Component of the signal that yields in a symmetric, positive-valued line shape.

(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).

(RF) Radio frequency refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna.
The RF pulses used in MRI are commonly in the 1-100 megahertz range, and their effect upon a body is potential heating of tissues and foreign bodies, such as metallic implants, mainly at the surface.
This is a principal area of concern for MRI safety limits caused by absorption (see specific absorption rate) of the applied pulses of RF energy.

The higher the frequency, the larger will be the amount of heat developed.
The more ionic the biochemical environment in the tissue, the more energy that will be deposited as heat.
This effect is well known for homogeneous model systems, but the complex structure of various human tissues makes detailed theoretical calculations very difficult, if not impossible. By scanning problems, it is important to verify the transmission frequency. If the RF transmitted into the patient was, e.g. 5000 Hz lower than the resonance frequency of the protons, no protons was excited, and no signal returns.

(SAR) The Specific Absorption Rate is defined as the RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg).
The SAR describes the potential for heating of the patient's tissue due to the application of the RF energy necessary to produce the MR signal. Inhomogeneity of the RF field leads to a local exposure where most of the absorbed energy is applied to one body region rather than the entire person, leading to the concept of a local SAR. Hot spots may occur in the exposed tissue, to avoid or at least minimize effects of such theoretical complications, the frequency and the power of the radio frequency irradiation should be kept at the lowest possible level. Averaging over the whole body leads to the global SAR.
It increases with field strength, radio frequency power and duty cycle, transmitter-coil type and body size. The doubling of the field strength from 1.5 Tesla (1.5T) to 3 Tesla (3T) leads to a quadrupling of SAR. In high and ultrahigh fields, some of the multiple echo, multiple-slice pulse sequences may create a higher SAR than recommended by the agencies. SAR can be reduced by lower flip angle and longer repetition times, which could potentially affect image contrast.
Normally no threatening increase in temperature could be shown. Even in high magnetic fields, the local temperature increases not more than 1°C. 2.1°C is the highest measured increase in skin temperature. Eddy currents may heat up implants and thus may cause local heating.

FDA SAR limits:

IEC (International Electrotechnical Commission) SAR limits of some European countries:
All limits are averaged over 6 minutes. (For more details: IEC 60601-2-33 (2002))

In most countries standard MRI systems are limited to a maximum SAR of 4 W/kg, so most scanning in level II is impossible.
For Level I, in addition to routine monitoring, particular caution must be exercised for patients who are sensitive to temperature increases or to RF energy.
For Japan different SAR limits are valid.

(T/R) Also called transceiver coil. An RF coil that acts as a transmitter (T) producing the B1 excitation field and as a receiver (R) of the MRI signal. Such a coil requires a T/R switching circuit to switch between the two modes. A body coil is typically a T/R coil, but smaller volume T/R coils (head/extremities) are often used at high field as a possibility of reducing RF power absorption (SAR).