MRI PHYSICS- PART I

Dr.Archana Koshy

• 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

• 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.

• 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

PROTONS IN A MAGNETIC FIELD

Bo

Parallel(low energy)

Anti-Parallel(high energy)

Spinning protons in a magnetic field will assume two states.

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.

• 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.

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

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.

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 .

TRANSVERSE MAGNETISATION

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 .

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

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.

Longitudinal relaxation

TRANSVERSE RELAXATION

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 .

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 .

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.

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.