Nuclear magnetic resonance (NMR) tomography has emerged as a powerful imaging technique in the medical field because of its high resolution capability and potential for chemical specific imaging. Although similar to the X-ray computerized tomography (CT), it uses magnetic fields and radio frequency signals to obtain anatomical information about the human body as cross-sectional images in any desired direction and can easily discriminate between healthy and diseased tissue.
2. Magnetic resonance imaging (MRI),
also known as nuclear magnetic
resonance imaging, is a technique for
creating detailed images of the human
body.
3. It uses magnetic fields and radio
frequency signals to obtain anatomical
information about the human body as
cross-sectional images in any desired
direction and can easily discriminate
between healthy and diseased tissue.
NMR images are essentially a map of
the distribution density of hydrogen
nuclei and parameters reflecting their
motion, in cellular water and lipids.
6. Basic Principle of NMR Imaging
System:
Fig a. Helium atom
Fig .b
Nuclear spin!!! the manipulation of
nuclear spin is the basis for MRI imaging.
7. If one places an atom within a magnetic field plane, i.e,
subject it to magnetic forces along two of the three
dimensions, then the nucleus will orbit around the third
(vertical) axis. This is called precession and is depicted in the
animation below:
8. The spin of nucleus is just like ends of bar magnet
in that it can have a positive or negative value.
All the negative spin atoms align themselves
downward on the Z axis (towards the feet of the
subject)
All the positive atoms align upward on the Z axis
(towards the subject’s head)
Each atom with a positive spin cancels out an
atom with a negative spin. There remain,
however, a few atoms that do not cancel one
another out. At room temperature, there are
always more positive spin atoms than negative
spin atoms. These unmatched atoms are the
important ones for MRI and FMRI.
9. The atoms achieve and equilibrium magnetization
value along the direction of the magnetic field,
i.e., the Z axis.
“spin lattice relaxation time” or (T1)
The amount of time it takes for the atoms to
return to their equilibrium value is called the
“spin lattice relaxation time” or (T1).
• “spin-spin relaxation time” or T2:
The amount of time it takes for the atoms to
return to their equilibrium magnetization value
along XY axis (transverse axis) is called the
“spin-spin relaxation time” or T2.
10. Advantages of MRI
1)CT provides details about the bone and tissue
structure of an organ whereas NMR highlights
the liquid-like areas on those organs and can
also be used to detect flowing liquids, like
blood.
2) A conventional X-ray scanner can produce an
image only at right angles to the axis of the
body, whereas the NMR scanner can produce
any desired cross-section.
