The science behind PET scanning

PET scan is a diagnostic imaging tool used in oncology, cardiology and neurology. It makes use of positron emitting radionuclides in the form of radiopharmaceuticals that is administered to patients. The bio-distribution of the radiopharmaceutical in the body would allow clinicians detect abnormalities and diseases through images of the body’s function and metabolism.

PET, which stands for Positron Emission Tomography, makes use of proton rich radionuclides labeled with certain biological compounds. Proton rich radionuclides emit radioactivity in the form of positrons. Positrons, when they travel through matter, lose energy and interact with surrounding electrons then annihilates, producing two gamma rays travelling at approximately opposite direction of each other. These gamma rays from positron annihilation are detected by the PET scanner which then reconstructs the information and produces the PET images.

Radionuclides used in PET scanning can be produced with the use of a cyclotron. A cyclotron is basically a particle accelerator. Charged particles, in the form of protons from a hydrogen atom, are injected into the ion source and then accelerated using a magnet known as “dees” in a well-defined direction. The proton gains speed because of the magnetic field which then causes the particles to accelerate in a circular orbit and consequently gain energy. The average magnetic field is roughly 1.6 Tesla. Once the proton has gained enough speed, they are extracted and directed towards the target material where a nuclear transformation takes place.

F-18 is the radionuclide of choice for most PET scans and F-18 Fluorodeoxy-glucose, better known as F-18-FDG, is still the gold standard in PET scanning. F-18-FDG is F-18 bounded to glucose, making it a glucose analogue, or basically “radioactive sugar.” F-18 is produced in the cyclotron by bombarding O-18 enriched water with the accelerated protons, making it unstable and radioactive. Being in a “radioactive state,” F-18 is short lived with a half-life of only a little under 110 minutes thereby making it an ideal radionuclide for diagnostic purposes as the radioactivity has been reduced to minimal levels immediately after the scan is finished.

True to its mandate of providing the best medical care, St. Luke’s Medical Center has its own cyclotron to produce radionuclides for medical purposes. The cyclotron was recently upgraded allowing higher amounts of radioactivity to be produced at a shorter period of time and catering to the increasing demand of patients. It is able to accelerate protons up to energies of 9.6MeV and has 5 targets. The cyclotron is self-shielded, with a wall that is approximately 70cm of concrete that eliminates the need for a housing or bunker. This reduces the radiation levels around the cyclotron area and the facility itself. The cyclotron is also fully automated and takes only a few clicks to run the cyclotron to produce radioactive materials needed for PET. Production of F-18-FDG at St. Luke’s is performed by in-house radiochemists, radio-pharmacists, and medical physicists.

PET scans can be done at PET Center of St. Luke’s Medical Center - Quezon City, the first PET facility in the country, or at St. Luke’s Medical Center - Global City, the second in the country. The PET/ CT scanners in St. Luke’s employ time-of-flight technology which generates improved image resolution compared to previous models while possibly reducing the amount of radioactivity administered to patients and decreased scanning time. The CT component of the PET/CT also improves the accuracy of localization as anatomical information is already available.

Medical staff at St. Luke’s is composed of board certified nuclear medicine physicians and radiologists who have had extensive experience in performing PET/CT imaging, most of whom trained abroad, and are ready to help and answer queries from doctors and patients interested in having PET/CT scans done. Their expertise cover a broad range of diseases in oncology, cardiology, and neurology, where most of which can be diagnosed using PET/CT scanning.

For more information, please call St. Luke’s Medical Center - Quezon City PET Center at 7230101 ext. 4157/4158 or St. Luke’s Medical Center - Global City Department of Nuclear Medicine and PET/CT at 7897700 ext. 1004/1152