Which among the following elements present in the human body forms the main…

Concept

Magnetic resonance imaging is an application of nuclear magnetic resonance. A nucleus that contains an odd number of protons or neutrons has a non-zero nuclear spin, so it behaves like a tiny bar magnet. When such a nucleus is placed in a strong static magnetic field, it precesses about that field at the Larmor frequency f = γB, where γ is the gyromagnetic ratio of that particular nucleus and B is the strength of the field.

A radio-frequency pulse tuned to that exact frequency is absorbed by these nuclei, and the energy they re-radiate while relaxing is the imaging signal. How strong that signal is from a tissue depends chiefly on how many such nuclei that tissue contains and on how sensitive the nucleus itself is to nuclear magnetic resonance, a sensitivity that grows with its gyromagnetic ratio; the field strength, the pulse sequence and the relaxation behaviour of the tissue shape it as well. The element on which a scanner is built is therefore the one that is both very abundant in the body and carries a spin-bearing nucleus of high intrinsic resonance sensitivity.

Applying it to the human body

1. About 60 to 70 percent of the human body is water, and the remaining tissue contains a large amount of fat; both are rich in hydrogen. Hydrogen accounts for roughly 10 percent of body mass and about 63 percent of all the atoms in the body.

2. The hydrogen-1 nucleus is a single proton with nuclear spin 1/2 and a gyromagnetic ratio of about 42.58 MHz per tesla, the largest among stable nuclei. In a 1.5 tesla clinical scanner it therefore precesses at about 63.9 MHz.

3. Hydrogen combines exceptionally high abundance in living tissue with the high intrinsic resonance sensitivity that comes from its large gyromagnetic ratio, so it yields by far the strongest signal, and clinical MRI is essentially proton imaging. Tissue contrast is produced by the different T1 and T2 relaxation times of these water and fat protons in different tissues.

Comparing the candidate nuclei

Hence the working of MRI in the human body rests on hydrogen, and specifically on the protons of hydrogen nuclei present in tissue water and fat. Sodium imaging does exist, but it needs special hardware and remains a specialist research technique rather than the basis of a routine scan.