Dtofmra

A pulse sequence is represented for 3D TOF MRA (see Fig. 3.6). A slab of several cm (usually about 5 cm) is obtained which contains up to 28-60 slice 3D volumes in axial plane through region of interest. The slice thickness is 0.7-1.0 mm, repetition time is 40 msec, and flip angle is 15-20° with FOV of 16-20 cm, depending on the patient size and region of interest. Depending upon the desired resolution

Figure 3.11: Three-dimensional TOF angiogram showing circle of Willis confirming the occlusion of the left internal carotid artery (left panel). Two-dimensional TOF angiograms demonstrating an internal carotid artery occlusion. A sagittal projection of right carotid bifurcation reveals a patent vessel post endarterectomy (top row on right). The sagittal projection of the left carotid bifurcation reveals stenosis of the proximal external carotid artery and occlusion of the internal carotid artery (bottom row on right).

Figure 3.11: Three-dimensional TOF angiogram showing circle of Willis confirming the occlusion of the left internal carotid artery (left panel). Two-dimensional TOF angiograms demonstrating an internal carotid artery occlusion. A sagittal projection of right carotid bifurcation reveals a patent vessel post endarterectomy (top row on right). The sagittal projection of the left carotid bifurcation reveals stenosis of the proximal external carotid artery and occlusion of the internal carotid artery (bottom row on right).

and imaging time, 128 x 128, 192 x 256, or 256 x 256 matrix can be used with NEX = 1. Very short echo times may be attained with flow compensation. These optimized scan parameters permit adequate penetration of inflowing, fresh, fully magnetized spins into the imaging volume. The resultant 3D data set initially is displayed as a series of slices, acquired in the axial plane. Later, it is subjected to the MIP ray tracing technique to create coronal and sagittal projections. A series of projections may also be generated to "rotate" the vascular structures around a single axis. Cine loop display can provide the perception of depth. Advantages of 3D techniques are appreciable as these techniques are more susceptible to saturation effects and less sensitive to slow flow. Thus, 3D volume acquisition techniques offer superior signal-to-noise ratios (SNR). 3D TOF MRA offers a prescription of very thin slices, thereby reducing the voxel size and decreasing the intravoxel dephasing. 3D TOF MRA maximizes the flow-related enhancement.

3.2.2.1 Optimization of Image Parameters of 3D TOF MRA

Optimization parameters are blood velocity, vessel orientation in relation to the slab, the size of the imaging volume, TR, slice thickness, voxel size, and flip angle. Flow velocity should ideally permit fresh, fully magnetized spins to traverse the entire imaging volume between successive RF pulses. This results in optimal signal enhancement because of in-flow effects. For instance, at normal flow velocity saturation effects will be minimal. At lower velocity, slow flowing blood becomes saturated as it moves through the imaging volume, and signal intensity decreases. Slow flow conditions may be encountered in the cases of vascular occlusive disease, venous thrombosis, and aneurysms with complex flow patterns.

3.2.2.1.1 Imaging Flow Orientation. It should be selected to minimize the saturation of moving spins as they course through the volume. For instance, axial orientation permits imaging of 'circle of Willis' using a small volume, thereby reducing the imaging time (see Fig. 3.11). In practice, coronal and sagittal orientations have been used to image both extracranial and intracranial carotid arteries in a single acquisition. Larger flip angles of 35-60° maximize signal in the extracranial carotids, but result in saturation of the intracranial vessels. Smaller flip angles of 15-30° improve visualization of the intracranial vessel because of the reduced saturation. As a result, trade-off is the decreased intensity of intravascular signal from the extracranial carotid arteries.

3.2.2.1.2 Repetition Time (TR). At short TR, stationary tissues exhibit greater saturation. It increases the tissue contrast between vessel and the surrounding tissues (see Fig. 3.12). However, at short TR, spins flowing through the imaging volume become saturated, resulting in loss of intravascular signal intensity. These saturation effects can be somewhat reduced by using a smaller flip angle or by shortening the T1 of blood through the use of MR contrast agents. Nonetheless, when the 3D acquisition is optimized for normal intracranial arterial flow (flip = 15-20°, TR = 40), slower flow will become saturated, reducing the delineation of venous anatomy and slow flow within aneurysm or diseased arteries. Despite this, 3D TOF MRA does not distinguish flowing spins from sub-acute hemorrhage. For instance, methemoglobin within a subacute hematoma has a short T1 and does not become saturated during the 3D acquisition. The

Figure 3.12: Three-dimensional TOF angiogram (left panel) shows cavernous angioma with visible methemoglobin due to short T1 due to simulated blood flow. For comparison, SPGR images are shown with high signal intensity center representing methemoglobin.

result is bright signal intensity in the images, which may simulate flow-related enhancement.

3.2.2.1.3 Echo Time (TE). Lower TE reduces motion-induced phase errors. Partial RF pulses reduce the minimum TE while these RF pulses preserve an acceptable slab profile. Very low TE may be achieved by removing flow compensation from the gradient waveform. Thus there is a trade-off between minimum echo time at the cost of flow compensation. This approach is currently used for clinical imaging.

3.2.2.1.4 Flip Angle. Flip angle has an effect on intravascular signal intensity and background suppression. Smaller arteries may be visualized at flip angles of 15-20° with TR of 40 msec. Stationary tissues exhibit greater saturation at a larger flip angle. For example, small 3D volumes of 28 slices show intravascular signal intensity of larger arterial structures at flip angles 20-35° with rapid flow. Arterial flow begins to saturate at flip angles greater than 40°. It results in reduced intravascular signal intensity (see Fig. 3.13).

3.2.2.1.5 Flow Compensation. Flow compensation is critical in 3D TOF MRA. Motion-induced phase dispersion results in signal void areas. These areas are frequently identified within the juxtasellar carotid arteries and proximal middle cerebral arteries. These signal void areas can be minimized by the use of shortest possible TE with flow compensation applied in the slice-select and read-out directions. This combined approach reduces the phase dispersion and

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