In an MRI or Magnetic Resonance Imaging procedure, the presence of hydrogen atoms is detected by subjecting them to a strong magnetic field that aligns the spin of the protons (i.e. the direction of their intrinsic magnetic moments) along the direction of the external field. These protons are then excited into a higher energy state by applying a radiofrequency source at a specifically tuned frequency. When the magnetic field is turned off, radiation is given off by the excited protons that can be picked up in sensitive detectors. For MRI scans, an cross-sectional image of tissue can be made by producing a well-calibrated magnetic field gradient across the extent of the tissue so that the magnetic field strength can be associated with a given location in the tissue. Since the proton signal frequency is proportional to the magnetic field, a given proton signal frequency can be assigned to a location in the tissue using a mathematical mapping procedure known as a Fourier transform. This provides the three-dimensional map of the tissue. The contrast between different types of tissue is revealed as the proton density is different in different types of tissue. What results from these scans are detailed images of what lies beneath our skin, yet without a single incision made or drop of blood shed.
When my daughter was young, I used to attempt to explain to her (when she asked) how various physical phenomena occurred. Sometimes she would be quite disappointed to understand how nature worked, almost as if I were ruining the magic or pulling back the veil to reveal the wizard. However for me, even knowing a bit how things work, never takes away my amazement and wonder at the practical applications mankind has been able to develop based on initially esoteric observations and studies.
