Positive-contrast techniques are being developed to increase the detection of magnetically labeled cells in tissues. We evaluated a post-processing positive-contrast technique, susceptibility-gradient mapping (SGM), and compared this approach with two pulse sequences, a gradient-compensation-based "White Marker" technique and an off-resonance-based approach, inversion recovery on-resonance water suppression (
IRON), for the detection of superparamagnetic
iron oxide (
SPIO) nanoparticle-labeled C6
glioma cells implanted in the flanks of nude rats. The SGM, White Marker and
IRON positive-contrast images were acquired when the labeled C6
glioma tumors were approximately 5 mm (small), approximately 10 mm (medium) and approximately 20 mm (large) in diameter along the largest dimension to evaluate their sensitivity to the dilution of the
SPIO nanoparticles as the
tumor cells proliferated. In vivo MRI demonstrated that all three positive-contrast techniques can produce hyperintensities in areas around the labeled flank
tumors against a dark background. The number of positive voxels detected around small and medium
tumors was significantly greater with the SGM technique than with the White Marker and
IRON techniques. For large
tumors, the SGM resulted in a similar number of positive voxels to the White Marker technique, and the
IRON approach failed to generate positive-contrast images with a 200 Hz suppression band. This study also reveals that
hemorrhage appears as hyperintensities on positive-contrast images and may interfere with the detection of
SPIO-labeled cells.