Net Magnetization

Net magnetization (M) of the hospital equipment called the nuclear magnetic resonance scanner when at rest is zero. In body tissues, or any specimen for that matter, before the application of a magnetic field, the magnetic moments of the nuclei making up the tissue are ran¬domly aligned, and have zero net magnetization. When an external fixed magnetic field is applied, after an interval, the individual magnetic moments align parallel or antipar¬allel with the applied net magnetic field, B. There is a slight prepon¬derance of nuclei aligned parallel with the magnetic field, and this gives the tissue a net magnetization (M). As an example for H, which has a large magnetic moment in a field of 14 kilogauss (KG), the fractional excess of parallel protons is only about 1 x 10-5 at room temperature. Nevertheless, as small as this slight excess is, it accounts for the small macroscopic net magnetic moment directed parallel with the external magnetic field, and it is this differential that accounts for the nuclear magnetic resonance signal on which the imaging is based.

Another important difference between the parallel and antiparallel protons is that the antiparallel protons are at a slightly higher energy level than the parallel protons. One way to conceptualize this differ¬ence in energy states is to consider the different levels of energy expended by two swimmers both tied to ropes and attempting to swim toward each other with one swimming downstream (with the static magnetic field) and the other expending more energy while swimming upstream (against the static magnetic field).

In NMR imaging, the energy differential between the parallel and antiparallel protons is directly proportional to external field strength of hospital lab equipment. At this point the important thing to note about these different energy levels is that the energy differential approximates the thermal energy exchanged between colliding molecules and thus is quite small, i.e., on the order of a few millicalories. At any given field strength, the two energy levels resonate. The energy exchange is at a very specific level, and the higher the magnetic field strength is, the greater the difference between the energies of the parallel and antiparallel protons.

Magnetic Wobble

The magnetic moment of the ensemble of protons in tissue is in stable alignment with the external static magnetic field. In actuality each one of these magnetic mo¬ments wobbles or rotates around the alignment of the static magnetic field in a process that is called precession. By way of analogy, the interaction between the proton angular magnetic moment and the external magnetic field, and the spinning mass of a top and the earth’s gravitation field are similar interactions, with the exception that the competing forces acting on a wobbling top are the earth’s static gravitational field trying to push the top down versus the top’s angular momentum attempting to keep the top upright. Thus, in a friction-free system, tops spinning on a flat surface possess a res¬onant precession or wobble about the direction of the local gravita¬tional field. In a similar manner, a spinning nucleus also processes or wobbles about an applied magnetic field with a resonant angular frequency, determined by a constant (the magnetogyric ratio) and the strength of the magnetic field. Each nuclear species possesses characteristic value for magnetogyric ratio but frequency and strength are related by the equation. The important relationship in this equation is that the an¬gular frequency for any nuclear species is characteristic and directly proportional to the static magnetic field. Thus, there is an interrela¬tionship between frequency and proton energy, such that both resonant frequency and proton energy are directly proportional to the magnetic field.