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Thread: Which factors can influence the signal intensity in MRI ?

  1. #1 Which factors can influence the signal intensity in MRI ? 
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    actually i know factors,but i dont know how they affect MRI.

    Pulse sequence
    magnet strength
    photon density
    tissue type (t1 and t2)
    voxel size




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    Forum Professor wallaby's Avatar
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    I'm going to preface this by saying that i'm not 100% confident in my answers, although i am currently studying Nuclear Magnetic Resonance so hopefully i won't be too far off base.

    The pulse sequence affects how long the equilibrium magnetization is restored in a sample, this in turn influences the signal decay. The time taken for the magnetization to return to equilibrium can be calculated for a given equilibrium magnetization, so deviations from this time will be the result of environmental factors.

    Tissue type, the environment in which a molecule is placed affects the rate at which the magnetization of the sample returns to equilibrium, another way to aid identification. The rate at which the signal decays will be characteristic of the tissue type, so measurement of the characteristic time for different samples will aid in identification of the sample.

    The strength of the magnetic field affects the calibration of the equipment, specifically it affects the larmor frequency. (stronger fields = larger larmor frequency) Faster precession of the magnetization about the magnetic field results in larger magnetic flux changes in the receiver coil and a stronger signal is obtained. (Typically these magnetic fields are just shy of 10T) From what i read on wiki, MRI makes use of gradients in the magnetic field to observe the differences in the rate of precession of molecules in three dimensions. Large differences at a specific location will indicate a sudden change in environment, say between the inside and the outside of an artery.

    No idea about photon density or voxel size.


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