PET Flashcards
F-18 FDG half life
fluoro-2-deoxyglucose
110 minutes
cyclotron
Rb-82 half life
rubidium-82
75 seconds half life
generator
PET
positron emission tomography
511 keV gamma rays emitted when a positron collides with an electron.
emitted photons are nearly collinear, traveling in opposite directions, almost 180 degrees apart.
Crystals employed for PET
BGO - highest stopping power but relatively poor energy resolution and timing resolution
Newer crystals: decrease dead time - allow for data acquired at much higher count rates
GSO
LSO - has intrinsic radioactivity
lutetium yttrium orthosixicate (LYSO) - has intrinsic radioactivity
CT QC for PET/CT: water phantom QA
daily
CT QC for PET/CT: Tube warm-up
daily
CT QC for PET/CT: air calibration (“fast QA”)
daily
CT QC for PET/CT: water phantom checks: thickness, accuracy, positioning
monthly
Rule of thumb for pixel per FWHM of resolution
at lesasat 3 pixels for every FWHM of resolution in the image
Rb-82 characteristics
monovalent cationic analog of potassium; extracted by Na+/K+ ATP pump
produced in generator by decay from strontium-82 (Sr-82) attached to an elution column
Generator is replaced q4wks
Sr-82 half life
25.5 days
Sr-82 mode of decay to:
Rb-82, bu electron capture
Rb-82 half life
75 seconds
daughter product is krypton-82, stable
Extraction of Rb-82 in blood affected by:
decreased by severe acidosis, hypoxia, and ischemia
also with increasing blood flow –> decreased extraction
Scout scanning
use to ensure patient is correctly positioned before injection
Fast transmission image or with a low-dose Rb-82 injection (10-20 mCi)
PET imaging parameters
Rest imaging should be performed before stress imaging to reduce the impact of residual stress effects (e.g., stunning and steal). For Rb-82, about 80% of the useful counts are acquired in the first 3 minutes, 95% of the useful counts are obtained in the first 5 minutes, and 97% are obtained in the first 6 minutes. The patient should be infused with Rb-82 for a maximum of 30 seconds.
PET imaging time wait
nl LV function, >50%, imaging start 70-90 secs after injection
reduced LV function, 30-50%, imaging begins 90-110 seconds after termination of infusion
<30%, 110-130 seconds
N-13 ammonia characteristics
At physiologic pH, ammonia is in its cationic form with a physical half-life of 10 minutes. Its relatively short half- life requires an on-site cyclotron and radiochemistry synthesis capability. The N-13 nitrogen decays by posi- tron emission. The daughter product is C-13 carbon, which is stable.
Ammonia - Passive diffusion
Ammonium ion by active Na/K transport mechanism
N-13 ammonia dosimetry
adult: 1.48 mSv total effective dose from 20 mCi
Critical organ is urinary bladder (6 mSv from 20 mCi)
Rb-82 dosimetry
1.75-7.5 mSv total effective dose for a maximal allowable activity of 60 mCi at both rest and stress
PET glucose metabolism
under fasting and aerobic conditions: long-chain fatty acids are preferred fuel in the heart (65-70%), glucose provides the rest
ischemia - glucose
F-18 FDG
produced in a cyclotron - bombardment of O-18 enriched water
decays by emission of positron with half-life ~110 minutes
63r keV kinetic energy window
Enters myocardial cells by same transport mechanism as glucose, in cell phosphorylated by hexokinase to FDG-6-phosphae, once phosphorylated, subsequent metabolism of FDG is minimal.
F-18 FDG
Whole body dosimetry from a 10 mCi dose is 7mSV
critical organ is urinary bladder, received 59 mSV
Myocardial substrate utilization
For the evaluation of myocardial viability with FDG, the substrate and hormonal levels in the blood need to favor utilization of glucose over fatty acids by the myocardium. This is usually accomplished by loading the patient with glucose after a fasting period of at least 6 hours to induce an endogenous insulin response. The temporary increase in plasma glucose levels stimulates pancreatic insulin pro- duction, which in turn reduces plasma fatty acid levels through its lipogeneic effects of adipocytes and also nor- malizes plasma glucose levels. The most common method of glucose loading is with an oral load of 25-100 g, but IV loading is also used.
