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Bone Mineral Densitometry I Breast Imaging Services I Cardiac Magnetic Resonance Imaging (MRI)

Computed Tomography I Contrast Study I Fluoroscopy I General Radiography I Magnetic Resonance Imaging (MRI)

Nuclear Medicine & PET Scan I Ultrasound

POSITRON EMISSION TOMOGRAPHY (PET)

	What is PET?
	How Does PET Work?
	PET in Cancer
	PET in Heart Disease
	PET in Neurological Disorders
	Benefits & Risks
	Limitations of PET
	Preparation for the Examination
	What Happens on the Day of the Examination?
	After the Examination
	Locations Where PET is Available

WHAT IS PET?

Positron emission tomography, also called PET or a PET scan, is a diagnostic nuclear medicine technique.

All cells need energy to sustain life. PET imaging is unique, which enables us to view and assess the human body from a functional and biochemical perspective, comparing normal and abnormal tissue metabolism, rather than morphological changes as seen by X-ray, CT and MRI.

These images are based upon subatomic particles, emitted from a radioactive substance administered to the patient.

HOW DOES PET WORK?

The most widely used radiotracer in PET imaging is fluorodeoxyglucose (FDG), labelled with F-18.

FDG is glucose analogue. Most of the diseased tissues have a higher rate of glucose utilisation, leading to higher FDG accumulation than healthier tissue. PET exploits this basic principle.

After absorption of these compounds, a scanner records the signals emitted by these tracers. A computer translates these signals into actual images - representing biological causes of normal organ function and failure of organ systems in disease. Affected areas stand out on the images, as regions of increased glucose (FDG) concentration (hot-spots).

PET IN CANCER

It is becoming quite evident that management of many types of cancers can be substantially influenced when PET is incorporated into the diagnostic algorithm. This is applicable to the diagnosis, staging, re-staging, monitoring or treatment response, attempt to locate an unknown primary and detection of recurrence of many malignancies.

The disease is a biological process and functional changes precede anatomical changes. Based on the differences of biological activity, PET evaluation of tissue metabolism can detect the probable presence or absence of malignancy much earlier than anatomical changes.

In addition, whole body imaging with PET examines all the organ systems in one procedure, evaluating not only primary but also metastatic disease.

PET has become an integral part in the management of lung, colorectal, head and neck, breast, oesophageal and uterine cancers, lymphomas and melanomas. It is also helpful in certain types of gastric, hepatobiliary, ovarian, pancreatic and renal malignacies.

PET IN HEART DISEASE

PET scanning allows detection and measurement of inadequate coronary blood flow during stress in coronary artery disease.

The quantitative analysis is not only helpful for early detection of coronary artery disease in high risk patients, but it also helps to monitor the effect of therapy and in risk modification.

The evaluation of myocardial viability (metabolism) is another key area. PET is the gold standard technique for assessment of myocardial viability.

In coronary artery disease, PET is used to:

	Evaluate impact of sudden chest pain on heart metabolism and function (stunning).
	Determine the patients most suitable for revascularisation (presence of hibernating myocardium). Patients with hibernating myocardium have very good chances of functional recovery, after an angioplasty or coronary bypass graft operation.

(PET is not a substitute for Coronary Angiography, neither can it be used as a routine screening test.)

PET IN NEUROLOGICAL DISORDERS

PET has a vital non-invasive role in:

	Differentiating radiation necrosis from tumour recurrence in post-therapy cases.
	Pre-surgical localisation of epileptic focus.
	Early diagnosis of Alzheimer's dementia and differentiating it from other types of dementia. Early and accurate diagnosis is a prerequisite of successful therapy, offering improved quality of life.

BENEFITS AND RISKS

	PET is a functional and metabolic imaging technique. It can detect biochemical alterations to suggest disease presence much earlier than anatomical changes, detectable by anatomical imaging methodologies as CT or MRI.
	With well-planned integration of PET into diagnostic and treatment plans of primary and recurrent malignancies, the numbers of unnecessary diagnostic and surgical interventions are expected to reduce. The diagnostic and surgical procedures with no additional benefits on the morbidity and mortality could be spared, saving cost and unnecessary trauma.
	The most commonly used radiopharmaceutical (FDG) is radiolabelled glucose (sugar), which is present in our bodies and necessary for our energy requirements. It is short-lived and is injected in minute quantity delivering less radiation exposure. It does not affect the normal body functions. (As an estimate, approximately one teaspoon of glucose is sufficient to scan the whole population of Singapore.)
	The radioactive substance may expose radiation to the foetus in pregnant patients or the infants of women who are breastfeeding. The risk to the foetus or infant should be considered in relation to the potential information gained from the PET examination. If you are pregnant, you should inform the PET imaging staff before the examination.

LIMITATIONS OF PET

It is worth remembering that a negative FDG PET scan does not rule out presence of malignancy. A few tumour types depend on protein metabolism rather than glucose. In such cases, a FDG PET scan can be false negative.

	PET can underestimate the severity of disease if blood sugar is too high.
	PET should not be used alone as a diagnostic tool. Its value is enhanced when it is part of a larger diagnostic work-up.

PREPARATION FOR THE EXAMINATION

	PET is usually done on an outpatient basis.
	At the time of scheduling please inform the staff about current medication you are taking and if you have diabetes, are pregnant/nursing or claustrophic.
	Bring along all your previous x-rays, ultrasound, CT or MRI films and any blood test reports.
	Wear comfortable clothing.
	Leave jewellery at home.
	Do not eat any caloric food for at least six hours before your scan. You are encouraged to drink more water. Please avoid all drinks with caffeine and sugar.
	Try not to smoke on the day of the scan.
	There should be no change in your medication.
	Diabetic patients need to regulate their blood glucose level to normal and must expect longer stay in the department.
	You can expect to be at the department for two to three hours. The actual scan itself takes far less time.
	Please be punctual. If you need to reschedule, notification must be given 48 hours in advance. This is because the radioactive tracer is expensive and is produced according to the number of scheduled patients.

WHAT HAPPENS ON THE DAY OF THE EXAMINATION?

	You will need to fill in a questionnaire on your medical history.
	An intravenous line will be set-up.
	You may be given a relaxant if necessary. Please bring along a companion with you.
	You will be given a bowel relaxant orally.
	Your blood glucose level will be checked.
	You will be given a small amount of the tracer - labelled glucose (FDG) via an intravenous administration into a peripheral vein.
	After that, you must wait for 45 to 90 minutes, while your body metabolises the FDG. You will be resting comfortably during this time.
	You will be asked to lie down on a table that slowly passes through the PET scanner. The scanner resembles a CT scanner. This scan time is approximately 20 minutes. In certain cases, the physician may need an additional scan two hours later. The second scan will not require any additional preparation.
	Some patients, specifically those with heart disease, may undergo a stress test prior to PET scan, using a pharmaceutical to alter the blood flow to the heart.

AFTER THE EXAMINATION

	You should feel fine. There are no side effects from the injected radiopharmaceutical.
	You may resume your normal diet following the procedure.
	You should drink additional fluids for several hours after the scan.
	There are no physical restrictions after the procedure.
	The scan results will then be sent to your doctor.

LOCATION WHERE PET IS AVAILABLE

Nuclear Medicine and PET Centre

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