Ultrashort T 2 * 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 (UTE) 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, 2009. © 2009 Wiley‐Liss, Inc.