In the dark, a tiny spark flashes. A gamma ray born inside a patient's body strikes a crystal and knocks out a shower of light. The detector's job is to figure out exactly where that happened. The more precise the answer, the sharper the image on the tomograph screen.

The new detector resembles a honeycomb, but not a flat one — the cells narrow toward the bottom. Each cell is a funnel: the top opening is 1.6 by 1.6 millimeters, and the bottom exit is 2 by 2 millimeters. Light born in the funnel collects in the narrow neck and falls onto a light-sensitive sensor. This geometry helps distinguish neighboring flashes better.

To test the detector, the team built a setup with a thin gamma-ray beam. The beam moved across the crystal with sub-millimeter precision and at various angles, like a needle stitching through the honeycomb. The collected data taught a computer to decode the signals. The flash location is determined to within 1 millimeter, and the ray's energy is measured with 11.8% accuracy.

For now, this is a lab prototype, but it points the way to sharper medical images. Single-photon emission computed tomography, that's the name of the method, could spot tumors and other disease foci earlier and more accurately.