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Iron oxide
General information
Ultrasmall superparamagnetic particles of iron oxide (USPIOs) of median diameter no less than 50 nm have been studied as blood-pool agents and are given intravenously to enhance liver imaging in patients with cirrhosis and to visualize lymph nodes. They consist of non-stoichiometric microcrystalline iron oxide cores, which are coated with dextrans (in ferumoxides) or siloxanes (in ferumoxsils). The most common form of iron oxide used is magnetite, which is a mixture of Fe 2 O 3 and FeO; a mixture of Fe 2 O 3 and Fe 3 O 4 can be used instead.
After injection, the particles accumulate in the reticuloendothelial system. Liver tumors have few or no reticuloendothelial cells and so there is a contrast between normal liver and tumor.
These contrast agents are well tolerated, with no serious adverse effects. The reported adverse effects include low back pain, vomiting and diarrhea, urticaria, flushing, dizziness, and muscle spasm [ ]. The incidence of adverse reactions is higher with these particulate iron oxide agents than with gadolinium chelates. However, that is not a great cause for concern, since few of the reactions are severe and all are self-limiting.
Ferristene
The USPIO contrast agent ferristene (Abdoscan, Amersham) is used in delineating the gastrointestinal tract after oral ingestion. Different rectal formulations of ferristene with different viscosities and iron concentrations have been evaluated in a phase II clinical study, in which ferristene enemas (200–500 ml) and intravenous gadodiamide (0.1 mmol/kg) were used in the evaluation and staging of rectal cancer in 113 patients [ ]. Five patients had 10 adverse events, including rectal pain, diarrhea, edema, a phobic reaction, nausea, and a rash; all recovered without further therapy. The high-viscosity formulation (70 g of granules/l) was better than the low-viscosity formulation in tumor staging, but the iron concentration (30 or 59 micrograms/ml) of the contrast agent was less important.
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