Eliminating susceptibility induced hyperintensities in T1w MPRAGE brain images at 7 T

At ultrahigh field, local susceptibility induced hyperintensities are pronounced in brain areas close to air-tissue boundaries in the inferior frontal lobe and temporal lobes on T1w MPRAGE images. Resulting from incomplete inversion, these artefacts can introduce biases in brain volumetry and erroneously suggest the existence of local tissular anomaly. We propose a straightforward approach to eliminate these artefacts by applying a shift (Δf_IR) to the center frequency of the adiabatic inversion pulse while widening the bandwidth of the latter by shortening the pulse duration (Δt_IR).

An MPRAGE sequence was customized allowing to change the duration (standard: 10,240 μs) and center frequency of the hyperbolic secant inversion RF pulse (IR). All measurements were performed on a 7 T whole body scanner (Siemens, Erlangen, Germany). 13 healthy volunteers (7 female and 6 male, average age (SD) = 38 ± 15 yrs) were recruited for the study, 3 of which were scanned for protocol optimization and the rest for performance evaluation. ΔB0 was mapped through the brain with a gradient echo sequence. The effects of Δf_IR and Δt_IR were studied separately and jointly to determine optimal parameter combinations to achieve the largest spatial extent of complete inversion throughout the brain.

Applying a positive Δf_IR restored inversion efficiency in the inferior frontal and temporal lobes, but also introduced undesired hyperintensities in the anterior temporal lobes. Widening the bandwidth alone could also partially reduce hyperintensities in the frontal area but with a limited efficiency. By simultaneously applying a positive Δf_IR of 300 Hz and shortening Δt_IR by 40%, these artefacts were eliminated across the whole cerebrum.

A robust elimination of susceptibility induced hyperintensities near air-tissue boundaries in T1w MPRAGE 7 T brain images is demonstrated. This technique only requires limited MR sequence modifications.