Ultrashort T2* relaxometry for quantitation of highly concentrated superparamagnetic iron oxide (SPIO) nanoparticle labeled cells.

A new method was developed to measure ultrashort T(2)* relaxation in tissues containing a focal area of superparamagnetic iron oxide (SPIO) nanoparticle-labeled cells in which the T(2)* decay is too short to be accurately measured using regular gradient echo T(2)* mapping. The proposed method utilizes the relatively long T(2) relaxation of SPIO-labeled cells and acquires a series of spin echo images with the readout echo shifted to sample the T(2)* decay curve. MRI experiments in phantoms and rats with SPIO-labeled tumors demonstrated that it can detect ultrashort T(2)* down to 1 ms or less.