(FORTUNE Small Business) – Originally intended to treat brain maladies, Stereotaxis's navigation system--which uses magnets to direct tiny catheters through the narrowest of blood vessels--tapped into a bigger market when the company switched its focus to the heart. After the system received FDA approval, revenue more than tripled, to $18.8 million in 2004.

BRAINSTORM: Founded by neurosurgeons in the late 1980s, Stereotaxis didn't take off until 1997, when new CEO Bevil Hogg--a South African entrepreneur who had co-founded Trek Bicycle--saw potential in the cardiac market. Clinical trials proved that the device could reach areas of the heart previously unavailable to catheter-based procedures.

HEART DRIVE: The St. Louis company went public last fall, and the system has won endorsement from medical experts, among them Dr. Warren Jackman at the University of Oklahoma Health Sciences Center, one of the top experts in arrhythmias. So far, 35 hospitals in the U.S. and Europe have installed the $1.5 million system, and Stereotaxis has additional orders for more than $21 million. --PATRICIA B. GRAY