Susceptibility-resistant variable-flip-angle turbo spin echo imaging for reliable estimation of cortical thickness: a feasibility study

The thickness of the human cerebral cortex, which provides valuable information in the studies of normal and abnormal neuroanatomy, is commonly estimated using high-resolution, volumetric magnetization-prepared rapid gradient echo (MP-RAGE) magnetic resonance imaging due to its strong T₁-weighted contrast and high signal-to-noise ratio. However, the accuracy of cortical thickness estimates using MP-RAGE is potentially contaminated by susceptibility-induced signal loss particularly at regions in close proximity to air-filled cavities. The purpose of this work is to investigate the feasibility of susceptibility-resistant variable-flip-angle (VFA) three-dimensional turbo/fast spin echo imaging for reliable estimation of cortical thickness of the human brain, wherein 1) radio-frequency (RF) pulse refocuses susceptibility-induced spin de-phasing, 2) the VFA refocusing pulse train is applied for a tissue-specific prescribed signal evolution along the echo train, 3) the desired T₁-weighted contrast is achieved by composite restore pulses at the end of the refocusing pulse train, and 4) blood signals are suppressed using the VFA scheme combined with increasing moments of flow-sensitizing gradients while dura mater signals are attenuated due to short T₂ relaxation time, which alleviates potential failure in brain segmentation. Numerical simulations of the Bloch equation are performed in both MP-RAGE and the proposed method for comparison. In vivo studies are performed in 14 healthy volunteers at 3 T. Image processing is then performed using the Freesurfer, resulting in mean and standard deviations of cortical thickness for the entire cortical surfaces. Statistical analysis demonstrates that particularly in the inferior prefrontal and temporal regions heavily affected by susceptibility-induced signal loss conventional MP-RAGE, if compared with the proposed method, significantly under-estimates cortical thickness. It is expected that the proposed pulse sequence, which is resistant to susceptibility-induced signal loss and attenuates the signal intensity of blood and dura mater, can be a potentially promising alternative to conventional MP-RAGE in reliably estimating cortical thickness for the entire brain.