The subject is largely consist of antiferromagnetic. Every antiferromagnetic has two hydrogen nuclei or protons. When a people enter into the powerful magnetic field of the scanner, the magnetic moments of some of these protons changes, and aligns with the field direction.

A radio frequencys transmitter is opened in brief, generating an electromagnetic field in an MRI machine. The photons of this place have just the right energy, known as the resonance frequency, to flip the spin of the aligned protons in the body. As the intensity and duration of application of the field enhancment, more aligned spins are affected. After the field is closed, the protons decay to the original spin-down state and the difference in energy between the two states is released as a photon. These photons, which produce the electromagnetic signal that the scanner detects. The frequency the protons resonate at depends on the magnetic field strength. As a result of energy conservation, this also command of the released photons frequency. The photons released when the field is removed have an energy, therefore a frequency, due to the amount of energy the protons absorbed when the field was active.
This relationship between field-strength and frequency, which allows nuclear magnetic resonance use for imaging. Extra magnetic fields are uesd during the scan to make the magnetic field strength rely on the position within the patient.

Position information can be renewed from the resulting signal by t a fourier transform use. These fields are created by passing electric currents by specially-wound solenoids, known as gradient coils. Since these coils are in the bore of the scanner, there are large forces between them and the main field coils, producing most of the noise that is heard during operation. Without efforts to dampen this noise, it can approach 130 decibels (dB) with strong fields.
An image can be structureted because the protons in different tissues return to their equilibrium state at different rates, which is a difference that can be detected. Five different tissue variables — spin density, T1 and T2 relaxation times and flow and spectral shifts can be used to construct images. By changing the parameters on the scanner, this effect is used to create contrast between different types of body tissue or between other properties, as in fMRI and diffusion MRI.
Contrast agents may be injected intravenously to improve the blood vessels appearance, tumors or inflammation. Contrast agents may also be directly injected into a joint in the case of arthrograms, MRI images of joints. Unlike Computed Tomography, MRI uses no ionizing radiation and is generally a very safe procedure. Nonetheless the strong magnetic fields and radio pulses can effect metal implants, including cochlear implants and cardiac pacemakers In the case of cardiac pacemakers, the results can sometimes be lethal, so patients with such implants are generally not eligible for MRI.
MRI is used to image every part of the body, and is particularly useful for tissues with many hydrogen nuclei and little density contrast, such as the brain, muscle, connective tissue and most tumors.

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