Detector crystals in a positron emission tomography (PET) apparatus gantry are cooled by directing cooling gas flow into a cooling duct bounded by the crystals and a cover defining the patient scanning field within the gantry. The cooling gas cools the crystals. Cooling gas may also be directed radially outwardly from the cooling duct into spatial gaps defined between detector enclosures that include the crystals, further isolating heat generated by other components within gantry from the detector crystals. Cooling gas is provided by a cooling system that may be incorporated within the gantry, external the gantry or a combination of both. Cooling gas can be provided by directing air within the gantry in contact with internal gantry cooling tubes and routing cooled air directly into the cooling duct with a powered fan.