Tissue Iron Detection & Quantification with MRI

Endogenous tissue iron, other than that in blood, occurs primarily in two MRI detectable forms: ferritin iron and hemosiderin iron [1]. Ferritin is a water soluble protein with an iron core and is the body’s main iron storage protein. Hemosiderin iron is water insoluble and consists largely of ferrihydrite. Ferritin may either be diffusely dispersed or localized in specific cells, depending on the tissue type. Hemosiderin is often the result of ferritin degradation and typically occurs in micron-sized aggregations. Ferritin is the predominant iron form in most healthy tissues, but hemosiderin may be present in high concentrations as a result of pathology (e.g., iron overload disorders). The MRI detectability of ferritin and hemosiderin is a result of both their relative abundances as well as their high magnetic susceptibilities. As a consequence of their susceptibilities, they generate substantial magnetic field inhomogeneities when placed in the strong magnetic field of an MRI scanner. These field inhomogeneities cause proton spin dephasing and increase the transverse relaxation rate. Thus ferritin and hemosiderin act effectively as natural contrast agents. Iron-sensitive MRI techniques provide useful non-invasive approaches for the diagnosis and monitoring of a variety of diseases with associated changes in iron concentration and the spatial pattern of iron deposition.