Effects of MP2RAGE B1+ sensitivity on inter-site T1 reproducibility and hippocampal morphometry at 7T

Roy A M Haast; Jonathan C Lau; Dimo Ivanov; Ravi S Menon; Kâmil Uludağ; Ali R Khan

doi:10.1016/j.neuroimage.2020.117373

Effects of MP2RAGE B₁⁺ sensitivity on inter-site T₁ reproducibility and hippocampal morphometry at 7T

Neuroimage. 2021 Jan 1:224:117373. doi: 10.1016/j.neuroimage.2020.117373. Epub 2020 Sep 17.

Authors

Roy A M Haast¹, Jonathan C Lau², Dimo Ivanov³, Ravi S Menon⁴, Kâmil Uludağ⁵, Ali R Khan⁴

Affiliations

¹ Centre for Functional and Metabolic Mapping, Robarts Research Institute, Western University, 1151 Richmond St. N., London, ON N6A 5B7, Canada. Electronic address: rhaast@uwo.ca.
² Centre for Functional and Metabolic Mapping, Robarts Research Institute, Western University, 1151 Richmond St. N., London, ON N6A 5B7, Canada; Department of Clinical Neurological Sciences, Division of Neurosurgery, Western University, 1151 Richmond St. N., London, ON N6A 5B7, Canada.
³ Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, PO Box 616, 6200 MD, Maastricht, Netherlands.
⁴ Centre for Functional and Metabolic Mapping, Robarts Research Institute, Western University, 1151 Richmond St. N., London, ON N6A 5B7, Canada; Brain and Mind Institute, Western University, 1151 Richmond St. N., London, ON N6A 5B7, Canada; Department of Medical Biophysics, Schulich School of Medicine and Dentistry, Western University, 1151 Richmond St. N., London, ON N6A 5B7, Canada.
⁵ IBS Center for Neuroscience Imaging Research, Sungkyunkwan University, Seobu-ro, 2066, Jangan-gu, Suwon, South Korea; Department of Biomedical Engineering, N Center, Sungkyunkwan University, Seobu-ro, 2066, Jangan-gu, Suwon, South Korea; Techna Institute and Koerner Scientist in MR Imaging, University Health Network, 100 College St, Toronto, ON M5G 1L5, Canada.

PMID: 32949709
DOI: 10.1016/j.neuroimage.2020.117373

Abstract

Most neuroanatomical studies are based on T₁-weighted MR images, whose intensity profiles are not solely determined by the tissue's longitudinal relaxation times (T₁), but also affected by varying non-T₁ contributions, hampering data reproducibility. In contrast, quantitative imaging using the MP2RAGE sequence, for example, allows direct characterization of the brain based on the tissue property of interest. Combined with 7 Tesla (7T) MRI, this offers unique opportunities to obtain robust high-resolution brain data characterized by a high reproducibility, sensitivity and specificity. However, specific MP2RAGE parameter choices - e.g., to emphasize intracortical myelin-dependent contrast variations - can substantially impact image quality and cortical analyses through remnants of B₁⁺-related intensity variations, as illustrated in our previous work. To follow up on this: we (1) validate this protocol effect using a dataset acquired with a particularly B₁⁺ insensitive set of MP2RAGE parameters combined with parallel transmission excitation; and (2) extend our analyses to evaluate the effects on hippocampal morphometry. The latter remained unexplored initially, but can provide important insights related to generalizability and reproducibility of neurodegenerative research using 7T MRI. We confirm that B₁⁺ inhomogeneities have a considerably variable effect on cortical T₁ estimates, as well as on hippocampal morphometry depending on the MP2RAGE setup. While T₁ differed substantially across datasets initially, we show the inter-site T₁ comparability improves after correcting for the spatially varying B₁⁺ field using a separately acquired Sa2RAGE B₁⁺ map. Finally, removal of B₁⁺ residuals affects hippocampal volumetry and boundary definitions, particularly near structures characterized by strong intensity changes (e.g. cerebral spinal fluid). Taken together, we show that the choice of MP2RAGE parameters can impact T₁ comparability across sites and present evidence that hippocampal segmentation results are modulated by B₁⁺ inhomogeneities. This calls for careful (1) consideration of sequence parameters when setting acquisition protocols, as well as (2) acquisition of a B₁⁺ map to correct MP2RAGE data for potential B₁⁺ variations to allow comparison across datasets.

Keywords: 7 Tesla MRI; Brain morphometry; MP2RAGE; Quantitative T(1); Transmit bias field.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Adult
Brain / physiology*
Hippocampus / physiology*
Humans
Image Processing, Computer-Assisted* / methods
Magnetic Resonance Imaging* / methods
Male
Reproducibility of Results
Sensitivity and Specificity

Grants and funding

366062/CIHR/Canada