Positive-contrast cellular MR imaging using susceptibility weighted echo-time encoding technique (SWEET)

The MRI (Magnetic Resonance Imaging) detection and tracking of superparamagnetic iron-oxide particle (SPIO)-labeled cells is expected to play an important role in cell-replacement therapies. SPIO-loaded cells reduce MR signal caused by dephasing due to the magnetic field inhomogeneity near the cells. This signal reduction is typically used for detecting/tracking the labeled cells when it is contrasted to the surrounding tissues (i.e. negative-contrast). However, the negative-contrast can be confused with the signal voids caused by other sources such as tissue inhomogeneity or partial volume effect. Therefore, selective and positive-contrast imaging for SPIO-labeled cells is warranted with the suppression of signals from background tissues. We were motivated to investigate a simple method to detect SPIO-labeled cells with positive-contrast by using susceptibility weighted echo-time encoding technique (SWEET). Human cervix carcinoma cells (KB) were labeled with dextran-coated iron-oxide particles (Feridex), and placed in the gel phantom. Consequently, the difference between the conventional spin-echo images and the echo-shifted images using SWEET rendered the positive signal manifestation from the labeled-cells. Gradient-echo sequence was used to confirm the location of labeled cells. We demonstrated that the method can be readily implemented without the use of dedicated pulse sequence programming. The method can separate out the magnetic susceptibility effect of SPIOloaded cells from the main field-inhomogeneity and chemicalshift artifact.