Chapter 43, Problem 43.1DQ

To determine

Why protons are used in MRI of body tissues rather than neutrons.

Answer to Problem 43.1DQ

Unlike free protons ( ${}_1^1\text{H}$) which are present in organic molecules and water, free neutrons are not present in the body tissue and hence neutrons are not used in MRI even though they have magnetic dipole moment and undergo spin flip upon electromagnetic radiation absorption.

Explanation of Solution

The predominant components in any living body are organic molecules and water. The major content of these chemical entities are the hydrogen atoms or in other words the protons. Thus, the free proton content is huge enough to make proper response towards magnetic field and hence used in the MRI scanning.

However, in the case of neutrons, even though they have magnetic dipole moment and can undergo spin flip by absorbing electromagnetic radiation, they cannot be used in MRI. This is because there are virtually no free neutrons in the body and hence there will not be any proper response that can be expected due to neutrons, when a body tissue is subjected to the magnetic field of MRI.

Conclusion:

Therefore, unlike free protons ( ${}_1^1\text{H}$) which are present in organic molecules and water, free neutrons are not present in the body tissue and hence neutrons are not used in MRI even though they have magnetic dipole moment and undergo spin flip upon electromagnetic radiation absorption.