The concentration of Hydrogen atoms in water molecules or in some groups of fat molecules within tissue. Initial MR signal amplitudes are directly related to H+ density in the tissue being imaged.

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 definition of a scan is to form an image or an electronic representation. The MRI scan uses magnetic resonance principles to produce extremely detailed pictures of the body tissue without the need for X-ray exposure or other damaging forms of radiation.
MRI scans show structures of the different tissues in the body. The tissue that has the least hydrogen atoms (e.g., bones) appears dark, while the tissue with many hydrogen atoms (e.g., fat) looks bright. The MRI pictures of the brain show details and abnormal structures (brain MRI), for example, tumors, multiple sclerosis lesions, bleedings, or brain tissue that has suffered lack of oxygen after a stroke. A cardiac MRI scan demonstrates the heart as well as blood vessels (cardiovascular imaging) and is used to detect heart defects with e.g., changes in the thickness and infarctions of the muscles around the heart. With MRI scans, nearly all kind of body parts can be tested, for example the joints like knee and shoulder, lumbar, thoracic and cervical spine, the pelvis including fetal MRI, and the soft parts of the body such as the liver, kidneys, and spleen. The MRI procedure includes three to nine imaging sequences and may take up to one hour.

See also Lumbar Spine MRI, MRI Safety and Open MRI.

(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).

Hydrogen nuclei magnetic moments are randomly oriented in the absence of an external magnetic field and are considered to have a net magnetization of zero. 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, which is commonly referred to as the vector B0. 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, this difference is all that counts. The magnetic moments of these protons are added together and are referred to as net magnetization vector (NMV) or the symbol 'M'.

See also Magnetization Transfer Contrast.