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Time-of-Flight Magnetic Resonance Angiography (TOF-MRA) is a non-contrast MRI technique that visualizes flowing blood using flow-related enhancement.
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https://www.youtube.com/watch?v=haax8yadse8
It is widely used in intracranial and neck vascular imaging and is especially useful when gadolinium contrast is contraindicated (e.g., in renal impairment).
This phenomenon is known as inflow enhancement.
| Parameter | Description |
|---|---|
| Sequence type | Gradient Echo (GRE), usually 3D or 2D |
| Flip angle | Moderate to high (20–60°) |
| TR/TE | Short (e.g., TR ~30 ms, TE ~3–7 ms) |
| Saturation bands | Applied above/below the imaging plane to suppress unwanted flow (e.g., venous) |
Time-of-flight effect in spin-echo imaging. The 90° pulse (blue) excites all the spins in the slice of thickness Δz. (a) Stationary blood will experience both a 90° and a 180° (red) pulse yielding a spin-echo signal (purple). (b) Slow-flowing blood may not completely leave the slice between the 90° and 180° pulses, resulting in a spin-echo signal only from the blood experiencing both (purple). (c) Fast-flowing blood will only experience the 90° pulse and no spin echo will occur resulting in zero signal. This is known as wash-out.
McRobbie DW, Moore EA, Graves MJ, Prince MR. Go with the Flow: MR Angiography. In: Cambridge University Press eBooks. ; 2017:251-268. doi:10.1017/9781107706958.016
Time-of-flight effect in gradient-echo imaging. The α° pulse (red) excites all the spins in the slice of thickness Δz. (a) Stationary blood will experience both α° pulses yielding a partially saturated signal (purple). (b) Slow-flowing blood may not completely leave the slice during TR and only a proportion of the blood will experience both pulses (purple). However, unsaturated blood (blue) that has not experienced any prior RF pulses will enter the slice during TR. The signal will be a combination of the unsaturated (blue) and saturated (purple) blood. (c) Fast-flowing blood will be completely replaced during the TR period, resulting in maximal signal in the second echo.
McRobbie DW, Moore EA, Graves MJ, Prince MR. Go with the Flow: MR Angiography. In: Cambridge University Press eBooks. ; 2017:251-268. doi:10.1017/9781107706958.016
| TOF Type | Description | Common Use |
|---|---|---|
| 2D TOF | Multiple thin slices; sensitive to slow flow | Carotid bifurcation, peripheral vessels |
| 3D TOF | Volumetric slab; better spatial resolution, less sensitive to slow flow | Intracranial arteries |
| Multi-slab TOF | Combines multiple small slabs to cover large FOV | Circle of Willis, vertebrobasilar system |
Maximum intensity projection (MIP) algorithm. Parallel rays (dotted lines) are projected through the data volume. The maximum pixel brightness is then displayed on the projection images. This has the effect of collapsing all the data along the ray into a single image. The data volume may be rotated a few degrees and the MIP repeated. If the projection images are then displayed in a movie loop, they provide all the necessary visual cues to allow ‘3D’ perception of the data
McRobbie DW, Moore EA, Graves MJ, Prince MR. Go with the Flow: MR Angiography. In: Cambridge University Press eBooks. ; 2017:251-268. doi:10.1017/9781107706958.016