For many years, magnetic resonance techniques aided chemical analysis in the food and petrochemical industries. The development of large-bore homogeneous magnets and computer assisted imaging (as in CT scanning) extended its use to the mapping of hydrogen nuclei (i.e. water) densities and their effect on surrounding molecules in vivo. Since these vary from tissue to tissue, MRI can provide a detailed image of both head and body structures.
When a substance is placed in a magnetic field, spinning protons within the nuclei act like small magnets and align themselves within the field.
A superimposed electromagnetic pulse (radiowave) at a specific frequency displaces the hydrogen protons.
The transverse component of the magnetisation vector generates the MRI signal.
Consider the effect on:
(a) individual protons
The T1 component (or spin-lattice relaxation) depends on the time taken for the protons to realign themselves with the magnetic field and reflects the way the protons interact with the 'lattice' of surrounding molecules and their return to thermal equilibrium.
The T2 compound (spin-spin relaxation) is the time taken for the protons to return to their original 'out of phase' state and depends on the locally 'energised' protons and their return to electromagnetic equilibrium.
Protons aligned but Protons spin in phase spinning out of phase (i.e. 'resonate')
A variety of different radiofrequency pulse sequences (saturation recovery (SR), inversion recovery (IR) and spin echo (SE) combined with computerised imaging produce an image of either proton density or of T1 or T2 weighting depending on the sequence employed.
Normal MRI images (T1/T2 weighting in relation to normal grey/white matter) axial views - head Sagittal view - head
Cervicodorsal spine (sagittal view)
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