What is PET scan?
A positron emission tomography (PET) scan is an imaging test that allows your doctor to check for diseases in your body. The scan uses a special dye containing radioactive tracers. These tracers are either swallowed, inhaled, or injected into a vein in your arm depending on what part of the body is being examined. Certain organs and tissues then absorb the tracer. When detected by a PET scanner, the tracers help your doctor to see how well your organs and tissues are working. The tracer will collect in areas of higher chemical activity, which is helpful because certain tissues of the body, and certain diseases, have a higher level of chemical activity. These areas of disease will show up as bright spots on the PET scan. The PET scan can measure blood flow, oxygen use, how your body uses sugar, and much more. A PET scan is typically an outpatient procedure. This means you can go about your day after the test is finished.
PET Scan compared to other tests
MRI usually cannot differentiate between post-RT changes and tumor recurrence but both can be very well differentiated in PET-CT as tumor recurrence shows good FDG uptake whereas post-RT changes show faint/no FDG uptake.
In most of the cases, MRI examines a part of the body rather than wholebody, in PET-CT we can evaluate wholebody in single go with relatively lesser time.
With PET-CT we can measure the metabolic activity of tumor in terms of SUV max which corresponds with the tumor aggressiveness. While assessing for response of the tumor, present SUV max when compared with previous SUV max gives us measurability of response which is not possible with MRI. MRI can measure the response in size which is also possible with PET-CT as CECT is done simultaneously with PET.
Because of highly powerful magnetic property, in patients with metallic implants/stents /pacemakers MRI cannot be done, but PET-CT can be done in these patients.
PET-CT scan acquisition can be finished in around 15-20 minutes whereas MRI sometimes takes hours of time depending on the protocol.
PET-CT is less claustrophobic as scanner length is less compared to MRI and noise during scan acquisition is also very less compared to MRI scanner.
In MRI, motion artifacts are high and can alter the scan interpretation, so patient should lie down without slight movement for longer time. Whereas in PET-CT as we have motion correction software, we can allow slight movements during PET scan. Normal movement can be allowed for the parts already acquired, though usually not recommended.
Importance of PET scan
Oncomedicine and Radiation medicine
- PET-CT is an accurate scan for initial metastatic work-up (initial staging), RT planning, response assessment of Chemo or Radiotherapy, restaging, recurrence evaluation and follow-up evaluation.
- PET scan can also be used for biopsy site evaluation in cases of recurrent metastatic cases.
Oncosurgery and General surgery
- In Oncosurgery, PET-CT is mainly useful for initial staging and restaging, to check whether tumor is resectable and to rule out metastatic disease both at the initial diagnosis and after Neoadjuvant Chemotherapy.
Head & Neck and Maxillofacial surgery
- PET-CT is useful for the initial staging, RT planning, response assessment, restaging, recurrence evaluation and follow-up evaluation of Head and Neck malignancies.
Neurology and Neurosurgery
- Apart from oncology, PET scan is useful for epilepsy focus evaluation, if surgical excision of the focus is planned already.
- PET scan can be useful in diagnosis of Dementia and movement disorders (Parkinson’s plus disorders).
- PET-CT is useful for the initial staging, RT planning, response assessment, restaging, recurrence evaluation and follow-up evaluation of Gastrointestinal malignancies.
- In case of HCC, as FDG uptake is low grade/variable, PET scan is more useful for recurrence evaluation.
- Viability: To assess the viability of particular territory or entire myocardium to identify the vascular territories requiring revascularization (cardiac stent).
- For helping in diagnosis and therapy response of cardiac Sarcoidosis.
- PET-CT is useful for the initial staging, RT planning, response assessment, restaging, recurrence evaluation and follow-up evaluation of Genitourinary tract malignancies.
- In case of RCC, as FDG uptake is low grade/variable, PET scan is more useful for recurrence evaluation.
- In Prostate cancer patients Ga-68 PSMA/F-18 PSMA PET scan can give accurate results than FDG PET scan.
- In metastatic Prostate cancer patients not responding to/not tolerating other systemic treatments, Lu-177-PSMA treatment can be given on daycare basis.
- PUO : In cases of PUO, PET scan can identify the infective or malignant focus,
- TB: To see the extent of disease at the time of diagnosis and especially to rule out the residual disease after completion of therapy.
- Sarcoidosis: For helping in diagnosis and therapy response.
- Infective imaging: With F-18 FDG labeled leucocytes, infection imaging for diagnosis and therapy response can be done, more useful for cardiac valve replacement, pacemaker or bone implant infections.