Optimization Table

In TOF MRA, arteries and veins are often displayed simultaneously, since the inflow enhancement is equally effective for directly opposed flow directions. As a result, there may be an interfering overlap of arterial and venous vessels in the MIP reconstruction which hampers the detection or assessment of vessel lesions. This drawback can be overcome by applying additional presaturation slabs that saturate and dephase spins before they enter the image slice. A selective arteriograph is generated if the presaturated area is placed distal to the imaging volume, i.e. at the entry side of the veins. In this case, inflowing venous spins are saturated in a manner similar to stationary spins and therefore do not produce a bright signal. Conversely, presaturation of arterial inflow permits the selective depiction of veins. In order to achieve sufficient suppression even of fast flowing blood, presaturation slabs of several centimeters thickness must be positioned close to the imaging vol ume. In case of 2D TOF sequences traveling presat-uration slabs may be employed that move along with the imaged slice but at a constant distance from the actually imaged slice.

For all TOF techniques, it is important to position the imaging volume (slices or slabs) perpendicular to the vessel to minimize the length of the vessel section in the slice or slab and to reduce the saturation effects. As intracranial vessels run in different directions, it is not possible to simultaneously orientate the imaging volume perpendicular to all vessels. Consequently, the 3D multi-slab technique has proven to be the best method for investigating brain arteries. In order to guarantee optimal coverage of the arterial vessel tree with minimal slab thickness it is advisable to tilt the slabs slightly from the axial to the coronal orientation (Fig. 11). The resulting MIP reconstruction gives excellent visualization of the brain arteries (Fig. 12).

The depiction of venous brain vessels using the TOF technique can be optimized by choosing a sagittal slice orientation tilted slightly towards coronal and axial directions (Fig. 13).As a result of this spatial orientation, saturation of blood in the sagittal sinus that could occur due to long-range flow within one single slice is prevented. Inflow enhancement is sufficient for all veins, regardless of their flow direction (Fig. 14).

The flip angle of a GRE sequence considerably

Brain Mra Protocol

Fig. 12. MIP reconstruction from a 3D multi-slab TOF MRA dataset of the brain arteries

Fig. 11. Typical position of a 3D TOF multi-slab. The presaturation slab above the imaging volume suppresses the signal of venous flow

Fig. 12. MIP reconstruction from a 3D multi-slab TOF MRA dataset of the brain arteries

Fig. 11. Typical position of a 3D TOF multi-slab. The presaturation slab above the imaging volume suppresses the signal of venous flow

Mra Images 2dtof Tof

Fig. 13. Imaging volume of a 2D TOF MRA of the brain veins. By tilting the sagittal slices in an axial and coronal orientation, saturation of venous blood in the sinus sagittalis is avoided. The saturation slab located beneath the imaging volume suppresses the arterial flow signal

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Fig. 13. Imaging volume of a 2D TOF MRA of the brain veins. By tilting the sagittal slices in an axial and coronal orientation, saturation of venous blood in the sinus sagittalis is avoided. The saturation slab located beneath the imaging volume suppresses the arterial flow signal

Sinus Sagittalis Inferior

Fig. 14. MIP reconstruction of a 3D TOF MRA dataset of the venous brain vessels

Flip Angle Tone Tof
Fig. 15. By linearly varying the flip angle across the volume (TONE), the progressive saturation of the flow signal occurring with 3D TOF MRA can be reduced

influences the vessel-background contrast. Large flip angles generate a high signal at the entry side of the blood, but also provoke rapid signal decrease along the course of the vessel due to saturation. Small flip angles produce a lower vessel-background contrast but less saturation. 2D TOF typically uses flip angles in the range of 30° - 70°, whereas 3D TOF employs lower angles between 15° and 20° to reduce saturation. In the technique referred to as TONE (Tilted, Optimized, None-Saturating Excitation) [5], the flip angle is varied linearly in the slice direction, beginning with small values at the entry side and ending with high values at the exit side of the volume (Fig. 15). In this way, saturation across 3D slabs can partially be compensated to achieve a more uniform signal distribution along the course of the vessel. The extent of the flip angle variation depends on flow direction, flow velocity (slow, medium, fast) and slab thickness. Therefore, sequence protocols that utilize TONE are optimized for specific vessel regions.

Further suppression of background signal can be achieved using magnetization transfer contrast

(MTC) [6, 7]. By applying a radio pulse at a much different frequency from that of water resonance, protons of motionally restricted macromolecules are saturated. In contrast mobile water protons that generate the MR signal in vessels and tissues are not affected by this off-resonance pulse. By cross-relaxation and chemical exchange, the saturation is transferred to the neighboring free protons, thus significantly reducing the measured MR signal. This effect can be seen in the gray and white brain matter (reduction of signal by 15-40%) but not in blood. Consequently, the contrast between the vessel and the surrounding tissue is enhanced. This is particularly useful for depiction of small vessels and slow flowing blood which are made appreciably more visible.

Unfortunately, fat does not exhibit magnetization transfer. Therefore, to avoid enhanced fat contrast, MTC should be used in combination with fat suppression techniques. Effective suppression of the signal of fat may be achieved either by direct frequency-selective saturation of the fat signal or by selective excitation of water (Fig. 16). The dis

Gadolinium Contrast Images Mra

Fig. 17. TOF MRA pre (left) and post (right administration of gadolinium contrast agent. After contrast injection, the suppression of veins is no longer effective

Arteriograph

Fig. 17. TOF MRA pre (left) and post (right administration of gadolinium contrast agent. After contrast injection, the suppression of veins is no longer effective advantage of fat saturation in comparison with water excitation is that water protons instead of fat protons could be accidentally affected by the pre-saturation pulse in regions outside the shim volume (i.e. in the less homogeneous magnetic field) resulting in unwanted reduction of inflow effect.

Unlike unenhanced MRA, contrast-enhanced MRA (CE MRA) is usually achieved through the application of an exogenous gadolinium-based MR contrast agent which shortens the T1 relaxation time of proton spins in its immediate vicinity. After intravenous administration, the T1 of blood is strongly reduced, thereby counteracting problems associated with the saturation of spins flowing across the volume. As a consequence, the contrast between blood vessels and surrounding tissues is improved, primarily for small vessels and those with slow flow. There are, however, two major disadvantages arising from contrast agent administration: First, if soft tissue in the imaging area shows considerable contrast enhancement, the background signal will also be enhanced. Second, due to the fast T1 relaxation of blood protons, suppression of either the signal from the arteries or veins using presaturation pulses will malfunction (Fig. 17). In such cases, evaluation of source images or multiplanar reconstructions may yield more information than MIP reconstructions.

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Responses

  • Mungo
    What do they measure for brain mra?
    8 years ago
  • willard
    How to position slices for mra brain?
    7 years ago

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