Summary

In clinical practice, unenhanced MRA techniques are mainly employed for imaging of the intracra-nial vasculature. Although they are also applicable to imaging of blood vessels in the neck, arm and leg, CE MRA is now firmly established as the dominating approach in the extracranial and peripheral regions. A summary of the different areas of application for the different MRA techniques is given in Table 8.

Table 8. Application areas of MRA techniques

3D-TOF

2D-TOF 3D-PC

2D-PC Magnitude contrast CE MRA

Intracranial:

- Arteries

***

*

*

- Veins

*

*** **

**

Carotids

**

**

***

Peripheral vessels

**

* ***

*** method of choice; ** second-best alternative or for additional information; * working technique, but with sub-optimal results TOF MRA: Time-of-Flight MRA; PC MRA: Phase Contrast MRA; CE MRA: Contrast Enhanced MRA

Table 9. Benefits and limitations of TOF and phase contrast MRA

TOF-MRA

Phase contrast MRA

Advantages

Simple to implement, robust

No saturation effects

High spatial resolution

Excellent background suppression

Shorter acquisition time (in 3D)

Enables quantitative flow measurement

Disadvantages

Reduced sensitivity to slow flow

Prior knowledge about flow rates required

Restrictions to size and orientation

Very long acquisition times for 3D

of the imaging volume

techniques

Short T1 tissue may be mistaken

Susceptible to phase errors

for flowing blood

TOF and phase contrast MRA are based on very different physical and technical mechanisms. As a result, there are advantages and limitations to both methods, which have to be taken into account when deciding on the appropriate method for a given vasculature territory (Table 9).

The TOF technique is robust and easy to implement. In the intracranial territory, it is established as a screening method that allows good-quality visualization of the vascular anatomy. 3D TOF acquisitions are particularly appropriate for imaging of arterial flow. 3D TOF MRA provides high spatial resolution with a significantly shorter acquisition time than that required for 3D phase contrast MRA. Problems due to saturation effects can partly be avoided with the use of multi-slab and TONE techniques. 2D TOF MRA is advantageous when imaging veins, because of its higher sensitivity to slow flow. On the other hand, it should be noted that the quality of TOF MRA is strongly influenced by flow velocity, the course of the vessels, and the size and orientation of the imaging volume. The use of very short TE values minimizes phase effects while poor background suppression can be improved by applying magnetization transfer. However, stationary tissue with short T1 may sometimes be mistaken for flowing blood.

Unlike TOF MRA, phase contrast MRA is not dependent on inflow effects. Therefore, the size and orientation of the imaging volume can be chosen arbitrarily. The method allows large sections of vessels to be depicted almost without saturation effects. 2D phase contrast MRA requires only a short acquisition time to generate a projection image of one single thick slice while 3D phase contrast acquisitions require a long time, since four datasets have to be acquired to obtain full flow information in all three orthogonal directions. Phase contrast MRA is sensitive to slow flow rates and is applicable mainly for imaging veins. Background suppression is excellent and short T1 tissues do not appear in the reconstructed angiograms. Because the signal intensity is directly proportional to the velocity of local blood flow, phase contrast MRA permits quantitative evaluations, such as flow quantification or determination of flow direction. Problems with phase contrast MRA may arise because prior knowledge about blood flow rates is required in order to avoid aliasing artifacts and flow voids.

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