Naviscan PET Systems, a San Diego maker of high-resolution positron-emission tomography (PET) scanners, raised $15 million in a third round of funding led by QuestMark Partners.

Returning investors included Sanderling Ventures, Mayo Medical Ventures and Walker Ventures. The company plans to use the new funding to continue clinical validation and commercialization of its PEM Flex Solo II PET scanner, which it claims can image tumors less than two millimeters in diameter.

Naviscan, previously based in Rockville, Md., last raised $6.5 million in Nov. 2005. See the company’s release on its latest funding here.