Children's Hospital, Boston
At 5 weeks old, a baby's heart is the size of a walnut. The hand of a surgeon can't work at tolerances that small. But through a tiny incision in an infant's leg, a filament can be threaded up through the vein, two chambers of the heart, and into a valve that's too narrow. A balloon inflates to expand the opening that lets oxygen reach the lungs. The procedure that fixed Janise Burge's heart left a scar measured in millimeters.
Photograph by Samantha Appleton