Table 1. Summary of functional MRI techniques.
| Functional MRI technique | Tissue property measured | MRI pulse sequence | Contrast agent use | Derived imaging biomarkers | Possible pathophysiological correlates | Current status of clinical evidence in head and neck RT |
|---|---|---|---|---|---|---|
| DWI | ||||||
| Gaussian | Water diffusivity | Diffusion-encoded gradient is required, mostly acquired by fat-suppressed single-shot EPI with at least two b-values (usually 0–1,000 sec/mm2) | No | Apparent diffusion coefficient ADC (mm2/sec) | Cellularity (cell density), cell size and structure EES, cell membrane integrity, necrosis, neoangiogenesis | Some evidences have been established while need to be further strengthened |
| Non-Gaussian | Non-Gaussian water diffusivity accounting for tissue complexity like microvascularity and cell membrane restriction to diffusion | Diffusion-encoded gradient is required, mostly acquired by fat-suppressed single-shot EPI but with extended b-values (<200 sec/mm2 required for IVIM, >1,000 sec/mm2 for others) | No | Non-Gaussian diffusion coefficients and additional parameters; DDKI and Kapp for DKI, D, D* and f for IVIM, DDC and α for SEM | Limited indications suggested by a small number of pilot studies | |
| Perfusion MRI | ||||||
| DCE-MRI | Tissue permeability, EES and plasma volume | T1w spoiled gradient echo sequences | Gd contrast agent | Semi-quantitative parameters: bolus arrival time, time-to-peak, wash-in rate, wash-out rate and maximum enhancement etc; quantitative Tofts parameters: Ktrans, kep, ve, vp | Tissue perfusivity, permeability, EES fraction, vascular density, plasma volume, neoangiogenesis, hypoxia | Some evidences have been established while need to be further strengthened |
| DSC-MRI | Tissue perfusivity reflected by blood flow induced susceptibility and T2* change | T2* weighted gradient echo EPI | Gd contrast agent | Semi-quantitative parameters similar to DCE-MRI, quantitative parameters: CBF, CBV, MTT | Blood flow, blood volume, vascularity | No clinical study and no evidence so far for head and neck RT |
| ASL | Tissue perfusivity reflected by magnetically labeled blood flow | Labeling RF pulse is required, usually acquired by EPI | No | Quantitative parameters: CBF, CBV, MTT | Blood flow, blood volume, vascularity | Limited indications suggested by a small number of pilot studies |
| MRS | Metabolite composition | STEAM, PRESS | No | Height, width and offsets of metabolite peaks on MR spectrum | Metabolism in various pathophysiological processes, e.g., proliferation | Limited indications suggested by a small number of pilot studies |
| BOLD MRI | Deoxyhaemoglobin induced tissue T2* reduction | T2* weighted gradient echo EPI | Carbogen breathing may be required | T2*W contrast or absolute T2* value | Tissue oxygenation, hypoxia | Few technical feasibility studies and no clinical evidence yet |
| MRE | Biomechanical properties such as elasticity and viscosity | Motion-encoded gradient echo sequences | No | Elasticity and viscosity values | To be established, potentially fibrosis and necrosis | Under technical development, no clinical study and no clinical evidence for head and neck RT |
| Spin-lock MRI | Proton exchange between free water and macromolecules induced relaxation time change | Spin-lock RF pulse is required, acquisition can be done by various sequences | No | Spin-lattice relaxation time in the rotating frame T1ρ | To be established | No clinical study and no clinical evidence so far for head and neck RT |
| CEST MRI | Chemical exchange processes between free water and mobile exchangeable (exogenous o endogenous) agents | Saturation RF pulses at offsets are required, acquisition can be done by various sequences | No for endogenous CEST; contrast agent still in research phase for exogenous CEST | Usually asymmetric magnetization transfer ratio MTRasym | To be established, potentially, metabolism in various, pathophysiological processes | No clinical study and no clinical evidence so far for head and neck RT |
*, means pseudo-diffusion coefficient. MRI, magnetic resonance imaging; DWI, diffusion-weighted imaging; EPI, echo planar imaging; ADC, apparent diffusion coefficient; DKI, diffusion kurtosis imaging; IVIM, intravoxel incoherent motion; SEM, stretched exponential model; EES, extra-vascular extra-cellular space; CBV, cerebral blood volume; CBF, cerebral blood flow; MTT, mean transit time; RT, radiation therapy; ASL, arterial spin labeling; MRS, MR spectroscopy; MRE, MR elastography; PRESS, pointed-resolved spectroscopy; STEAM, stimulated echo acquisition mode; CEST, chemical exchange saturation transfer; MTRasym, asymmetric magnetization transfer ratio.