Positive contrast with alternating repetition time SSFP (PARTS): a fast imaging technique for SPIO-labeled cells.

There has been recent interest in positive-contrast MRI methods for noninvasive tracking of cells labeled with superparamagnetic iron-oxide nanoparticles. Low-tip-angle balanced steady-state free precession sequences have been used for fast, high-resolution, and flow-insensitive positive-contrast imaging; however, the contrast can be compromised by the limited suppression of the on-resonant and fat signals. In this work, a new technique that produces positive contrast with alternating repetition time steady-state free precession is proposed to achieve robust background suppression for a broad range of tissue parameters. In vitro and in vivo experiments demonstrate the reliability of the generated positive contrast. The results indicate that the proposed method can enhance the suppression level by up to 18 dB compared with conventional balanced steady-state free precession.