Abstract

Functional magnetic resonance imaging (fMRI) studies with ultra-high field (UHF, 7+ Tesla) technology enable the acquisition of high-resolution images. In this work, we discuss recent achievements in UHF fMRI at the mesoscopic scale, on the order of cortical columns and layers, and examine approaches to addressing common challenges. As researchers push to smaller and smaller voxel sizes, acquisition and analysis decisions have greater potential to degrade spatial accuracy, and UHF fMRI data must be carefully interpreted. We consider the impact of acquisition decisions on the spatial specificity of the MR signal with a representative dataset with 0.8 mm isotropic resolution. We illustrate the trade-offs in contrast with noise ratio and spatial specificity of different acquisition techniques and show that acquisition blurring can increase the effective voxel size by as much as 50% in some dimensions. We further describe how different sources of degradations to spatial resolution in functional data may be characterized. Finally, we emphasize that progress in UHF fMRI depends not only on scientific discovery and technical advancement, but also on informal discussions and documentation of challenges researchers face and overcome in pursuit of their goals.This article is part of the theme issue ‘Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity’.

Highlights

  • Functional magnetic resonance imaging has been a prolific tool for cognitive and neuroscientific research since its introduction in the early 1990s [1,2,3]

  • Imaging systems at 3 Tesla (3T) have become standard in both clinical and research applications, and, in pursuit of high-resolution imaging facilitated by ultra-high field (UHF) strengths, dozens of 7 Tesla (7T) systems have been installed globally [4]

  • As the technology has improved by way of increasing field strength-dependent signal-to-noise ratio (SNR) [5,6,7], functional contrastto-noise ratio (CNR) [8,9] and spatial specificity, more details of the functional architecture of the brain are available for study

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Summary

Introduction

Functional magnetic resonance imaging (fMRI) has been a prolific tool for cognitive and neuroscientific research since its introduction in the early 1990s [1,2,3]. Since the contrast in SE EPI acquisitions requires well-calibrated RF pulses and the flip angle varies significantly throughout the cortex—especially at UHF and with surface coils—only a portion of the brain volume covered by the pulse sequence (roughly 50%) provided adequate CNR for analysis This limitation is not a problem for applications focused on a single visual area and well-placed slices with a well-calibrated coil can provide beautiful images in which the fMRI contrast is dominated by small veins. On the other hand, are more likely to be uncontaminated ( there are large veins running parallel to the GM/WM boundary whose contributions to the depth-dependent fMRI response have yet to be quantified [98]) This is a logical consequence of the vascular structure [99], and a key insight to bear in mind when looking at all published laminar profiles reliant on the BOLD signal, regardless of the acquisition approach

Acquisition considerations
Characterizing blurring
Closing remarks
24. Zimmermann J et al 2011 Mapping the
41. Huber L et al 2015 Cortical lamina-dependent
Findings
87. Huber L et al 2020 Layer-dependent functional

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