What causes hydrogen atoms to line up during an MRI?

In an MRI, hydrogen atoms in the body align due to the strong magnetic field generated by the MRI machine. This magnetic field causes the protons in hydrogen atoms, which are abundant in water and fat tissues, to align with the magnetic field direction. When a radiofrequency pulse is applied, these protons are temporarily knocked out of alignment. As they return to their original alignment, they emit signals that are detected and used to create images of the body's internal structures.