Positive Contrast Imaging of Iron-Oxide Labeled Human Embryonic Stem (hES) Cell and Fibroblast using SWEET

Introduction: The MRI detection of superparamagnetic iron-oxide particles (SPIO) -labeled cells is important in cell-replacement therapies, which can help monitoring the migration/proliferation of cells following the transplantation. Cells loaded with SPIOs cause signal reduction (negative contrast) associated with dephasing due to the magnetic field inhomogeneity near the cells, therefore the signal void are typically used as the means for detection of the cells. Negative contrast agents have drawback of being confused with the signal void caused by other sources such as tissue inhomogeneity or partial volume effects [1, 2]. Selective and positive contrast for SPIO-labeled cells is warranted with suppression of background tissue, and recently, several methods to elicit positive contrast have been suggested [2-4]. Off-resonance imaging using spectrally selective RF pulse has the advantage of detecting the positive signal proximal to the cells [2, 3], however, requiring special sequence programming. We were motivated to provide simple means of detecting SPIO-labeled cells by using susceptibility weighted echo-time encoding technique (SWEET) whereby the subtraction of two sets of image volumes acquired at slightly-shifted echo time generates positive contrast at the cell position [5].