Biomedical Engineering Reference
In-Depth Information
Figure 3.25: A method of multislice gradient echo is represented using short TR
and larger flip angles with sufficient SNR (on left panel). However, gadolinium
contrast agent shortened the blood with T1 recovery with less saturation effects
(on right panel).
3.2.7.1 Multiple Overlapping Thin-Slab Acquisition (MOTSA)
It is a combination of 2D TOF and 3D TOF techniques for the purpose of reducing
the saturation effects associated with a thick slab. In this method, multiple thin
slabs used, which overlap by 25-50%. Extracting the central slices of each slab
creates the final imaging volume and discards the peripheral slices, which are
more affected by saturation effects. The main drawback of this technique is the
appearance of “Venetian blind” artifact at the points where the slabs overlap.
3.2.7.2 Tilt Optimized Nonsaturated Excitation (TONE)
In this technique, flip angle (
α
) is increased progressively as the flowing spins
move into the imaging volume by using increasing RF pulses. A large flip angle
(
α
) yields higher SNR. Thus, larger flip angle counteracts the saturation effects
of slow-flowing blood in deeper slices. This allows better visualization of distal
vessels and the slow-flowing vessels. In common practice, ramped flip angle
excitation pulse is used. In our commonly used scheme, the center flip angle is
30
◦
and the flip angle at each end varies by 30%. As a result, flip angle changes
20
◦
at the entry slice and 40
◦
at the exit slice.
