PET/CT

PET

positron emission tomography

beta positive emission

Positron emission

tomography

3 dimensional imaging

cyclotron

A charged particle accelerator which repeatedly propels a beam of charged particles (protons) in a circular path. These energetic particles then hit a target material and get absorbed in to the nucleus, converting the target in to the different species

Fluorine-18 FDG

•Flurodeoxyglucose is a positron emitting isotope with a short half life(110 mins)
• Behaves like a natural glucose in the body
• tumours & abnormal cells have increased glucose metabolism

How is Fluorine 18 FDG acquired?

• Atoms of F18 decay by emitting charged particles
• A positron is a positively charged particle similar to an electron (e+ ) (β +)
• Positron collides with electron in the shell of another atom (up to 0.6mm away)
• Collision results in annhilation reaction.
• Energy is released in form of 2 gamma rays each 511keV at 180 degrees (approx.) to each other
• 2 gamma rays released by annhilation reaction will hit the array of detectors and create a signal.
•The two photons/gamma rays would have to hit the detectors 180 degrees from each other (directly opposite) in a certain amount of time from each other for it be counted as a true even. - (true coincidence)
• The more coincidence events along one LOR the more activity is in that area.

Why combine PET & CT ?

• To detect structure and function simultaneously.
• Greater detail with a higher level of accuracy; because both scans are performed at one time without the patient having to change positions, there is less room for error.
• Greater convenience for the patient who undergoes two exams (CT & PET) at one sitting, rather than at two different times.

Patient preparation PET/CT

• Fasting,water only for 6 hours before appointment( clearer image-dye)
• Blood sugar levels should be 8mmol/ml
• IDD(Insulin dependent diabetics)normal diet,normal morning insulin.(may test blood sugars before scan)
• Patient should be relaxed before procedure starts .
• Diet and bowel preparation can be given

Distribution of FDG - brain

High uptake in the grey matter

Distribution of FDG-Myocardium

Variable uptake

Distribution of FDG-Lungs

Low uptake

Distribution of FDG-mediastinum( The area between the lungs. The organs in this area include the heart and its large blood vessels, the trachea, the esophagus, the thymus, and lymph nodes but not the lungs)

Low uptake

Distribution of FDG-Liver

Low uptake

Distribution of FDG- Gastrointestinal tract( esophagus, stomach, colon)

Variable activity

Distribution of FDG-Urinary tract

Excretes FDG

Distribution of FDG-Muscular system

Low uptake at rest

How to reduce radiation dose

• Administer correct activity
• Hydrated patient
• Frequent voiding of the bladder especially when the scan is completed.
• Ensure preparation instructions are followed

PET/CT benefits

• Elimination of invasive procedures.
• Avoidance of unnecessary surgery.
• Whole-body survey for unknowns.
• Elimination of multiple tests.
• Timely determination of therapy effectiveness.
• Early diagnosis of disease before structural changes have
occurred.
• Direct replacement of stable biological counterparts without disturbing natural metabolic pathways – quantify physiological function in tissue.
• Target specificity with tracers.

PET/CT Disadvantages

• Time-consuming.
• PET scanning can give false results if chemical balances
within the body are not normal.
• Because the radioactive substance decays quickly and is effective for only a short period of time, timing of injection and scanning is important
• Gantry size for obese patients.
• Access to cyclotron (cost / timing)

Clinical Applications of PET/CT

• Oncology
• Cardiology
• Neurology