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. The measured T-2* values were about 10% higher than those from the ultrashort TE (LITE) technique. The shorter the TE, the less the measurements deviated from the UTE T-2* mapping. Combined with the regular T-2* mapping, this technique is expected to provide quantitation of highly concentrated iron-labeled cells from direct cell transplantation. Magn Reson Med 61:761-766, 2009. (C) 2009 Wiley-Liss, Inc.