Purpose To develop a fully data‐driven retrospective intrascan motion‐correction framework for volumetric brain MRI at ultrahigh field (7 Tesla) that includes modeling of pose‐dependent changes in polarizing magnetic (B 0 ) fields. Theory and Methods Tissue susceptibility induces spatially varying B 0 distributions in the head, which change with pose. A physics‐inspired B 0 model has been deployed to model the B 0 variations in the head and was validated in vivo. This model is integrated into a forward parallel imaging model for imaging in the presence of motion. Our proposal minimizes the number of added parameters, enabling the developed framework to estimate dynamic B 0 variations from appropriately acquired data without requiring navigators. The effect on data‐driven motion correction is validated in simulations and in vivo. Results The applicability of the physics‐inspired B 0 model was confirmed in vivo. Simulations show the need to include the pose‐dependent B 0 fields in the reconstruction to improve motion‐correction performance and the feasibility of estimating B 0 evolution from the acquired data. The proposed motion and B 0 correction showed improved image quality for strongly corrupted data at 7 Tesla in simulations and in vivo. Conclusion We have developed a motion‐correction framework that accounts for and estimates pose‐dependent B 0 fields. The method improves current state‐of‐the‐art data‐driven motion‐correction techniques when B 0 dependencies cannot be neglected. The use of a compact physics‐inspired B 0 model together with leveraging the parallel imaging encoding redundancy and previously proposed optimized sampling patterns enables a purely data‐driven approach.
SCN1A gene mutations disrupt sodium channel (NaV1.1) function, causing childhood epilepsy which can be severe. Predicting functional consequences in these children is challenging and new prognostic imaging biomarkers are needed. Sodium MRI directly assesses brain sodium and is a potential in vivo imaging biomarker. Using a multiecho sodium sequence, at 7T, we found children with SCN1A mutations had increased relative sodium concentrations across a large number of brain regions compared to controls. The cause of this change is likely complex, and may reflect an interplay between neuronal/axonal/glial dysfunction – with disruptions in microstructure and physiology – and possible medication effects.
Motivation: Brain T2* values can provide information about tissue specific maturation, and can also provide knowledge important for optimising echo times for fMRI acquisition. Goal(s): To characterise tissue specific T2* values in the neonatal brain at 7T. Approach: Whole brain T2* maps from 14 neonates were generated using a 3D multi-echo acquisition. Results: Median T2* values were: cortical grey matter: 58 ms, deep grey matter: 70 ms and white matter: 86 ms. Values differ markedly from those described in adults and measured at standard field strengths. Impact: We describe tissue-specific T2* values in the neonatal brain at 7T, which may provide new information about brain development in health and disease, and provide a basis for optimising fMRI sequences for neonates at 7T.
Abstract Background and Aims Histopathological diagnosis is the gold standard in many acquired inflammatory, infiltrative and amyloid based peripheral nerve diseases and a sensory nerve biopsy of sural or superficial peroneal nerve is favoured where a biopsy is deemed necessary. The ability to determine nerve pathology by high‐resolution imaging techniques resolving anatomy and imaging characteristics might improve diagnosis and obviate the need for biopsy in some. The sural nerve is anatomically variable and occasionally adjacent vessels can be sent for analysis in error. Knowing the exact position and relationships of the nerve prior to surgery could be clinically useful and thus reliably resolving nerve position has some utility. Methods 7T images of eight healthy volunteers' (HV) right ankle were acquired in a pilot study using a double‐echo in steady‐state sequence for high‐resolution anatomy images. Magnetic Transfer Ratio images were acquired of the same area. Systematic scoring of the sural, tibial and deep peroneal nerve around the surgical landmark 7 cm from the lateral malleolus was performed (number of fascicles, area in voxels and mm 2 , diameter and location relative to nearby vessels and muscles). Results The sural and tibial nerves were visualised in the high‐resolution double‐echo in steady‐state (DESS) image in all HV. The deep peroneal nerve was not always visualised at level of interest. The MTR values were tightly grouped except in the sural nerve where the nerve was not visualised in two HV. The sural nerve location was found to be variable (e.g., lateral or medial to, or crossing behind, or found positioned directly posterior to the saphenous vein). Interpretation High‐resolution high‐field images have excellent visualisation of the sural nerve and would give surgeons prior knowledge of the position before surgery. Basic imaging characteristics of the sural nerve can be acquired, but more detailed imaging characteristics are not easily evaluable in the very small sural and further developments and specific studies are required for any diagnostic utility at 7T.
Motivation: There is currently no high contrast structural T1-weighted protocol available for neonatal imaging at ultrahigh field (7T), as the longer T1 relaxation times make existing protocols optimized for older subjects unsuitable. Goal(s): To produce submillimeter T1-weighted images for neonates at 7T in a clinically feasible scan time. Approach: We optimised a T1-weighted MP2RAGE protocol using the genetic algorithm for constrained optimisation while accounting for transmit field inhomogeneities. Results: 3D whole-brain images with 0.65-0.8mm isotropic resolution were acquired in under 6-7 minutes from 3 infants. Quantitative T1 maps were produced using an in-house fitting algorithm. Impact: We describe the first neonatal optimised MP2RAGE protocol for acquiring high-contrast submillimeter-resolution images with full-brain coverage that are relatively insensitive to transmit field inhomogeneities.
Abstract Introduction Quantitative MRI is important for non-invasive tissue characterisation. In previous work we developed a clinically feasible multi-contrast protocol for T 1 -weighted imaging based on the MP2RAGE sequence that was optimised for both children and adults. It was demonstrated that a range of Fluid And White Matter Suppression (FLAWS) related contrasts could be produced while maintaining T 1 -weighted uniform image (UNI) quality, a challenge at higher field strengths. Here we introduce an approach to use these images to calculate effective proton density (PD*) and quantitative T 1 relaxation maps especially for shorter repetition times (TR MP2RAGE ) than those typically used previously. Methods T 1 and PD* were estimated from the analytical equations of the MP2RAGE signal derived for partial Fourier acquisitions. The sensitivity of the fitting results was evaluated with respect to the TR MP2RAGE and B 1 + effects on both excitation flip angles and inversion efficiency and compared to vendor T 1 maps which do not use B 1 + information. Data acquired for a range of individuals (aged 10-54 years) at the shortest TR MP2RAGE (4000ms) were compared across white matter (WM), cortical grey matter, and deep grey matter regions. Results The T 1 values were insensitive to the choice of different TR MP2RAGE . The results were similar to the vendor T 1 maps if the B 1 + effects on the excitation flip angle and inversion efficiency were not included in the fits. T 1 values varied over development into adulthood, especially for the deep grey matter regions whereas only a very small difference was observed for WM T 1 . Effective PD maps were produced which did not show a significant difference between children and adults for the age range included. Conclusion We produced PD* maps and improved the accuracy of T 1 maps from an MP2RAGE protocol that is optimised for UNI and FLAWS-related contrasts in a single scan at 7T by incorporating the excitation flip angle and inversion efficiency related effects of B 1 + in the fitting. This multi-parametric protocol made it possible to acquire high resolution images (0.65mm iso) in children and adults within a clinically feasible duration (7:18 min:s). The combination of analytical equations utilizing B 1 + maps led to T 1 fits that were consistent at different TR MP2RAGE values. Average WM T 1 values of adults and children were very similar (1092ms vs 1117ms) while expected reductions in T 1 with age were found for GM especially for deep GM.
Magnetic Resonance Imaging (MRI) can provide detailed information about the neonatal brain when it is rapidly developing and thus vulnerable to injuries. However, at standard MRI field strengths MRIs (1.5 and 3 Tesla (T)) there are limitations in tissue contrast and resolution. MRI imaging at ultra-high field (7T) provides significant gains in signal-to-noise-ratio and tissue contrast, which can markedly increase sensitivity to anatomy and pathology.1 Experience of scanning neonates at 7T is limited and there are specific challenges due to differences in tissue composition and the ultra-high magnetic field environment. The aim of this study was to determine feasibility of 7T neonatal imaging and explore the potential gains in anatomical and pathological sensitivity.
Methods
17 neonatal scans (median corrected age 40+3 weeks) were acquired with ethical approval (19/LO/1384) using a 7T Siemens MAGNETOM Terra system. Scanner software was modified to mitigate potential risks of temperature instability in neonates.2 Infants were scanned in natural sleep following feeding, swaddled using a vacuum-evacuated bag and hearing protection with lateral immobilisation. Pulse-oximetry, heart rate and axillary temperature were continuously monitored by neonatal staff throughout scanning using 7T compatible equipment. 15 infants also had MRI scans on a 3T Philips Achieva system for comparison. Sequences acquired included high-resolution T2-weighted images in three orthogonal planes, susceptibility-weighted images, T1 and T2 quantitative maps, fMRI and spectroscopy.
Results
Images were successful acquired in 16 neonates (1 infant was imaged on 2 occasions) (table 1). All infants tolerated scanning on the 7T system and vital sign monitoring was stable throughout scanning over 52 minutes (range 25–80). There was no significant difference in temperature before and after 7T scanning (p-value=0.2). Additional detail of anatomical or pathological features were seen in 7T images compared to 3T, with image quality classified as equivalent or superior following neuroradiology review. These included improved visualization of the hippocampus, cerebellar vermis and occipital cortical folding. 7T images also provided improved visualization of cystic septi in periventricular leukomalacia and the haemorrhagic origin of cystic lesions in preterm infants.
Conclusion
We provide evidence for feasibility of 7T scanning in neonates and highlight the potential benefits for clinical care and research through improving image quality by increasing resolution and tissue contrast. In addition to improving diagnostic sensitivity, this could have provided new insight into the pathophysiological processes underlying poorly understood conditions such as neurodevelopmental disorders.
References
Yamada K, Yoshimura J, Watanabe M, Suzuki K. Application of 7 tesla magnetic resonance imaging for pediatric neurological disorders: early clinical experience. Journal of Clinical Imaging Science 2021;11:65. https://doi.org/10.25259/JCIS_185_2021 Malik Malik SJ, Hand JW, Satnarine R, Price AN, Hajnal JV. Specific absorption rate and temperature in neonate models resulting from exposure to a 7T head coil. Magn Reson Med. 2021;86:1299– 1313. https://doi.org/10.1002/mrm.28784
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