3. FELIX BLOCH & EDWARD PURCELL
DISCOVERED MAGNETIC RESONANCE PHENOMENON IN 1947.
RECEIVED NOBEL PRIZE IN 1952.
USED FOR CHEMICAL AND PHYSICAL MOLECULE ANALYSIS.
RAYMOND DAMADIAN IN 1971 SHOWED MAGNETIC RELAXATION OF
TUMOR AND NORMAL TISSUE DIFFERED.
4. 92 ELEMENTS OCCUR NATURALLY ON EARTH
HUMAN BODY IS BUILT OF ONLY 26 ELEMENTS
OXYGEN,CARBON,NITROGEN,HYDROGEN ELEMENT CONSTITUTE
96% OF HUMAN BODY MASS.
OXYGEN IS 65% OF BODY MASS; CARBON IS 18.5%, HYDROGEN
9.5%, NITROGEN 3.2%.
LET US IGNORE ALL ELEMENTS BUT HYDROGEN.
5.
6.
7. The additional spin or wobble of magnetic moment of hydrogen
produce by the influence of Bo causes the spinning of proton in a
regular manner called “precession”.
This cause the magnetic moment to follow a circular path around Bo. i.e.
precessional path.
The speed at which they wobble around is the precessional frequency.
8. Wo=Bo.μ
The gyromagnetic ratio (μ) is a constant of proportionality expressed as the
precessional frequency of hydrogen at 1TESLA(T) and has the value 42.75MHz/T .
thus at 1.5T f is 42.57x1.5= 63.86MHz at 1.0T f is
42.57x1=42.57MHz and
at 0.5T f is 42.57x0.5= 21.28MHz
9. Resonance is an energy transition occurs when an object is subjected to a
frequency same as its own.
Induced:
by applying RF pulse at same frequency as precessing magnetic moment
hydrogen spins,
At 90° to the magnetic field and NMV.
Result of resonance:
1) phase coherence
2) energy absorption
10.
11.
12. As a result of resonance the NMV is precessing in phase in transverse plane.
Induction of voltage in receiver coil constitutes the MR signal with larmor frequency
which is in accordance to the law of electromagnetism.
When the RF excitation pulse is removed, nmv realign with bo.
This leads to relaxation process.
The induction of reduced signal is free induction decay(fid).
13.
14.
15. Contrast between tissues occurs because there is a different signal intensity between
different tissues.
Signal intensity depend on amplitude of signal.
This is controlled by two parameters:
1) Extrinsic contrast parameter
2) Intrinsic contrast parameter
Here, t1 recovery,t2 decay and pd mechanism mainly contribute contrast in images.
16. Proton density(PD) is the number of mobile hydrogen protons per unit volume of that tissue.
T1 and T2 relaxation depends on three factors:
1) Inherent energy of tissue
2) How closely the molecules are packed
3) How well molecular tumbling rate matches the larmor frequency of hydrogen.
composition of fat and water in regard to recovery and decay is important phenomena for t1 & t2
images.
17. T1 RECOVERY
Caused by spin lattice energy transfer.
It is the time it takes for 63% of longitudinal magnetization to
recover in that tissue.
T1 recovery in fat: short
T1 recovery in water: long
T1 contrast is controlled by TR.
For good T1 contrast TR must be short.
T2 DECAY
Caused by spin-spin energy transfer.
It is the time it takes for 63% of transverse magnetization to
decay due to dephasing.
T2 decay in fat: short
T2 decay in water: long
T2 contrast is controlled by TE.
For good T2 contrast TE must be long.
18. Proton density contrast refers to difference in signal intensity between tissue.
Tissue with high PD are bright on a PD contrast images.
Tissue with low PD are dark on a PD contrast images.
19.
20.
21. Here contrast depends mainly on the difference between
T1 times of fat and water and all tissues with
intermediate signal.
The TR applied should be short so that neither fat nor
water totally recovers.
22. Here contrast depends mainly on the
difference in the T2 time of fat and water.
The TE applied is long so that both fat and
water get time to decay.
23. In a PD weighted image, differences in proton densities are
demonstrated.
To achieve this a long TR combined with a short TE should be
selected.
By this T1 and T2 effects are diminished and thus PD contrast
predominates.
24. It is the decay of signal strength followed by RF pulse.
Two factors are responsible for this effect:
1) spin-spin relaxation
2) magnetic field inhomogeneity;
i)macroscopic inhomogeneity
ii) microscopic inhomogeneity
It is faster than T2 decay.
