Introduction to PET/CT Imaging

Todd M. Blodgett, MD

Alex Ryan, MD

Marios Papachristou, MD

PET Biochemistry

FDG is taken up by many malignancies due to overexpression of glut-1
Some malignancies may not have significant upregulation of glut-1 and may not be visualized well on FDG PET

PET Physics

PET imaging uses positron emitting radioisotopes such as F-18
F-18 is attached to glucose, resulting in FDG
FDG is taken up by cells, preferentially by many malignant cells
FDG is a marker of cellular metabolism but is not itself metabolized

AC Algorithms

CT is used for attenuation correction with PET/CT scanners
Obviates the need for a separate transmission scan leading to overall decease in scan times

Typical Scan Protocol

Patients are injected with FDG and remain in a quiet room for 1-2 hours
Patients scanned CT first then PET

PET/CT Increasingly Important in

Diagnosis, staging, and restaging of cancer
Image-guided radiation therapy planning
Treatment monitoring

Graphic depicts a typical PET/CT scanner, which houses both CT and PET scanners in a single unit.

Graphic shows the metabolic pathway for FDG. It enters the cell, undergoes the first step of cellular metabolism, but is not further metabolized. FDG is trapped in the cell long enough to appear on images of the patient.

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
- PET and CT scanners are housed in single device
- Single gantry passes patient through both scanners without interim repositioning
- 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 transmembrane transporter glut-1 is overexpressed
Not all malignant cells overexpress glut-1 transporter
- These malignancies may not take up significantly elevated levels of glucose or FDG
FDG enters the cell and is a substrate for hexokinase, 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

History
- First PET/CT scanner became operational in 1998, and first commercial scanners appeared in 2001
- Current state of the art incorporates multi detector row CT with high resolution PET scanners
  - Larger patient ports (70 cm or larger)
  - Aids in radiation planning and accommodates increasing dimensions of average patient in USA
Technical considerations
- Smaller detectors contribute to improved resolution
  - For example, 4×4 mm lutetium oxyorthosilicate detectors offer slightly higher PET resolution than 6×6 mm detectors
- Gadolinium oxyorthosilicate and lutetium oxyorthosilicate scintillators
  - Result in lower rates of both scattered photons and random coincidences compared with bismuth germinate scintillators
  - Generally offers improved whole-body 3D imaging