Fusion Technique with PET and CT Scan
FDG-PET scan is highly sensitive in detecting malignant lesions in tumors that are hyperglycemic. However, benign entities such as infectious and inflammatory processes can be problematic because they also exhibit increased in FDG uptake. Moreover, it is easy to see how normal interpretation difficult. This provides only limited anatomic landmarks. Hence, even with knowledge of the normal distribution of FDG uptake, it is often difficult to confidently identify an area as normal or pathologic with PET alone. With PET scan alone, improved diagnostic accuracy cannot be acquired, but a correlation with CT scan images which provide an excellent anatomic landmarks and PET scan images is the best way to improve and accurately study a pathology. However, correlating images from a CT study to a PET study can be difficult given that the position of the patient and the time interval between scans vary.
PET and CT Scan Fusion Technique
To optimize the combined benefits of anatomic information from CT scan images and metabolic information from PET, a combined PET and CT scanner were developed. With the combined PET and CT scanner, the patient undergoes a CT scan followed immediately by a PET scan during the same imaging session. This method provides more precise co-registration of the two modalities by addressing problems associated with repositioning and prolonged time between scanning methods.
Clinical Procedure of Florine 18-FDG PET / CT
Like standard CT, protocols for PET / CT continue to evolve as the technology improves. Also like standards CT, decisions must be made before each specific PET/CT examination as to wether intravenous or oral contrast material is required. In addition, radiologist must decide whether the CT portion of the examination is for attenuation correction and localization only, in which case scans are acquired at a lower dose at about 40 to 80 mAs is typical, or whether the full range of CT information is needed, in which case the mAs is similar to standard CT examinations. These decisions are based on the clinical questions posed and whether the patient already has undergone a recent, diagnostic CT. A typical PET / CT procedure consists of the following steps.
- A detailed medical background and history is obtained
- The patient fasts for at least 6 hrs. before the scan.
- Oral contrast material is given, if indicated.
- The patient receives an intravenous injection of 370 to 555 MBq or 10 to 15mCi of FDG.
- Approximately 30 to 60 minutes after the intravenous FDG injection the patient is placed on the examination table.
- The patient is positioned in the scanner in the same manner as for standard CT examinations example, arms of the patient are raised for chest and abdomen studies, sponges are used to ensure the patient’s comfort, immobilization devices are used when needed.
- Scout are acquired and cross sectional slices are planned.
- Intravenous contrast material is administered, if indicated. Injection protocols mimic those used in standard CT protocols of the same body part.
- The patient is given clear breathing intructions like shallow breathing is the most common. The patient is asked to breath in the same manner for both CT and PET portions of the examination. Motion correction such as respiratory gating is recommended when available.
- The CT scan is performed.
- Attenuation correction factors are generated from the CT data to be applied to the PET data.
- The PET scan is obtained as a series of acquisitions at overlapping bed positions. Acquisition parameters are usually facility and camera specific.
- Reconstruction of PET data occurs depending on facility protocol.
- Reconstructed PET and CT imaged are available for viewing on a workstation.
- Delayed or repeat images may be helpful in select cases.
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