How a PET CT Scan Works
A PET CT scan is a medical imaging test that combines two technologies into one. PET (positron emission tomography) detects metabolic activity inside the body, while CT (computed tomography) creates detailed cross-sectional images of your anatomy. Together, they produce a map that shows both where something unusual is happening and exactly where it sits in the body.
Before the scan, a small amount of a radioactive substance called a tracer is injected into a vein in your arm. The most common tracer is FDG, a modified form of glucose. Cancer cells tend to use glucose faster than normal cells, so they absorb more of the tracer and light up on the PET images. The CT portion then overlays the structural detail, giving the doctor a precise picture.
When Doctors Order a PET CT Scan
Doctors recommend a PET CT scan in several situations. The most common ones include:
Cancer staging: Once cancer has been diagnosed through a biopsy, a PET CT helps determine how far it has spread. This is called staging, and it directly influences treatment decisions. Knowing whether the cancer is confined to one area or has reached the lymph nodes, bones, or other organs changes the approach.
Treatment monitoring: During or after chemotherapy, radiation, or other treatments, a PET CT can show whether the cancer is responding. If the metabolic activity in a tumour has decreased, it suggests the treatment is working.
Recurrence detection: For patients who have completed treatment, a PET CT can detect cancer that has come back, sometimes before it shows up on other imaging tests.
Evaluating a suspicious finding: If another scan or blood test has raised concern, a PET CT can help clarify whether a lesion or mass is likely cancerous or benign.
What a PET CT Shows That Other Scans May Not
A regular CT scan or MRI shows the size and shape of structures in the body. They can identify a lump, but they cannot always tell whether that lump is active cancer, scar tissue from a previous treatment, or something else entirely. A PET CT adds the metabolic dimension. It shows how actively cells in a particular area are functioning.
This is especially useful for small lesions that may not look unusual on a CT scan alone but are metabolically active. It also helps distinguish between residual scar tissue after treatment and a tumour that is still growing.
Types of PET CT Scans
While FDG is the most widely used tracer, it is not the only one. Different tracers target different types of cancer cells. For example, PSMA PET CT uses a tracer that binds to a protein found on prostate cancer cells, making it far more accurate for prostate cancer than FDG. FAPI PET CT targets fibroblast activation protein, which is found around many types of tumours. The choice of tracer depends on the type of cancer and the clinical question the doctor needs answered.
You can read more about the different PET CT scans available and how each one works.
What Happens During the Scan
On the day of the scan, you will receive the tracer injection and then wait for about 45 to 60 minutes while it distributes through your body. During this time, you will be asked to sit or lie quietly and avoid physical activity, as muscle movement can affect the tracer distribution.
The scan itself usually takes 20 to 30 minutes. You will lie on a table that moves slowly through the scanner. The machine is open on both ends, so it is less enclosed than an MRI machine. The process is painless.
After the scan, the tracer leaves your body naturally through urine over the next several hours. Drinking plenty of water helps clear it faster.
Is It Safe?
The radiation exposure from a PET CT is carefully controlled and falls within established safety limits. The tracer delivers a small dose that is comparable to what you might receive from a couple of years of natural background radiation. For most patients, the clinical benefit of the scan far outweighs this small exposure. If you are pregnant, breastfeeding, or have concerns about radiation, discuss them with your doctor before the scan is scheduled.
When to Discuss a PET CT with Your Doctor
If you have been diagnosed with cancer or are undergoing treatment, your oncologist will recommend a PET CT when it is clinically useful. If you are experiencing symptoms or have received abnormal results on another test, ask your doctor whether a PET CT could provide additional clarity. The decision to scan should always be a clinical one, made by your treating team based on your specific situation.