Cardiac FDG PET for viability assessment

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For most essential points to best prepare for the procedure, please download the patient information brochure (PDF).
For a more elaborate explanation of the procedure and related points, please continue to read ahead.

Brief introduction

Principle

Concepts related to functioning of heart, its blood supply (coronary arteries), associated abnormalities, and their evaluation and treatment have been covered in the article on myocardial perfusion imaging (MPI).

Completely normally functioning heart muscle cells ('cardiomyocytes') tend to use fatty acids for energy production provided they have adequate blood supply and relatively less workload (e.g., in resting state). If workload increases or blood supply reduces, they tend to switch to glucose for energy production. RPs used in MPI (MIBI and tetrofosmin) are concentrated in cells with active mitochondria, which in turn are present in energy-consuming cells (e.g., cardiomyocytes). Fluorodeoxyglucose (FDG) is a radioactive molecule that behaves similarly to glucose. Thus, a cardiomyocyte that is 'alive' would take up at least MIBI or FDG.

Parts of myocardium that receive less blood on sustained basis reduce their blood supply and motion in order to reduce the oxygen requirement, and so as to not get damaged. This state is known as 'hibernation'. Thus, hibernating myocardium can seem to be non-viable ('dead') on MPI (which detects blood supply and heart motion) as well as echocardiography (which detects heart motion). However, hibernating myocardium continues to take up glucose, and that is how its presence can be ascertained by FDG PET.

If the myocardium served by stenosed coronary artery is adjudged to be non-viable, performing revascularization (PCI and/or CABG) would prove unhelpful despite the associated risks and financial cost. Hence, in certain situations it is imperative to establish viability of the myocardium before attempting revascularization. The size of hibernating myocardium (as delineated by FDG PET) predicts magnitude of improvement in symptoms and survival following revascularization.

Indications (conditions in which advised)

Detecting and quantifying hibernating myocardium before attempting PCI in non-emergency cases.
Detecting and quantifying hibernating myocardium before attempting CABG.

Considerations / steps before procedure

Prior appointment is a must as FDG spontaneously decays, and is not readily available at the diagnostic facility.
Patient must not be pregnant.
If breast feeding, please inform the staff for further directions.
6 hours’ fasting before scan.
No diabetes related drugs (oral medications or insulin) for 6 hours before scan.
Please let the staff know at the time of appointment if the patient is unable to lie still for ~20 min, has breathing difficulty, or might need special medical attention for the duration of visit.
Have at least 1/2 litre water within an hour before reporting for the scan. There is no need to hold urine.
There is no need to hold urine before reporting for the procedure.
Wear loose-fitting but warm clothes.
Do not bring expensive jewellery or accessories along with you for the scan.
Proof of doctor's request is necessary for starting the procedure.
The patient must be accompanied by a responsible carer.

What to expect as part of procedure?

Important steps

Depending upon the clinical situation, cardiac FDG PET could be performed alone, or could be combined with resting MPI study, the steps for which are outlined here. Following are the steps of cardiac FDG PET alone.

Patient registration and consent.
Handing over medical documents to the staff.
Blood glucose level is measured through finger prick method.
25 to 50 g glucose is to be had orally.
Blood glucose level is measured again.
If needed, insulin is injected just deep to the skin.
Blood glucose measurement and/or insulin injection may have to be repeated a few times to ascertain desired response.
Upon ascertaining desired changes in blood glucose level, FDG is injected.
Waiting time of ~1 hour (± 15 min) to allow sufficient time for FDG to be taken up and eliminated by organs in the chest region. Pass urine just before the scan, and also before that (if needed).
Patient lies on their back on the scanner table, and the region of heart is scanned for 12 to 15 minutes.
Scan is briefly reviewed for adequacy and quality. Occasionally, a scan may be repeated. If everything is to staff’s satisfaction, then the patient may leave.

Discomfort / risks during procedure

Pain during IV cannula insertion.
There is risk of extravasation at the time of IV cannula insertion or FDG injection. [Extravasation means that blood leaks through the walls of vein punctured by cannula’s needle].
Oral glucose can cause nausea.
Insulin can cause sudden lowering of blood glucose level ('hypoglycaemia'), which in turn is associated with symptoms like excessive sweating, giddiness, light-headedness, etc.
FDG does not cause any discomfort. It is injected in very minute quantity, so it does not alter function of any organ.

Time taken for the entire patient visit

Typical: 5 hours.
Be prepared for: 7 hours (can be further prolonged in case of increased blood glucose level or other unforeseen events like malfunctioning of scanner, delayed delivery of FDG, etc.).

What to expect after procedure?

Precautions / special care

Maintain at least 1 metre-distance from pregnant ladies and children (below 10 years) for at least 5 hours after the scan.
Drink plenty of water throughout the day (3 to 4 litres).
Pass urine as frequently as possible.
Look for symptoms of hypoglycaemia described above. If need be, have a carbohydrate-rich meal.

Discomfort / risks

Possible hypoglycaemia.
Both FDG and X-rays expose the patient to ionizing radiation. However, radiation dose received as part of the entire procedure is not expected to cause any obvious harmful effect. Additionally, benefit from information received through the procedure greatly outweighs any theoretical risk associated with this radiation exposure.

Time taken for report generation

Typical (for self-paying patients): 12 hours.
Be prepared for: 36 hours (especially if some patient data or past scan data are missing).
Credit patients will receive their reports from the respective referring hospitals.

Additional reading

For additional information, please visit the following links.

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