Biomedical Engineering Reference
In-Depth Information
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, ax-
ial 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 ori-
entations 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 be-
cause 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 sur-
rounding 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 arte-
rial 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
