Contrast agents incorporating super-paramagnetic iron-oxide (SPIO)
nanoparticles have shown promise as a means to visualize labeled cells
using MRI. Labeled cells cause significant signal dephasing due to the
magnetic field inhomogeneity induced in water molecules near the cell.
With the resulting signal void as the means for detection, the particles
are behaving as a negative contrast agent, which can suffer from
partial-volume effects. Disclosed is a new method for imaging labeled
cells with positive contrast. Spectrally-selective RF pulses are used to
excite and refocus the off-resonance water surrounding the labeled cells
so that only the fluid and tissue immediately adjacent to the labeled
cells are visible in the image. Phantom, in vitro, and in vivo
experiments show the feasibility of the new method. A significant linear
correlation (r=0.87, p<0.005) between the estimated number of cells
and the signal has been observed.