Coherent gradient echo sequences can measure the free induction decay (FID), generated just after each excitation pulse or the echo formed prior to the next pulse. Coherent gradient echo sequences are very sensitive to magnetic field inhomogeneity. An alternative to spoiling is to incorporate residual transverse magnetization directly into the longitudinal steady state. These GRE sequences use a refocusing gradient in the phase encoding direction during the end module to maximize remaining transverse (xy) magnetization at the time when the next excitation is due, while the other two gradients are, in any case, balanced.
When the next excitation pulse is sent into the system with an opposed phase, it tilts the magnetization in the -a direction. As a result the z-magnetization is again partly tilted into the xy-plane, while the remaining xy-magnetization is tilted partly into the z-direction.
A fully refocused sequence with a properly selected and uniform f would yield higher signal, especially for tissues with long T2 relaxation times (high water content) so it is used in angiographic, myelographic or arthrographic examinations and is used for T2* weighting. The repetition time for this sequence has to be short. With short TR, coherent GE is also useable for breath hold and 3D technique. If the repetition time is about 200 msec there's no difference between spoiled or unspoiled GE. T1 weighting is better with spoiled techniques.
The common types include GRASS, FISP, FAST, and FFE.
The T2* component decreases with long TR and short TE. The T1 time is controlled by flip angle. The common TR is less than 50 ms and the common TE less than 15 ms
Other types have stronger T2 dependence but lower SNR. They include SSFP, CE-FAST, PSIF, and CE-FFE-T2.
Examples of fully refocused FID sequences are TrueFISP, bFFE and bTFE.

Contrast is the relative difference of signal intensities in two adjacent regions of an image.
Due to the T1 and T2 relaxation properties in magnetic resonance imaging, differentiation between various tissues in the body is possible. Tissue contrast is affected by not only the T1 and T2 values of specific tissues, but also the differences in the magnetic field strength, temperature changes, and many other factors. Good tissue contrast relies on optimal selection of appropriate pulse sequences (spin echo, inversion recovery, gradient echo, turbo sequences and slice profile).
Important pulse sequence parameters are TR (repetition time), TE (time to echo or echo time), TI (time for inversion or inversion time) and flip angle. They are associated with such parameters as proton density and T1 or T2 relaxation times. The values of these parameters are influenced differently by different tissues and by healthy and diseased sections of the same tissue.
For the T1 weighting it is important to select a correct TR or TI. T2 weighted images depend on a correct choice of the TE. Tissues vary in their T1 and T2 times, which are manipulated in MRI by selection of TR, TI, and TE, respectively. Flip angles mainly affect the strength of the signal measured, but also affect the TR/TI/TE parameters.
Conditions necessary to produce different weighted images:
T1 Weighted Image: TR value equal or less than the tissue specific T1 time - TE value less than the tissue specific T2 time.
T2 Weighted Image: TR value much greater than the tissue specific T1 time - TE value greater or equal than the tissue specific T2 time.
Proton Density Weighted Image: TR value much greater than the tissue specific T1 time - TE value less than the tissue specific T2 time.

See also Image Contrast Characteristics, Contrast Reversal, Contrast Resolution, and Contrast to Noise Ratio.

A contrast medium (or contrast agent) is a chemical substance introduced to the anatomical or functional region being imaged, to increase the differences between different tissues or between normal and abnormal tissue, by altering the relaxation times.
The chemical composition of the contrast media determines the specific usage. Similar to nuclear imaging is the intention in development of MR contrast media a high affinity to different organs or even tumors (e.g. necrosis avid contrast agent).
In 'contrast' to nuclear imaging contrast agents MR contrast media do not contain radiopharmaceuticals and the concentrations are about 100 times higher. Nuclear imaging contrast agents are direct contrast agents;; they are directly visible caused by their radioactivity. MR contrast agents affect the targeted tissue; they are indirect contrast agents.
See also Contrast Agents, the info sheet gives an overview and more in-depth information about different types of MRI contrast medium.

The characteristic time between significant fluctuations in the local magnetic field experienced by a spin due to molecular motions. For values of the correlation time such that the magnetic field as a function of time has large Fourier components near the resonance frequency, the T1 relaxation time will be shortened.

Eovist® (other brand name Primovist™) is a organ specific MRI contrast agent for the imaging, detection and characterization of liver conditions, including liver tumors, cysts, as well as other malignant and benign lesions. It is a water-soluble ethoxybenzyl derivative of Gd-DTPA. This compound is taken up by the hepatocytes (approximately 30% of the dose goes to the hepatocytes) and is equally excreted renal and biliary in humans. Excretion of Gd-EOB-DTPA in the bile may also permit visualization of both the gall bladder and the bile ducts.
Eovist® brightens the signal of T1 weighted MR images immediately after contrast administration. Dynamic and accumulation phase imaging can also be performed after bolus injection of Eovist®. The hepatocytes uptake will increase the signal intensity of normal liver parenchyma at 10 to 20 minutes after injection. This results in improved lesion-to-liver contrast because malignant tumors (metastases, the majority of hepatocellular carcinomas) do not contain either hepatocytes or their functioning is hampered.

WARNING: Gadolinium-based contrast agents increase the risk for nephrogenic systemic fibrosis (NSF) in patients with acute or chronic severe renal insufficiency (glomerular filtration rate less than 30 mL/min/1.73m²), or acute renal insufficiency of any severity due to the hepato-renal syndrome or in the perioperative liver transplantation period.

See also Drug Development and Approval Process USA, Contrast Medium, Hepatobiliary Contrast Agents, Tumor Specific Agents and Molecular Imaging.