Date post: | 21-Feb-2017 |
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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.