K 162MR Imaging of Vascular Volume and Cellular Uptake in Experimental Choroidal Melanoma with Superparamagnetic Monocrystalline Iron Oxide Nanoparticles (MION) as Contrast Agent
M. Krause1, J. Xiong1, K. K. Kwong2, E. S. Gragoudas1, L. H. Y. Young1
Introduction: Functional magnetic resonance imaging (MRI) allows high tissue resolution and accurate measurement of physiologic parameters in vivo. Vascular volume and cellular uptake was investigated in a rabbit choroidal melanoma model using MION as a new intravascular contrast agent.
Methods: Experimental pigmented choroidal melanomas were established in 11 New Zealand albino rabbits. 6 of 11 tumors were treated with photodynamic therapy (PDT) before MRI. The in vivo MRI experiments were performed on a 1.5 Tesla GE Signa system. 3D-spoiled gradient echo- (SPGR)- pulse sequences were used to acquire T1 and T2 weighted axial images (5/22/35º and 50/61/10º (TE/TR/Flip angle)). At first, a baseline set of images was collected. 1.5mg/kg MION was then injected i.v. and images were obtained <1h and > 24h after the injection. Signal intensities were measured within tumor, ciliary body, choroid, and iris and relative signal intensities were determined for these tissues in relation to vitreous.
Results: Significant changes of relative T2 signal intensity (p=0.04) were found in untreated melanomas <1h after injection of MION. After 24h and 48h, the changes were still present. In tumors treated with PDT, relative T2 changes after MION application were significantly lower compared to untreated tumors. Corresponding results of T1 sequences were less pronounced.
Conclusions: In choroidal melanomas, MION can be used for functional MRI of vascular volume and cellular uptake. T2 signal intensities differ substantially between untreated and treated melanomas, indicating reduced vascular volume and compromised cellular function after PDT.
1Massachusetts Eye and Ear Infirmary, Department of Ophthalmology, Harvard Medical School, Boston, MA, USA