Final answer:
The primary downsides of 3D imaging with PET include the exposure to radiation and its limitation to imaging only metabolically active tissues, alongside issues such as high cost and potential inadequate availability of PET scanning equipment.
Step-by-step explanation:
The downside of 3D imaging with PET scans includes a few significant disadvantages. Firstly, PET scans involve the release of radiation since they use radiopharmaceuticals, which emit short-lived radioisotopes that decay by β+ radiation. Patients are exposed to this radiation, which, even though it is relatively short-lived and considered safe, can still be a concern, especially with repeated exposures. Secondly, the technique has limitations as it can only view metabolically active tissues. This means it's not effective in imaging areas without metabolic activity, such as when trying to image the brain of a deceased individual. Additionally, PET scanning equipment might not be widely available due to its high cost, and there is a need for specialized facilities with adequate shielding to protect against the emitted radiation.