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Reduced FOV Excitation Using Spatial-Spectral RF Pulses and Second-Order Gradients : Experimental Verification
| Content Provider | Semantic Scholar |
|---|---|
| Author | Ma, Chao King, Kevin F. Liang, Zhi-Pei |
| Copyright Year | 2011 |
| Abstract | INTRODUCTION: Multidimensional, spatially selective RF pulses have wide applications in MR imaging/spectroscopy experiments. A key limitation of multidimensional RF pulses is their long duration needed to traverse the multidimensional excitation k-space. Conventional efforts to address this problem include the use of multiple RF transmit coils for parallel excitation [1-2]. Recently, high-order, non-linear gradients are exploited to reduce multidimensional RF pulse length [3-6]. We have recently proposed a new method for reduced FOV excitation using spatial-spectral RF pulses and second-order gradients [7]. The method leverages the unique spatial dependence of the second-order gradients to excite a circular region-of-interest (ROI) in a thin slice using a 2D spatial-spectral RF pulse. Based on the design method presented in [7], this work presents the first experimental results of the method on a 3.0T commercial MRI scanner. METHOD: The goal is to excite a circular ROI of radius R, thickness d and flip angle θ (i.e. |Mxy(x,y,z)|=M0 sinθ, if x+y≤R,|z|≤d/2; 0, otherwise). A static second-order gradient in the form of z−(x+y)/2 (Z2 gradient) is applied during excitation. Such a Z2 gradient establishes a unique relationship between resonance frequency and spatial location, which is especially suitable for circular ROI excitation. Notably, in the target slice (|z| ≤ d/2), the resonance frequency is approximately a function of the distance to the origin: f=-(γ/4π)GZ2r, r=(x+y). With the Z2 gradient, a spatial-spectral RF pulse is used to achieve the target excitation pattern. Especially, the slice-selectivity of the pulse avoids undesired excitation outside the target slice, and the frequency-selectivity of the pulse achieves spatial selectivity in the radial direction within the target slice. Therefore, with the Z2 gradient, a 2D spatial-spectral pulse can achieve 3D spatial selectivity! Suppose the main-lobe width of the spatial-spectral RF pulse is BW, and the first excitation replicate is at ±frep along the frequency axis. Then, the radius of the excited ROI is [BW/(γGZ2/2π)], and the first excitation side-lobe is at r=[frep/(γGZ2/4π)]. Based on the above relationships, the target circular ROI can be excited by properly choosing BW and frep of the spatial-spectral pulse and the Z2 gradient strength GZ2 (more details of the design procedure of the proposed method can be found in [7]). |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://cds.ismrm.org/protected/12MProceedings/files/2207.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
