Ability to do work, measured in joules (J).

The initial energy level of a spinning proton is depended of thermodynamics conditions (see Boltzmann distribution). In a magnetic field, each spin can exist in one of a number of distinct states having different energy levels. Each level is given a magnetic quantum number, m.

(MRI) Magnetic resonance imaging is a noninvasive medical imaging technique that uses the interaction between radio frequency pulses, a strong magnetic field and body tissue to obtain images of slices/planes from inside the body. These magnets generate fields from approx. 2000 times up to 30000 times stronger than that of the Earth. The use of nuclear magnetic resonance principles produces extremely detailed pictures of the body tissue without the need for x-ray exposure and gives diagnostic information of various organs.
Measured are mobile hydrogen nuclei (protons are the hydrogen atoms of water, the 'H' in H20), the majority of elements in the body. Only a small part of them contribute to the measured signal, caused by their different alignment in the magnetic field. Protons are capable of absorbing energy if exposed to short radio wave pulses (electromagnetic energy) at their resonance frequency. After the absorption of this energy, the nuclei release this energy so that they return to their initial state of equilibrium.
This transmission of energy by the nuclei as they return to their initial state is what is observed as the MRI signal. The subtle differing characteristic of that signal from different tissues combined with complex mathematical formulas analyzed on modern computers is what enables MRI imaging to distinguish between various organs. Any imaging plane, or slice, can be projected, and then stored or printed.
The measured signal intensity depends jointly on the spin density and the relaxation times (T1 time and T2 time), with their relative importance depending on the particular imaging technique and choice of interpulse times. Any motion such as blood flow, respiration, etc. also affects the image brightness.
Magnetic resonance imaging is particularly sensitive in assessing anatomical structures, organs and soft tissues for the detection and diagnosis of a broad range of pathological conditions. MRI pictures can provide contrast between benign and pathological tissues and may be used to stage cancers as well as to evaluate the response to treatment of malignancies. The need for biopsy or exploratory surgery can be eliminated in some cases, and can result in earlier diagnosis of many diseases.

See also MRI History and Functional Magnetic Resonance Imaging (fMRI).

Once hydrogen protons are placed in the presence of an external magnetic field, they align themselves in one of two directions, parallel or anti parallel to the net magnetic field.
The strength of the external magnetic field and the thermal energy of the atoms are the factors, which affect the direction of alignment of the hydrogen protons. The high-energy protons are strong enough to align themselves against or anti parallel to the magnetic field, whereas the lower energy protons will align themselves with or parallel to the magnetic field.
As the magnetic field increases, there are fewer protons, which are strong enough to align anti parallel to the magnetic field. There are always a larger number of protons aligned parallel with the magnetic field, so once the parallel and anti parallel protons cancel each other out, only the small number of low energy protons left aligned with the magnetic field create the overall net magnetization of the patient's body.

The relaxation effect is the transition of an atom or molecule from a higher energy level to a lower one. The return of the excited proton from the high energy to the low energy level is associated with the loss of energy to the surrounding tissue. The T1 and T2 relaxation times define the way that the protons return to their resting levels after the initial radio frequency (RF) pulse. The T1 and T2 relaxation rates have an effect of the signal to noise ratio (SNR) of MR images.
The relaxation process is a result of both T1 and T2, and can be controlled by the dependency of one of the two biological parameters T1 and T2 in the recorded signal. A T1 weighted spin echo sequence is based on a short repetition time (TR) and a change of it will affect the acquisition time and the T1 weighting of the image. Increased TR results in improved SNR caused by longer recovering time for the longitudinal magnetization. Increased TE improves the T2 weighting, combined with a long TR (of several T1 times) to minimize the T1 effect.