Table 1.
Summary of MR imaging techniques for which changes during the course of radiotherapy have been investigated.
| MR imaging technique | qMRI metric | Tissue characteristics | Studies investigating changes during RT |
|---|---|---|---|
| DWI | ADC | Tissue cell density | Rectum (14–25) Cervix (26–39) Head-and-neck (40–48) Esophagus (49–57) Brain (58) Lung (59) Liver (60) Prostate (61, 62) Sarcoma (48) |
| DCE-MRI | semi-quantitative measurements (e.g. peak enhancement) quantitative parameters derived with pharmacokinetic modeling (e.g. Ktrans, ve) |
Perfusion and vascular permeability of tumor microenvironment | Cervix (32, 63–65) Head-and-neck (44, 66, 67) Esophagus (50) Liver (68) |
| IVIM | f, D, D* | Tissue perfusion and cell density | Cervix (69–74) Esophagus (75, 76) Head-and-neck (77, 78) Brain (79, 80) |
| Relaxometry | T2, T1, PD, T2* | Tissue relaxation times | Prostate (61, 62) Brain (81, 82) |
| Spectroscopy | e.g. choline to creatine ratio | Metabolism | Brain (83–86) Cervix (35) Head-and-neck (87) |
| OE-MRI | O2 concentration | Hypoxia | Lung (88) |
| Saturation transfer MRI (MT, CEST) | e.g. MTR, qMT, MTRasym, MTRamide | Tissue macromolecular content (e.g. lipids, proteins, peptides) | Brain (81, 89, 90) Head-and-neck (91) |
| Fat composition | PDFF, %PDFF | Fat content | Bone marrow (92) |
| Radiomics | Histogram features, local textural features | Tissue heterogeneity | Cervix (93, 94) Rectum (95, 96) Head-and-neck (97) Sarcoma (98) Pancreas (99) |
Papers were searched on PubMed with search terms “early response” and “radiotherapy” or “radiation oncology” as well as measurements during treatment mentioned in title or abstract. Only studies in humans and in English were included. Reference list of the included papers were checked to identify other relevant papers.
DWI, diffusion-weighted imaging; ADC, apparent diffusion coefficient; DCE-MRI, dynamic contrast-enhanced MRI; Ktrans, volume transfer constant between blood plasma and extravascular extracellular space; ve, fractional volume of extravascular extracellular space; IVIM, intravoxel incoherent motion imaging; f, perfusion fraction; D, diffusion coefficient; D*, pseudo-diffusion coefficient; T2, T2 relaxation time mapping; T1, T1 relaxation time mapping; PD, proton density mapping; R2*, R2* mapping; OE-MRI, oxygen-enhanced MRI; MT, magnetization transfer; CEST, chemical exchange saturation transfer; MTR, magnetization transfer ratio; qMT, quantitative magnetization transfer; MTRasym, magnetization transfer asymmetry; MTRamide, magnetization transfer ratio of amide protons; PDFF, proton density fat fraction.