(Above) Transmission electron microscopy (TEM) image showing accumulation of magnetic particles inside the fluid-filled regions (vacuoles) of a rat's T cells, which are agents of the immune system. Regions of the cells containing the magnetic particles are
shown magnified below the main image.
The magnetic particles produce a signal that can be picked up by a magnetic resonance imaging (MRI) machine. They are introduced into the vacuoles through a natural process known as endocytosis, and typically many such particles enter each vacuole. The accumulated particles act as a single larger particle which produce a signal that can be picked up by a MRI scanner.
(A) Magnetic resonance image of microscope slide containing magnetically labeled T cells in a gelatin solution. MRI scans of cells without the particles produced no contrast. (B) All of the T cells were labelled with a fluorescent molecule. This is a fluorescence microscopy image of the same slide, to ensure that the MRI scanner was picking up actual cells and not just air bubbles or other artifacts. Note that only 20% of the T cells were labelled with magnetic particles. (C) A comparison of the two images. The cells picked up by the MRI scanner overlap with the fluorescence microscopy image. Note that the cells in the MRI image appear as larger blobs. This is because the magnetic particles in each vacuole produce signals 50 times greater than their size. (They occupy about 1 micron of space in the vacuole, meaning that an MRI scanner with 50-micron resolution can detect them.) Unlike fluorescence microscopy, however, MRI can pick up these cells in the interior of a living body.
This research is reported by Stephen J. Dodd, Mangay Williams, Joseph P. Suhan, Donald S. Williams, Alan P. Koretsky, and Chien Ho in the January 1999 issue of Biophysical Journal. All images copyrighted 1999 by the Biophysical Journal.