Synthetic quantitative MRI through relaxometry modelling

Martina F. Callaghan, Siawoosh Mohammadi, Nikolaus Weiskopf
2016 NMR in Biomedicine  
Quantitative MRI (qMRI) provides standardized measures of specific physical parameters that are sensitive to the underlying tissue microstructure and are a first step towards achieving maps of biologically relevant metrics through in vivo histology using MRI. Recently proposed models have described the interdependence of qMRI parameters. Combining such models with the concept of image synthesis points towards a novel approach to synthetic qMRI, in which maps of fundamentally different physical
more » ... roperties are constructed through the use of biophysical models. In this study, the utility of synthetic qMRI is investigated within the context of a recently proposed linear relaxometry model. Two neuroimaging applications are considered. In the first, artefact-free quantitative maps are synthesized from motion-corrupted data by exploiting the over-determined nature of the relaxometry model and the fact that the artefact is inconsistent across the data. In the second application, a map of magnetization transfer (MT) saturation is synthesized without the need to acquire an MT-weighted volume, which directly leads to a reduction in the specific absorption rate of the acquisition. This feature would be particularly important for ultra-high field applications. The synthetic MT map is shown to provide improved segmentation of deep grey matter structures, relative to segmentation using T 1 -weighted images or R 1 maps. The proposed approach of synthetic qMRI shows promise for maximizing the extraction of high quality information related to tissue microstructure from qMRI protocols and furthering our understanding of the interrelation of these qMRI parameters. KEYWORDS magnetization transfer, relaxometry, synthetic quantitative MRI 1 | INTRODUCTION Quantitative MRI (qMRI) provides standardized measures of specific physical parameters that are sensitive to the underlying tissue microstructure. The standardized nature of these parameters facilitates comparison across sites and time points, which greatly improves the sensitivity and efficiency of multi-centre and longitudinal studies. 1 qMRI is also the first step towards achieving maps of biologically rele-vant metrics through in vivo histology using MRI. 2 However, since qMRI metrics are sensitive to multiple biological factors (e.g. fibre density, water, myelin and iron content), multiple parameters are needed to improve biological specificity. 3 The multi-parameter mapping (MPM) protocol 1 is one such quantitative imaging approach, in which data are combined to calculate maps of the longitudinal relaxation rate (R 1 ), the effective transverse relaxation rate (R 2 *), the magnetization transfer saturation (MT) and the effective proton density (PD*). Given a set of qMRI parameters, such as are generated with the MPM protocol, it is possible to synthesize images with arbitrary contrast weighting through use of the appropriate MRI signal model. [4] [5] [6] This provides a flexible and time efficient approach to investigating tissue integrity and pathology, e.g. by generating multiple inversion recovery images covering a range of inversion times. Such a synthetic approach has been proposed as a first step towards the adoption of This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
doi:10.1002/nbm.3658 pmid:27753154 pmcid:PMC5132086 fatcat:gdeqyoesdbgh5ib77hn7yfpldm