MRI uses strong magnetic fields and radio waves to produce detailed images of the inside of the body. Protons in the body align with the magnetic field, and radio waves excite the protons causing them to emit signals. The signals are detected by coils and used to construct an image on a computer. Different tissues can be distinguished based on proton density and relaxation times after excitation. Gradient fields are used to localize the source of the signals within the body.

1. MRI PHYSICS- PART IDr.Archana Koshy

2. •Our bodies are made up of roughly 63%
water
•MRI machines use hydrogen atoms which
act like little magnets, having a north and
south pole
•The atoms inside our body are aligned in
all different directions

4. • Nuclei line up with magnetic moments either in a parallel or anti-
parallel configuration.
• In body tissues more line up in parallel creating a small additional
magnetization M in the direction of B0.

5. • Frequency of precession of magnetic
moments given by Larmor equation .
g ~ 43 mHz/Tesla
f = Larmor frequency (mHz)
g = Gyromagnetic ratio (mHz/Tesla)
B0 = Magnetic field strength (Tesla)
f = g x B0

6. PROTONS IN A MAGNETIC FIELD
Bo
Parallel
(low energy)
Anti-Parallel
(high energy)
Spinning protons in a magnetic field will assume two
states.

7. MRI and Radio Frequencies
• The RF coil produces a radio frequency simultaneously
to the magnetic field
• This radio frequency vibrates at the perfect frequency
(resonance frequency) which helps align the atoms in
the same direction
• The radio frequency coil sent out a signal that
resonates with the protons. The radio waves are then
shut off.
• The protons continue to vibrate sending signals back
to the radio frequency coils that receive these signals.

8. • The signals are then ran through a computer and go
through a Fourier equation to produce an image.
• Tissues can be distinguished from each other based
on their densities.

9. SPINNING PROTONS – TINY MAGNETS ALIGNS ON THE APPLICATION OF
EXTERNAL MAGNETIC FORCE .

10. MAGNETIZATION VECTOR
 The spins can be broken down into two perpendicular components:
a longitudinal or transverse component.
 In a B0 magnetic field, the precession
corresponds to rotation of the transverse
component along the longitudinal axis.

11. LONGITUDINAL
MAGNETIZATION
• External magnetic field is directed along X axis
• Protons align on the parallel and anti parallel to the
external magnetic field (along positive and negative sides )
• Forces of protons on negative and positive side cancel each
other
• Few protons remain on the positive side which arent
cancelled .
• Forces of these protons add up together to form a
magnetic vector along z axis .

13. TRANSVERSE
MAGNETISATION

15. MR SIGNAL
• Transverse magnetisation vector formed has a
precession frequency .
• On movement , it produces electric current .
• The coils receive this current as MR signal.
• Strength of the signal depends upon magnitude
of the transverse magnetisation .
• MR signals are Fourior transformed into MR image
by computers .

18. LOCALISATION OF SIGNAL
• In order to localise the area from where the signals
are originating, three more magnetic fields are
superimposed.
1. Slice selection gradient - Z axis
2. Phase encoding gradient –Y axis
3. Frequency encoding gradient –X axis

20. RELAXATION
• Recovery of protons back towards equilibrium after having
been disturbed by RF excitation.
• Relaxation times of protons and heterogenous distribution
of tissue proton densities determine the contrast in an MR
image .
• When RF pulse is switched off , TM reduces and LM
increases.

21. Longitudinal relaxation

22. TRANSVERSE RELAXATION

25. T1
• Time taken by LM to recover after RF pulse is
switched off , to its original value .
• Time taken when LM reaches back to 63% of its
original value .
• Depends upon tissue composition ,structure and
surroundings .
• If lattice has magnetic field, which fluctuates at
Larmor frequency ,transfer of thermal energy
from protons to the lattice is easy and fast .

26. T2
• Time taken by TM to disappear .
• Depends on inhomogeneity of external
magnetic field .
• If liquid is impure and has larger molecules ,
they move at a slower rate .
• Maintains homogenity of magnetic field
• As a result, protons go out of phase very fast .
• Hence fat has shorter T2 .

28. SPIN ECHO SEQUENCE
• Most commonly used pulse sequence.
• The pulse sequence timing can be adjusted to give T1-
weighted, Proton or spin density, and T2-weighted images.
• Dual echo and multi echo sequences can be used to obtain
both proton density and T2-weighted images
simultaneously.
• The two variables of interest in spin echo sequences is the
repetition time (TR) and the echo time (TE).
• All spin echo sequences include a slice selective 90 degree
pulse followed by one or more 180 degree refocusing
pulses as shown in the diagram.

30. GRADIENT ECHO SEQUENCE
• Alternative technique to spin echo sequences
, differing from it in two principal points:
1. Utilization of gradient fields to generate
transverse magnetisation.
2. Flip angles of less than 90°.
• The flip angle is usually at or close to 90 degrees
for a spin echo sequence .
• Commonly varies over a range of about 10 to 80
degrees with gradient echo sequences.