Superparamagnetic Iron Oxide

Superparamagnetic iron oxide (SPIO) are liver-specific contrast agents that have an affinity for the reticuloendothelial system (e.g., Kupffer cells of the liver). They result in low signal in hepatic parenchyma on T1- and T2-weighted images. Hepatic metastases do not have Kupffer cells and are generally hyperintense on T2-weighted images, so they are made more conspicuous by SPIO agents.

Some SPIO contrast agents (e.g., Endorem, Feridex) have been associated with side effects like lumbar pain and hypotension. They are infused slowly to avoid these side effects, and scanning is generally started 30 minutes after start of infusion.

A newer SPIO (Resovist, approved in Europe) can be injected as a bolus and scanning can start 10 minutes after start of injection. Resovis can also cause some enhancement of lesions on T1-weighted images, although to a much lesser extent than traditional gadolinium agents, allowing the extraction of additional information using dynamic imaging.

SPIO-enhanced imaging are equal to or more sensitive compared with traditional gadolinium agents. SPIO-enhanced imaging is also significantly more sensitive than PET in the detection of hepatic metastases; however, PET is more specific.

References

Rappeport ED, Loft A. Liver metastases from colorectal cancer: imaging with superparamagnetic iron oxide (SPIO)-enhanced MR imaging, computed tomography and positron emission tomography. Abdom Imaging. 2007;32(5):624-34.