(SPIO) Relatively new types of MRI contrast agents are superparamagnetic iron oxide-based colloids (median diameter greater than 50nm). These compounds consist of nonstoichiometric microcrystalline magnetite cores, which are coated with dextrans (in ferumoxide) or siloxanes (in ferumoxsil). After injection they accumulate in the reticuloendothelial system (RES) of the liver (Kupffer cells) and the spleen. At low doses circulating iron decreases the T1 time of blood, at higher doses predominates the T2* effect.
SPIO agents are much more effective in MR relaxation than paramagnetic agents. Since hepatic tumors either do not contain RES cells or their activity is reduced, the contrast between liver and lesion is improved. Superparamagnetic iron oxides cause noticeable shorter T2 relaxation times with signal loss in the targeted tissue (e.g., liver and spleen) with all standard pulse sequences. Magnetite, a mixture of FeO and Fe2O3, is one of the used iron oxides. FeO can be replaced by Fe3O4.
Use of these colloids as tissue specific contrast agents is now a well-established area of pharmaceutical development. Feridex®, Endorem™, GastroMARK®, Lumirem®, Sinerem®, Resovist® and more patents pending tell us that the last word in this area is not said.
Some remarkable points using SPIO:

See also Superparamagnetism, Superparamagnetic Contrast Agents and Classifications, Characteristics, etc..

A usual variation of sequential plane imaging technique that can be used with selective excitation technique that does not affect adjacent slices. Since, in SE imaging TR longer than TE, the machine would be idling most of the time, if a single slice would be acquired; multiple slice imaging was introduced early on. Adjacent slices are imaged while waiting for relaxation of the first slice toward equilibrium, resulting in decreased image acquisition time for the set of slices. The maximum number of slices of a pulse sequence depends on the repetition time.

Contrast enhanced GRE sequences provide T2 contrast but have a relatively poor SNR. Repetitive RF pulses with small flip angles together with appropriate gradient profiles lead to the superposition of two resonance signals.
The first signal is due to the free induction decay FID observed after the first and all ensuing RF excitations.
The second is a resonance signal obtained as a result of a spin echo generated by the second and all addicted RF-pulses.
Hence it is absent after the first excitation, it is a result of the free induction decay of the second to last RF-excitation and has a TE, which is almost 2TR. For this echo to occur the gradients have to be completely symmetrical relative to the half time between two RF-pulses, a condition that makes it difficult to integrate this pulse sequence into a multiple slice imaging technique. The second signal not only contains echo contributions from free induction decay, but obviously weakened by T2-decay. Since the echo is generated by a RF-pulse, it is truly T2 rather than T2* weighted. Correspondingly it is also less sensitive to susceptibility changes and field inhomogeneities.
Companies use different acronyms to describe certain techniques.
Different terms (see also acronyms) for these gradient echo pulse sequences:
CE-FAST Contrast Enhanced Fourier Acquired Steady State,
CE-FFE Contrast Enhanced Fast Field Echo,
CE-GRE Contrast Enhanced Gradient-Echo,
DE-FGR Driven Equilibrium FGR,
FADE FASE Acquisition Double Echo,
PSIF Reverse Fast Imaging with Steady State Precession,
SSFP Steady State Free Precession,
T2 FFE Contrast Enhanced Fast Field Echo (T2 weighted).
In this context, 'contrast enhanced' refers to the pulse sequence, it does not mean enhancement with a contrast agent.

Quick Overview

A combination of B1 inhomogeneity, poor slice profile, and insufficient spoiler gradients between the refocusing pulse and the readout interval of a spin echo sequence results in a FID signal being detected along with the echo.
Since the FID is not phase encoded (normally the phase encoding occurs before the refocusing pulse), it is not dispersed along the phase encoding axis, but appears as a line across the center of the image.

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T2 weighted imaging relies upon local dephasing of spins following the application of the transverse energy pulse. The contrast of a T2 weighted image is predominantly dependent on T2 and the T2 dependence will be increased by using a long echo time.
Fat has a shorter T2 time than water and relaxes or decays more readily than water. Since the amount of transverse magnetization in fat is small, fat generates very little signal on a strong T2 weighted contrast image and appears intermediate to dark. The T2 weighting is stronger with a longer TE. Water has a very high T2 constant, therefore has very high T2 signal and thus appears bright on a T2 contrast image. Cerebral white matter (fat containing) is less intense than grey matter. Flowing blood (flow effects) and haematomas (haemoglobin, haemosiderin) have a variable signal intensity on MR images.
Images created with TR's and TE's to enhance T2 contrast are referred to as T2 weighted images. Both T1 and T2 weighted images are acquired for most medical MRI examinations.