Recent publications have reported increased signal intensity in the dentate nucleus and globus pallidus on unenhanced T1-weighted MRI of patients who have received multiple contrast-enhanced MRI scans, and several autopsy studies have documented the presence of gadolinium in these same brain areas. (1-11). However, little is known about the differences among Gadolinium-based contrast agents (GBCAs) in this regard. A comparative study in rats was performed to systematically examine the potential deposition, concentration and distribution pattern of gadolinium in the brain of rats after multiple intravenous injections of linear and macrocyclic GBCAs at extended doses. Additionally the histopathology of brain and skin tissue was investigated. In order to address the localization of Gd in brain on a near-microscopic level an imaging method, highly sensitive to and specific for Gd, was applied.Fifty healthy, male Wistar-Han rats were randomly divided into one control (saline) and four treatment groups (linear GBCAs: gadodiamide and gadopentetate dimeglumine; macrocyclic GBCAs: gadobutrol and gadoteridol, 10 animals per group). Each animal received twenty daily intravenous injections at a dose of 2.5 mmol Gd/kg body weight. Eight weeks after the last application, the animals were sacrificed and skin and brain were sampled. Histopathology of skin and brain (n=3 per group) and analysis of Gd tissue concentration by ICP-MS were performed (n=4 per group). The Gd distribution in the cerebellum was determined in 30 µm thick brain tissue kryo-sections using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS, n=3 per group).

Only in the gadodiamide group (4/10 animals) and not in any of the other groups macroscopic nephrogenic systemic fibrosis (NSF) - like skin lesions were observed. No morphological changes in the brain tissue of all investigated samples could be detected by routine H&E microscopic examination, immunohistochemistry and special stains The following Gd-concentrations were found in skin / brain (in µmol Gd/kg wet tissue): gadodiamide: 1472 ± 115 / 11.1 ± 5.1, gadopentetate dimeglumine: 80.8 ± 6.2 / 13.1 ± 7.3, gadobutrol: 1.1 ± 0.5 / 0.7 ± 0.4 and gadoteridol: 1.7 ± 0.8 / 0.5 ± 0.2. The detected residual Gd in the brain was very low (<0.0002%ID/g) in all groups, but always above the limit of quantification. The measured Gd-concentrations after injection of linear GBCAs (gadodiamide, gadopentetate dimeglumine) in the brain were significantly higher than after macrocyclic agents (gadobutrol, gadoteridol).

The spatially resolved LA-ICP-MS analysis (60 x 90 µm2 resolution) of linear GBCAs showed regions of high Gd-signal in the white matter and the deep cerebellar nuclei including the lateral cerebellar nucleus, which is equivalent to the dentate nucleus in humans.

In the repeated extended dose animal model very low gadolinium concentrations in the brain were detected eight weeks after the last application. They were significantly higher for the linear than for the macrocyclic GBCAs which is in good accordance with the known differences in thermodynamic and kinetic complex stabilities of the GBCA classes. In none of the groups were histopathological findings detected in the brain. Surprisingly, the major fraction of the deposited gadolinium from linear GBCAs was not only located in the deep cerebellar nuclei but also throughout the white matter of the brain. This was not observed for the macrocyclic agents.