Cosine-modulated acquisition cleans spectra for better respiratory cine

Cihat Eldeniz^{1}, Yasheng Chen^{1}, and Hongyu An^{1}

A 3D stack-of-stars Golden Angle MRI sequence was previously proposed as a self-gated MR acquisition method [1]. Figure 1 illustrates the k-space sampling pattern. In this sequence, Cartesian encoding is utilized along the kz direction, forming a stack, while radial spokes are used in the kx-ky plane to form stars (hence the name stack-of-stars). For a given azimuthal angle, a stack-of-spokes is acquired first. After that, the next stack-of-spokes is acquired for another azimuthal angle. Thus, in Figure 1, the red stack of spokes is acquired for Angle 0, and then the blu stack of spokes is acquired for Angle 1. The golden angle (GA) is referred to as the 111.25° angular increment between two adjacent stack-of-spokes. The GA acquisition provides a rapid and nearly uniform coverage of the kx-ky space during any period of time. The typical acquisition time for each stack of spokes is Nz×TR and it usually ranges from 120 to 200 ms. Because respiration rate is slower than this sampling rate, it is assumed that all spokes in a stack of spokes are acquired at the same respiratory phase. Since every stack of spokes samples the k-space center (kx=ky=kz=0), signal variations across different radial angles at this point are assumed to be caused by respiratory motion and can be used to derive motion [2-4]. This type of approach is referred to as self-gated or self-navigated. Signal within the frequency range of 0.1–0.5 Hz and 0.5–2.5 Hz were assumed to represent respiratory and cardiac motion, respectively [3]. However, we have found that this assumption does not hold true. As Figure 2 clearly demonstrates, even in a stationary phantom, the pseudo-periodicity of the Golden Angle approach leads to a coherent signal pattern that can be misinterpreted as a motion signal. To overcome this problem, we developed a data acquisition scheme that generates a controlled frequency modulation pattern. The idea is to push all acquisition related frequency components to the far end of the spectrum. In this respect, we apply a scheme that is similar to amplitude modulation. First, the azimuthal angle $$$\phi$$$ is chosen as $$$\phi = \pi+(k\pi/N)cos(k\pi) $$$ where $$$ k = 0,1,...,N-1$$$ with $$$N$$$ being the number of radial lines. The modulation frequency is chosen so as to obtain the peaks at the highest possible location, that is, at the far end of the spectrum. However, in order to take advantage of the golden angles, each of the cosine-modulated angles were quantized onto the golden-angle grid. For instance, using $$$N=2216$$$ lines of cosine-modulated angles was enough to cover the grid for a 2000-line acquisition of the regular golden-angle acquisition after quantization. The remaining 216 lines were appended to the end of the acquisition and were later discarded.

For both acquisitions, the reconstruction was performed after filtering out the spectral content beyond 0.6 Hz, detrending and then binning the data.

[1] R. Grimm et al., MICCAI 2013, vol. 8151, pp. 17–24.

[2] R. Grimm et al., Med. Image Anal., vol. 19, no. 1, pp. 110–120

[3] L. Feng et al., MRM., Mar. 2015.

[4] B. Li et al., Proceedings of the ISMRM 23rd Annual Meeting, Toronto, Canada, 2015, p. 3806.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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