Purpose This study investigates the implications of all degrees of freedom of within‐scan patient head motion on patient safety. Methods Electromagnetic simulations were performed by displacing and/or rotating a virtual body model inside an 8‐channel transmit array to simulate 6 degrees of freedom of motion. Rotations of up to 20° and displacements of up to 20 mm including off‐axis axial/coronal translations were investigated, yielding 104 head positions. Quadrature excitation, RF shimming, and multi‐spoke parallel‐transmit excitation pulses were designed for axial slice‐selection at 7T, for seven slices across the head. Variation of whole‐head specific absorption rate (SAR) and 10‐g averaged local SAR of the designed pulses, as well as the change in the maximum eigenvalue (worst‐case pulse) were investigated by comparing off‐center positions to the central position. Results In their respective worst‐cases, patient motion increased the eigenvalue‐based local SAR by 42%, whole‐head SAR by 60%, and the 10‐g averaged local SAR by 210%. Local SAR was observed to be more sensitive to displacements along right–left and anterior–posterior directions than displacement in the superior–inferior direction and rotation. Conclusion This is the first study to investigate the effect of all 6 degrees of freedom of motion on safety of practical pulses. Although the results agree with the literature for overlapping cases, the results demonstrate higher increases (up to 3.1‐fold) in local SAR for off‐axis displacement in the axial plane, which had received less attention in the literature. This increase in local SAR could potentially affect the local SAR compliance of subjects, unless realistic within‐scan patient motion is taken into account during pulse design.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Improving the signal-to-noise-ratio (SNR) of magnetic resonance imaging (MRI) using denoising techniques could enhance their value, provided that signal statistics and image resolution are not compromised. Here, a new denoising method based on spectral subtraction of the measured noise power from each signal acquisition is presented. Spectral subtraction denoising (SSD) assumes no prior knowledge of the acquired signal and does not increase acquisition time. Whereas conventional denoising/filtering methods are compromised in parallel imaging by spatially dependent noise statistics, SSD is performed on signals acquired from each coil separately, prior to reconstruction. Using numerical simulations, we show that SSD can improve SNR by up to ~45% in MRI reconstructed from both single and array coils, without compromising image resolution. Application of SSD to phantom, human heart, and brain MRI achieved SNR improvements of ~40% compared to conventional reconstruction. Comparison of SSD with anisotropic diffusion filtering showed comparable SNR enhancement at low-SNR levels (SNR = 5–15) but improved accuracy and retention of structural detail at a reduced computational load.