Nuclear Medicine · PET

PET/CT and PET/MRI One-Liners

High-yield one-liners on PET/CT and PET/MRI interpretation, physiological uptake patterns, common pitfalls and oncological applications.

Rapid-revision one-liners covering PET/CT and PET/MRI principles, physiological and pathological uptake patterns, response assessment criteria and common interpretation pitfalls.

FDG mechanism
F-18 fluorodeoxyglucose is a glucose analogue transported by GLUT transporters and phosphorylated by hexokinase to FDG-6-phosphate, which is trapped intracellularly because it is not a substrate for glucose-6-phosphatase.
SUV definition
Standardised uptake value (SUV) equals tissue activity concentration (kBq/mL) divided by injected dose (kBq) per body weight (g); SUVmax is the single hottest voxel within a region of interest.
Physiological brain uptake
Intense, symmetric grey-matter FDG uptake is normal due to obligate cerebral glucose metabolism; reduced cortical uptake may indicate encephalitis, ischaemia or sedation.
Physiological renal and bladder uptake
FDG is excreted renally; intense ureteric and bladder activity is normal but may obscure pelvic or ureteric lesions, necessitating delayed imaging or diuresis.
Brown fat uptake
Symmetric supraclavicular, cervical, paravertebral and mediastinal FDG uptake in cold or anxious patients represents brown adipose tissue; mitigated by warming, anxiolysis or propranolol.
SUV threshold caveat
SUVmax greater than 2.5 is commonly used as a malignancy threshold but is neither sensitive nor specific; active inflammation, infection and granulomatous disease can exceed this value.
PET/MRI advantage
PET/MRI provides superior soft-tissue contrast in brain, liver, pelvis and musculoskeletal regions with approximately 70% lower radiation dose than PET/CT, but uses segmentation-based attenuation correction that may introduce artefacts at tissue-air interfaces.
Ga-68 DOTATATE
Ga-68 DOTATATE binds somatostatin receptor subtype 2 with higher affinity than In-111 pentetreotide, offering superior spatial resolution and sensitivity for well-differentiated neuroendocrine tumour staging.
Ga-68 PSMA in biochemical recurrence
Ga-68 PSMA PET/CT detects recurrent prostate cancer at PSA levels below 0.5 ng/mL with detection rates of 40-60%, significantly outperforming conventional imaging.
Deauville scale
The 5-point Deauville scale compares residual FDG uptake to liver and mediastinal blood-pool activity for interim and end-of-treatment response assessment in Hodgkin and aggressive non-Hodgkin lymphoma.
Post-treatment PET timing
FDG-PET should be performed at least 6-8 weeks after the last chemotherapy cycle and at least 12 weeks after radiotherapy to minimise false-positive inflammatory uptake.
Metabolic tumour volume and TLG
Metabolic tumour volume (MTV) and total lesion glycolysis (TLG = MTV x SUVmean) are volumetric PET parameters with independent prognostic value in lymphoma, cervical and head-and-neck cancers.
False-positive FDG causes
Active infection, granulomatous disease (tuberculosis, sarcoidosis), post-surgical or post-radiation inflammation, brown fat and reactive lymph nodes are the principal false-positive mimics.
False-negative FDG causes
Low-grade malignancies (typical carcinoid, lepidic-predominant adenocarcinoma, mucinous neoplasms), lesions below 7 mm and hyperglycaemia-induced competitive inhibition are key false-negative causes.
PET/MRI attenuation correction
Unlike PET/CT which uses CT-derived mu-maps, PET/MRI uses segmentation or atlas-based methods; susceptibility artefacts near metallic implants or air-bone interfaces may underestimate attenuation and SUV.
Focal thyroid FDG uptake
Focal incidental thyroid FDG uptake carries a 30-50% malignancy risk and warrants dedicated ultrasound evaluation; diffuse uptake more commonly indicates thyroiditis.
FDG in unknown primary
Whole-body FDG-PET/CT is recommended in cervical nodal metastasis with unknown primary, identifying the primary site in approximately 30-40% of cases and altering management in over 50%.
Dual-time-point imaging
Delayed imaging at 2 hours can help differentiate malignant from inflammatory uptake; malignant lesions typically show rising SUV over time whereas inflammatory lesions plateau or decrease.
PSMA physiological distribution
Normal PSMA uptake is seen in lacrimal and salivary glands, liver, spleen, kidneys, small bowel and sympathetic ganglia (especially coeliac and stellate); ganglion uptake is a common pitfall mimicking nodal disease.
PET/MRI in paediatrics
PET/MRI is particularly advantageous in paediatric oncology due to substantially reduced radiation exposure and superior CNS, bone-marrow and pelvic soft-tissue assessment.

Caution

SUV thresholds and tracer indications vary by institutional protocol, scanner calibration and clinical context. Always verify current NCCN, ESMO or SNMMI guidance before applying quantitative cut-offs.

Exam pearl

In viva, always state the Deauville score with reference to liver and mediastinal blood pool, and remember that brown fat uptake is symmetric and anatomically predictable.

Viva questions

What is the biochemical basis of FDG trapping in cells?
FDG is transported by GLUT transporters and phosphorylated by hexokinase to FDG-6-phosphate, which cannot be further metabolised by glycolysis or dephosphorylated efficiently, resulting in intracellular trapping proportional to glucose metabolism.
How does the Deauville 5-point scale work?
Score 1 is no uptake, 2 is uptake at or below mediastinal blood pool, 3 is uptake above blood pool but at or below liver, 4 is moderately increased uptake above liver, and 5 is markedly increased uptake or new lesions. Scores 1-3 are generally considered complete metabolic response.
Why might PET/MRI underestimate SUV near metallic implants?
PET/MRI uses segmentation-based attenuation correction rather than CT-derived mu-maps. Susceptibility artefacts near metal cause signal loss on MRI, leading to underestimation of tissue attenuation and consequently underestimated SUV values.
What is the minimum recommended interval between chemotherapy completion and FDG-PET for response assessment?
At least 6 to 8 weeks after the last chemotherapy cycle, and at least 12 weeks after completion of radiotherapy, to reduce false-positive inflammatory uptake.
Name three causes of false-negative FDG-PET.
Low-grade tumours such as typical carcinoid or lepidic adenocarcinoma, mucinous neoplasms with low cellularity, and lesions below the spatial resolution threshold of approximately 7 millimetres.

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