Applications of RF Pulse Designs: Inner Volume Imaging, SMS, B1 Shimming & pTx
1University of Glasgow, United Kingdom
Synopsis
This talk reviews a few popular RF pulse design applications: inner volume imaging, simultaneous multislice, $$$B_{1}$$$ shimming, and parallel transmission.
Target Audience
MRI scientists and pulse sequence developersOutcomes/Objectives
Following the educational talk on radio frequency (RF) pulse design theory, this talk will provide insight on a few interesting RF pulse applications:- Inner volume imaging: for higher resolution and/or accelerated imaging
- Simultaneous multislice (SMS) imaging: for accelerated imaging
- $$$B_{1}$$$ Shimming: for $$$B_{1}$$$ transmit field homogenization
- Parallel transmit: for homogenization of $$$B_{1}$$$, added degrees of freedom to pulse design, safer imaging at high fields, and much more!
Inner Volume Imaging
The imaging field of view (FOV) is often chosen to cover the extent of the body region being imaged and avoid FOV foldover aliasing, requiring finer sampling in data encoding k-space, $$$FOV=\frac{1}{\Delta k}$$$. This causes practical limitations to the temporal and spatial resolution achievable in some applications. When the goal is to image a smaller region within the body, inner volume (IV) excitation imaging is a tool to combat the need for higher resolution at faster speeds [1].One of the first IV techniques uses a pair of orthogonal, slab-selective RF pulses in a spin echo excitation and refocusing pair [1]. Only the rectangular volume that intersects the two planes is refocused into a spin echo, while everything else dephases. This later was named zonally magnified or "ZOOM" and incorporated a fast EPI readout [2,3]. Additional IV methods use spatially-selective 2D RF pulses [4-6] which in return incorporate 2D excitation trajectories such as EPI or spirals. Furthermore, fully tailored 3D volume excitations have been developed incorporating a variety of 3D trajectories [7-8].
Simultaneous Multislice
Simultaneous multislice (SMS) imaging, also known as multiband (MB) imaging is an effective means of accelerating imaging by exciting multiple slices separated by some distance. This reduces the total number of excitations required to obtain a full volume, with the speedup known as the MB factor. SMS is particularly useful for applications where speed is critical such as cardiac imaging, where a time series is collected such as fMRI and contrast enhanced MRI, and where acquisitions are long such as diffusion tensor imaging [9].Conventional SMS imaging operates on the principle of the Fourier shift theorem, where multiple slices are excited at different phases [10]. Summing these pulses creates a conventional multiband pulse:
$$b_{MB}(t)=A(t)\cdot\sum_{n=1}^{N}e^{i(\gamma G_{z}\cdot z_{n}\cdot t+\phi_{n})}$$
where $$$A(t)$$$ is the slice select envelope, $$$N$$$ is the MB factor, $$$\gamma$$$ is the gyromagnetic ratio, and $$$z_{n}$$$ is the slice position at each $$$n$$$th slice with phase offset $$$\phi_{n}$$$. $$$A(t)$$$ is typically a standard slice-selective RF pulse shape which can be a truncated sinc function or a Shinnar Le-Roux (SLR) pulse [11]. During excitation, a trapezoidal slice-select gradient with amplitude $$$G_{z}$$$ common to all simultaneous slices is played. Due to the summing of multiple RF waveforms, SMS is often limited by either the RF amplifier hardware or power deposition contributing to the specific absorption rate or SAR (shown in Figure 1). In general, both the power deposition and peak amplitude of SMS pulses scale roughly linearly with the number of slices [12]. In this section of the talk, I’ll discuss some methods that have been proposed to address SMS peak amplitude and power limitations to leverage the benefits of MB imaging [12-16].
$$$B_{1}$$$ Shimming
As the Larmor frequency increase with field strength, the sinusoidal RF wavelength decreases and leads to constructive and destructive interference of the $$$B_{1}$$$ field, creating flip angle inhomogeneity within larger body regions such as the torso or even head at ultra-high fields (UHF, ≥7 T) [17, 18]. To combat this problem, the idea of a spatially-varying $$$B_{1}$$$ field is proposed via multi-port excitations, more commonly known as parallel transmission (pTx) [19]. Analogous to parallel imaging with multiple receive channels, pTx has been described as "Transmit SENSE’" with multiple RF channels [20].$$$B_{1}$$$ shimming is the simplest form of pTx, where the amplitude and phase of each transmit channel is set independently. This creates a combined $$$B_{1}$$$ field,
$$b(\textbf{r},t)=b_{1}(t)\cdot\sum_{c=1}^{C}w_{c}s_{c}(\textbf{r})$$
where $$$b(\textbf{r},t)$$$ is the combined $$$B_{1}$$$ shimmed RF field, $$$b_{1}(t)$$$ is the complex RF waveform constant for all $$$C$$$ transmit channels, $$$w_{c}$$$ are the complex channel weights or "shims’", and $$$s_{c}(\textbf{r})$$$ are the single channel transmit RF field maps. Typically, $$$B_{1}$$$ shimming is optimized for flip angle homogeneity within a volume or slice of interest, although specific applications have led to the derivation of other optimization cost functions [21].
Parallel Transmit
From the equation for $$$B_{1}$$$ shimming above, we might notice that we can leverage additional degrees of freedom with parallel transmission by altering the RF waveform per transmit channel $$$b_{1,c}(t)$$$ ,$$b(\textbf{r},t)=\sum_{c=1}^{C}s_{c}(\textbf{r})\cdot b_{1,c}(t)$$
This spatially and time-varying form is known as "dynamic pTx", or simply "pTx" for short. Since pTx was first proposed nearly two decades ago, a wealth of innovative pulse design solutions have been developed. A few notable examples include iterative pTx design in the spatial domain [22], 2D slice-selective pTx with spokes trajectories [23, 24], 3D non-selective k-t points trajectories [25], and recently the calibration-free universal pulses [26].
Although pTx is most commonly associated with $$$B_{1}$$$ or flip angle homogeneity, it can also be used in the "Transmit SENSE’" fashion to allow for reduced sampling of excitation k-space, leading to shorter RF pulses [27]. Therefore, pTx can be beneficial for other pulse designs discussed in this talk such as SMS and inner volume imaging.
In single transmit MRI, the SAR delivered by an RF pulse is proportion to the integrated RF power, yet in the case of pTx the superimposed $$$B_{1}$$$ fields can yield local SAR hotspots [28]. In early days of pTx, local SAR was only evaluated with electromagnetic (EM) field simulations for each voxel of a tissue model, which is computationally limiting for pTx pulse design and scanner SAR monitoring. In 2011 the method of virtual observation points (VOPs) was presented by Eichfelder et al., which establishes a worst-case upper SAR bound to cluster various regions within an EM model [29]. VOPs are now commonly used for pTx pulse design optimizations and are integrated into the pTx SAR monitoring framework of modern scanners with pTx capability. Although local SAR hotspots can be an initial cause for concern, pTx can in fact be used to craft safer MRI scanning but creating implant-friendly imaging schemes [30, 31] or enabling sophisticated techniques such as dark mode imaging [32].
Acknowledgements
References
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