PET-CT vs CT vs MRI: How They Differ and When Each Is Used in Cancer

CT and MRI show anatomy: where the tumor is, how big it is and how it relates to nearby organs. PET-CT adds information about activity — how intensely the tissue takes up glucose. It isn’t a “more accurate version of CT” but a different tool for a different question, so one scan doesn’t replace another.

How the three scans differ

CT (computed tomography) is a series of X-ray images that a computer assembles into a 3D picture. It’s fast and widely available, and shows the lungs, liver, lymph nodes and bones well. It uses X-ray radiation.

MRI (magnetic resonance imaging) uses a magnetic field, with no radiation. It sees soft tissues best: the brain and spinal cord, liver, pelvis and the soft tissues of the limbs. The scan takes longer and is noisier than CT.

PET-CT (positron emission tomography combined with CT). Before the scan, radioactively labeled glucose (FDG, fluorodeoxyglucose) is injected into a vein. Tissues with an active metabolism — including many tumors — take it up more strongly and “light up” on the image. The CT in the same machine shows exactly where that uptake is.

When each scan is used

The general logic is this.

• CT is the main workhorse for staging (working out how far the disease has spread) in many tumors and for monitoring treatment. Lesions are most often measured on CT before and after treatment cycles.
• MRI is used when a specific area needs to be seen in detail: the brain, spine, liver, pelvis. Sometimes MRI is ordered if CT has given an unclear picture.
• PET-CT, according to Cancer Research UK, helps with diagnosis and staging, choosing treatment, looking for recurrence and planning radiotherapy. After treatment it sometimes helps tell scar tissue from active tumor.

Which scan you need is determined by the type of tumor and the question your doctor is trying to answer right now. Having a PET-CT “just in case” instead of the CT you were sent for is a bad idea: you may get an unclear finding and not get the measurements that are needed. If you have several reports and it’s not clear what they add up to, I’ll go through your documents in a consultation.

Why contrast is needed and who should be careful with it

Contrast makes blood vessels and organs stand out more on the image, and many tumor lesions are harder to see without it.

• For CT, iodine-based contrast is usually used. It carries a small risk to the kidneys, so your kidney function may be checked with a blood test beforehand. If you’ve ever had an allergy to contrast, be sure to tell your doctor in advance.
• For MRI, gadolinium-based contrast is used. You also need to mention kidney disease, diabetes and any past reactions to contrast.
• Before an MRI, be sure to report any implants: a pacemaker, cochlear implant, brain aneurysm clips, neurostimulator, artificial heart valves, metal fragments, a drug pump. The magnetic field can interact with them. Many modern devices are MRI-compatible, but the exact model needs to be known.

If you are pregnant or might be, say so before any scan: CT and PET-CT are avoided during pregnancy where possible.

Why PET-CT is sometimes wrong

PET-CT doesn’t show “cancer” — it shows increased glucose uptake. Other processes cause that too. A review in RadioGraphics (2012) lists among the causes of false-positive uptake:

• inflammation and infections — pneumonia, abscess, tuberculosis, sarcoidosis, pancreatitis;
• healing after surgery, biopsy or a fracture;
• inflammation after radiation therapy and chemotherapy;
• brown fat (in the neck, above the collarbones, in the armpits);
• some benign tumors.

The opposite also happens — a false-negative result, when a tumor is present but there’s no uptake:

• very small lesions (according to the same review, roughly up to 6 mm) may not be detected;
• some tumors naturally take up little glucose: carcinoids, some low-grade lymphomas, some liver and kidney tumors, mucinous (mucus-producing) and necrotic tumors.

A finding on PET-CT is a reason for discussion, not a final conclusion. If the treatment plan depends on the result, a doubtful lesion often needs to be confirmed by biopsy.

That’s why, if PET-CT is done to assess treatment effect, the doctor takes into account when chemotherapy, surgery or radiation ended: inflammation after treatment can itself cause uptake.

What SUV means and why it isn’t a verdict

SUV (standardized uptake value) is a number that shows how actively a lesion has taken up the tracer compared with the average across the body. Patients often ask me what SUV is “bad”. There is no universal threshold.

A systematic review in AJR (2010) looks at what SUV depends on besides the tumor itself:

• blood sugar level — high blood sugar competes with the tracer and lowers the SUV;
• the time between injecting the tracer and scanning;
• body weight and composition;
• the scanner model and image processing settings;
• lesion size — SUV is underestimated in small lesions.

The practical conclusion: SUV values from different centers and different scanners can’t be compared directly. To monitor treatment, I ask, where possible, for repeat PET-CT scans to be done at the same center. A high SUV on its own says nothing about stage or prognosis — it’s read together with the whole picture.

How response to treatment is assessed: RECIST in a nutshell

So that doctors around the world understand “the tumor has shrunk” or “has grown” in the same way, the RECIST 1.1 criteria are used. The radiologist chooses up to 5 measurable lesions (no more than 2 per organ) and adds up their sizes.

Stable disease isn’t a bad result: with many types of treatment it’s an expected and workable outcome. More on how stage and spread affect the plan in my article on TNM staging. And I wrote about why tumor markers don’t replace imaging in my article on tumor markers.

How to prepare for a PET-CT

According to Cancer Research UK:

• don’t eat for 4–6 hours before the scan; plain water is usually fine;
• if you have diabetes, call the center in advance: your meals or appointment time may need to be adjusted — high blood sugar distorts the result;
• for 24 hours beforehand you may need to avoid strenuous exercise;
• usual medicines are generally taken as normal unless you’re told otherwise;
• allow about 2 hours: the tracer is injected about an hour before the scan, and you need to rest quietly during that hour;
• after the scan, drink plenty of fluids and avoid close contact with pregnant women, babies and young children for the rest of the day;
• if you are breastfeeding, let the center know a few days in advance.

For CT with contrast, you’re sometimes asked not to eat for 2–4 hours. The exact preparation rules always come from the center where you’re having the scan.

What to do with your scan results

The radiologist’s report is a description, not a ready-made treatment plan. It’s interpreted by your oncologist together with the histology and other findings. What I advise after any scan:

• collect not only the report but the images themselves (a disc or a download link) — you’ll need them for comparison and for a second opinion;
• keep all your scans in one folder by date, including the very first ones — they are what later scans will be compared with;
• at a repeat scan, ask the radiologist to compare it with previous images and to state which one exactly.

The phrase “no change” or “stable disease” in a report usually describes the result compared with the previous scan, so it’s important to know which one it was compared with.

Questions to ask your doctor

• What question are we answering with this scan?
• Is contrast needed, and do I have any contraindications to it?
• Will we compare the result with previous images, and where should they be?
• If an unclear lesion shows up on PET-CT, what will we do next?
• Where is it best to have repeat scans so the results are comparable?

Sources

1. PET-CT scan — Cancer Research UK
2. CT scan — Cancer Council Australia
3. MRI scan — Cancer Council Australia
4. Image-guided Biopsy: What the Interventional Radiologist Needs to Know about PET/CT — Kobayashi K. et al., RadioGraphics (RSNA), 2012
5. A Systematic Review of the Factors Affecting Accuracy of SUV Measurements — Adams M.C. et al., American Journal of Roentgenology, 2010
6. RECIST 1.1 — EORTC; Eisenhauer E.A. et al., European Journal of Cancer, 2009