An in-vivo study of BOLD laminar responses as a function of echo time and static magnetic field strength

Irati Markuerkiaga,David G Norris,José P Marques,Lauren J Bains

doi:10.1038/s41598-021-81249-w

Abstract

Layer specific functional MRI requires high spatial resolution data. To compensate the associated poor signal to noise ratio it is common to integrate the signal from voxels at a given cortical depth. If the region is sufficiently large then physiological noise will be the dominant noise source. In this work, activation profiles in response to the same visual stimulus are compared at 1.5 T, 3 T and 7 T using a multi-echo, gradient echo (GE) FLASH sequence, with a 0.75 mm isotropic voxel size and the cortical integration approach. The results show that after integrating over a cortical volume of 40, 60 and 100 mm3 (at 7 T, 3 T, and 1.5 T, respectively), the signal is in the physiological noise dominated regime. The activation profiles obtained are similar for equivalent echo times. BOLD-like noise is found to be the dominant source of physiological noise. Consequently, the functional contrast to noise ratio is not strongly echo-time or field-strength dependent. We conclude that laminar GE-BOLD fMRI at lower field strengths is feasible but that larger patches of cortex will need to be examined, and that the acquisition efficiency is reduced.

Highlights

Layer specific functional MRI requires high spatial resolution data
An assessment of the potential value of gradient echo (GE)-BOLD laminar fMRI at static field strengths below the gold standard of 7 T requires a characterisation of the laminar activation profile as a function of TE, and measurement of the relative sensitivity when data are acquired in the physiological noise regime
The profiles obtained at 1.5 T (Fig. 2a), which represent to our knowledge the first layer specific functional studies at this field, show qualitatively the same expected behaviour as those obtained at higher fields (Fig. 2b,c) but with a greater variance

Summary

Introduction

Layer specific functional MRI requires high spatial resolution data. To compensate the associated poor signal to noise ratio it is common to integrate the signal from voxels at a given cortical depth. Most layer specific BOLD-fMRI studies in humans in recent years have been performed at 7 T to obtain a higher sensitivity to activation[3,4,5,6,7,8,9,10,11,12,13] Many of these layer specific studies at 7 T further improved the signal to noise ratio (SNR) by integrating the signal over a patch of cortex at a given cortical depth. I.e. those that result in signal modulations unrelated to neuronal changes induced by the functional task, become important, the signal variation will become correlated over voxels and sensitivity will not improve by summation of new voxels In this physiological noise r egime[2,14,15,16,17] the sensitivity gain in moving to higher static field strengths will be lower than for the thermal noise regime. An assessment of the potential value of GE-BOLD laminar fMRI at static field strengths below the gold standard of 7 T requires a characterisation of the laminar activation profile as a function of TE, and measurement of the relative sensitivity when data are acquired in the physiological noise regime

Objectives

Methods

Results

Conclusion