Magnitude Contrast Technique

The concept of magnitude contrast angiography is analogous to x-ray digital subtraction angiography (DSA). The basic idea is to acquire two datasets: one flow-rephased and one flow-dephased [8]. First a flow-compensated measurement is performed using GMR in order to image flowing blood with high signal intensity. In the second acquisition, flow-sensitizing bipolar gradient pulses are applied specifically to induce velocity-dependent phase shifts of moving spins. If the flow-sensitizing gradient is strong enough, the spins within a voxel possessing different velocities may totally dephase resulting in dark vessel signal (Fig. 22). Since stationary tissue appears the same in both acquisitions, subtraction of one dataset from the other results in the signal of the stationary tissue being cancelled leaving only the moving blood as visible (Fig. 23). Interleaving the acquisition of both datasets can diminish the impact of motion artifacts on the subtraction process.

The signal intensity in the subtracted image depends only on the velocity component along the flow-sensitizing gradient which is normally applied in the frequency-encoding direction. Therefore, the imaging volume should be oriented in such a way that the main flow direction in the vessel of interest is parallel to the read-out direction.





rephased —

. maximum signal □

dephased —

. total signal loss ■








Fig. 22. Left: Using flow compensation (GMR), the phases of all spins regardless of their velocity are aligned. Thus, the signal is maximal, the vessel appears bright. Right: Using strong flow-sensitizing gradients, complete dephasing of the spins occurs due to their different velocities. In this case, the signal is nulled, the vessel appears black

rephased acquisition

dephased acquisition

Subtraction image

bright vessels

bright vessels

dark vessels

dark background

Fig. 23. Principle of magnitude contrast angiography. First a flow-rephased dataset is acquired followed by a flow-de-phased acquisition. After pixel by pixel subtraction (similar to x-ray digital subtraction angiography (DSA)) only the signal of flowing blood remains, while background signal is nulled

Information about all three orthogonal flow directions can be obtained by repeating the flow-encoded acquisition with altered gradient orientations. As a result, a total of four acquisitions has to be performed (one rephased, three dephased), which impacts on the overall acquisition time.

Although not in widespread use, magnitude contrast MRA can be considered applicable for imaging of peripheral vessels (arm, leg) since it allows larger sections of arteries to be visualized. If flow is unidirectional, a single pair of rephased and dephased acquisitions is sufficient, thereby reducing the overall acquisition time.

Magnitude contrast MRA requires a spectrum of flow velocities within a voxel. Laminar flow with its parabolic flow profile is therefore readily detected. The signal acquired is a direct result of the velocity distribution in each voxel, ensuring complete background suppression. The method is well adapted for depicting slow flow with good spatial resolution, covering larger sections of vessels. One disadvantage arises from the fact that this technique does not provide any information on flow direction or flow velocity.

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