... but what if we were to open up that really thin volume into a bigger volume? Turn that slice into a slab with a gradient echo acquisition of a volume? Well... if you do that, then you've got a 3D Time of Flight MR angiogram...
Why bother with this technique? It has a couple of advantages over 2D TOF, including the better signal-to-noise and better evaluation of tortuous vessels inherent in a 3D technique. Speed is variable depending on the amount of resolution you want or need...the speed of the 3D TOF sequence is directly proportional to the number of partitions in the slab (and therefore the number phase-encoding steps)... more partitions = smaller voxels = higher spatial resolution = longer time.
So... in considering how 2D TOF works (see the 10/14/2012 post), how is it that a flowing blood proton bombarded with RF pulses at the bottom of the slab still has signal at the top? Shouldn't it get saturated and lose signal? Isn't TOF inherently limited to thin slices?
|Blood becomes more saturated as it moves through the slab.|
For this reason, 3D TOF is much less sensitive to slow flow than 2D TOF, and 3D TOF is better for higher velocity arteries, such as the circle of Willis (above) or the aorta. 2D TOF is better for systems with slow flow (such as the peripheral extremities or abdominal vessels)
|Theoretical schematic of increasing flip angles through the slab|
3D TOF can also be combined with gadolinium contrast agents, which would selectively shorten the T1 relaxation time of the blood and allow the larger flip angles..... but the benefit of a non-contrast study is lost.
Some techniques vary the flip angle through the slab, with a smaller flip angle early in the slab to prevent too much saturation, and a larger flip angle toward the end of the slab, where residual magnetization is less of an issue (right).
Magentic transfer techniques can also be used to increase the contrast between relatively homogenous fluid (blood) and heterogeneous fluid with large macromolecules (tissue).
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4. "The Physics of Clinical MR Taught Through Images" Runge, Nitz, Schmeets, et al. (2005)