(FID) A free induction decay curve is generated as excited nuclei relax. The amplitude of the FID signal becomes smaller over time as net magnetization returns to equilibrium. If transverse magnetization of the spins is produced, e.g. by a 90° pulse, a transient MR signal will result that will decay toward zero with a characteristic time constant T2 (or T2*); this decaying signal is the free induction decay.
The signal peaks of the echoes fall onto this T2 decay curve, while at each echo the signals arise and decay with T2*. The typical T2 relaxation times being of the order of 5-200 ms in the human body. The first part of the FID is not observable (named the 'receiver dead time') caused by residual effects of the powerful exciting radio frequency pulse on the electronics of the receiver.

(FLAIR) Fluid attenuation inversion recovery is a special inversion recovery sequence with long TI to remove the effects of fluid from the resulting images. The TI time of the FLAIR pulse sequence is adjusted to the relaxation time of the component that should be suppressed. For fluid suppression the inversion time (long TI) is set to the zero crossing point of fluid, resulting in the signal being 'erased'.
Lesions that are normally covered by bright fluid signals using conventional T2 contrast are made visible by the dark fluid technique FLAIR is an important technique for the differentiation of brain and spine lesions.

See also Inversion Recovery.

(Gd) Gadolinium is a Lanthanide element that is paramagnetic in its trivalent state.
This paramagnetic substance is used for MR imaging because of the effect of strongly decreasing the T1 relaxation times of the tissues to which gadolinium has access. When injected during magnetic resonance imaging, gadolinium will tend to change signal intensities by shortening the T1 time in its surroundings.
The relaxivity of gadolinium is an important measure of its efficacy, which is dependent on the chemical properties of the complex. The gadolinium ion cannot be used in its chloride, sulfate, or acetate forms because of poor tolerance and low solubility in water in the neutral pH range. Although toxic by itself, gadolinium can be given safely in a chelated form such as DTPA, that still retains much of its strong effect on relaxation times (relaxivity).

See also Dotarem®, Gadovist®, MultiHance®, Omniscan®, OptiMARK®, and Contrast Agents, the info sheet gives an overview and more in-dept information about different types of MRI contrast agents.

(IRSE) Form of inversion recovery imaging in which the signal is detected as a spin echo. For TE short compared to the T2 relaxation time, there will be only a small effect of T2 differences on image intensities; for longer TE's, the effect of T2 may be significant.

(Mn-DPDP) This agent, mangafodipir trisodium, is a hepatocyte specific MRI contrast agent. Manganese is very toxic, so it has to be chelated and put in the form of a vitamin B6 analog, which is taken up by normal hepatocytes to some extent.
Teslascan® was developed in the early 1980's, went through clinical trials in the early 1990's, and was approved in 1997. One problem with assessing the efficacy of this agent is the fact that the phase III trials finished in the early 1990's, and the techniques used for MR today are very different from the techniques used almost a decade ago.
This contrast agent shortens the T1 relaxation time. On T1 weighted pictures it makes a normal liver look brighter. Since metastases, for example, do not generally take up this agent, the contrast between the enhancing liver and the non-enhancing lesions will increase on T1 weighted pictures. It does not have much effect on T2 weighted images.