These techniques use longer decrease in repetition time TR. As a result, the saturation effects lead to signal loss with the use of larger flip angles. This approach using longer decrease in TR improves the SNR. Other ways are also used to decrease saturation effects. Common use of paramagnetic contrast agent such as gadolinium chelate causes spin-lattice relaxation time (T1) shortening of blood (see Fig. 3.25). Consequently, the T1 recovery (from short T1 to normal T1 values) is faster with less saturation effects. In routine, multiple overlapping thin-slab acquisition (MOTSA) tilt optimized nonsaturated excitation (TONE) are also promising techniques to reduce saturation effects.
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).
18.104.22.168 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.
22.214.171.124 Tilt Optimized Nonsaturated Excitation (TONE)
In this technique, flip angle (a) is increased progressively as the flowing spins move into the imaging volume by using increasing RF pulses. A large flip angle (a) 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.
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