Table 2. . Emerging MRI modalities for assessing intratumor heterogeneity in glioblastoma.
| Modality | Biological properties visualized | Limitations† |
|---|---|---|
| DWI | Signal drop in regions of free-moving (Gaussian) diffusion | Should not be assessed without corresponding T2-weighted or proton density image as the T2-signal can affect DWI signal (T2-shinethrough); nonquantifiable |
| ADC | Quantifiable; high signal may indicate: cystic lesions, edema, low cellularity, necrosis; low signal may indicate: cytotoxic edema, cellularity. | Cannot differentiate tumor cells from inflammatory cells |
| DTI | White matter integrity based on infiltration, displacement or disruption; with this, one can distinguish tumor bulk from the surrounding margin | Provides details solely pertaining to isotropic and anisotropic movement of 1H (protons). Limited microenvironment information |
| T1-weighted DCE | BBB disruption, blood vessel permeability, ‘leaky’ vasculature | Model-dependent: based on either flow-limited or permeability limited conditions. Model details can be found in work by Tofts et al. [49] |
| T2*-weighted DSCI | Regional‡ rCBV | Due to BBB disruption, T2*-signal drops with recirculation of blood. As the rCBV model assumes an intact BBB, leakage correction is essential to account for damaged and leaky BBB in GBM |
| 1H-MRS CSI | Metabolic changes; microenvironment: high Cho = membrane turnover (cellularity); low NAA = damaged neurons; lactate and lipids = anaerobic, acidic conditions | Improved spectra resolution comes at the cost of increased acquisition time (longer time in the scanner); scanning time further increases when obtaining 3D data |
†A limitation relevant to these imaging modalities, the emerging ones in particular, is a lack of validation and standardization.
‡Though various metrics can be derived using T2*-weighted imaging, only rCBV is focused on as it is the one most analyzed, showing greatest promise in terms of potential standardization [55].
ADC: Apparent diffusion coefficient; BBB: Blood–brain barrier; Cho: Choline; DCE: Dynamic contrast enhanced; DSCI: Dynamic susceptibility contrast imaging; DTI: Diffusion tensor imaging; DWI: Diffusion-weighted imaging; GBM: Glioblastoma; 1H-MRS CSI: Magnetic resonance spectroscopy chemical shift imaging; NAA: N-acetylaspartate; rCBV: Relative cerebral blood volume.