Multiecho pseudo‐golden angle stack of stars thermometry with high spatial and temporal resolution using k‐space weighted image contrast

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MR imaging provides excellent soft tissue contrast and when used to guide focused ultrasound (FUS), provides the ability to localize, plan, monitor and verify treatments 1. FUS has been used to noninvasively treat uterine fibroids as well as breast, prostate, liver and brain cancer 2. As FUS can locally heat tissue very quickly, at rates greater than 1°C/s, the monitoring of treatments 6 requires a high spatial and temporal resolution. Also, because the energy is delivered from a large transducer aperture to a small focus, a large field of view (FOV) is required to monitor any possible energy deposition away from the focus. The FUS beam will likely travel through several different tissue types during treatment where a portion of the beam will be reflected and transmitted at each tissue interface depending on the impedance difference between the tissues. Each tissue type will also absorb a different amount of the ultrasound energy. For example, 90% of the ultrasound energy through the skull is reflected or absorbed 8.
Monitoring of interventional treatments can be done using 2D or 3D MRI sequences where the method chosen is often governed by the trade‐off between the needed temporal and spatial resolution and required FOV. Currently, clinical monitoring of MR‐guided FUS (MRgFUS) treatments is limited to a single (or relatively few) 2D slices 2 providing a limited FOV. For example, 2D monitoring of the ultrasound focus during transcranial MRgFUS treatments is severely limited and can miss heating outside of the slices monitored, such as near the skull surface, in grating lobes, or in any points of unintended energy deposition due to beam aberration 14.
MR temperature imaging does have some limitations, which are more apparent when using 2D imaging such as partial volume effects, which cause temperature underestimation 15. These effects can be reduced using smaller voxels and band‐limited (sinc) interpolation 15, but these options are not readily available in 2D MRI which has slices that are thicker and interpolation cannot be used in the through slice direction. Furthermore, it can also be difficult to properly position a single 2D slice to capture the entire focus and to limit slice crosstalk, multiple 2D slices often have a gap between each slice where any temperature changes will not be measured. Respiration and motion artifacts will also introduce errors to the temperature monitoring.
3D MR thermometry can overcome many of the FOV, partial volume, and coverage gap limitations, which are inherent in 2D imaging but unfortunately, standard 3D sequences typically require too much time to acquire k‐space to be clinically viable. Temporal resolution can be increased by methods involving undersampling, such as temporally constrained reconstruction 16, model predictive filtering 17, Kalman filtering 18, parallel imaging 19, or using a sequence designed for increased speed such as segmented echo‐planar imaging (seg‐EPI) 20.
While a 3D seg‐EPI offers several advantages, it has limitations. The chemical shift artifact, field inhomogeneity, and field variation due to motion artifacts are increased due to the low bandwidth in the phase encoding direction. The chemical shift typically requires imaging with fat saturation, while the respiration artifact can be corrected to a limited extent depending on the orientation of the 3D slab 22. Increasing the EPI factor, or number of lines collected per TR, will increase the temporal resolution while further escalating the chemical shift and respiration artifacts and decreasing the signal‐to‐noise ratio (SNR). Seg‐EPI sequences also typically have image distortions along the phase encode direction.
Non‐Cartesian 3D sequences, such as stack of stars (SOS) and stack of spirals 23, have several advantages that have been explored for use in thermometry.

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