A "microscopic" introduction

The nuclei of many elements have a property called spin magnetic moment. In simple terms, this means that they behave like many tiny compass needles. In the presence of an external magnetic field they tend to align with the direction of the field.

I said that they are "similar" to small compasses. Not identical. The differences are these

1. The needle of a compass can point in any direction, while the microscopic compass of nuclei can only point in a discrete set of directions (this phenomenon is common to all quantities of the microscopic world, and is called quantization). In the simplest case (called spin 1/2) only two.
2. At room temperature a compass aligns as the Earth's magnetic field dictates, while a much stronger field is needed to align nuclear magnetic moments, which at room temperature point in a random direction among those allowed to them. Without the external field, indeed, one direction is as good as another.

How it works

• For simplicity, let us consider the nuclei of H (extremely abundant in all organic tissues) or of ¹³C. They have "spin 1/2", that is, for these nuclei the "mini-compasses" can point in only two directions.
• To fix ideas, let us call these directions up and down.

• If we apply a very strong magnetic field in the up direction, a majority of the "mini-compasses" on the nuclei will align with the field, so as to take on a more stable configuration in terms of energy.
• Some of the nuclei will always remain pointing down, because of thermal agitation, which moves them randomly.
• The field, however, has had the effect of unbalancing the population in favour of the up direction.
• Using quantum mechanics, one can calculate the energy difference between the nuclei oriented up and those oriented down. This energy is, of course, larger the stronger the magnetic field but, with the fields that can be obtained in the laboratory, it corresponds to the energy of radio-frequency electromagnetic waves.

• In summary: by sending an electromagnetic wave at the right frequency (that is, in resonance with the energy difference), we manage to move some "mini-compasses" from up to down.
• In other words, counting the difference between the number of up nuclei and that of down nuclei, we will obtain no effect with waves off resonance, while we will observe a sudden decrease if we use the right frequency. This frequency is called the Larmor precession frequency, after its discoverer.

Applications

Nuclear magnetic resonance is a powerful technique that makes it possible to investigate the chemical composition of materials, and to obtain detailed images of the soft parts of the human body (images that cannot be obtained with radiography).

Starting from the principles set out above, a few more processing steps must be taken to arrive at a complete image.