In gamma cameras, which component helps determine the direction of incoming photons and image quality?

• A. Crystal
• B. Collimator
• C. Light pipe
• D. PMTs

Answer: (B)

The directionality of detected photons—and thus the image quality—comes from the collimator. In a gamma camera, photons emitted from the patient pass through the collimator before reaching the scintillation crystal. The collimator is made of many small, long holes with septa between them; only photons traveling along or near the axis of a hole can reach the crystal. This gates photons by their incoming direction, mapping where in the patient the photon originated and shaping the image.

The geometry of the holes—their size, length, and septal thickness—drives the trade-off between resolution and sensitivity: longer, narrower holes improve spatial resolution (sharper images) but reduce sensitivity (fewer photons detected), while shorter, wider holes increase sensitivity but blur detail. The crystal then converts the gamma energy into light, and the light is collected by light pipes and detected by PMTs to form the image, but these parts don’t determine the photon’s original direction.