The impact of B1+ correction on MP2RAGE cortical T1 and apparent cortical thickness at 7T

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.