As the name implies TOF MRA is based the principle of flow-related enhancement, a time-of-flight flow phenomenon discussed in a prior Q&A. Stationary tissues in an imaged volume become magnetically saturated by multiple repetitive RF-pulses that drive down their steady-state magnetization levels. "Fresh" blood flowing into the imaged volume has not experienced these pulses and thus has a high initial magnetization. The signal from inflowing blood thus appears paradoxically bright compared to background tissue. A maximum intensity projection (MIP) rendering technique with leveling, filtering, and optional surface display is then used to create the MR angiogram.
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Although nearly all vendors use the generic terminology "time-of-flight", Philips calls their 2D-TOF and 3D-TOF sequences M2D Inflow and M3D Inflow, respectively. Philips also refers to 3D volumes as "chunks" rather than the more commonly used nomenclature of "slabs" or "sections".
Saloner D. An introduction to MR angiography. Radiographics 1995;15:453-465. (Older review; good discussion of TOF and PC MRA)
You mentioned that TOF MRA could be performed in either 2D or 3D mode. Which should I use?
What are time-of-flight effects?