Smart Medical Imaging

EPSRC Centre for Doctoral Training

Research

2019_031 - Enhanced neonatal brain development MRI at ultra-high field

1st supervisor: Joseph Hajnal
2nd supervisor: Enrico De Vita

MRI is extremely valuable in studying normal and abnormal brain development in neonates. The developing Human Connectome Project (dHCP), led jointly by King’s and Imperial, aims to map early life development by acquiring MRI anatomical and functional data from fetuses and neonates. Ongoing analysis of dHCP data is hoped to provide new insights into structural and functional development. However there remain key unknowns relating to the effects of prematurity and perinatal trauma on brain development that existing MRI approaches are unlikely to resolve.

One challenge of imaging neonates, is achieving sufficient spatial resolution to visualise small brain structures, maintaining reasonable acquisition times and acceptable noise levels. The dHCP uses a clinical 3T MRI scanner technology primarily designed for imaging adults combined with bespoke neonatal brain receiver coils and purpose designed patient handling1. However, even the dedicated acquisition protocol providing anatomical resolution of 0.8mm isotropic is still only just able to resolve key structures and for example is not able to differentiate cortical layers.
MR sensitivity increases approximately with field strength. As the associated technical challenges are overcome, UHF-MRI is only recently starting to be applied to clinical and biomedical research. For instance 7T MRI can provide information on focal brain lesions in epileptic subjects and in multiple sclerosis not visible at lower fields2,3; ultra high resolution brain imaging is also revealing astonishing anatomical details particularly with enhanced T2* contrast that makes UHF particularly well suited for studying deep nuclei characterised by high iron content4,5.
The objective of this project is to explore the benefits of UHF-MRI for enhancing the available image data quality for neonatal subjects. This will provide a closer look at the brain during a period of exuberant development, when it is both different from adult brain (with different MRI properties) and is changing fast. We aim to produce a 7T protocol that exceeds the dHCP neonatal protocol in sensitivity and resolution, and use this to explore critical facets of brain development and damage, such as patters of deep grey nuclei damage in hypoxic ischemic injury and subtle vascular changes associated with prematurity6.
Few studies scanned children at 7T, with the youngest subjects we are aware of being 5 years old7.The highest field at which neonates have been scanned is 4.7T8,9.

Since neonatal imaging has never been attempted at 7T our first task will be to demonstrate that safe operation can be assured. To the best of current knowledge there are no known inherent health risks associated with exposure to strong static magnetic fields10.

The key risk that must be addressed for any novel MRI application is subject heating during imaging, quantified by the specific absorption rate (SAR) of radiofrequency energy, intrinsically higher at higher frequencies. Safety can be assured by modelling of RF transmitter coils and comparing with detailed experiments on phantoms – an approach taken for imaging all other populations. It should be noted that there are no inherent additional risks associated with smaller subjects. On the contrary, smaller subjects are less prone to RF heating effects for many RF coil designs11 and their higher surface-to-volume ratios make neonates far less prone to systemic heating from RF fields than adults. Further, RF inhomogeneity effects, problematic for imaging larger fields of view at UHF, will be much less pronounced in neonates.

References
1. Hughes EJ, Winchman T, Padormo F, et al. A dedicated neonatal brain imaging system. Magn. Reson. Med. 2017;78:C1.
2. Harrison DM, Oh J, Roy S, et al. Thalamic lesions in multiple sclerosis by 7T MRI: clinical implications and relationship to cortical pathology. Multiple sclerosis. 2015;21(9):1139-1150.
3. De Ciantis A, Barba C, Tassi L, et al., 7T MRI in focal epilepsy with unrevealing conventional field strength imaging. Epilepsia 2016;57:445–454.
4. Stucht D, Danishad KA, Schulze P, et al., Highest Resolution In Vivo Human Brain MRI Using Prospective Motion Correction. PLoS ONE 2015; 10(7): e0133921.
5. Cho ZH, Min HK, Oh SH, et al., J Neurosurg. 2010; 113(3):639-47.
6. Malamateniou, C., Counsell, S. J., Allsop, J. M., Fitzpatrick, J. A., Srinivasan, L., Cowan, F. M., …Rutherford, M. A. (2006). The effect of preterm birth on neonatal cerebral vasculature studied with magnetic resonance angiography at 3 Tesla. NeuroImage, 32(3), 1050–1059
7. Harris AD, Singer HS, Horska A, et al., GABA and glutamate in children with primary complex motor stereotypies: an 1 H-MRS study at 7T. American Journal of Neuroradiology 2016, 1–6. http://doi.org/10.3174/ajnr.A4547
8. De Vita E, Bainbridge A, Cheong JLY, et al., Localised 4.7 Tesla proton magnetic resonance spectroscopy in neonatal encephalopathy: implementation, safety, and preliminary interpretation of results. Imaging Decisions 2005; 9(4):31-41.
9. De Vita E, Bainbridge A, Cheong JLY, et al., Magnetic Resonance Imaging of Neonatal Encephalopathy at 4.7T. Pediatrics 2006;118(6):e1812-21.
10. Formica D, Silvestri S. Biological effects of exposure to magnetic resonance imaging: an overview. Biomed. Eng. Online 2004;3:11
11. Malik SJ, Beqiri A, Price AN, et al., Specific absorption rate in neonates undergoing magnetic resonance procedures at 1.5 T and 3 T. NMR Biomed. 2015;28:344–352.