 3.
Turn off the Y “phase encode
gradient”
4. All nuclei resume precessing at
the same frequency
5. All nuclei retain their
characteristic Y coordinate
dependent phase angles
6. A “read out” gradient is applied
along the X axis, creating a
distribution of precessional
frequencies along the X axis.
7. The signal in the RF coil is now
sampled in the presence of the X
gradient.

37. • While the frequency encoding
gradient is on, each voxel
contributes a unique
combination of phase and
frequency. The signal induced
in the RF coil is measured while
the frequency encoding
gradient is on.

38. • 8. The cycle is repeated with a different setting of the Y phase
encoding gradient. For a 256 x 256 matrix, at least 256 samples of
the induced signal are measured in the presence of an X frequency
encoding gradient. The cycle is repeated with 256 values of the Y
phase encoding gradient.
• 9. After the samples for all rows are taken for every phase-encode
cycle, 2D Fourier Transformation is then carried out along the phaseencoded columns and the frequency-encoded rows to produce
intensity values for all voxels.

39. K- SPACE
• The Fourier transformation acts on the observed “raw data” to form an image. A
conventional MRI image consists of a matrix of 256 rows and 256 columns of voxels
(an “image matrix”).
•
The “raw data” before the transformation ALSO consists of values in a 256 x 256
matrix
• The raw data matrix is also called K-SPACE.
• The 2DFT of k-space produces an image.
• Each value in the resulting image matrix corresponds to a grey scale intensity
indicative of the MR characteristics of the nuclei in the voxels. Rows and
columns in the image are said to be “frequency encoded” or “phase encoded”.

40. k-space
Image space
y
ky
FT-1
kx
x
FT
Acquired Data
MRI task is to acquire k-space image then
transform to a spatial-domain image. kx is
sampled (read out) in real time to give N
samples. ky is adjusted before each readout.
Final Image
MR image is the magnitude
of the Fourier transform of
the k-space image

41. • The top row of k-space would be measured in the presence of a
strong positive phase encode gradient.
• A middle row of k-space would be measured with the phase
encode gradient turned off.
• The bottom row of k-space would conventionally be measured in
the presence of a strong negative phase encode gradient.
• While the frequency encoding gradient is on, the voltage in the RF coil is
measured at least 256 times. The 256 values measured during the first RF
pulse are assigned to the first row of the 256 x 256 “raw data” matrix. The
256 values measured for each subsequent RF pulse are assigned to the
corresponding row of the matrix.

42. RF BANDWIDTH
• There are actually two RF bandwidths are associated with MRI:
Transmit and Receive
•
RF Transmit bandwidth ~ +1 kHz affects
slice thickness
•
RF Receive bandwidth ~ +16 kHz is
sometimes adjusted to optimize
signal-to-noise in images

43. SURFACE COILS
• Surface coils are placed directly on the area of interest, and have
different shapes corresponding
to the part to be examined.
• They are receiver coils only, most of the received signal coming
from tissues near by; deeper structures cannot be examined with
these coils

44. BASIC MRI TECHNIQUE
• 1. PLACE A PATIENT IN A UNIFORM MAGNETIC FIELD
• 2. DISPLACE THE EQUILLIBRIUM MAGNETIZATION VECTOR WITH RF – PULSE.
• 3. OBSERVE THE SIGNAL AS THE MAGNETIZATION VECTOR RETURNS TO
EQULLIBRIUM

46. MR Signal
When RF pulse is given
NMV rotates around transverse plane It passes across Receiver Coil
Inducing voltage in it thus producing a signal
• RF Removed  Signal decreased  Amplitude of MR Signal decreased
• Free Induction Decay "FID":
• Free (No RF Pulse)
• ID (because of Decay of Induced signal in Receiver Coil)

47. IMAGING
NMV can be separated in to
Individual Vectors of tissue present in the patient
• Such as Fat, CSF & Muscle

48. IMAGING
High Signal Low Signal Intermediate
White
Black
Large transverse Small transverse
component of
component of
magnetization
magnetization
Grey

49. CONTRAST MECHANISMS
High Signal Low Signal Intermediate
White
Black
Large transverse Small transverse
component of
component of
magnetization
magnetization
Grey

50. CONTRAST MECHANISMS (PARAMETERS)
Image contrast controlled by:
1- Extrinsic Contrast parameters:
TR, TE & Flip Angle
2- Intrinsic Contrast parameters:
T1 Recovery, T2 Decay, Proton Density, Flow & Apparent Diffusion Coefficient

51. CONTRAST MECHANISMS (RELAXATION PROCESS)
after removal of RF pulse
Signal induced in Receiver Coil decreased
T1-relaxation time
NMV recovers and realign to B0 this process called "T1 Recovery"
T1 time is
an intrinsic contrast parameter that inherent to tissue being imaged
T2- relaxation time
2-Nuclei loss Precessional coherence or diphase and NMV decay in
the transverse plane this process called "T2 Decay“
T2 Decay is
an intrinsic contrast parameter and is inherent to the tissue being imaged

52. CONTRAST MECHANISMS (T1 RECOVERY)
Short TR  DIFFERNCE IN SIGNAL INTENSITY  T1 contrast
(T1 Weighted)
• TR 300-600 ms

53. CONTRAST MECHANISMS (DEFINITIONS)
Repetition Time "TR“:
Time from application of one RF pulse
To the application of the next
(it affects the length of relaxation period
after application of one RF excitation pulse
to the beginning of the next)
SHORT TR : <500ms
LONG TR : > 1500ms

54. CONTRAST MECHANISMS (DEFINITIONS)
Echo Time "TE"
Time between RF excitation pulse and
collection of signal
(it affects the length of relaxation period
after removal of RF excitation pulse
and the peak of signal received in receiver coil)

55. • TR determines T1 contrast
• TE determines T2 contrast.

56. CONTRAST MECHANISMS (DEFINITIONS)
Flip Angle
Angle throw which the NMV moved as result of a RF excitation pulse

57. CONTRAST MECHANISMS (T1 RECOVERY)
T1 Recovery
Caused by exchange of energy from
nuclei to their surrounding environment or lattice
"Spin Lattice Energy Transfer"
and realign in B0
this occur in exponential process
at different rates in different tissue
NB: Molecules are constantly in motion; Rotational and Transitional

58. CONTRAST MECHANISMS (T1 RECOVERY)
T1 in Fat
T1 in Water
absorb energy quickly
T1 is very short
i.e. nuclei dispose
their energy to
surrounding fat tissue
and return to B0 in
very short time
inefficient at receiving
energy
T1 is longer
i.e. nuclei take allot
longer to dispose
energy to surrounding
water tissue
FAT
WATER

60. PROPERTIES OF BODY TISSUES
Tissue
T1 (ms)
T2 (ms)
Grey Matter (GM)
950
100
White Matter (WM)
600
80
Muscle
900
50
4500
2200
250
60
Cerebrospinal Fluid (CSF)
Fat
Blood
1200
T1 values for B0 ~ 1Tesla.
T2 ~ 1/10th T1 for soft tissues
100-200

63. CONTRAST MECHANISMS (T2 DECAY)
T2 Decay
Caused by exchange of energy from one nucleus to another
"Spin-spin Energy Transfer“
as result of intrinsic magnetic fields of nuclei interlacing with each other
this energy exchange  loss of coherence or dephasing
and as result NMV decay in transverse plane
T2 is exponential process
occur at different rates in different tissues