Pushing the limits of high‐resolution functional MRI using a simple high‐density multi‐element coil design

Abstract

Recent studies have shown that functional MRI (fMRI) can be sensitive to the laminar and columnar organization of the cortex based on differences in the spatial and temporal characteristics of the blood oxygenation level-dependent (BOLD) signal originating from the macrovasculature and the neuronal-specific microvasculature. Human fMRI studies at this scale of the cortical architecture, however, are very rare because the high spatial/temporal resolution required to explore these properties of the BOLD signal are limited by the signal-to-noise ratio. Here, we show that it is possible to detect BOLD signal changes at an isotropic spatial resolution as high as 0.55 mm at 7 T using a high-density multi-element surface coil with minimal electronics, which allows close proximity to the head. The coil comprises of very small, 1 × 2-cm(2) , elements arranged in four flexible modules of four elements each (16-channel) that can be positioned within 1 mm from the head. As a result of this proximity, tissue losses were five-fold greater than coil losses and sufficient to exclude preamplifier decoupling. When compared with a standard 16-channel head coil, the BOLD sensitivity was approximately 2.2-fold higher for a high spatial/temporal resolution (1 mm isotropic/0.4 s), multi-slice, echo planar acquisition, and approximately three- and six-fold higher for three-dimensional echo planar images acquired with isotropic resolutions of 0.7 and 0.55 mm, respectively. Improvements in parallel imaging performance (geometry factor) were up to around 1.5-fold with increasing acceleration factor, and improvements in fMRI detectability (temporal signal-to-noise ratio) were up to around four-fold depending on the distance to the coil. Although deeper lying structures may not benefit from the design, most fMRI questions pertain to the neocortex which lies within approximately 4 cm from the surface. These results suggest that the resolution of fMRI (at 7 T) can approximate levels that are closer to the spatial/temporal scale of the fundamental functional organization of the human cortex using a simple high-density coil design for high sensitivity.

Maximizing sensitivity for fast GABA edited spectroscopy in the visual cortex at 7 T

“…Small surface coils tend to yield the highest SNR within their sensitive area, but their receive sensitivity is not very uniform and decreases with distance from the coils 41. The high density surface coils used in the present setup allow for high sensitivity and SNR up to at least a 4‐5 cm distance from the coil elements 30, 31. In the coil setup design, a choice was made to increase the visual angle by increasing the screen size.…”

“…The high density receive arrays with small coil elements fit tightly on the participant's head, and are used to produce high resolution, high SNR, fMRI and MRI scans, with a receive depth extending several centimeters from the coil elements 30, 31…”

Safety of Ultra-High Field MRI: What are the Specific Risks?

“…This rapid growth in the number of UHF-MRI sites has quickly led to the development of scanning protocols [1][2][3] and the required hardware (brain [4][5][6], cardiac [7,8], body [9,10], spine [11,12], breast [13,14], and eye [15] ) to enable the use of UHF-MRI in both fundamental and clinical research studies as well as in pilot studies for pure clinical use [16][17][18]. A recent review article provides a comprehensive overview of the current status of high field MRI [19 •• ].…”

A study-specific fMRI normalization approach that operates directly on high resolution functional EPI data at 7Tesla

“…To obtain a temporal resolution of about 3 seconds most studies have been performed on specific brain areas (Barry et al, 2011; De Martino et al, 2011; Harmer et al, 2012; Heidemann et al, 2012; Petridou et al, 2012; Polimeni et al, 2010; Sanchez-Panchuelo et al, 2010), but whole brain coverage can be achieved by using acceleration in the slice (Moeller et al, 2010) or second phase encoding (3D) direction (Poser et al, 2010). …”