PET-CT and PET-MRI: Why Scanners Are Combined, and What Changes

A PET image on its own can look like a cloud of bright patches with no map underneath. It tells you that something is metabolically active, but the body around it is nearly invisible. Hybrid scanners solve this by taking a second scan, either CT (computed tomography, which uses X-rays) or MRI (magnetic resonance imaging, which uses a strong magnet and radio waves), in the same session. This page explains why CT became the standard partner, what PET-MRI offers and gives up, what the combination changes for the person on the table, and where the limits and costs lie. The signal that PET detects is explained in the physics of positron emission and annihilation.

Why CT was added to PET

The idea of combining the two was put forward in the early 1990s by David Townsend, Ron Nutt and colleagues. In a historical account in Seminars in Ultrasound, CT and MR, Townsend writes that the first prototype became operational in 1998 at the University of Pittsburgh, and that the first commercial PET/CT scanner to be announced was the GE Discovery LS in early 2001. His summary of the benefit is short: the co-registered anatomy localised functional abnormalities and clarified equivocal situations. By 2006, he adds, PET-only scanners could no longer be obtained from the major suppliers. More on the early years is in the short history of positron imaging.

That change was quick and complete. The International Atomic Energy Agency (IAEA) handbook Nuclear Medicine Physics says the spatially registered anatomical information from the CT component provided the impetus for widespread acceptance of PET in oncology, and that combined scanners have effectively replaced stand-alone PET for clinical use. RadiologyInfo.org, the patient site of the Radiological Society of North America and the American College of Radiology, states that combined PET/CT scanners perform almost all PET scans today and may provide more accurate diagnoses than the two scans done separately.

The second job: attenuation correction

CT does more than draw a map. As explained on the physics page, some of the photons that PET relies on are absorbed or deflected inside the body, and the scanner needs to know how much tissue lay in the way to correct for it. Before hybrid machines, that information came from a separate transmission scan, using a radioactive source, that took a long time to collect. The IAEA handbook describes CT-based correction as having very low noise, fast acquisition, insensitivity to radioactivity in the body and no need to replace decaying sources.

The handbook adds a point many people find surprising: the main advantage is speed, not better accuracy, because the older transmission sources were perfectly adequate when scanned long enough. With multi-detector CT, low-noise images of the whole body can be obtained in a few seconds, which substantially shortens the overall examination. Shorter scans improve comfort, reduce the chance of motion, allow "arms up" positioning to be tolerated, and let a centre scan more people in a day.

There is one technical wrinkle. CT uses X-rays far less energetic than the 511 keV photons of PET. A 2022 review by Alqahtani notes that CT photon energies are typically 100 to 140 keV, so the CT attenuation map has to be scaled up to 511 keV before it is used. A review of attenuation correction for PET-MR by Krokos and colleagues agrees that, once converted correctly, CT-based values give highly accurate results.

What a hybrid scan changes for the patient

In practice, you lie on one bed that moves through two linked machines. The handbook explains that PET and CT data are acquired one after the other, not at the same time, so the CT picture is taken at one moment and the PET at another. The PET portion still typically needs many minutes of data collection, and the handbook warns that movement between the two scans can misalign the images, blur detail and produce characteristic artefacts. Staying still, and breathing quietly if asked, is therefore part of the job.

The CT part comes in two broad flavours. For many oncology scans, the handbook says, state-of-the-art CT is not necessary: low-dose protocols are favoured, with reduced X-ray tube current and typically no intravenous contrast dye. Where a diagnostic-quality CT is wanted as well, the settings and the dose go up. The radiation difference is real. The FDA's 2023 briefing on PET drugs cites literature giving an average CT dose of 5.0 mSv and a total of 14 mSv for a standard FDG PET/CT with attenuation-correction CT, versus 15.4 mSv for CT and 24.4 mSv in total for a diagnostic-quality CT. The unit, the millisievert (mSv), and what these numbers mean are covered in radiation dose in context. For a walk-through of the day itself, see what to expect on scan day.

The hardware has also become friendlier over time. According to the handbook, early PET/CT systems were separately developed PET and CT components that operated independently, and later designs removed the PET transmission sources and moved toward greater integration. In some designs this allowed a larger patient opening, which helps people of larger body size, and in others a gap between the two gantries that gives staff better access to the patient during scanning. Details vary by manufacturer and by centre, so it is reasonable to ask what the machine you will use looks like.

PET-MRI: what is different

PET-MRI pairs the PET detectors with a magnetic resonance scanner. In a 2017 review of where PET/MRI might replace PET/CT, Ehman and colleagues describe its main attractions as better soft-tissue contrast and no ionising radiation from the MRI part, plus the ability to acquire PET and MRI data together. They argue that MRI is the accepted test of choice for most non-traumatic anatomical questions in the brain, and that the pelvis and children's imaging, where reduced radiation matters, are areas of potential advantage. Krokos and colleagues likewise list lower radiation to the patient among the benefits already seen. Note that PET-MRI still involves a radioactive tracer, so the tracer's own exposure remains; only the CT part is removed.

The engineering is harder. The IAEA handbook points out that conventional photomultiplier tubes, the usual light sensors in PET detectors, are bulky and highly sensitive to magnetic fields, so PET-MR systems use semiconductor alternatives such as avalanche photodiodes. A second problem is attenuation correction. MRI measures something quite different from X-ray absorption. As Ramalho and colleagues put it, the MR signal reflects proton density instead of photon attenuation, so converting it to the numbers PET needs is challenging. Krokos and colleagues add that tissues giving no MRI signal, such as bone and air, can cause errors in bone and in lesions near bone. Ehman and colleagues note that MRI has particular difficulty with lung, and that PET/MRI may miss small lung nodules of under 5 to 10 mm that PET/CT can show, which is one reason PET/CT is still preferred for many chest questions.

Time is the third difference. Ramalho and colleagues explain that patient positioning takes longer because MR coils are placed on the body and MR sequences are long; they give about 15 to 20 minutes for the PET data of a whole-body exam but 60 to 90 minutes to cover the same territory with a full range of MR sequences. Ehman and colleagues say exams may take an hour or longer.

PET-CT and PET-MRI side by side (summary of the sources cited here)

Feature	PET-CT	PET-MRI
Second scan uses	X-rays	Magnet and radio waves
Radiation from the second scan	Yes, from low to substantial depending on protocol	None from the MRI part
Basis of attenuation correction	Measured X-ray absorption, scaled to 511 keV	Estimated from MR images, harder near bone and lung
Typical session length	Shorter; CT takes seconds	Often an hour or more
Availability	Almost all PET scans today	Not available everywhere

Limits, cost and availability, stated honestly

PET-MRI is the rarer technology. Ehman and colleagues counted approximately 30 PET/MRI scanners in the United States against more than 1,600 PET/CT systems at the time of their 2017 paper; that figure is dated, and the proportion has probably shifted. A 2023 review by Krokos and colleagues notes that thirteen years after the first PET-MR installation the scanners still made up a very small share of the hybrid PET systems installed. RadiologyInfo.org describes PET/MRI as an emerging technology not currently available everywhere. Ramalho and colleagues mention the "financial concerns of replacing a very expensive imaging modality with an even more expensive one", and Ehman and colleagues list higher cost and complexity, the need for technologists who know both nuclear medicine and MRI, and indications that were still being developed.

These points are general. We do not state prices, because costs differ widely between health systems and between centres, and they are not what decides your examination. Your referring doctor and imaging team choose the scanner that fits your question and what is available. Nor does a hybrid scan remove the need for expert reading. The CT or MRI can be reported alongside the PET, and what each adds to the final report is discussed in reading a PET report. For how these scanners are used in specific conditions, see PET in cancer care and PET in dementia and memory evaluation.

A practical note: an MRI scanner contains a powerful magnet. If your scan involves one, ask your centre what screening applies to you and what the session will involve, because procedures differ.

The short version

CT was added to PET for two reasons: it shows where abnormal activity sits, and it supplies a fast, low-noise map for correcting the PET data. PET-MRI trades the CT radiation for better soft-tissue contrast, but it takes longer, is harder to correct for attenuation near bone and lung, costs more and is available in far fewer places. Which scanner you use depends on your clinical question and your local equipment, and your imaging team has the final word.