A PET scan generally exposes a person to **more radiation than a CT scan alone**, but the difference depends on the specific type of CT scan and whether the PET is combined with CT.
To understand why, it helps to know how each scan works and the sources of radiation involved. A CT (computed tomography) scan uses X-ray beams to create detailed 3D images of the inside of the body. The radiation dose from a CT scan varies depending on the body part scanned and the scan settings, typically ranging from about 2 to 10 millisieverts (mSv). For example, a head CT might be on the lower end, while an abdominal CT might be higher.
A PET (positron emission tomography) scan, on the other hand, involves injecting a small amount of a radioactive tracer into the body. This tracer emits positrons that interact with electrons, producing gamma rays detected by the scanner to reveal metabolic activity in tissues. The radiation dose from the tracer itself is usually around 5 to 7 mSv. PET scans alone do not produce X-rays, so the radiation comes solely from the tracer.
Often, PET scans are combined with CT scans in a single session (PET/CT) to provide both metabolic and anatomical information. In this case, the total radiation dose is the sum of the PET tracer dose plus the CT scan dose, which can reach around 25 mSv or less depending on the protocol. This combined approach offers a more comprehensive picture but also increases radiation exposure compared to either scan alone.
In summary:
| Scan Type | Radiation Dose (approximate) | Source of Radiation |
|—————-|——————————|————————————|
| CT scan | 2 to 10 mSv | X-ray beams |
| PET scan alone | 5 to 7 mSv | Radioactive tracer injected |
| PET/CT scan | Up to ~25 mSv | Radioactive tracer + CT X-rays |
The PET tracer radiation dose is relatively low and passes through the body within a few hours. Advances in PET/CT technology have also allowed reductions in radiation dose by optimizing tracer amounts and CT scan settings without compromising image quality.
While PET scans involve radiation, the amount is considered low and justified by the valuable metabolic information they provide, especially in cancer detection, staging, and treatment monitoring. CT scans provide detailed anatomical images but do not show metabolic activity. Combining both scans helps doctors see not only what tissues look like but also how they function.
Therefore, a PET scan alone usually involves **more radiation than a single CT scan of some body parts**, but less than a full diagnostic CT of certain areas. When combined as PET/CT, the radiation dose is higher than either alone but offers a powerful diagnostic tool that balances risk and benefit carefully.
Radiation exposure from these scans is monitored and minimized as much as possible, and they are only recommended when medically necessary. The risk from the radiation is generally low compared to the benefits of accurate diagnosis and treatment planning.
