The blood oxygen level dependent (BOLD) response, as measured with fMRI, offers good spatial resolution compared to other non-invasive neuroimaging methods. The use of a spin echo technique rather than the conventional gradient echo technique may further improve the resolution by refocusing static dephasing effects around the larger vessels, so sensitizing the signal to the microvasculature. In this work the width of the point spread function (PSF) of the BOLD response at a field strength of 3 Tesla is compared for these two approaches. A double echo EPI pulse sequence with simultaneous collection of gradient echo and spin echo signal allows a direct comparison of the techniques. Rotating multiple-wedge stimuli of different spatial frequencies are used to estimate the width of the BOLD response. Waves of activation are created on the surface of the visual cortex, which begin to overlap as the wedge separation decreases. The modulation of the BOLD response decreases with increasing spatial frequency in a manner dependent on its width. The spin echo response shows a 13% reduction in the width of the PSF, but at a cost of at least 3-fold reduction in contrast to noise ratio. Good spatial resolution is one of the principle advantages of fMRI compared to other neuroimaging methods, such as MEG or EEG. However, compared to most MRI techniques, the typical resolution of the blood oxygenation level dependent (BOLD) response is quite poor. The resolution is limited by physiologic rather than technical considerations, with the vascular response to neural activity extending over several millimeters. The change in deoxyhaemoglobin content in the draining veins and venules leads to inaccurate localization of neural activity (1), and also to poor precision by widening the spatial extent or point spread function (PSF) of the response, resulting in an inability to resolve activity from close sources (2,3). In general, localization and resolution are not related; for example, it is perfectly possible to have very high resolution signal in the wrong location. However, in this case, deoxyhaemoglobin changes in venous vessels distant from the site of neuronal activation will degrade both measures.Recent work suggests that at a field strength of 3T, a spin echo (SE) sequence could improve the spatial resolution of the BOLD response compared to the standard gradient echo (GE) technique (4,5). To understand this we need to consider the relative signal contribution from both the intra-and extravascular spaces. The extravascular signal change is due to the dephasing effect of local field gradients surrounding the blood vessels. Water protons surrounding capillaries will move a considerable distance relative to the capillary diameter during the echo time and, hence, will experience a range of field gradients. This dynamic dephasing is a random process that cannot be refocused by a spin-echo. Water protons surrounding large vessels, however, will tend to remain in the same magnetic field during the echo time, resulting in little dynamic d...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.