Quick answer: PET-CT and MRI answer different questions. PET-CT shows how active tissues are: cancer cells often use more sugar, so they light up, which makes PET-CT good for finding cancer throughout the whole body, checking whether it has spread and seeing whether treatment is working. MRI shows soft tissue in fine detail without radiation, which makes it better for looking closely at specific areas such as the brain, spinal cord, liver, prostate, pelvis and breast. Many people with cancer have both at different points in their care.
I work with both scanners, and patients often ask me why they need «another scan» when they have just had one. The answer is usually that each test sees something the other cannot.
Function vs anatomy
- PET-CT combines two scans in one session. The PET part uses a radioactive tracer, most often FDG, a form of glucose, to show metabolic activity: areas using a lot of sugar appear as «hot spots.» The CT part shows anatomy so the hot spots can be located precisely. Because it detects changes at the cellular level, PET can sometimes show disease before it is visible on CT or MRI.
- MRI uses a strong magnet and radio waves to produce very detailed images of soft tissue: the exact size, shape and boundaries of a tumor and its relationship to nearby structures. It uses no ionizing radiation.
One way to picture it: PET-CT is a map of the whole body showing where something is active; MRI is a close-up photograph of one area showing exactly what it looks like.
PET-CT vs MRI at a glance
| PET-CT | MRI | |
|---|---|---|
| Shows | Metabolic activity plus anatomy | Detailed soft-tissue anatomy |
| Coverage | Usually the whole body in one scan | Usually one body area per exam |
| Radiation | Yes: about 22.7 mSv for a whole-body PET-CT | None |
| Contrast | Radioactive tracer; sometimes CT contrast | Sometimes gadolinium |
| Preparation | Fasting, low-carb diet, no exercise, blood sugar control | Usually little; screening for metal and implants |
| Time | About 2 hours total, including 30 to 60 minutes of rest after the injection | Usually 30 to 60 minutes in the scanner |
| Machine | Short, open ring | Longer, louder tube |
| Image resolution | Lower than CT or MRI, but very sensitive to activity | High detail |
When PET-CT is usually chosen
- Staging: finding out whether a cancer has spread to lymph nodes or other organs
- Checking response to treatment: active tumors light up; tumors that respond to treatment become less active, sometimes before they shrink
- Looking for recurrence: checking whether cancer has come back after treatment
- Finding the most active area to target for a biopsy or radiation therapy planning
- Cancers where it is commonly used include lymphoma, lung cancer, head and neck cancers, colorectal cancer and melanoma
See [link: PET Scan: What to Expect From Start to Finish].
When MRI is usually chosen
- Brain and spinal cord tumors, where MRI shows the most detail, and where the brain’s normal high sugar use limits FDG PET
- Prostate cancer in the prostate itself, and rectal, cervical and uterine cancers in the pelvis
- Liver lesions, to characterize spots seen on other scans
- Breast cancer: MRI for high-risk screening or to assess the extent of a known cancer
- Bone and soft tissue tumors of the limbs
- When radiation should be avoided, for example in pregnancy or when many follow-up scans are needed
See [link: Brain MRI] and [link: Breast MRI: Who Needs It and What to Expect].
Why you might need both
The two tests complement each other. A common sequence is an MRI to look closely at the main tumor and a PET-CT to check the rest of the body for spread. Later, either may be repeated to see how treatment is working. Neither is «better» overall; your care team chooses based on the type of cancer, its location and the question they need to answer.
From the scanner room: [TU EXPERIENCIA: un recorrido típico (sin datos identificables) de un paciente oncológico entre RM y PET-TC y por qué se pide cada prueba. Qué diferencias notan los pacientes entre las dos.]
Limitations to keep in mind
PET-CT:
- Not every hot spot is cancer: inflammation, infection and normal organs such as the brain, heart and bladder also take up tracer.
- Some slow-growing cancers do not take up much FDG and can be missed; other tracers, such as PSMA for prostate cancer, are used in those cases. See [link: PSMA PET Scan for Prostate Cancer: A Patient Guide].
- Results depend on good preparation, especially blood sugar. See [link: How to Prepare for a PET Scan (Diet, Diabetes, Exercise)].
- It involves more radiation than most imaging tests.
MRI:
- Usually covers one area at a time, so it is less practical for checking the whole body.
- Takes longer and is louder, which can be hard for some patients.
- Some implants and devices require special checks or rule it out. See [link: Can You Have an MRI With Metal Implants, Tattoos or a Pacemaker?].
What about PET-MRI?
Some centers have PET-MRI scanners that combine PET with MRI instead of CT, giving activity and detailed soft-tissue images in one session with less radiation than PET-CT. It is an emerging technology and not yet available everywhere.
Tips from the technologist
[TU EXPERIENCIA: 2-3 consejos propios. Ideas: llevar los estudios previos de las dos pruebas para comparar, no programar PET y RM con contraste en el mismo día sin preguntar, preguntar al equipo qué pregunta quiere responder cada prueba.]
Frequently asked questions
Is a PET scan more accurate than an MRI for cancer?
It depends on the cancer and the question. PET-CT is better for finding activity and spread throughout the body; MRI is better for looking closely at certain organs and soft tissues.
Why did my doctor order a PET scan after an MRI?
Often to check whether the cancer has spread beyond the area the MRI looked at, or to see how active a finding is.
Can an MRI detect cancer spread?
Within the area scanned, yes. Some centers also use whole-body MRI for specific situations, but PET-CT is more commonly used to look for spread throughout the body.
Which has more radiation, PET-CT or MRI?
MRI uses no ionizing radiation. A whole-body PET-CT is about 22.7 mSv. See [link: How Much Radiation Is in a CT Scan?].
Can PET and MRI be done on the same day?
Sometimes. Ask your center, because preparation and timing differ between the two tests.
This article is for general information and does not replace advice from your doctor or care team.