High‐resolution imaging of magnetisation transfer and nuclear Overhauser effect in the human visual cortex at 7 T

Abstract

The aim of this study was to optimise a pulse sequence for high-resolution imaging sensitive to the effects of conventional macromolecular magnetisation transfer (MT(m)) and nuclear Overhauser enhancement (NOE), and to use it to investigate variations in these parameters across the cerebral cortex. A high-spatial-resolution magnetisation transfer-prepared turbo field echo (MT-TFE) sequence was designed to have high sensitivity to MT(m) and NOE effects, whilst being robust to B0 and B1 inhomogeneities, and producing a good point spread function across the cortex. This was achieved by optimising the saturation and imaging components of the sequence using simulations based on the Bloch equations, including exchange and an image simulator. This was used to study variations in these parameters across the cortex. Using the sequence designed to be sensitive to NOE and MT(m), a variation in signals corresponding to a variation in MT(m) and NOE across the cortex, consistent with a reduction in myelination from the white matter surface to the pial surface of the cortex, was observed. In regions in which the stria was visible on T2*-weighted images, it could also be detected in signals sensitive to MT(m) and NOE. There was greater variation in signals sensitive to NOE, suggesting that the NOE signal is more sensitive to myelination. A sequence has been designed to image variations in MT(m) and NOE at high spatial resolution and has been used to investigate variations in contrast in these parameters across the cortex.

Relationships between cortical myeloarchitecture and electrophysiological networks

“…A phase-sensitive inversion recovery T 1 weighted image was acquired and used for MEG coregistration. MT data were obtained from z-spectra acquired using an MT-TFE sequence (46). MT imaging provides contrast based on the exchange of magnetization between free water and protons bound in macromolecules.…”

Establishing upper limits on neuronal activity–evoked pH changes with APT‐CEST MRI at 7 T

“…Four‐pool (free water in tissue, APT‐CEST, nuclear Overhauser Enhancement and magnetization transfer) Bloch‐McConnell equations were solved numerically , assuming the following gray‐matter pool parameters: free‐water pool (T 1 /T 2 = 1.9 s/55 ms), APT pool (T 2 = 10 ms, Δω = 3.5 ppm, M 0 = 0.13%, R = 22.2 Hz), nuclear Overhauser Enhancement pool (T 2 = 0.3 ms, Δω = − 3.5 ppm, M 0 = 3%, R = 10 Hz), and magnetization‐transfer pool (T 2 = 10 µs, Δω = − 2.4 ppm, M 0 = 3%, R = 50 Hz) . The T 1 values of other than water pools (i.e., APT‐CEST, nuclear Overhauser Enhancement, and magnetization transfer) were fixed to 1 s .…”

Comparison of pulsed three-dimensional CEST acquisition schemes at 7 tesla: steady state versus pseudosteady state

“…Four‐pool (water, APT, NOE, and super‐lorentzian lineshape MT) Bloch‐McConnell equations were solved numerically assuming white matter parameters (Table ) similar to those reported in Mougin et al . The T1 values of all pools other than water were fixed to 1 s .…”

“…The PS protocol was as follows : saturation prepulse (a train of 40 RF‐spoiled 25 ms sinc‐gauss pulses interleaved with a sinusoidal‐modulated GR‐spoiler, duty cycle 50%); readout (axial acquisition, three‐shot TFE, TFE factor of 550, intershot interval 2 s, TR/TE/FA = 2.3 ms/1.05 ms/10°, phase encoding spirals out from the center of k‐space in y–z); two dummy scans, time per volume 15.9 s, scan time 10 min 18 s. An FA of 10° for PS was chosen as a trade‐off between sensitivity and point spread function. The parameters for both sequences were chosen to match the total acquisition scan times as closely as possible.…”