FDG PET/CT Scans

📌 Key Point: FDG PET/CT combines metabolic imaging with anatomical imaging. It uses 18F-fluorodeoxyglucose (18F-FDG), a radioactive glucose analogue, to highlight tissues with increased glucose uptake. This makes it particularly useful in oncology, but it also has important roles in infection, inflammation, and selected neurological and cardiac indications.

📖 About

• PET = Positron Emission Tomography, which shows physiological / metabolic activity.
• CT = Computed Tomography, which shows structural anatomy.
• When fused together, PET/CT allows clinicians to see where an abnormality is and how metabolically active it is.
• The tracer most commonly used is 18F-FDG, which behaves like glucose and is taken up by cells via glucose transporters.
• Malignant cells often have increased glucose metabolism, but infection, inflammation, healing tissue, and even normal physiological tissues can also be FDG-avid.

⚙️ How FDG PET/CT Works

• After intravenous injection, FDG enters cells in a similar way to glucose.
• Inside the cell it is phosphorylated to FDG-6-phosphate.
• Unlike normal glucose, it is not metabolised further efficiently and becomes relatively trapped within the cell.
• Tissues with high glucose utilisation therefore accumulate more tracer and appear as areas of increased signal on PET imaging.
• The CT component helps with attenuation correction and provides anatomical localisation.

🧬 Why FDG Uptake Happens

• Malignancy: many tumours show increased glycolysis, often referred to as the Warburg effect.
• Inflammation: activated neutrophils, macrophages, and lymphocytes are metabolically active and may take up FDG avidly.
• Infection: infected tissue and inflammatory response both contribute to high uptake.
• Normal physiological uptake: brain, myocardium, kidneys/urinary tract, liver, bowel, and brown fat may all show expected tracer activity.

🩺 Main Clinical Uses

• Oncology: diagnosis support, staging, restaging, treatment response assessment, and recurrence detection in selected cancers.
• Lymphoma: especially important for staging and response assessment in FDG-avid lymphoma subtypes.
• Lung cancer: useful for staging and identifying metabolically active nodal or distant disease.
• Cancer of unknown primary: may help identify an occult primary site or additional disease burden.
• Infection and inflammation: helpful in selected cases such as fever of unknown origin, vascular graft infection, large-vessel vasculitis, and some forms of prosthetic infection.
• Neurology: used in selected brain disorders, including some dementia pathways and epilepsy work-up.
• Cardiology: selected roles include myocardial viability assessment and assessment for cardiac inflammatory disease in specialist settings.

🎗️ Oncology Applications

🔥 Infection and Inflammation Applications

• Fever of unknown origin (FUO): can help localise occult inflammatory, infective, or malignant causes when routine tests are unrevealing.
• Large-vessel vasculitis: may show increased uptake in inflamed aortic and major arterial walls.
• Sarcoidosis: useful for mapping active inflammatory disease and identifying biopsy targets.
• Prosthetic or device infection: selected use in specialist practice, interpreted carefully alongside clinical and microbiological data.
• Osteomyelitis and deep infection: may help define extent of metabolically active infection in appropriate cases.

🧠 Neurology and Cardiology

• Brain FDG PET can demonstrate regional cortical hypometabolism patterns that support the assessment of some neurodegenerative conditions.
• It may also be used in epilepsy work-up to identify interictal hypometabolic cortex in selected patients.
• Cardiac FDG PET may be used to evaluate myocardial viability and certain inflammatory conditions such as cardiac sarcoidosis in specialist centres.

🧾 Patient Preparation

• Patients are usually asked to fast before the scan to reduce competition from circulating glucose.
• Blood glucose should be checked because hyperglycaemia can reduce scan quality and lesion detectability.
• Strenuous exercise should be avoided beforehand, as recent muscular activity increases FDG uptake in skeletal muscle.
• Patients are usually kept warm and resting during the uptake phase to reduce brown fat and muscle uptake.
• Hydration is commonly encouraged, and patients are often asked to empty the bladder before imaging.

🕒 What Happens During the Scan

1. 🩸 A small amount of 18F-FDG is injected intravenously.
2. ⏳ There is a resting uptake period, commonly around 45–60 minutes.
3. 🛏️ The patient then lies on the scanner table while CT and PET images are acquired.
4. 🧠 Depending on the indication, imaging may be whole-body or targeted to a specific region.
5. 📄 The images are interpreted in conjunction with clinical history, prior imaging, and laboratory data.

📊 How PET/CT Is Reported

• Reports describe the distribution and intensity of FDG uptake.
• Uptake is often assessed visually and may be supported by semiquantitative measures such as the SUV (Standardised Uptake Value).
• SUVmax is commonly quoted, but it should never be interpreted in isolation.
• Interpretation depends on pattern recognition, anatomical correlation, timing, prior treatment, and the underlying clinical question.

📏 Standardised Uptake Value (SUV)

• SUV is a semiquantitative estimate of tracer uptake in a lesion.
• Higher SUV can suggest greater metabolic activity, but it does not reliably distinguish cancer from infection or inflammation on its own.
• SUV is affected by multiple variables including body habitus, timing after injection, serum glucose, scanner calibration, uptake time, and reconstruction methods.
• For this reason, pattern, context, and comparison with prior scans matter more than a single SUV number.

👀 Normal Physiological FDG Uptake

• Brain: normally intensely FDG-avid due to high glucose utilisation.
• Myocardium: variable physiological uptake.
• Liver and spleen: usually mild to moderate background uptake.
• Kidneys, ureters, bladder: tracer is excreted renally, so urinary activity is expected.
• Bowel: variable uptake may be seen.
• Brown fat: can be FDG-avid, especially if the patient is cold or anxious.
• Muscles: recent activity, shivering, talking, chewing, or tension can increase uptake.

⚠️ Common Pitfalls

• False positives: infection, inflammation, post-operative change, radiotherapy effect, fracture healing, sarcoidosis, and reactive nodes can all be FDG-avid.
• False negatives: very small lesions, low-grade tumours, hyperglycaemia, or lesions near areas of intense physiological uptake may be missed.
• Urinary tracer excretion can obscure pelvic lesions.
• Brown fat uptake can mimic nodal disease.
• Recent treatment may alter uptake and complicate interpretation.

🚫 Limitations

• FDG PET/CT is not cancer-specific; uptake simply reflects increased glucose metabolism.
• Some tumours are only weakly FDG-avid or variably avid.
• Spatial resolution is limited, so microscopic disease and very small metastases may not be seen.
• It involves ionising radiation from both the radiotracer and the CT component.
• It should be used to answer a clear clinical question, not as a blanket screening test.

☢️ Radiation and Safety

• FDG PET/CT uses a small amount of radioactive tracer and a CT scan, so there is radiation exposure.
• The investigation is generally considered safe when appropriately indicated.
• Pregnancy and breastfeeding require specific consideration and local specialist advice.
• Patients are commonly advised to limit close prolonged contact with pregnant women and very young children for a short period after the scan according to local instructions.

💉 Diabetes and FDG PET/CT

• Good glucose control matters because elevated blood glucose reduces tumour-to-background contrast.
• Diabetic patients often need tailored preparation instructions regarding fasting and medication timing.
• Insulin timing is important because recent insulin can drive FDG into muscle and fat, degrading image quality.

🧑‍⚕️ When FDG PET/CT Is Especially Helpful

• When conventional imaging has found an abnormality but cannot determine its full extent.
• When biopsy needs to be targeted to the most metabolically active site.
• When clinicians need to distinguish active disease from treated scar tissue in selected settings.
• When systemic inflammatory or infective disease is suspected but the source remains occult.

❗ When Caution Is Needed

• Immediately after surgery, chemotherapy, radiotherapy, or invasive procedures, because inflammatory change may cause misleading uptake.
• In low-grade or indolent tumours that may not be strongly FDG-avid.
• When the clinical team is relying too heavily on a single SUV number rather than the whole pattern.

🩻 FDG PET/CT Compared with Other Imaging

🧠 Clinical Interpretation Pearls

• Always interpret FDG PET/CT in the context of the history, examination, blood tests, and other imaging.
• Do not equate “FDG-avid” with “malignant” automatically.
• Look for pattern: symmetrical nodal uptake, diffuse marrow activity, physiological bowel uptake, or post-treatment inflammatory change can all be misleading.
• PET/CT is best used when it changes management: confirming stage, directing biopsy, refining prognosis, or clarifying whether disease is active.

🧑‍⚕️ Management Relevance

• May upstage or downstage cancer and therefore alter surgery, chemotherapy, radiotherapy, or palliative planning.
• May identify the safest or highest-yield biopsy site.
• May support a diagnosis of inflammatory disease when combined with clinical findings.
• May prevent unnecessary invasive procedures if it clarifies disease distribution.

📝 Teaching Commentary

FDG PET/CT is powerful because disease is often detectable by its metabolism before gross anatomical change becomes obvious. That is why it is so useful in staging lymphoma, evaluating selected solid tumours, and uncovering occult inflammatory or infective pathology. The trap is that FDG is not tumour-specific: activated inflammatory cells are often just as glucose-hungry as malignant cells. Good clinicians therefore read PET/CT as a physiology-plus-anatomy test, not as a glowing map of “cancer versus no cancer”. The real skill lies in asking the right clinical question, preparing the patient properly, and interpreting the scan in context rather than overcalling every hot spot.