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PET/CT

PET/CT. By Sam McNulty. What is PET/CT. An imaging modality that allows the clinician to better differentiate benign vs malignant structural abnormalities seen on CT as well as see possible malignancies where no structural abnormalities are seen. PET. Stands for positron emission tomography

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PET/CT

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  1. PET/CT By Sam McNulty

  2. What is PET/CT • An imaging modality that allows the clinician to better differentiate benign vs malignant structural abnormalities seen on CT as well as see possible malignancies where no structural abnormalities are seen.

  3. PET • Stands for positron emission tomography • Fluorine-18-deoxyglucose (FDG), a radionuclide labeled glucose analogue is injected and the pt is imaged

  4. Indications • Breast cancer: staging of distant metastasis, restaging, and monitoring response to treatment (when a change in therapy is anticipated based on results) • Cervical cancer: staging as adjunct to conventional imaging • Colorectal cancer: diagnosis, staging, and restaging • Esophageal cancer: diagnosis, staging, and restaging • Head and neck cancer (non-thyroid, non-CNS): diagnosis, staging, and restaging • Lymphoma: diagnosis, staging, and restaging

  5. Indications con’t • Melanoma: diagnosis, staging, and restaging • Non small cell lung cancer: diagnosis, staging, and restaging • Solitary pulmonary nodules: characterization • Follicular cell thyroid cancer: restaging of recurrent or residual disease previously treated by thyroidectomy and radioiodine ablation in the setting of serum thyroglobulin > 10 ng/ml and a negative I-131 whole body scan • Myocardial viability: primary or initial diagnosis or following an inconclusive SPECT prior to revascularization • Refractory seizures (brain): pre-surgical evaluation only

  6. How does it work? • In the FDG decay process, positrons are emitted. When a positron is emitted it travels for a short distance from the site of origin (on the order of 1-3 millimeters) gradually losing energy to the tissue through which it passes. When most of the positron’s kinetic energy has been lost, it undergoes a process called annihilation. In annihilation, the positron reacts with an electron in the immediate area and the result is the emission of two very high energy (511 keV each) photons. The two 511 keV photons are emitted in opposite directions at approximately 180 degrees from each other. These two photons interact with the PET detector ring at near opposite sites which define a line within the body along which the annihilation occurred. With computer processing, this line between the two emitted photons permits fairly precise localization of the annihilation reaction and thus defines a tissue site in the body where the positron emission occurred (i.e. an area of FDG activity).

  7. Huh? • Malignant cells take inherently have a higher metabolism than non-malignant cells. They have a higher mitotic rate as well as more ineffecient aerobic metabolism leading to more anaerobic metabolism • Through these mechanisms they will take up the FDG at a faster rate and this will can be seen on the scan as the FDG decays.

  8. PET/CT • PET and CT scans are now often done at the same time. • The CT and PET data sets are fused or “coregistered” electronically by the scanner’s computer system and presented to the interpreter on a work station. The data can then be simultaneously and interactively viewed as CT data, PET data, and superimposed CT and PET data in any percentage combination of these data sets desired (e.g. 100% PET data, 100% CT data, 50% CT / 50% PET data).

  9. PET/CT • Doing both scans at the same time offers the advantage over just the PET scan alone because the CT gives exquisite anatomic detail as well as physiologic information.

  10. Fused PET/CT images

  11. SUV • When a PET camera is appropriately calibrated, it is able to assess the amount of FDG activity per volume (millicuries/millileter or mCi/mL) of tissue and this can be expressed in “SUV” units • SUV=Mean region of interest activity(mCi/mL) =administered activity (mCi)/body weight (g) expressed in g/mL • SUV over 2.5 is considered suspicious for malignancy

  12. Limitations of PET/CT • FDG is not cancer specific and will accumulate in any areas of high rates of metabolism and glycolysis. • Therefore, increased uptake can be expected in all sites of hyperactivity at the time of FDG administration (e.g. muscles and nervous system tissues); at sites of active inflammation or infection (e.g. sarcoidosis, arthritis, infection etc.); and at sites of active tissue repair (e.g. surgical or traumatic wounds, healing fractures, etc.).

  13. Brain/CNS • Brain tissue is always highly metabolically active so detection of brain malignancies, particularly in gray matter, is limited with FGD PET. • Hypometabolism is also typical in epileptogenic foci during the interictal period. FDG activity may also be decreased in one or both frontotemporoparietal association complexes in Alzheimer’s dementia.

  14. Cardiac • When compared to conventional SPECT imaging, PET/CT using Rubidium- 82 coupled with a 64-slice CT scanner is a faster, easier, and more accurate method to measure myocardial blood flow.

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