(
USPIO) The class of the ultrasmall
superparamagnetic iron oxide includes several chemically and pharmacologically very distinct materials, which may or may not be interchangeable for a specific use. Some ultrasmall
SPIO particles (median diameter less than 50nm) are used as
MRI contrast agents (
Sinerem®,
Combidex®), e.g. to differentiate metastatic from inflammatory lymph nodes.
USPIO shows also potential for providing important information about angiogenesis in cancer tumors and could possibly complement
MRI helping physicians to identify dangerous arteriosclerosis plaques.
Because of the disadvantageous large
T2*//T1 ratio,
USPIO compounds are less suitable for arterial
bolus contrast enhanced magnetic resonance angiography than
gadolinium complexes. The tiny ultrasmall superpara
magnetic iron oxides do not accumulate in the RES system as fast as larger particles, which results in a long plasma half-life.
USPIO particles, with a small median diameter (less than 10 nm), will accumulate in lymph nodes after an intravenous injection by e.g. direct transcapillary passage through endothelial venules. Once within the nodal parenchyma, phagocytic cells of the mononuclear phagocyte system take up the particles.
As a
second way, USPIOs are subsequently taken up from then interstitium by lymphatic vessels and transported to regional lymph nodes. A lymph node with normal phagocytic function takes up a considerable amount and shows a reduction of the
signal intensity caused by T2 shortening effects and
magnetic susceptibility. Caused by the small uptake of the USPIOs in metastatic lymph nodes, they appear with less signal reduction, and permit the differentiation of healthy lymph nodes from normal-sized, metastatic nodes.
See also
Superparamagnetic Contrast Agents,
Superparamagnetic Iron Oxide,
Very Small Superparamagnetic Iron Oxide Particles,
Blood Pool Agents,
Intracellular Contrast Agents.