Positron Emission Tomography (PET)Medical editor Trond Bogsrud MD
Oslo University Hospital
Positron Emission Tomography (PET) is a nuclear medical examination method. PET is a well-documented, well-established and very useful tool in oncological imaging.
Oncological imaging for:
- Staging the primary diagnosis and recurrence
- Evaluating the effect of aggressive chemotherapy treatment
- Evaluating the effect of completed treatment, including differentiating scar tissue from viable residue tissue
- Suspicion of recurrence (for example, increased level of tumor marker in the blood)
- To provide concrete diagnostic information that will provide a basis for the choice of the best possible treatment.
PET has a very high sensitivity and can register absorption of radiopharmaceutical agents in extremely low concentrations. Since the central atoms in biochemical compounds (carbon, oxygen, nitrogen) all have positron-emitting isotopes that can be produced in small hospital cyclotrons, it is possible to mark a number of central molecules such as oxygen, water, amino acids, various metabolites, hormones, and neurotransmitters.
For clinical PET, dextrose is usually used where a hydroxide group is replaced by 18F (18-flourine), a compound that is called 18F-FDG (flourine-18 labeled deoxyglucose). 18F-FDG has a high affinity for cells with increased metabolism, for example cancer cells. The substance is transported into the cells and phosphorylates glucose to 18F-FDG-phosphate, but no further break-down occurs. Because cell membranes are impermeable to phosphorylated deoxyglucose, an intracellular accumulation of the substance occurs.
- Small tumors ( < 0,5 cm) and tumors with low to moderate absorption can escape detection.
- Inflammatory conditions will produce increased absorption.
- For patients with diabetes (especially those requiring insulin) and non-fasting patients, high muscular absorption will reduce the sensitivity for tumor detection.
- Some tumor types have low FDG absorption (for example, prostate and bronchoalveolar carcinoma).
Sources of error
- Infections and inflammatory conditions (including post-operative changes) will result in increased absorption.
- Normally, the intestine can have a high absorption.
- Myocardium often displays high absorption, also in fasting patients.
- 18 F-FDG is excreted through the kidneys and FDG in the urinary tract can be misinterpreted.
- Absorption in brown fat tissue can be misinterpreted as a tumor if PET is not compared with CT. PET/CT combined in the same apparatus gives better specificity than PET alone.
- Radio-pharmaceutical agent: 18F-FDG is formed by radiating a heavier natural variant of oxygen with protons. This occurs in a cyclotron. Fluorine-18 (18F) is produced at the hospital cyclotron located at Rikshospitalet .
Patient preparation depends on the clinical diagnosis.
- Fast for at least 6 hours before the examination in order to increase the absorption of 18F-FDG. But the patient should drink plenty (2-4 glasses per hour. Water, tea, or coffee without sugar or cream/milk added can be substituted for water.
- Measurement of s-glucose is performed before injection of 18F-FDG.
- After intravenous injection of 18F-FDG, it is very important that the patient lies relaxed in a quiet room without talking and avoiding all forms of stimuli, in order to avoid non-specific absorption of 18F-FDG in the muscles.
- Tranquilizers and painkillers are often administered prior to the injection.
- The patient should be warm and comfortable prior to the injection in order to prevent absorption in the brown fat, which may affect the interpretation.
There will be other precautions for neurological and cardiological diagnoses.
- The patient must lie completely still while the images are being taken.
- A whole-body examination takes approximately 25 minutes.
- For PET, tissue absorption is displayed by positron-emitting, radiopharmaceutical preparations.
Registration of emission
- The positron is considered a positively charged electron.
- When the positron leaves the radioactive core, it will travel up to a few millimeters before it collides and fuses with an electron and is transformed into energy; this is called annihilation.
- The mass of the positron and the electron is transformed into energy in the form of two photons, each of 511 keV, which are emitted in diametrically opposing directions (180°).
- A ring detector around the patient will catch the photons.
- The two photons will encounter the ring detector at the same time (coincident detection), and because they have moved in exactly opposite directions, the detection will precisely localize the radiation focus (for example, a lymph node with tumor tissue).
- A modern PET-camera with ring detector can map the entire body in 20 minutes.
- The PET-scanners have integrated CT, so that the information from PET is accurately localized anatomically.
Examples of findings
- Anal cancer: Anal tumor and metastasis in lymph node
- Hodgkin's lymphoma (HL): HL with involvement of: soft tissue in the larynx , vertebra L4 , os pubis L and femur
- Cancer of the rectum: Adenocarsinom in rektosigmoideum liver metastases
- Intracranial tumors: Astrocytoma grade II/III, left parietal lobe high-grade glioblastoma, right frontal lobe
- Lung cancer: Lung tumor lung cancer with lymph node spread
- Sarcoma: Soft tissue sarcoma in the left thorax
- Cancer in the esophagus: Tumor in the distal esophagus
- Colon cancer: Metastasis-suspect lesion in adrenal gland
- At the end of the examination, the radioactivity is small, but the patient should keep a distance (about 3 meters) from children and pregnant ladies the day of the scan.
- The result will normally be available the following day.