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1 results

  • 38 - The Physics of MRI

  • from PART IV - MAGNETIC RESONANCE IMAGING
  • Edited by J. Christian Fox, University of California, Irvine
  • Book:
    Clinical Emergency Radiology
    Published online:
    07 December 2009
    Print publication:
    29 September 2008, pp 517-520

  • Summary

    All magnetic resonance imaging (MRI) instruments contain a homogeneous magnetic field that is required to establish longitudinal magnetization of the protons within it. Many instruments achieve this by using permanent magnets that directly create magnetic fields, which are oriented along an axis extending between the two poles of a magnet. The secret of MRI lies in nuclear magnetic resonance (NMR) technology first pioneered by Felix Bloch and Edward Purcell in 1946. The intensity of MRI signals traditionally correlates to three characteristics of the tissue being imaged: proton density, T1 relaxation time, and T2 relaxation time. Properties of the tissue's molecular environment directly influence the T1 and T2 relaxation values. The issue of using MRI in pregnancy often arises as well, particularly when the safety of CT is called into question due to the risk of exposing a growing fetus to ionizing radiation.