MRI OF SINGLE MAMMALIAN CELLS has been demonstrated by Carnegie Mellon researchers (Chien Ho, 412-268-3395), opening possibilities for improving treatment of organ rejection and studying how natural killer cells attach to tumors. In the present work, the researchers took pictures of rat T cells (components of the immune system) embedded in a gelatin solution. To prepare the cells for MR imaging, they added magnetic iron-oxide particles coated with dextran, a glucose-based molecule. Through a natural process known as endocytosis, the particles were absorbed by each cell and taken up in a fluid-filled region known as the vacuole. Many such particles entered each vacuole, and collectively they produced a detectable magnetic signal over a region 50 times greater than the vacuole's 1 micron size. Therefore, an MRI scanner with 50-micron resolution could detect a single vacuole in a T cell, itself only 7-10 microns in size. Previous studies have shown that dextran-coated iron-oxide particles do not affect a T cell's function and could act as a safe "MRI contrast agent" inside bodies. While this technique may not be able to track single cells in a living body, owing to its less favorable environment for MR imaging, the researchers believe the method can be developed into a sensitive, noninvasive means of detecting unwanted accumulation of T cells in transplanted organs, a sign of tissue rejection. More generally, this technique may enable MRI to detect the presence and movement of any other type of cell (including tumor-killing cells) in living things, as long as one can design a suitable MRI contrast agent that can enter the cell without affecting its function. (Dodd et al., Biophysical Journal, January 1999; images at Physics News Graphics.)