A coil is a large inductor with a considerable dimension and a defined wavelength, commonly used in configurations for MR imaging. The frequency of the radio frequency coil is defined by the Larmor relationship.
The MRI image quality depends on the signal to noise ratio (SNR) of the acquired signal from the patient. Several MR imaging coils are necessary to handle the diversity of applications. Large coils have a large measurement field, but low signal intensity and vice versa (see also coil diameter). The closer the coil to the object, the stronger the signal - the smaller the volume, the higher the SNR. SNR is very important in obtaining clear images of the human body. The shape of the coil depends on the image sampling. The best available homogeneity can be reached by choice of the appropriate coil type and correct coil positioning. Orientation is critical to the sensitivity of the RF coil and therefore the coil should be perpendicular to the static magnetic field.

RF coils can be differentiated by there function into three general categories:
The RF signal is in the range of 10 to 100 MHz. During a typical set of clinical image measurements, the entire frequency spectrum of interest is of the order 10 kHz, which is an extremely narrow band, considering that the center frequency is about 100 MHz. This allows the use of single-frequency matching techniques for coils because their inherent bandwidth always exceeds the image bandwidth. The multi turn solenoid, bird cage coil, single turn solenoid, and saddle coil are typically operated as the transmitter and receiver of RF energy. The surface and phased array coils are typically operated as a receive only coil.

See also the related poll result: '3rd party coils are better than the original manufacturer coils'

Resovist® is an organ-specific MRI contrast agent, used for the detection and characterization of especially small focal liver lesions.
Resovist® consists of superparamagnetic iron oxide (SPIO) nanoparticles coated with carboxydextran, which are accumulated by phagocytosis in cells of the reticuloendothelial system (RES) of the liver. The uptake of Resovist® Injection in the reticuloendothelial cells results in a decrease of the signal intensity of normal liver parenchyma on both T2- and T1 weighted images.
Most malignant liver tumors do not contain RES cells and therefore do not uptake the iron particles. The resulting imaging effect is an improved contrast between the tumor (bright) and the surrounding tissue (dark).
Resovist® can be injected as an intravenous bolus, which allows immediate imaging of the liver and reduces the overall examination time. A dynamic imaging strategy after bolus injection supports to characterize lesions. In comprehensive clinical trials, it demonstrated an excellent safety profile.
In 2001, Resovist® was approved for the European market.

See also Superparamagnetic Iron Oxide.

Resovist® competed with Primovist™, the other liver imaging agent of Bayer Schering Pharma AG. Due to this reason, the production of Resovist® has been abandoned in 2009.

Sinerem® is the brand name (same as Combidex®) for an ultrasmall superparamagnetic iron oxide (USPIO) to detect metastatic disease in lymph nodes. Metastatic nodes show less uptake of this MRI contrast agent, which results in less signal decrease and allows the differentiation of normal lymph nodes from normal-sized, metastatic nodes.
Lymph node imaging with Sinerem® is performed 24 to 36 hours after slow infusion. Normal lymph nodes turn black post contrast, namely on T2* weighted images. Metastatic lymph nodes remain unchanged in signal intensity.
Indication and Diseases: Cancer, Imaging for diagnosis, Lymphatic disorders.
See Ferumoxtran, and Classifications, Characteristics, etc.

Guerbet decided in 2007 to withdraw its Marketing Authorisation Application (MAA) for Sinerem.

(N) The SI units is moles/m³.
Definition: The concentration of nuclei in tissue processing at the Larmor frequency in a given region; one of the principal determinants of the strength of the NMR signal from the region.
For water, there are about 1.1 x 105 moles of hydrogen per m³, or 0.11 moles of hydrogen/cm³.
The signal intensity measured is related to the square of the xy-magnetization, which in a SE pulse sequence is given by
Mxy = Mxy0(1-exp(-TR/T1)) exp(-TE/T2)
where Mxy0 = Mz0 is proportional to the proton or spin density, and corresponds to the z-magnetization present at zero time of the experiment when it is tilted into the xy-plane.
True spin density is not imaged directly, but must be calculated from signals received with different interpulse times. The spin density contrast can be generated by using a long TR and sampling the data immediately after the RF pulse (with a TE as short as possible).

The spins flow with the blood through a slice and experience a RF pulse. If they flow out of the slice by the time the signal is recorded (because the repetition time (TR) is asynchronous with the pulsatile flow), the flowing blood produces intravascular signal void by 'time of flight' effects, turbulent dephasing and first echo dephasing. The liquid flow occasionally produces an intravascular high signal intensity due to flow related enhancement, even echo rephasing and diastolic pseudogating.

See also Flow Artifact and Flow Effects.