Positron Emission Tomography (PET) scans have long been used for brain imaging. Now clinicians have also started using them to detect cancer. The accuracy of these 3-D imaging systems could reduce the need for biopsies and invasive surgery. However, each machine costs up to $2 million; as a result, only a few hundred machines in the world are up and running. Acton, MA-based PhotoDetection Systems has developed a PET system that could cut the cost in half-and detect tumors with greater resolution. Like other PET machines, the new system uses a crystal to convert gamma rays (the high-energy photons emitted from an injected radioisotope in the patient's body) to light. But instead of using more than a thousand pricey photosensors to read the light, it relies on an array of optical fibers to pinpoint the position of the light emerging from the crystal. The machine can then detect tumors as small as four millimeters in diameter, as well as larger but less active tumors that would normally go undiagnosed. The company hopes to have a machine ready for testing by mid-2003.