(PRESTO) PRESTO is a 3 dimensional ultrafast gradientecho sequence that combines the whole brain coverage with T2* weighted imaging. PRESTO is useful for BOLD and perfusion imaging studies. In combination with parallel imaging techniques, PRESTO provides higher temporal resolution and more coverage compared to traditional multislice imaging. In addition, the sensitivity to susceptibility artifacts and flow phenomena is reduced, compared with EPI techniques, enabling MRI scans throughout the brain including the skull base.
(TOF) The time of flight angiography is used for the imaging of vessels. Usually the sequence type is a gradientechosequences with short TR, acquired with slices perpendicular to the direction of blood flow.
The source of diverse flow effects is the difference between the unsaturated and presaturated spins and creates a bright vascular image without the invasive use of contrast media. Flowing blood moves unsaturated spins from outside the slice into the imaging plane. These completely relaxed spins have full equilibriummagnetization and produce (when entering the imaging plane) a much higher signal than stationary spins if a gradientecho sequence is generated. This flow related enhancement is also referred to as entry slice phenomenon, or inflow enhancement.
Performing a presaturationslab on one side parallel to the slice can selectively destroy the MR signal from the in-flowing blood from this side of the slice. This allows the technique to be flow direction sensitive and to separate arteriograms or venograms. When the local magnetization of moving blood is selectively altered in a region, e.g. by selective excitation, it carries the altered magnetization with it when it moves, thus tagging the selected region for times on the order of the relaxation times.
For maximum flow signal, a complete new part of blood has to enter the slice every repetition (TR) period, which makes time of flight angiography sensitive to flow-velocity. The choice of TR and slice thickness should be appropriate to the expected flow-velocities because even small changes in slice thickness influences the performance of the TOF sequence.
The use of sequential 2 dimensional Fourier transformation (2DFT) slices, 3DFT slabs, or multiple 3D slabs (chunks) are depending on the coverage required and the range of flow-velocities.
3D TOF MRA is routinely used for evaluating the Circle of Willis.
Echo spacing is the distance in time between the echoes in multiple echosequences like, e.g. echo planar imaging, fast spin echo. A short echo space produces compact sequence timing and less image artifacts.
The shorter the rise time, the faster the gradients and therefore the echo spacing. Gradients with a shorter echo spacing will have a better resolution and more slices per TR.