Diffusion-weighted imaging
DWI sensitizes the MR signal to water motion using diffusion-encoding gradients. Signal behavior depends on diffusion weighting, tissue properties and sequence timing. ADC maps quantify an apparent diffusion coefficient and are interpreted alongside high-b-value images to separate diffusion effects from T2-related signal.
Diffusion tensor imaging
DTI extends diffusion imaging by sampling multiple directions and fitting a tensor model. Derived measures can describe directional diffusion, and tractography algorithms use local directional information to estimate pathways through white matter. Tractography is a model-based visualization rather than a direct photograph of axons.
Time-of-flight MRA
TOF MRA commonly exploits inflow effects: unsaturated blood entering an imaging volume can appear bright relative to repeatedly excited stationary tissue. Flip angle, slab thickness, flow direction, saturation and velocity influence vessel conspicuity.
Phase-contrast MRA
Phase-contrast techniques encode velocity information into phase. They can be used for angiographic visualization and flow-related measurements when acquisition parameters, including velocity encoding, are appropriately selected.
Learning the tradeoffs
Advanced MRI is especially sensitive to acquisition choices. Spatial resolution, SNR, distortion, motion, susceptibility, flow direction and post-processing all influence the final visualization. Understanding those dependencies is more durable than memorizing a single protocol.