Gradient echo acquisition for superparamagnetic particles with positive contrast (GRASP): sequence characterization in membrane and glass superparamagnetic iron oxide phantoms at 1.5T and 3T.

Iron oxides are used for cell trafficking and identification of macrophages in plaque using MRI. Due to the negative contrast, differentiation between signal loss caused by iron and native low signal in tissue may be problematic. It is, therefore, preferable to achieve positive contrast. The purpose of this study was to test the efficacy of a new MRI sequence GRASP (GRe Acquisition for Superparamagnetic Particles) to generate a positive signal in phantoms containing iron. Membrane phantoms were constructed containing Ferumoxide at 7 concentrations. Standard GRE sequences were modified with user controlled z-gradient rephasing (+/- 100%). CNR values were determined as a function of echo time (TE) and % rephasing at 1.5T and 3T. T(2)* values were determined using multiple double-echo GRE. The GRASP sequence generated positive signal enhancement in phantoms containing iron. For all rephasing values <or=30%, positive contrast was observed. The CNR generated at 1.5T was greater than the values at 3T for all concentrations tested. Correlation between CNR at 0% and 100% rephasing was observed at 1.5T(R = 0.84). Additionally, correlation between field change across the volume and CNR was observed. In conclusion, GRASP sequences may be used to generate positive signal enhancement in the presence of iron using MRI.