Week 13: PET CT Fusion Imaging

Question 1

PET

Answer 1

• positron emission tomography (PET)
• nuclear medicine study that uses molecular imaging methods that provide metabolic detail
• provides functional information regarding how the cells of the body operate (CT provides structural info)
• CT and PET can be used in combination to make a complete diagnosis, provide information on both the cancers location and the metabolism

Question 2

PET/CT Scanner

Answer 2

• composed of a multi-detector CT scanner in conjunction with (but separate from) a PET scanner
• during the study the patient passes through the CT scanner and then into another imaging field of the PET scanner
• the straights of CT/PET complement eachother

Question 3

FDG-PET Imaging

Answer 3

• PET creates an image from the radiation given off when positrons encounter electrons in the body
• this is done by giving the patient a radiopharmaceutical with a short half life made up of a radionuclide (F18) linked to a pharmaceutical agent (deoxyglucose)
• FDG is a glucose analog that is taken up by cells (many normal tissue use glucose for energy)
• FDG uptake makes a map of glucose metabolism
• malignant tumours often favour the glycolytic pathway for metabolism

Question 4

FDG PET Interpretation

Answer 4

• FDG-PET scans are interpreted mainly by qualitative means
• an area of abnormality is detected by comparison with background activity
• another method is also used, standard uptake value (SUV), when areas are judged to be equivocal by qualitative analysis alone

Question 4

FDG Imaging Pitfalls

Answer 4

-tumour cells not the only cells that exhibit increased FDG uptake
- because FDG maps
glucose metabolism, its
distribution can be altered
by any physical activity
- it is important to recognize and understand normal variants of FDG uptake and benign disease to avoid mistaking them for pathologic processes
- the timing of a patient’s last
meal and level of hydration
before a PET study can affect
the PET results (patients must fast for 4-6 hours before PET scan)
- may interfere with insulin dependent patients (insulin should be given as far from the
time of FDG injection as feasible)

Question 5

Standard Uptake Value (SUV)

Answer 5

• an index of glucose metabolism
• the number represents the relative uptake of radionuclides in tissues

Question 5

Basic Steps for PET/CT Procedure

Answer 5

• obtain medical history
• patent is instructed to fast before scan
• oral C is given if indicated
• patient is given IV injection of 370-555 MBq of FDG
• 30-60 minutes after FDG injection patient is positioned on the scan table
• scouts are acquired and cross sectional slices are planned
• IV CM is administered if indicated
• patient if given breathing instructions for both CT and pET portions of the exam
• CT is performed
• attenuation correction factors are generated from the CT data to be applied to the PET data
• PET scan is performed
• reconstruction of PET data occurs depending on facility protocol
• reconstructed PET and CT images are available for viewing and should be checked before the patient leaves
• delayed or repeat images may be helpful in some cases

Question 6

PET/CT Fusion

Answer 6

• clinical interpretation can be difficult with PET alone because it provides limited anatomical landmarks
• PET/CT combines the benefits of anatomical information from CT with metabolic information from PET
• decisions must be made before PET/CT exam: is oral or IV contrast required?, is the correlative CT portion
  of the examination only
  needed for attenuation
  correction and localization,
  or is more detailed anatomic
  CT information needed?