What do you know about fundamental MRI? Discuss how it is beneficial in ENT practices.Write down the basic principles of fundamental MRI? Mention the application in Otolaryngology?

  • f-MRI is one of the most versatile tool in cognitive neurosciences.
  • The grey matter is active during sensory, motor and cognitive activity . It can be defined based on the accompanying local increase in blood flow and oxygenation.
  • f-MRI is very sensitive to increase in the local blood flow during neuronal activities.
  • These are two types of f-MRI:
    1. Per fusion MRI: It measure the change in blood flow.
    2. Blood-MRI : Due to the higher ratio of the oxygenated hemoglobin to deoxygenated haemoglobin in local capillaries, the brain activity, there arisen an image control.
  • f-MRI has higher spatial resolution (˜1-10mm) but the temporal resolution are slower (several seconds) than the neuronal response (ms).

Basic principle of f-MRI

  • The radio frequency pulse excites the nuclear spins in the tissue.The excited nuclear spin when returning to low energy state(relax) excites an energy. The frequency of the excited energy is made to vary spatially which is detected by MRI.
  • The local environment for the water molecules vary across a heterogeneous tissue such as brain and thus the relaxation rate also varies.
  • The MRI becomes more sensitive to this variation in the relaxation rate if we vary the inter pulse delay or ‘TR’ in the pulse sequence. “T1 weighted” clinical scan generates image contract using this principle.
  • T1 weighted-
    • differentiates grey matter from the white matter.
    • Does not distinguish active brain from resting brain.
    • Infection of gadolinium DIPA and scanning may give some functional information.
  • There are huge numbers of spins that are observed simultaneously tissue. Each of the nuclei has different relation to other molecules and experiences a slightly different varying local magnetic field.  These shifting fields allows an exchange of energy between the nuclei spins which leads to the faster loss of the net signal from all of the nuclei together. This is known as spin-spinner or T2 relation time T2 weight MRI uses this  intrinsic properly of nuclei(T2) in particular chemical environment to generate the image contrast.
    • It differentiates the water content between different region of the brain.
  • The rate of decay of signal in faster if there are local magnetic field variation that the molecule can diffuse though over the time course of a single TE.
  • The resonance frequencies change as molecules move through different magnetic field which lower the coherence of the nuclear spins and the lead to more rapid decay of net signal.
    • In the presence of local magnetic field inhomogeneities the rate of signal decay is expressed by the T2 relaxation time.If the local magnetic field change rapidly e.g. Tissue adjacent to blood vessels filled c  para-magnetic deoxyhaemoglolan the T2 <T2. Thus determine the dramatic signal control provided by blood breakdown produce in tissue for clinical MRI.It is also the mechanism underlying generation of BOLD f-MRI images.

    Application in ENT:

    1. Imaging normal auditory system:
      • Information about organization about human auditory cortex i.e topological representation in 10 auditory cortex
      • For auditory language processing, left hemisphere dominance has been recognized.
    2. Defining functional relation between different regions of brain. e.g. localization of specific processing region
    3. Imaging plasticity: Short term plasticity or learning has become particularly fruitful of MRI research.
    4. Imaging drug response: Attractive for studying effect of drugs.
    5. For establishing diagnosis and prognostic and monitoring treatment.
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