Physical principles of the method
PET is based on the detection of a radiopharmaceutical (RP) administered to the patient, which emits positrons ($\beta^+$). A positron travels a minimal distance in tissues and collides with an electron ($\beta^-$). Annihilation occurs — the mass of the particles converts into energy, generating two photons ($\gamma$) with an energy of 511 keV. These photons fly apart in opposite directions at strictly 180 degrees.
Tomograph detectors surround the patient's body and record these gamma quanta. The key principle is coincidence detection: the system registers a signal only if two photons strike opposite detectors simultaneously. This makes it possible to localize the site of annihilation on a straight line connecting the two detectors. Since PET does not require collimators (radiation restrictors), the method possesses tremendous sensitivity.
Radiopharmaceuticals and the role of FDG
In oncology, 18F-fluorodeoxyglucose (FDG) is most commonly used — an analogue of regular glucose with attached radioactive fluorine. The molecule enters the cell via transport proteins and is phosphorylated by the enzyme hexokinase. Afterward, the modified FDG loses its ability to undergo further metabolism, gets trapped in a chemical "trap," and accumulates inside the cell.
The mechanism is based on the Warburg effect (aerobic glycolysis): malignant cells are characterized by upregulated glucose consumption and produce lactate even under normal oxygen supply. On scans, areas with aggressive glycolysis glow the brightest.
To quantitatively assess this "glow," the SUV (Standardised Uptake Value) parameter is used. It is calculated as a correction for the administered dose of radioactivity and the patient's body weight. In clinical practice, clinicians generally rely on $SUV_{max}$ — the maximum tracer uptake in a single voxel within the region of interest.
Non-FDG tracers and theranostics
In addition to glucose, PET allows for the visualization of numerous other biological targets:
- Cell proliferation markers: the thymidine analogue FLT (labeled with fluorine-18) is phosphorylated by thymidine kinase and retained in the cell, helping to differentiate cancer from benign lesions in the lungs and colon.
- Amino acid metabolism markers: methionine (MET) labeled with carbon-11 is taken up depending on the rate of cell division and microvessel density. It exhibits extremely high sensitivity (up to 95%) in detecting gliomas.
- Hypoxia markers: FMISO (fluoromisonidazole) is retained in areas with oxygen deprivation, which is characteristic of aggressive metastases and helps predict the likelihood of local recurrence.
- Bone remodeling markers: sodium fluoride (Sodium Fluoride) detects lytic bone metastases.
A separate promising area is theranostics, which combines a diagnostic test and treatment. A striking example is the binding of a radiopharmaceutical to prostate-specific membrane antigen (PSMA). First, a diagnostic isotope is used to locate prostate cancer lesions, and then a beta-emitter (lutetium-177) or an alpha-emitter (actinium-225) is attached to the same molecule to deliver targeted radionuclide therapy.
Application in oncology and limitations
FDG PET/CT is routinely used in oncology at all stages:
- Staging at initial presentation to select the treatment strategy.
- Monitoring response to therapy (allows differentiation of viable residual tumor from post-therapeutic necrosis or fibrosis).
- Restaging when recurrence is suspected.
- Search for the primary tumor in patients with already detected metastases.
Despite its extremely high sensitivity, the method has limitations. First, PET cannot differentiate the original labeled molecule from its metabolites — contrast is created solely due to regional differences in tracer uptake. Second, the use of short-lived radionuclides requires a cyclotron to be located nearby. In addition, it is an expensive method that carries a significant radiation burden for the patient.
To remember the essence of FDG-PET, recall the phrase: "Tumors love sweets, but cannot digest them." The glucose analogue enters the cell, is phosphorylated by hexokinase, but is not metabolized further, becoming trapped and generating a bright signal.
Class and exam questions
Why is PET usually performed alongside CT or MRI?
PET has tremendous sensitivity to metabolic processes, but low spatial resolution. Hybrid imaging overlays the functional signal onto an precise anatomical map obtained via CT or MRI.
Does the scanner distinguish the original radiopharmaceutical molecule from its breakdown products?
No, the detectors register only the fact of positron annihilation. The image contrast is formed exclusively due to the difference in tracer concentration in various tissue areas.
What is the Warburg effect and why is it important for diagnostics?
This is the Warburg effect (aerobic glycolysis), in which malignant cells produce lactate even under conditions of normal oxygen supply. It is precisely because of this metabolic anomaly that tumors intensively uptake fluorodeoxyglucose.
What else to study on this topic
- Assessment of lipid metabolism using 11C-choline and 11C-acetate
- Specific hypoxia markers (FAZA, ATSM) in radiotherapy planning
- Use of microbubbles in ultrasound molecular imaging
- Optical imaging: fluorescence, bioluminescence, and near-infrared spectrum
- Visualization of cell death using Annexin V
Master the entire topic in the "Diagnostic Radiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Positron Emission Tomography" and 2,199 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 10 video lessons on the subject
Other topics in the "Diagnostic Radiology in Oncology" section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.