Religion And The Changes Through The Years

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Religion And The Changes Through The Years

Overview: Physics of Magnetic Resonance Microscopy
Magnetic resonance microscopy (MRM) is founded on the same physical principles as its clinical cousin, magnetic resonance imaging (MRI). Two crucial discoveries have made MRI possible. The 1952 Nobel Prize in Physics was awarded to Felix Bloch of Stanford and Edward M. Purcell of Harvard for their discovery of nuclear induction. Nuclei with unpaired nucleons (neutrons or protons) possess a magnetic moment arising from the angular momentum of these spinning nucleons. The interested reader can find a thorough quantum mechanical description in several excellent texts (e.g., A. Abragam, The Principles of Nuclear Magnetism (1978), P.T. Callaghan, Principles of Nuclear Magnetic Resonance Microscopy (1993)).
Classical Interpretation
A classical treatment of nuclear magnetic resonance is frequently used to give an intuitive understanding. Consider the unpaired protons of hydrogen in water. The proton is a charged particle with angular momentum. When a collection of these protons are placed in a strong magnetic field, the individual protons try to align with the external field. The angular momentum causes all of the protons to precess about the magnetic field much as the child's gyroscope precesses when placed on a pedestal. All the protons precess at a very explicit frequency, the Larmor frequency , given by the equation
where is a constant. Because the collection is precessing in synchrony at , the vector components parallel to the magnetic field B0 add to each other to generate a net magnetization M which also precesses at . Measuring the effect on a single proton would be very difficult because the magnitude is so small. Because M is the sum of many protons acting synchronously, it is large enough to measure. If an additional magnetic field B1 is applied at this same frequency, M can be forced away from the longitudinal (z) axis into the transverse plane. But once in the transverse plane, M continues to precess. As it does so, it will cause a time varying signal (at the Larmor frequency) in any loop of wire (antenna) through which M passes. This is the nuclear induction, which forms the basis for nuclear magnetic resonance.
Spatial Encoding for MR Microscopy
Spatial encoding for MR microscopy is founded on the same fundamental principle as MRI-the use of magnetic gradients to encode nuclear magnetic signals. In a typical two-dimensional study, a gradient applied al...

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