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Purpose: To detect the ultrastructural site of gadolinium retention in skin by using an animal model of nephrogenic systemic fibrosis and compare a linear, low-stability gadolinium chelate (formulated gadodiamide) with a macrocylic, high-stability gadolinium chelate (gadoterate meglumine)., Materials and Methods: Experimental procedures were performed according to rules and regulations laid down by the UK Home Office (Animal Procedures Act of 1986). Male Wistar rats were subjected to 5/6 subtotal nephrectomy (creatinine clearance, 25% normal). Gadolinium-based contrast agents, formulated gadodiamide (n = 9) and gadoterate meglumine (n = 11), were administered intravenously (2.5 mmol/kg for 5 days). After 28 days, skin was analyzed by means of morphometric and immunohistochemical techniques and electron microscopy. Data were compared with the Student t test. Skin gadolinium was located by means of energy-filtered transmission electron microscopy., Results: Formulated gadodiamide produced a 40-fold greater increase in gadolinium in skin than did gadoterate meglumine. An electron-dense filamentous material, detected within extracellular matrix, displayed a "halo" appearance, associated with collagen fibrils and electron-dense intracellular fragments of collagen fibrils within activated fibroblasts. Both electron-dense features demonstrated the presence of gadolinium but were much less apparent following gadoterate meglumine administration, where the presence of gadolinium was not detected. Formulated gadodiamide increased dermal cell count, dermal thickness, and collagen bundle density with enhanced immunostain for CD34, fibroblast-specific protein 1,4-hydroxy-prolyl-hydroxylase, and factor XIIIa. Circular staining for α-smooth muscle actin indicated new blood vessel formation. Skin of rats receiving gadoterate meglumine remained unchanged., Conclusion: Gadolinium retention in skin following formulated gadodiamide administration was located to the collagen fibril, in both the extracellular matrix and within activated fibroblasts., (© RSNA, 2012.)