Rapid and accurate navigators for motion and <scp>B</scp><sub>0</sub> tracking using QUEEN: Quantitatively enhanced parameter estimation from navigators

PurposeThis study leverages externally generated Pilot Tone (PT) signals to perform motion‐corrected brain MRI for sequences with arbitrary k‐space sampling and image contrast.Theory and MethodsPT signals are promising external motion sensors due to their cost‐effectiveness, easy workflow, and consistent performance across contrasts and sampling patterns. However, they lack robust calibration pipelines. This work calibrates PT signal to rigid motion parameters acquired during short blocks (˜4 s) of motion calibration (MC) acquisitions, which are short enough to unobstructively fit between acquisitions. MC acquisitions leverage self‐navigated trajectories that enable state‐of‐the‐art motion estimation methods for efficient calibration. To capture the range of patient motion occurring throughout the examination, distributed motion calibration (DMC) uses data acquired from MC scans distributed across the entire examination. After calibration, PT is used to retrospectively motion‐correct sequences with arbitrary k‐space sampling and image contrast. Additionally, a data‐driven calibration refinement is proposed to tailor calibration models to individual acquisitions. In vivo experiments involving 12 healthy volunteers tested the DMC protocol's ability to robustly correct subject motion.ResultsThe proposed calibration pipeline produces pose parameters consistent with reference values, even when distributing only six of these approximately 4‐s MC blocks, resulting in a total acquisition time of 22 s. In vivo motion experiments reveal significant () improved motion correction with increased signal to residual ratio for both MPRAGE and SPACE sequences with standard k‐space acquisition, especially when motion is large. Additionally, results highlight the benefits of using a distributed calibration approach.ConclusionsThis study presents a framework for performing motion‐corrected brain MRI in sequences with arbitrary k‐space encoding and contrast, using externally generated PT signals. The DMC protocol is introduced, promoting observation of patient motion occurring throughout the examination and providing a calibration pipeline suitable for clinical deployment. The method's application is demonstrated in standard volumetric MPRAGE and SPACE sequences.

Sequence‐agnostic motion‐correction leveraging efficiently calibrated Pilot Tone signals

Brackenier,

Cordero‐Grande,

McElroy

et al. 2024

Spherical echo‐planar time‐resolved imaging (sEPTI) for rapid 3D quantitative T2* and susceptibility imaging

Wang,

Liao,

Cao

et al. 2024

PurposeTo develop a 3D spherical EPTI (sEPTI) acquisition and a comprehensive reconstruction pipeline for rapid high‐quality whole‐brain submillimeter and QSM quantification.MethodsFor the sEPTI acquisition, spherical k‐space coverage is utilized with variable echo‐spacing and maximum kx ramp‐sampling to improve efficiency and signal incoherency compared to existing EPTI approaches. For reconstruction, an iterative rank‐shrinking B0 estimation and odd‐even high‐order phase correction algorithms were incorporated into the reconstruction to better mitigate artifacts from field imperfections. A physics‐informed unrolled network was utilized to boost the SNR, where 1‐mm and 0.75‐mm isotropic whole‐brain imaging were performed in 45 and 90 s at 3 T, respectively. These protocols were validated through simulations, phantom, and in vivo experiments. Ten healthy subjects were recruited to provide sufficient data for the unrolled network. The entire pipeline was validated on additional five healthy subjects where different EPTI sampling approaches were compared. Two additional pediatric patients with epilepsy were recruited to demonstrate the generalizability of the unrolled reconstruction.ResultssEPTI achieved 1.4 faster imaging with improved image quality and quantitative map precision compared to existing EPTI approaches. The B0 update and the phase correction provide improved reconstruction performance with lower artifacts. The unrolled network boosted the SNR, achieving high‐quality and QSM quantification with single average data. High‐quality reconstruction was also obtained in the pediatric patients using this network.ConclusionsEPTI achieved whole‐brain distortion‐free multi‐echo imaging and and QSM quantification at 0.75 mm in 90 s which has the potential to be useful for wide clinical applications.

Run‐time motion and first‐order shim control by expanded servo navigation

Riedel,

Ulrich,

Pruessmann

2024

PurposeTo provide a navigator‐based run‐time motion and first‐order field correction for three‐dimensional human brain imaging with high precision, minimal calibration and acquisition, and fast processing.MethodsA complex‐valued linear perturbation model with feedback control is extended to estimate and correct for gradient shim fields using orbital navigators (2.3 ms). Two approaches for sensitizing the model to gradient fields are presented, one based on finite differences with three additional navigators, and another projection‐based approximation requiring no additional navigators. A mechanism for noise decorrelation of the matrix and the data is proposed and evaluated to reduce unwanted parameter biases.ResultsThe rigid motion and first‐order field control achieves robust motion and gradient shim corrections improving image quality in a series of phantom and in vivo experiments with varying field conditions. In phantom scans, magnet drifts, forced gradient field perturbations and field distortions from shifts of a second bottle phantom are successfully corrected. Field estimates of the magnet drifts are in good agreement with concurrent field probe measurements. For in vivo scans, the proposed method mitigates field variations from torso motions while being robust to head motion. In vivo gradient field precisions were along with single‐digit micrometer and millidegree rigid precisions.ConclusionThe navigator‐based method achieves accurate, high‐precision run‐time motion and field corrections with low sequence impact and calibration requirements.