Who Should Not Have — a PET-CT Scan?

Having diabetes does not prevent a PET-CT, but it does require preparation. The FDG radiotracer used in most scans behaves like glucose, so a high blood glucose level on the day reduces image quality and can make results unreadable. Most imaging protocols require glucose to be within a specified range before the tracer is injected — if your reading is outside that range on the day, the scan is rescheduled rather than cancelled. Your diabetologist or oncologist can advise on adjusting insulin or oral medication timing in the days beforehand. Ask the scanning centre what level they require when you book.

Kidney disease affects only the CT contrast component of the scan, not the PET images themselves. If your kidney function is below the imaging centre's threshold, the contrast injection is omitted and a non-contrast CT is used instead for the technical correction the scan requires. This is a standard protocol modification — it does not cancel the scan, and the PET images remain interpretable for most clinical questions. Bring your most recent blood results showing creatinine or eGFR to the appointment so the team can make the decision on the day.

The pause duration depends on which radiotracer is used. Your imaging team will give you the specific instructions for your scan, and you should ask for these in writing before the appointment so you can plan. During the pause, milk should be expressed and discarded rather than stored or given to the baby. Do not assume the duration is the same as for other procedures or other tracers — confirm directly with the scanning centre when you book.

Pregnancy comes closest to an absolute bar. The radiotracer exposes a developing baby to radiation that serves no benefit to them, and in almost all situations the scan is deferred until after delivery. There is no other situation that constitutes an absolute contraindication in the same way. Every other barrier — high blood sugar, reduced kidney function, recent surgery, contrast allergy — can be worked around by modifying the protocol or adjusting the timing. The decision is always made by your oncologist and the imaging team together based on your individual clinical situation.

Tell them about pregnancy or the possibility of it, breastfeeding, diabetes and your usual glucose control, any known allergy to contrast dye, your most recent kidney function results if you have kidney disease, any surgery or biopsy in the past four to six weeks, and any anxiety about enclosed spaces. Giving this information when you book — not on the day — allows the team to modify the protocol or arrange sedation in advance rather than rescheduling the scan entirely. A cancelled appointment delays your result and, with it, your treatment decision.

It depends on what the scan was intended to answer. In many situations the PET-CT can proceed without the CT contrast component, which still provides the nuclear medicine images your oncologist needs. Where the scan must be deferred entirely, contrast-enhanced CT or MRI can address many of the same staging and response-assessment questions. Your oncologist will explain which alternative scan answers the clinical question and what, if anything, is lost by using it. Being unable to have the standard protocol does not automatically mean the clinical question goes unanswered.