This document provides an overview of PET/CT imaging and 18F-FDG PET/CT scans. PET/CT is a valuable diagnostic tool that combines functional PET imaging with anatomical CT imaging. 18F-FDG is the most commonly used radiotracer that is taken up by tissues with high glucose metabolism, such as cancer cells. A 18F-FDG PET/CT scan involves fasting the patient, injecting the radiotracer, and imaging the body to detect areas of abnormal radiotracer uptake that may indicate cancer or other diseases. Both quantitative SUV values and visual analysis are used to interpret 18F-FDG PET/CT scans.

1. Basics and principles
of PET/CT scan
Ola, Attieh, MD
Nuclear Medicine senior specialist

2. What is PET?
 PET stands for Positron Emission Tomography.
 An imaging technique that uses small amounts of
radiolabeled biologically active compounds (tracers) to
measure biochemical and physiological processes through
distribution of the tracer in the body.

3. PET/CT
 Uses both systems in one Gantry.
 Acquired images are combined into a co-registered (fused)
images.
 A valuable diagnostic tool, providing both functional
information by PET and anatomical details by CT scanner in a
single image set.

5. Basic Principle of PET (Positron Emission)
 Positron Emission occurs when the Proton rich isotope
(Unstable Parent Nucleus) decays and a Proton decays to a
Neutron, a Positron and a Neutrino.
 After traveling a short distance (3-5mm), the positron
emitted encounters an electron from the surrounding
environment. The two particles combine and "annihilate"
each other, resulting in the emission of two gamma rays in
opposite directions of 0.511 MeV each.

6. Annihilation reaction

7. Coincidence detection
 Patient being the radioactive source,
the positrons are annihilated in the
body tissue and produce two
photons (511 KeV) in opposite
direction (180o) which is detected in
a electronic time interval called
“Coincidence time window”.

8. PET image reconstruction

9. PET tracers
Isotope Half life
(min)
Positron Range
(mm)
Production Source
15O 2 1.5 Cyclotron
13N 10 1.4 Cyclotron
11C 20 1.1 Cyclotron
18F 110 1.0 Cyclotron
68Ga 68 2.9 Generator

10. PET radiopharmaceuticals
Tracer Fields of use
11C-choline Prostate cancer diagnosis
11C-PiB Plaque burden; Alzheimer’s disease
11C-methionine Amino acid transport; brain tumors diagnosis
15O-water Oxygen intake & distribution; perfusion imaging
18F-FDG Glucose utilization; tumors
18F-FLT Cell proliferation; cancer diagnosis
18F-NaF Bone mineralization; bone metastasis
68Ga-PSMA Prostate cancer diagnosis
68Ga-Dota Somatostatin receptor; neuroendocrine tumor

11. 18F-FDG
 18F is a cyclotron produced radionuclide through proton
bombardment of enriched 18O-water.
 18F is attached to a glucose analog to form 2-fluoro-2-deoxy-
D-glucose.
 Most widely used PET tracer.
 It is absorbed by various tissues as normal glucose would be.

12. 18F-FDG

13. 18F-FDG
 Taken up avidly by most tumors.
 Tumor imaging with FDG is based on the principle of
increased glucose metabolism of cancer cells due to:
 Over expression of Glucose transporters.
 Higher levels of Hexokinase.
 Down-regulation of Glucose-6-phosphatase.
 General increase in metabolism from high growth rates.

14. Patient Preparation
 Major goals is to minimize tracer uptake in normal tissues, such as
the myocardium and skeletal muscle, while maintaining uptake in
target tissues (neoplastic disease).
 Before arrival:
a) Avoid strenuous activity at least 24 hours before PET scan.
b) Fast and do not consume beverages, except for water, for at least 4–6 h
before the administration of F-18 FDG to decrease physiologic glucose levels
and to reduce serum insulin levels to near basal levels.
c) Oral hydration with water is encouraged.
d) IV fluids containing dextrose or parenteral feedings also should be withheld
for 4–6 h.

15. Patient Preparation
 Before injection:
 Remain seated after the injection throughout uptake phase
to avoid muscular uptake.
 Check blood glucose level since tumor uptake of 18F-FDG is
reduced in hyperglycemic states (FBS not greater than 150–
200 mg/dL).
 No regular insulin SC injected within 4 hrs of having FDG
administration since hyperinsulinemia cause increase FDG
uptake in skeletal muscle.

16. 18F-FDG PET/CT imaging
 IV. inject 0.14-0.2 mCi/Kg of F-18 FDG (10-20 mCi).
 60 minutes following IV. F-18 FDG, PET scan is performed.
 Skull base-to-mid-thigh or head-to-toe.
 PET scan time: 2-3 min/ bed position.
 CT scan: low mAs scan is adequate for attenuation correction
& anatomical localization.

17. Normal PET/CT scan images

18. Normal Distribution of F-18 FDG
 Normal physiologic uptake in every
viable tissue, including brain, lymphoid
tissue (e.g., tonsils), salivary glands,
thymus, myocardium (in some patients
despite prolonged fasting), breast, liver,
spleen, stomach, intestines, kidneys and
urinary bladder, muscle, BM, uterus,
ovaries, testicles.

19. Indications of 18F-FDG: Non Oncology
 Neurology and Brain:
 Epilepsy: pre-surgical localization of a epilepticus focus of activity in
patients with refractory seizures.
 Diagnosis of neurodegenerative dementia in patients with mild cognitive
impairment.
 Differentiation of Alzheimer’s Dementia from Frontotemporal Dementia.
 Differentiation of Progressive Supranuclear Palsy from Parkinson’s disease

20. Non-oncology, cont’d
 Myocardial perfusion and viability: assessment of myocardial
viability prior to planned revascularization, where diagnosis
of hibernating myocardium in patients with poor left
ventricular function is essential prior to revascularization
procedure.
 Infection and inflammation: large vessel vasculitis, vascular
graft infection and fever of unknown origin where
conventional imaging work-up is negative.

23. Indications of 18F-FDG: oncology
 Differentiating benign from malignant lesions.
 Searching for an unknown primary tumor when metastatic
disease is discovered as the first manifestation of cancer or
when the patient presents with a paraneoplastic syndrome.
 Staging known malignancies.
 Monitoring response of therapy on known malignancies.

24. Indications of 18F-FDG: oncology
 Residual tumor vs. post treatment fibrosis or necrosis.
 Detecting tumor recurrence, especially in the presence of
elevated levels of tumor markers.
 Selecting the region of a tumor for biopsy.
 Guiding radiation therapy planning.

26. Fig. 1. A Whole-body imaging by FDG-
PET/CT. NSCLC (squamous cell carcinoma) of
the left lung, centrally located (SUVmax 17.9;
8.8 cm) with right adrenal metastasis
(SUVmax 12.5, 3.6 cm), lymph node
metastasis and multiple bone lesions which
were not appreciable on CT alone.

27. Baseline MIP FDG-PET (a) in a 26–year male patient, presented with extensive HL involving lymph node groups
in both sides of the diaphragm with bone marrow involvement. MIP iPET after 2 cycles of ABVD (b) showing
complete metabolic resolution of the hypermetabolic lymph nodes and the bone marrow lesion CMR
(Deauville score 2). MIP FDG-PET after 4 additional cycles of ABVD (c) showed sustained CMR on EoT PET but
with the development of inflammatory changes in the base of the right lung (d).

28. Appropriate PET Scan Timing
 Post biopsy 1 week
 Post surgery 4-6 weeks
 Post CTx 4-6 weeks
 Post ERT 12 weeks
 Post immunotherapy 4-8 weeks

29. SUV- Standardized Uptake Value
 A semi-quantitative measure of the tracer uptake in a region
of interest that normalizes the lesion activity to the injected
dose and body weight.
 SUV does not have a unit.
 SUV should not be used alone to differentiate malignant
from benign processes.
 Malignant tumors have an SUV of > 2.5–3.0.

30. Limitations of PET/CT
 18F-FDG is not ‘specific’ radiotracer for imaging malignant
disease and will accumulate in any areas of high rates of
metabolism and glycolysis.
 Active muscle contraction during the uptake period.
 Activated brown fat.
 Normal wound healing.
 Sites of active inflammation or infection.

31. FDG muscular uptake

32. FDG brown fat uptake

33. Sarcoidosis
Vs.
Lymphoma

34. Malignancies with low FDG uptake:
 Low grade pulmonary adenocarcinoma spectrum lesions.
 Low grade lymphomas, including MALT, chronic lymphocytic leukemia or
small lymphocytic lymphoma, marginal zone lymphoma, peripheral T-cell
lymphoma.
 Some types of renal cell cancer.
 Prostate cancer.
 Pancreatic cancer.
 Mucinous neoplasm of ovaries.
 HCC.

35. Malignancies with low FDG uptake:
 Lobular breast cancer.
 Some soft tissue sarcomas.
 Well differentiated thyroid ca.
 Well differentiated neuroendocrine tumors.
 Uterine malignancies.
 Differentiated teratoma.
 Low grade glioma.

36. Thank You