Similar to magnitude contrast, phase contrast an-giography is based on the acquisition of two datasets that differ in the phase of moving spins [9,10].At first, a flow-rephased sequence (S^ is applied which defines the phase of transverse magnetization under conditions of total flow compensation. The second measurement (S2) is flow-sensitive, utilizing special flow-encoding gradients to provoke a measurable phase shift. Contrary to the magnitude contrast technique, the chosen gradient is weak enough to avoid complete phase dispersion arising from the velocity distribution of the spins. Complex subtraction of the two datasets S1 and S2 yields the phase difference ® as well as the difference vector AS, both of which depend on the velocity component of the spins along the flow-encoding gradient (Fig. 24).
There are two different approaches to utilizing flow-induced phase shifts to generate angiograph-ic images. In the so-called phase map images, it is the phase difference ® that is depicted as signal intensity. The sign of the phase shift encodes the
Fig. 24. Principle of phase contrast angiography: A flow-compensated (S,) and a flow-sensitive (S2) dataset are acquired. Complex subtraction yields the phase difference O and the difference vector AS. Both quantities depend on the local flow velocity and can be used for generating flow images
Fig. 24. Principle of phase contrast angiography: A flow-compensated (S,) and a flow-sensitive (S2) dataset are acquired. Complex subtraction yields the phase difference O and the difference vector AS. Both quantities depend on the local flow velocity and can be used for generating flow images flow direction. Non-moving tissue appears as a medium shade of gray while flowing blood is either brighter or darker, according to the direction of flow. As the intensity of each pixel is linearly proportional to flow velocity, phase images are particularly well suited for flow quantification and for identifying flow direction.
In the second approach Magnitude images, which reveal the length of the difference vector AS, are applied to anatomically image the vessels. However, while the brightness in each pixel is a measure of the local flow velocity, there is no information about the flow direction.
The difference vector AS increases with rising spin velocity, reaching a maximum at O= 180°. The corresponding critical velocity is called flow sensitivity or velocity encoding (venc) and is determined by the strength of the bipolar flow-encoding gradients. The manufacturers of MR scanners provide a set of sequences adapted to different velocity ranges. In clinical practice, it is important to estimate the maximum flow velocity expected to occur in the vessel in advance in order to choose the phase contrast sequence with an adequate venc value.
Fig. 25. In the phase image, the value of the phase shift O is depicted as signal intensity. In the range from -venc to +venc, the phase shift is directly proportional to the flow velocity. Flow that is faster leads to a turnover of the phase (aliasing)
With phase contrast, only those velocities ranging between -venc and +venc, corresponding to phase shifts between -180° and +180°, can be uniquely detected. If the flow velocity exceeds the venc value, there is an abrupt change of signal intensity in the phase image from bright to dark or vice versa (Fig. 25). Blood flow that provokes a phase shift of 190° cannot be distinguished from oppositely directed flow that generates a phase shift of -170°. This ambiguity is called aliasing.
In the magnitude image, maximum intensity is reached when the flow velocity equals the venc value (Fig. 26).At higher velocities, the signal intensity begins to decrease again. If the flow velocity in a voxel is exactly twice the venc value, no signal emanates from the voxel, and the vessel appears to be interrupted.
As an example, for a venc value of 40 cm/sec spins flowing at a rate of 40 cm/sec provoke a phase shift of 180° yielding maximum signal intensity. Spins that flow at 60 cm/sec possess a phase shift higher than 180°. Therefore, on the magnitude image they appear less bright than the spins flowing at 40 cm/sec, but equally as bright as spins flowing at a rate of 20 cm/sec. Likewise, if there are spins flow
ing at a rate of 80 cm/sec, they will provide zero signal and appear as stationary spins.
The phase contrast method is sensitive only for the velocity component along the flow-encoding gradient. In order to obtain information on all flow directions, one dedicated flow-encoding gradient for each orthogonal direction of space is required. Thus, a phase contrast sequence comprises a total of four acquisitions: one flow-compensated measurement and three flow-encoded acquisitions in the x-, y-, and z-directions. Interleaving the four datasets can reduce artifacts caused by patient motion.
Phase and magnitude images of the flow components in the three orthogonal directions are obtained by complex subtraction of flow-encoded and flow-compensated datasets (Fig. 27). The three magnitude subtraction images can be added to obtain a sum magnitude image that depicts blood flow with bright signal regardless of flow direction (Figs. 27,28).
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