Makindo Medical Notes

Note: The tracer usually chosen is 18F-fluorodeoxyglucose (FDG), a short half-life glucose analogue. It becomes concentrated in metabolically active tissues. PET can be combined with CT or MRI to provide high-quality images that integrate anatomical and physiological information.

About

• Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) are imaging techniques that identify structures based on their metabolic activity, complementing other imaging methods like CT and MRI.
• PET is particularly valuable for investigating malignancies and has a range of other diagnostic applications, including assessing infection and inflammation.

Positron Emission Tomography (PET)

• PET measures glucose uptake through the use of radiolabelled glucose, commonly 18F-fluorodeoxyglucose (FDG).
• After administration, FDG accumulates in tissues with high metabolic activity, such as tumors and the brain.
• The radioactive FDG decays rapidly, releasing positrons. When these positrons encounter electrons in tissues, they annihilate, producing pairs of high-energy photons that are detected by the PET scanner.
• By detecting these photons, PET generates detailed images that highlight areas of increased metabolic activity, aiding in diagnosis.
• PET is often combined with CT or MRI to enhance anatomical localization of metabolically active regions, offering both structural and functional insights.

Applications

• Oncology: PET is crucial for detecting lung tumors, staging cancers like lymphoma, melanoma, and oesophageal cancer, and assessing extrathoracic metastases, including bone and adrenal involvement.
• Infection & Inflammation: Identifies areas of acute inflammation and infection, such as tuberculosis (TB) and sarcoidosis, as well as sources of occult infection (e.g., pyrexia of unknown origin, PUO).
• Cardiology: Useful for assessing myocardial viability and perfusion in coronary artery disease.
• Neurology: Though limited by high background metabolic activity in the brain, PET is being explored with amyloid tracers for Alzheimer's disease diagnosis.
• Emerging Areas:
  • 11C-labelled Metomidate: Detects adrenal tumors and other adrenocortical origins.
  • Somatostatin Receptor Imaging: Useful in imaging neuroendocrine tumors using specific tracers.

Limitations of PET

• High Background Uptake: Areas like the brain, liver, kidneys, and bladder naturally have high FDG uptake, making it challenging to detect abnormalities in these regions.
• Not Ideal for Slow-Growing Tumors: Tumors with low metabolic rates, such as bronchoalveolar carcinomas, may not show up well on PET scans.
• Brain Metastases: Due to high normal brain activity, PET is less useful for detecting brain metastases compared to MRI.

Single Photon Emission Computed Tomography (SPECT)

• SPECT is similar to PET but uses different radioisotopes that emit gamma rays, which are detected by gamma cameras.
• SPECT is generally of lower spatial resolution than PET but benefits from longer-lived radioisotopes that are more widely available.
• Common applications of SPECT include:
  • Myocardial Perfusion Imaging: Assesses blood flow to the heart muscle to evaluate ischemic heart disease.
  • Bone Scans: Detects bone metastases and other skeletal abnormalities.
  • Brain Scans: Evaluates cerebral blood flow in conditions like stroke and epilepsy.

Comparison: PET vs. SPECT

• Resolution: PET generally provides higher resolution images due to the nature of positron detection, while SPECT has lower spatial resolution.
• Availability and Cost: SPECT is more widely available and less expensive compared to PET, making it suitable for a broader range of clinical settings.
• Radioisotopes: PET uses positron-emitting tracers like 18F-FDG, while SPECT uses gamma-emitting isotopes like Technetium-99m, which have longer half-lives.

References

• Radiopaedia - Positron Emission Tomography (PET)
• Radiopaedia - Single Photon Emission Computed Tomography (SPECT)
• National Institutes of Health (NIH) - PET and SPECT Imaging