Neurosurgery notes/Radiology/Nuclear medicine/PET scan

PET scan

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Nuclear medicine

Technique

  • IV injection of a positron-emitting radiopharmaceutical (15 O and 18 F) → wait to allow for systemic distribution → scanning for detection and quantification of patterns of radiopharmaceutical accumulation in the body.
  • Uptake of this compound followed by further breakdown occurs in the cells.
  • Tumour cells have a high metabolic rate, and hence this compound is also metabolised by tumour cells.

Mechanism

  • FDG is metabolised to FDG-6-phosphate which cannot be further metabolised by tumour cells → accumulates and concentrates in tumour cells. This accumulation is detected and quantified.
  • The positron-emitting isotope administered to the patient undergoes β+ decay in the body, with a proton being converted to a
      • Neutron,
      • Positron (the antiparticle of the electron, sometimes referred to as a β+ particle),
        • The positron travels a short distance and annihilates with an electron (within the cell) → formation of two high energy photons which travel in diametrically opposite directions.
      • Neutrino.
      PET scan
      PET scan

Isotope used

18 FDG (flurordeoxyglucose) PET

  • Most common used radionuclides
  • Brain uses high glucose hence cant be used to differentiate area of hypermetabolism
  • Assessment of brain tumors;
    • 1st tracer used for brain tumours
    • However, it has a low tumor-to-background ratio in brain, limiting its utility.
    • 18F-FDG uptake correlates with tumor grade
      • Low grade gliomas do not take up much compared to white matter
      • High-grade gliomas (grades III and IV) showing higher uptake than low-grade gliomas.
        • Therefore, in spite of its limitations, 18F-FDG PET-CT is used for imaging of high-grade glioma.
FDG PET hypometabolism
FDG PET hypometabolism

Amino acid PET (AA PET) 18FDOPA

  • Amino acid PET Radiotracers including 18F-FDOPA display superior contrast compared to 18F-FDG because of low uptake of amino acids in normal brain tissue.
  • Used in
    • Detection of low-grade gliomas.
      • 18F-FDG PET can be falsely negative, even in high-grade recurrent gliomas and, therefore, 18F-FDOPA PET can be an alternative imaging modality to rule out recurrence even when 18F-FDG PET is negative.
  • Cons
    • 18F-FDOPA tumor uptake cannot provide reasonable predictions about tumor grade and proliferation in recurrent tumors that have undergone treatments.
    • Difficult synthesis or need for an on-site cyclotron limits their widespread use.

False-positive FDG uptake

  • Granulomatous disease
  • Abscess
  • Surgical changes
  • Foreign body reaction e.g. talc pleurodesis
  • Excessive bowel uptake with metformin therapy
  • Inflammation (although at times e.g. evaluating for vasculitis, this may be the finding of interest)
  • Fat necrosis

Normal physiological uptake

  • Brain
  • Waldeyer ring, e.g. palatine tonsils symmetrically, especially when younger 7
  • Salivary glands
  • Skeletal muscle, especially after strenuous activity and laryngeal muscles following speech
  • Myocardium
  • Gastrointestinal tract, e.g. intestinal wall
  • Genitourinary tract: FDG is excreted via the renal system and passes into the collecting systems
  • Brown fat
  • Thymus
  • Bone marrow
  • Lactating breasts
  • Nipples
  • Testicles

Uses

  • Oncologic (Holzgreve et al 2021)
    • Detection, staging, response to treatment
    • Differentiation between radiation necrosis and recurrence
        • (Upper row) A 59-year-old male patient diagnosed with an IDH-wild-type glioblastoma (WHO CNS grade 4). Following resection and chemoradiation with temozolomide, the contrast-enhanced MRI (CE-T1w MRI) suggested tumor relapse in the right parietal region 7 months after completing radiotherapy. Accordingly, the dynamic FET PET scan revealed pathologically increased FET uptake right parietal (TBRₘₐₓ 4.2) and decreased time–activity curve;
        • (Lower row) A 37-year-old female patient diagnosed with an IDH-wild-type glioblastoma (WHO CNS grade 4). Following resection and chemoradiation with temozolomide, the contrast-enhanced MRI suggested tumor relapse in the left frontal region 7 months after completing radiotherapy. In contrast to the patient in the upper row, the FET uptake in the left frontal region was not pathologically increased (TBRₘₐₓ 1.6) with a steadily increasing time–activity curve, indicating reactive treatment-related changes. SUV = standardized uptake value.
        notion image
    • Practical and experimental roles of PET imaging in glioma management include
      • Grading tumors and estimating prognosis;
      • Localizing the optimum biopsy site,
      • Defining target volumes for radiotherapy (RT),
      • Assessing response to therapy,
      • Detecting tumor recurrence and distinguishing it from radionecrosis.
    • Lymphoma has a high glucose metabolism on FDG-PET, which can be more apparent than contrast enhancement.

Pros

  • The advantage over CT is PET gives information about function because of neuronal activity or blood flow

Cons

  • Motion artifacts result in an inaccurate anatomical co-registration of the CT and PET studies
  • Poor spatial resolution
    • So will combine with CT or MRI to give spatial reference
    • Due to The distance (2-3 mm) the positron travels before annihilation and the detector element size both contribute to relatively poor spatial resolution.
    • It should be considered that its sensitivity in lesions smaller than 1 cm in diameter is diminished due to the lower spatial resolution of PET scanners