JUNE 2003

Molecular imaging briefing

Molecular imaging is a matter of scale

By: James Brice

Radiologists, who are accustomed to gross pathology, may need reorientation to deal with the scale of molecular anatomy that is microscopic in comparison. Molecular imaging forces the interpreter to think small because the relevant biological processes occur on cellular and subcellular levels.

Without magnification, radiologists can identify differences in objects as small as 0.1 mm in diameter, according to Kunio Doi, Ph.D., director of the Kurt Rossman Laboratories for Radiologic Image Research at the University of Chicago. Mammographers can identify suspicious breast calcifications about that size.

Stem cells that may become a target for molecular imaging tracking are typically 10 microns in diameter (one millimeter equals 1000 microns). Reporter genes will document the activity of gene fragments and enzymes within the nuclei of cells. A cell nucleus is typically 100 nanometers in diameter (one micron equals 1000 nanometers).

The image agents that track biochemical behavior on this scale must themselves be small. Designers strive to generate as much signal as possible from minuscule volume. A dose may be made up of a fewer than 100 million molecules administered in nanomolar or picomolar concentrations.

With apologies to Mickey (the transgenic) Mouse, it is a small world after all.