Guobin Li1, Zhaopeng Li1, Chaohong Wang1, Yang Xin1, Weijun Zhang1, Xiaodong Zhou1, and Weiguo Zhang1
1Shanghai United Imaging Healthcare Co., Ltd, Shanghai, China, People's Republic of
Synopsis
To reduce pulsatile artifacts of blood flow in FSE imaging, a combined solution is proposed, in which two concomitant saturation bands are achieved at both side of each slice without any extra RF pulses and gradients. Furthermore, through a proper setting of slice acquisition order, flowing blood can be continuously and seamlessly saturated in multi-slice acquisition.
In MR imaging, especially at high filed, pulsatile flow of blood is a major source of image artifacts. Methods such as propeller acquisition, flow compensation, saturation band can be applied to mitigate the problem. In fast spin echo (FSE) imaging, non-Cartesian k-space trajectories may sacrifice sampling efficiency and contrast flexibility1-3. Though saturation bands can in principle be combined with any sequence, their efficiency can be compromised due to multiple factors: (1) To avoid perturbation of imaged signal, saturation band is usually placed outside of the imaged volume, making its distance to excited slices variable in multi-slice acquisition; (2) the phase of blood flow is often out of sync with image data acquisition when scan is not triggered; (3) extra saturation bands lengthen the minimum TR. To achieve simple and efficient suppression of pulsatile artifacts in FSE sequences, a method is proposed in this work, which realizes concomitant and seamless suppression of the blood signal in multi-slice acquisition.
The method comprises two parts: (1) concomitant saturation bands; (2) optimal slice acquisition order. There are two ways to realize concomitant saturation bands in FSE sequence without any extra saturation RF pulses and spoiling gradients. In the first one, the thickness of the excitation slice is greater than that of the refocusing slice. As demonstrated in Figure 1, in a through-plane vessel, fresh blood sees the excitation pulse first before arriving at the imaged slice. Since the excited blood signal may miss several refocusing pulses, the crusher and readout gradients around those pulses act as spoiling gradients. When the blood flows into the imaged slice, its accumulated de-phasing can hardly be re-phased by the rest of the refocusing pulses. Consequently in such cases, the FSE sequence is always accompanied by two saturation bands at both sides of each slice, which suppress the in-flow blood. Alternatively, the thickness of refocusing slice can be made greater than the excitation slice using Gauss-shaped refocusing pulses. In such cases, fresh blood sees the transition band of the refocusing pulses first, which yields a tip angle much smaller than 180 degree. By using the concomitant excitation effect of the refocusing pulses, blood can be continuously saturated along the echo train. Since systolic blood flow is much faster than during the diastolic phase. The proposed concomitant saturation bands may not be sufficient to suppress systolic blood flow. A further improvement to continuously suppress blood signal in multi-slice acquisition is proposed: slices are acquired sequentially along the direction of blood flow in each TR; to avoid cross-talk due to the increased excitation or refocusing thickness, image volume is split into several sections (as shown in Figure 2). The proposed method was implemented on a 3T whole body MR scanner, and volunteer images are acquired with consent: brain imaging, TR/TE/TI = 2000ms/12ms/860ms, echo train length = 5, 2 sections. In the conventional FSE: excitation thickness = refocusing thickness, slice acquisition order: head->foot. In the proposed FSE: excitation thickness = 2*refocusing thickness, slice acquisition order: foot->head. As shown in Figure 3, CSF was completely suppressed with inversion recovery. However, conventional FSE images were severely contaminated by artifacts from the arterial vessels (see the bright blood signals). In contrast, images acquired with the proposed method were much cleaner since the blood signals were significantly suppressed by using the concomitant saturation bands and the foot-to-head slice acquisition order (see the dark hole of the vessels).
The proposed method can be combined with both spin echo and fast spin echo sequences for any contrast. It efficiently suppresses flow artifacts without any extra RF pulses and gradients. Some limitations of the method are: (1) In-plane blood flow cannot be suppressed; (2) image volume has to be split into several sections to avoid cross talk, which means that fewer slices can be excited in a TR, prolonging the scanning time in some cases.
Acknowledgements
No acknowledgement found.References
1. Hennig J et al, Magn Reson Med, 3: 823–833, 1986
2. Pipe JG, Magn Reson Med, 42:963-9, 1999
3. Theilmann R J. et al, Magn Reson Med. 51:768–774