The impact of B1+ correction on MP2RAGE cortical T 1 and apparent cortical thickness at 7T

Abstract

Determination of cortical thickness using MRI has often been criticized due to the presence of various error sources. Specifically, anatomical MRI relying on T 1 contrast may be unreliable due to spatially variable image contrast between gray matter (GM), white matter (WM) and cerebrospinal fluid (CSF). Especially at ultra-high field (≥ 7T) MRI, transmit and receive B 1 -related image inhomogeneities can hamper correct classification of tissue types. In the current paper, we demonstrate that residual B 1+ (transmit) inhomogeneities in the T 1 -weighted and quantitative T 1 images using the MP2RAGE sequence at 7T lead to biases in cortical thickness measurements. As expected, post-hoc correction for the spatially varying B 1+ profile reduced the apparent T 1 values across the cortex in regions with low B 1+ , and slightly increased apparent T 1 in regions with high B 1+ . As a result, improved contrast-to-noise ratio both at the GM-CSF and GM-WM boundaries can be observed leading to more accurate surface reconstructions and cortical thickness estimates. Overall, the changes in cortical thickness ranged between a 5% decrease to a 70% increase after B 1+ correction, reducing the variance of cortical thickness values across the brain dramatically and increasing the comparability with normative data. More specifically, the cortical thickness estimates increased in regions characterized by a strong decrease of apparent T 1 after B 1+ correction in regions with low B 1+ due to improved detection of the pial surface . The current results suggest that cortical thickness can be more accurately determined using MP2RAGE data at 7T if B 1+ inhomogeneities are accounted for.