18F-FDG PET(-CT) imaging
Positron-emission tomography (PET) is a nuclear functional imaging modality that is used to observe metabolic processes in the body. The method is based on the detection of gamma rays that are indirectly emitted during the decay of radionuclides. Radionuclides are isotopes with short half-lives such as fluorine-18 (18F). These radionuclides are incorporated in substances normally used by the body such as glucose or glucoses analogues (such as fluorodeoxyglucose) creating radiotracers. The most well-known radiotracer is 18F-fluorodeoxyglucose (18F-FDG). 18F-FDG is injected into the patient’s body where it is predominantly being used in cells with a high glucose metabolism such as brain cells, liver cells but also cancer cells. This enables visualization of cell activity in tumors subsequently enabling staging, re-staging and response monitoring in many tumor types.
Diffusion-weighted MRI (DW-MRI)
Diffusion-weighted MRI (DW-MRI) is a functional imaging modality that allows for tissue characterization by deriving image contrast from variations in the free diffusion (i.e. thermally driven random mobility or Brownian motion) of water molecules, which is a marker for microstructural density. The apparent diffusion coefficient (ADC) can be calculated to quantify these differences in diffusion or microstructural density in a certain volume of interest over time. The ADC is inversely correlated with tissue cellularity and as chemoradiotherapy can result in the loss of cell membrane integrity, tumor response can be detected as an increase in tumor ADC. This makes DW-MRI eligible for response monitoring in tumors.
Dynamic contrast-enhanced MRI (DCE-MRI)
Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is the acquisition of serial MRI images while intravenously administering a contrast agent. Unlike conventional enhanced MRI, which simply provides a snapshot of enhancement at one point in time, DCE-MRI permits a fuller depiction of the contrast kinetics within tumors, and thus provides insight into the nature of the tissue properties related to perfusion. DCE-MRI is used mostly to characterize masses, stage tumors, and monitor therapy.
Patient with a cT4aN2M0 mid-esophageal squamous cell carcinoma with a complete histopathologic response to treatment (tumor regression grade [TRG] 1) (red contours). Diffusion-weighted images (a, d and g), corresponding T2-weighted magnetic resonance images (b, e and h), and corresponding apparent diffusion coefficient (ADC) maps (c, f and i) before neoadjuvant chemoradiotherapy (nCRT) (a–c), during nCRT (d–f), and after nCRT (g–i). ADC calculations revealed an ADC increase of 48% after 2 weeks of nCRT and an increase of 44% 7 weeks after nCRT. Reference: (van Rossum et al., 2015)