PET-MRI

For Medical Students

What It Shows

Positron emission tomography combined with magnetic resonance imaging shows where an injected tracer is taken up, overlaid on the anatomy. 5.2.12 covers the basic principles.

The uptake reflects metabolic activity rather than structure. 5.13.1 covers bone marrow oedema.

It matters because no structural study can say whether a lesion is active, and activity is often the clinical question. 5.11.7 covers bone marrow lesions.

How It Is Acquired

Inject a tracer and acquire the emission data while the magnetic resonance images are acquired. 5.2.12 covers the basic principles.

Register the two datasets, so the uptake can be localised to a structure. 5.2.23 covers the reconstruction.

Record the tracer used, since different tracers report different processes. 5.13.7 covers bone tumours.

Record the time from injection, because uptake changes with it. 5.20.1 covers structured reporting.

Account for the radiation dose the tracer carries. 5.2.24 covers the radiation dose.

What Normal Looks Like

Low uptake in quiescent bone and soft tissue. 5.5.6 covers the bones.

Physiological uptake at the sites known to take up that tracer. 5.13.1 covers bone marrow oedema.

Registration that places the uptake inside the structure it belongs to. 5.2.23 covers the reconstruction.

Symmetry between sides where the joint is normal. 5.20.1 covers structured reporting.

What Abnormal Looks Like

Focal uptake at a site of structural change, marking it as active. 5.11.7 covers bone marrow lesions.

Uptake with no structural abnormality, which structural imaging would have missed. 5.13.2 covers stress reactions.

Structural change with no uptake, which suggests an inactive or old lesion. 5.13.5 covers osteonecrosis.

Diffuse synovial uptake in an inflamed joint. 5.14.1 covers synovitis.

MRCS Level

How It Is Acquired

In a viva the expected answer separates activity from structure.

The expected points are the tracer, the simultaneous acquisition, the registration, the timing, and the dose.

Say that the combination gives both in one sitting, which two sequential studies do not. 5.2.23 covers the reconstruction.

Say that uptake is not specific to a diagnosis. 5.14.7 covers septic arthritis.

Say that availability and cost confine this to research and to selected cases. 5.1.5 covers the cost-effectiveness of imaging.

What Abnormal Looks Like

The reasoning expected is that structural imaging shows the consequence of a process and metabolic imaging shows the process, so the two answer different questions about one lesion. 5.11.7 covers bone marrow lesions.

Bone marrow change is the clearest case, because its appearance is the same whether it is progressing or resolving. 5.13.1 covers bone marrow oedema.

Uptake rises in infection, in inflammation, in healing and in tumour, so the finding localises activity without naming its cause. 5.14.7 covers septic arthritis.

Simultaneous acquisition registers better than two separate studies, which is the technical argument for the combination. 5.2.23 covers the reconstruction.

How Good It Is

Combined metabolic and structural imaging is the only way to report activity and cause in one study, and it reports activity without specificity.

Its best contribution is separating an active lesion from an inactive one. 5.13.5 covers osteonecrosis.

It is limited by cost, by availability, by the tracer dose, and by uptake that is not specific to any diagnosis. 5.1.5 covers the cost-effectiveness of imaging.

No figures appear for accuracy on this page, under decision 028, and no uptake values, tracer doses or timing intervals appear either: those are published and local protocol quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the question it answers is whether to wait or to act. 5.13.2 covers stress reactions.

An inactive lesion with structural change is a different problem from an active one. 5.13.5 covers osteonecrosis.

Infection against aseptic loosening after arthroplasty is the case most often raised for it. 5.17.9 covers the revision arthroplasty.

The cost and the availability mean the question is usually answered by aspiration, bloods and time. 5.1.3 covers choosing the correct investigation.

A knee specialist is more likely to read a report of one than to request one. 5.20.8 covers common reporting errors.

Where It Misleads

The commonest error is reading uptake as a diagnosis. 5.14.7 covers septic arthritis.

Physiological uptake is read as pathology. 5.5.6 covers the bones.

A registration error places the uptake in the wrong structure. 5.2.23 covers the reconstruction.

Metal degrades the magnetic resonance component and the attenuation correction together. 5.2.19 covers the artefacts.

Uptake after recent surgery is read as infection. 5.20.7 covers reporting the post-operative knee.

Fellowship Level

Where It Misleads

At fellowship level this appears in referrals from other specialties and in research rather than as a knee study. 5.13.7 covers bone tumours.

The tracer determines the question, and a study acquired for one question gets reread for another. 5.20.8 covers common reporting errors.

Normal uptake over time after surgery has not been established for most knee procedures. 5.20.7 covers reporting the post-operative knee.

The dose is justified by a decision that changes, so a study confirming what was already planned did not justify it. 5.2.24 covers the radiation dose.

Quantifying uptake carries the same standardisation problem as the rest of this chapter. 5.18.1 covers the quantitative MRI.

The honest position is that this technique answers a question nothing else can and is reached for so rarely in knee practice that most surgeons will meet it as a report rather than as a request. 5.1.5 covers the cost-effectiveness of imaging.