Model-based Spiral Trajectory Correction using Scanner-specific Gradient Calibration

Craig H. Meyer^{1}, Samuel W Fielden^{1}, Josef Pfeuffer^{2}, John P. Mugler III^{3}, Alto Stemmer^{2}, and Berthold Kiefer^{2}

Across all scanners, the variation (standard deviation) of the gradient delays along the three physical axes ranged from 0.3 to 0.9 ms. Some of this variation would be expected to arise from the different models of gradient systems among scanners. For example, considering only seven 3T scanners, the variation of gradient delays was smaller, ranging from 0.1 to 0.4 ms. The eddy-current terms were relatively small among all scanners (reflecting that the baseline eddy-current compensation performed well for the spiral waveforms), although there were systematic differences among scanner models. Considering again the seven 3T scanners, the maximum coefficient of variance for eddy-current terms was less than 10% and the mean was 7%. Comparing model-based reconstruction to reconstruction using measured trajectories, anisotropic gradient delays alone provided an average reduction of 3.4% in RMSE relative to nominal isotropic delays, and anisotropic gradient delays plus eddy current terms provided an average reduction of 37.3% in RMSE. Thus, the gradient delays on these systems were relatively isotropic.

Visually, it was difficult to discern differences between spiral images reconstructed using measured trajectories and those reconstructed using model-based parameters including gradient delays and eddy-current terms for each axis. Representative images are shown comparing transverse (Fig. 1), coronal (Fig. 2) and sagittal (Fig. 3) images from measured trajectories to the corresponding images from model-based trajectory correction. The difference images illustrate the small remaining error between the two image sets. The eddy-current terms corrected image scaling errors resulting from variations in the magnitude of the gradient transfer function as a function of gradient temporal frequency [3]. Spiral abdominal images of a volunteer are shown in Fig. 4.

1. Meyer CH et al. Magn Reson Med 1992; 28:202-213.

2. Tan H, Meyer CH. Magn Reson Med 2009; 61:1396-1404.

3. Addy NO, Wu HH, Nishimura DG. Magn Reson Med 2012; 68:120-9.Figure 1. Transverse phantom images. Top row, from left to right: reconstructed using measured trajectory; nominal isotropic delay model; anisotropic delay model; anisotropic delay model plus eddy current terms. Bottom row: Difference images relative to measured trajectory image.

Figure 2. Coronal phantom images. Top row, from left to right: reconstructed using measured trajectory; nominal isotropic delay model; anisotropic delay model; anisotropic delay model plus eddy current terms. Bottom row: Difference images relative to measured trajectory image.

Figure 3. Sagittal phantom images. Top row, from left to right: reconstructed using measured trajectory; nominal isotropic delay model; anisotropic delay model; anisotropic delay model plus eddy current terms. Bottom row: Difference images relative to measured trajectory image.

Figure 4. Spiral abdominal images reconstructed with the anisotropic delay model plus eddy current terms.

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

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