Monday, October 15, 2012

Problems with 2D TOF: Flow-related dephasing

In yesterday's (10/14/12) post about 2D Time-of-Flight MRA, there was one key parameter that was missing... the TE.

So what TE should be used for a 2D TOF?

This brings up another important point about flow-related enhancement in MRA.  In the previous discussion, we sort of assumed that all the inflowing blood was all coherently phased.  T2-weighted effects takes advantage of differential phase decay in different microenvironments... that's its strength... but does flowing blood stay in a coherent phase?  Do we need to worry about these effects?

The answer is no, the blood is not in a coherent phase -- and yes, we need to be concerned about this.

The very nature of a gradient sequence (which is necessary for repetitive quick imaging in MRA) is that it has a gradient of field strengths... and this change in the magnetic microenvironment causes dephasing of the inflowing blood and loss of flow-related enhancement on a T1-weighted gradient.

The problem is compounded by the fact that -- no surprise -- the amount of dephasing depends on the amount of time that the blood is exposed to the gradient field... but the velocity of laminar blood flow in a tube is not uniform.  The blood at the center of the tube moves most quickly, experiences the least gradient, and has the least flow-related enhancement; the more sluggish blood at the periphery dephases the most.  This usually results in an overall loss of flow-related enhancement within the voxel (intravoxel dephasing).

There are a few ways to help correct for this:

1) Gradient Moment Nulling  (i.e. "Canceling the change in the gradient magnetic moment")

Spin-spin dephasing in stationary protons can be accounted for with a bipolar gradient, in which an equal and opposite gradient is applied before the echo is listened for (opposite gradients within one TE).  This has the effect of canceling the dephasing.

... but the problem is that this doesn't work for flowing blood very well.

To compensate for this, you can use a "flow-compensation gradient" or "gradient moment nulling," which is basically the same bipolar gradient, but in reverse... so... 1. application of original gradient, 2. application of opposite gradient (bipolar), 3. second application of opposite gradient, 4. application of original gradient, and then 5. listen for echo (all gradients occur within one TE). This sequence corrects for the phase shift.

(ref 3)

Problems with this technique?  It assumes constant velocity of blood and can't account for turbulent flow very well.  It also requires a much longer TE, lengthening the scan time, which is already lengthy in TOF technique.

2) Short TEs

Another, more simple, way of trying to account for flow-related dephasing is just to not allow the spins enough time to dephase... i.e. use as short a TE as possible. TEs less than 10 msec are not uncommon. This also has the added benefit of decreasing scan time.

3) Use as small a voxel as possible

The smaller the voxel, the less chance for spins within the voxel to dephase one another... and the higher the spatial resolution....... but the longer the scan time and decreased S:N.  2D TOF techniques tend to use larger voxel sizes than 3D techniques, and therefore, there is more opportunity for flow-related dephasing in this technique.

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