PET/CT is not only for cancer: brain, heart and hidden infection

Ask most people what a PET scan is for and they will say cancer. That is fair — oncology is the majority of what any PET centre does. But the underlying principle, that metabolism changes before structure does, applies just as well elsewhere, and in three other fields PET answers questions nothing else can.

The brain: which dementia is it?

An MRI shows the brain's shape. It reveals shrinkage, strokes, tumours and fluid. What it cannot show is how well the surviving tissue is actually working — and in degenerative disease, function falls years before shrinkage appears.

FDG PET of the brain maps that function directly, and the different dementias have distinct signatures:

• Alzheimer's disease typically reduces metabolism in the parietal and temporal regions and the posterior cingulate, often before there is anything to see on MRI.
• Frontotemporal dementia affects the frontal and anterior temporal lobes, and tends to present with personality and behaviour change rather than memory loss — a distinction that matters enormously to families trying to understand what is happening.
• Dementia with Lewy bodies involves the occipital cortex, which the others spare.
• Vascular cognitive impairment produces a patchy, scattered pattern following the territory of damaged vessels.

Why does the distinction matter if none of them is curable? Because the answers differ. Cholinesterase inhibitors help in some and not others. Certain antipsychotics are dangerous in Lewy body dementia. Frontotemporal dementia in a younger patient carries different genetic implications for the family. And — not least — knowing that a parent's behaviour is a disease rather than a choice changes how a family copes with it.

PET is also used to identify potentially reversible causes of cognitive decline that have been mistaken for dementia.

The brain: where are the seizures starting?

About a third of people with epilepsy continue to have seizures despite appropriate medication. For focal epilepsy, surgery on the responsible area can be curative — but only if that area can be located precisely.

MRI finds a structural cause in many cases. When it does not, FDG PET frequently can. Between seizures, the responsible zone consumes less sugar than the tissue around it and appears as a cool spot. In MRI-negative epilepsy, PET is often the study that makes surgery possible at all.

For this scan, tell us when your most recent seizure was — a very recent one changes the interpretation — and continue your medication exactly as prescribed.

The heart: is this muscle dead, or just starved?

After a heart attack, part of the heart muscle stops contracting properly. The critical question before bypass surgery or angioplasty is whether that muscle is scar — permanently dead — or hibernating: alive, but so short of blood supply that it has shut down to survive.

On an echocardiogram both look identical. They demand opposite decisions. Operating on scar exposes a patient with a weak heart to surgical risk for no benefit. Not operating on hibernating muscle leaves recoverable function on the table.

Hibernating muscle is still alive and still consumes glucose, so it takes up FDG. Scar does not. Segments that light up will usually regain contractile function once blood flow is restored, and that is the evidence a cardiac surgeon wants before accepting the risk.

This study uses a specific glucose-and-insulin preparation rather than simple fasting, so our technologist will call you the day before with exact instructions.

PET/CT is also the reference test for cardiac sarcoidosis, where inflammation in the heart muscle can cause dangerous rhythm disturbances and needs a very different treatment from ordinary heart failure.

Hidden infection: fever of unknown origin

Someone has had a fever above 38.3 °C for more than three weeks. Blood cultures are negative. Ultrasound, chest X-ray and routine CT are unremarkable. Inflammatory markers are high and nobody knows why. This situation is more common than it should be, and it is miserable for the patient.

White blood cells rushing to fight infection burn glucose exactly as tumour cells do. FDG PET/CT therefore screens the entire body in a single pass and shows where the process is, rather than testing one organ at a time in sequence.

Typical culprits it uncovers: a deep abscess, spinal infection (spondylodiscitis), infected prosthetic joints or vascular grafts, endocarditis, large-vessel vasculitis, tuberculosis at an unexpected site — and, in a meaningful minority, an occult cancer or lymphoma that was causing the fever all along.

Just as usefully, PET tells the treating physician where to biopsy. Sampling the most metabolically active site substantially improves the chance of a diagnostic result rather than a repeat procedure.

Tell us about recent antibiotics, steroids and any surgery in the past six weeks — all three change the picture, and steroids in particular can suppress the very signal we are looking for.

Inflammation you can watch

For large-vessel vasculitis — Takayasu arteritis and giant cell arteritis — PET/CT shows which arteries are actively inflamed right now, rather than the permanent damage inflammation has already caused. That distinction is what allows a physician to decide whether to keep a patient on steroids or taper them safely.

The same applies to sarcoidosis, IgG4-related disease and other systemic inflammatory conditions: PET maps how many organs are involved and how active each one is, and it does so quantitatively enough that follow-up scans mean something.

The common thread

In every one of these, PET is not a better photograph. It answers a question about function that no photograph can address — is this tissue working, is it alive, is it inflamed right now. That is a fundamentally different kind of information, and it is why molecular imaging has spread well beyond the cancer centre.

If your neurologist, cardiologist or physician has suggested a PET/CT and you were surprised because you do not have cancer, this is why. Call us on +91 93169 25516 if you would like the specific protocol explained before you come in.