Saxena, now an assistant professor of chemistry at the University of Pittsburgh, remarks that while using two distinct pieces of apparatus "might seem counterintuitive at first glance, in many circumstances the use of one set-up in NMR leads to an uneasy balance between effective signal encoding and sensitive signal detection. By separating the two, not only can the signal fidelity be vastly improved, but also many new schemes that use much more powerful and sensitive detection and coding methods and principles can be envisaged."
The basic steps:
NMR and MRI work because many atomic nuclei have magnetic moments, acting like toy bar magnets with north and south poles. In a magnetic field these spinning nuclei orient themselves along the field lines, with spins up or down. Slightly more energy is required to maintain the spin-down state.
In the encoding phase, a radiofrequency (RF) pulse matched to the energy difference between the two states knocks the target nuclei atilt, causing their spin axes to precess around the field lines like off-center toy gyroscopes. The exact precession rate is characteristic of each chemical species ?? ubiquitous hydrogen is the most commonly used species in NMR and MRI -- and is also affected by the chemical and physical surroundings.
For MRI, an extra step encodes additional data. In addition to the uniform magnetic field, additional magnets are turned on briefly to superimpose fields that are stronger in one direction than the other. When the target nuclei are subjected to RF pulses, the differences in field strength are reflected in changed angles and speeds of precession. Together with the timing of the pulses, the gradient fields give each spinning nucleus a unique set of coordinates corresponding to its position.