Resolution limit of cylinder diameter estimation by diffusion MRI: The impact of gradient waveform and orientation dispersion. (20th March 2017)

• Record Type: Journal Article
• Authors: Nilsson, Markus; Lasič, Samo; Drobnjak, Ivana; Topgaard, Daniel; Westin, Carl‐Fredrik
• Is Part Of: NMR in biomedicine. Volume 30:Number 7(2017:Jul.)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2017-03-20
• Subjects: Axon diameter -- diffusion imaging -- double diffusion encoding -- microstructure -- oscillating diffusion encoding -- q‐trajectory encoding -- resolution limit -- single diffusion encoding; Nuclear magnetic resonance -- Periodicals; Magnetic Resonance Spectroscopy -- Periodicals; 574
• DOI: 10.1002/nbm.3711
• Languages: English
• ISSNs: 0952-3480

Abstract

Abstract : Diffusion MRI has been proposed as a non‐invasive technique for axonal diameter mapping. However, accurate estimation of small diameters requires strong gradients, which is a challenge for the transition of the technique from preclinical to clinical MRI scanners, since these have weaker gradients. In this work, we develop a framework to estimate the lower bound for accurate diameter estimation, which we refer to as the resolution limit. We analyse only the contribution from the intra‐axonal space and assume that axons can be represented by impermeable cylinders. To address the growing interest in using techniques for diffusion encoding that go beyond the conventional single diffusion encoding (SDE) sequence, we present a generalised analysis capable of predicting the resolution limit regardless of the gradient waveform. Using this framework, waveforms were optimised to minimise the resolution limit. The results show that, for parallel cylinders, the SDE experiment is optimal in terms of yielding the lowest possible resolution limit. In the presence of orientation dispersion, diffusion encoding sequences with square‐wave oscillating gradients were optimal. The resolution limit for standard clinical MRI scanners (maximum gradient strength 60–80 mT/m) was found to be between 4 and 8 μm, depending on the noise levels and the level of orientation dispersion. For scanners with a maximum gradient strength of 300 mT/m, the limit was reduced to between 2 and 5 μm. Abstract : Accurate quantification of the axon diameter demands strong gradients and can thus be challenging for clinical MRI scanners. We define the resolution limit as the lower bound for accurate diameter estimation and predict its value for diffusion encoding techniques that go beyond the conventional single diffusion encoding sequence. In standard clinical MRI scanners (maximum gradient strength 60‐80 mT/m), the resolution limit was found to be between 4 and 8 μm, depending on noise levels and the level of orientation dispersion.