(
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:
•
A minimum delay of about 10 min. between injection (or infusion) and
MR imaging, extends the examination time.
•
Cross-section
flow void in narrow blood vessels may impede the differentiation from small
liver lesions.
•
Aortic pulsation artifacts become more pronounced.
See also
Superparamagnetism,
Superparamagnetic Contrast Agents and
Classifications, Characteristics, etc..
•
] and 3 definitions [
]
Further Reading:
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