Optimization Table

The correct choice of the flow sensitivity of a phase contrast sequence is critical to the quality of vessel depiction. If the chosen venc value is too high, the acquired signal-to-noise ratio is poor due to the small difference between signals. Conversely, venc values that are two low lead to aliasing artifacts and flow voids which may be misconstrued as stenosis. Typical flow velocities in some larger vessels, which may indicate the appropriate venc values to use, are given in Table 7.

Blood flow in arteries and veins is rarely constant during the heart cycle. Therefore, given that phase contrast sequences require several minutes of acquisition time, it is not the maximum but the mean flow velocity averaged over the heart cycle that is relevant. In the brain the vessels are highly tortuous and thus blood flow is hardly ever directed along one single phase encoding direction. Hence, in practice, the maximum signal intensity is obtained when the chosen venc value is about half the maximum flow velocity, as demonstrated in Fig. 29.

Due to the irregular course and position of vessels in the brain, it is reasonable to encode one single flow sensitivity in all three orthogonal directions. Conversely, in the peripheral arteries, there is one main flow direction, but a great variation of flow velocities. Consequently, in the peripheral arteries a "multi-venc" measurement can be performed in which three flow velocities (e.g. 30, 60,

Table 6. Options for improving phase contrast MRA

Adapting flow sensitivity (venc) to maximum flow velocity

Encoding different flow velocities (multivenc) or different flow directions

Contrast agent improves flow signal

2D acquisition provides one single projection within a short acquisition time, 3D acquisition permits MIP postprocessing ECG triggering can be applied in cases of pulsatile flow Presaturation pulses can separate arteries and veins

Table 7. Flow velocities in some large vessels (according to Siemens application manual Magnetom Vision)

Vessel

Flow velocity (cm/s)

Ascending aorta

50 -

■ 100

Descending aorta

100

Aortic stenosis

150

- 500

Aortic valve insufficiency

150

- 200

Common carotid artery

60 -

■ 80

Carotid artery stenosis

100

- 500

Middle cerebral artery

60

Basilar artery

40 -

50

Femoral artery

60 -

80

Popliteal artery

35 -

40

Vena cava

5-

40

Portal vein

5 -

10

Carotid Artery Velocity Table
Fig. 29. Phase contrast study on a flow phantom with pulsating flow. Left: venc = vmax; Right: venc = vmax/2. Due to vessel tortuosity and the time varying flow velocity, the signal intensity is higher when the flow sensitivity (venc) of the sequence is half the maximum flow velocity

Fig. 30. Phase contrast angiography before (left) and after (right) administration of a gadolinium contrast agent. Due to T1 shortening, the signal-to-noise ratio increases, thereby improving the detection of small vessels and 90 cm/sec) are encoded simultaneously but in only one encoding direction. All three acquired images are then combined to form a sum image.

In CE MRA, the signal-to-noise ratio of the blood increases after contrast agent administration because the greater T1 shortening of the spins leads to more signal. Phase contrast angiography, unlike TOF angiography, can benefit from this ef fect without the penalty of increased background signal. The additional signal afforded by the application of a contrast agent (already at a dose of 4 ml) is particularly beneficial for the visualization of small vessels with low flow velocity when working at lower field strength (Fig. 30).

As with TOF MRA, both 2D and 3D phase contrast MRA techniques are available. However, un-

Tof Mra From Low Field
Fig. 31. Phase contrast angiogram of the intracranial vessels

like TOF MRA, phase contrast MRA does not impose any restrictions on image orientation, because the method is not dependent on inflow effects.

2D phase contrast sequences are well suited for imaging vessels in a large volume. Because only moving spins contribute to the measured signal, the background signal is very effectively suppressed. Even thick single slices of about 100 mm can be imaged and the overall acquisition times are relatively short (about 1-2 min). The result is a projection image in which all the vessels in the excited slice volume are depicted (Fig. 31). A drawback, however, is that it is not possible to reconstruct projections from another perspective.

With 3D phase contrast MRA datasets, as well as with stacks of 2D phase contrast acquisitions, MIP reconstructions can be acquired in any projection, in a manner similar to that performed in 3D TOF MRA. However, depending on the number of partitions, the acquisition time for a 3D phase

Table 8. Application areas of MRA techniques contrast sequence is significantly greater than that of a 3D TOF sequence, because four data volumes need to be acquired rather than just one.

The possibility to combine ECG- or peripheral pulse triggering with the 2D phase contrast sequence may allow the acquisition of time-resolved images of pulsating blood flow in vessels. A series of cine velocity images can be obtained that spans the cardiac cycle. If an optimal depiction of pulsating blood flow is required rather than time resolution, it is advantageous to confine the acquisition to the cardiac interval of maximum flow velocity.

As with TOF MRA, phase contrast techniques can be combined with presaturation pulses in order to eliminate unwanted vessels from the reconstruction. However, because phase contrast sequences are independent of inflow effects, the separation of arteries and veins is not as successful as in TOF-MRA.

Due to the flow encoding gradients, TE values are prolonged compared with those employed in TOF sequences, because the bipolar gradient pulses have to be applied within the TE interval. As a result, phase contrast sequences are sensitive to phase errors caused by, for example, susceptibility effects or turbulent flow.

The sensitivity for flow is proportional to the flow encoding gradient area (gradient amplitude * gradient length). Therefore, the encoding of low flow velocities requires longer TE values. Shortening of TE values can be achieved with stronger gradients, by keeping the gradient area constant. In this way, phase contrast MRA benefits from stronger gradient systems.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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