MRI of blood volume with superparamagnetic iron in choroidal melanoma treated with thermotherapy

Functional magnetic resonance imaging (MRI) with a new intravascular contrast agent, monocrystalline iron oxide nanoparticles (MION), was applied to assess the effect of transpupillary thermotherapy in a rabbit model of choroidal melanoma. 3D-spoiled gradient recalled sequences were used for quantitative assessment of blood volume. The MRI-parameters were 5/22/35° (time of repetition (TR)/echo delay (TE)/flip angle (FA)) for T1- and 50/61/10° for T2-weighted sequences. Images were collected before and at different times after MION injection. In all untreated tissues studied, MION reduced the T2-weighted signal intensity within 0.5 h and at 24 h (all p ≤ 0.012), whereas no significant changes were detected in treated tumors. T1-weighted images also revealed differences of MION-related signal changes between treated tumors and other tissues, yet at lower sensitivity and specificity than T2. The change of T2-weighted MRI signal caused by intravascular MION allows early distinction of laser-treated experimental melanomas from untreated tissues. Further study is necessary to determine whether MRI can localize areas of tumor regrowth within tumors treated incompletely.