TERMINOLOGY
Abbreviations
Fluorodeoxyglucose(FDG), standard uptake value (SUV), Hounsfield unit ( HU), attenuation correction (AC), fine-needle aspiration( FNA)
Positive predictive value(PPV), negative predictive value (NPV)
IMAGING ANATOMY
General Anatomic Considerations
Hardware approach to image fusion allows accurate registration of anatomic and metabolic images
1. PET and CT scanners are housed in single device
2. Single gantry passes patient through both scanners without interim repositioning
3. Motion between the CT and PET portions of a PET/CT scan will cause significant misregistration
PET BIOCHEMISTRY
FDG Uptake
Enhanced glycolysis in many malignant cells leads to increased FDG uptake
Glucose transmembrace transporter glut-1 is overexpressed
Not all malignant cells overexpress gult-1 transporter elevated levels of glucose or FDG
FDG enters the cell and is a substrate for hexohinase, the first enzyme of glycolysis
Hexokinase phosphorylates FDG to FDG-6 phosphate
Metabolic activity of FDG ceases at that point and FDG remains trapped in the cell long enough to image the patient
PET PHYSICS
Hardware
Technical considerations
Smaller detectors contritute to improved resolution
Gadolinium oxyorthosilicate and lutetium oxyorthosilicate scintillators
1. Result in lower rates of both scattered photons and random coincidences compared with bismuth germinate scintillators
2. Generally offers improved whole-body 3D imaging
Radiotracers
Radioisotope most commonly used in PET imaging is fluorine-18
Positron emitters are neutron-deficient isotopes that achieve stability through nuclear transmutation of a proton into a neutron
Imaging Considerations
Limitations of spatial resolution in PET imaging
1. Range
After emission, a positron loses energy through interactions with surrounding tissues until it annihilates with an electron
2. Coincidence detection
Two annihilation gamma rays are emitted in nearly opposite directions and are detected in coincidence
Physical effects contribute 2 mm or less to degradation of image resolution
1. Spatial resolution of PET scanner is limited by multiple other factors and is, at best, 6-8 mm with current technology
(Ryan, Almusa , Papachristou, Paidisetty, SPECIALITY IMAGING PET/CT Oncologic Imaging with Correlative Diagnostic CT, Amirsys, Canada 2009 )