Muscle Uptake on Your PET Scan: — What Causes It and Whether It Matters

Not necessarily. Radiologists routinely note muscle uptake and account for it when reading the scan. Whether it affects your results in a meaningful way depends on where the uptake is relative to the areas your oncologist needs to evaluate. Your oncologist will tell you whether the findings they needed to see were readable. If there is genuine uncertainty about a particular area, a repeat scan with better preparation may be recommended — but that is a decision made with your full clinical picture, not from the report alone.

Yes. The muscles you use to speak — in your throat, jaw, and around your larynx — absorb FDG when they are active. In the neck region, where lymph nodes are assessed in many cancer staging scans, this uptake can make interpretation harder. Most PET centres ask patients not to talk during the uptake period for exactly this reason. Reading quietly or sleeping are both fine.

Tell the team before the scan begins. They can note it in the report, which helps the radiologist interpret any muscle uptake they see. In some cases the team may suggest rescheduling so the scan is as readable as possible. That decision depends on how urgent the scan is and which part of your body is being assessed. Do not say nothing — the team needs that information to give your oncologist an accurate report.

Shivering is your body generating heat by contracting muscles repeatedly. Those contractions use glucose, and because FDG behaves like glucose, shivering muscles absorb it. Cold also activates brown adipose tissue — a type of fat that burns energy to produce warmth — and this appears as uptake in the neck and shoulders. Neither is disease. Both can be prevented by staying warm from the moment you arrive until your scan is complete.

Radiologists are trained to recognise the patterns that separate muscle uptake from disease. Muscle uptake tends to be bilateral, follows the shape of known muscle groups, and does not produce the focal, asymmetric signal typical of a tumour. Where there is any genuine uncertainty, the radiologist will say so in the report, and your oncologist will decide whether further investigation is needed. A report describes what was seen — what it means for your situation is a conversation to have with your treating team.

Possibly, at the injection site and in nearby lymph nodes. Vaccine injections cause a local immune reaction that increases metabolic activity — both muscle and lymph nodes in the area will absorb FDG. This is a known issue in PET preparation. Tell the team which arm was injected and when. If the affected area is important to your cancer assessment, your oncologist may recommend waiting before scanning, or will review the results with this context in mind.