  Machine learning head models (MLHMs) are developed to estimate brain
deformation for early detection of traumatic brain injury (TBI). However, the
overfitting to simulated impacts and the lack of generalizability caused by
distributional shift of different head impact datasets hinders the broad
clinical applications of current MLHMs. We propose brain deformation estimators
that integrates unsupervised domain adaptation with a deep neural network to
predict whole-brain maximum principal strain (MPS) and MPS rate (MPSR). With
12,780 simulated head impacts, we performed unsupervised domain adaptation on
on-field head impacts from 302 college football (CF) impacts and 457 mixed
martial arts (MMA) impacts using domain regularized component analysis (DRCA)
and cycle-GAN-based methods. The new model improved the MPS/MPSR estimation
accuracy, with the DRCA method significantly outperforming other domain
adaptation methods in prediction accuracy (p<0.001): MPS RMSE: 0.027 (CF) and
0.037 (MMA); MPSR RMSE: 7.159 (CF) and 13.022 (MMA). On another two hold-out
test sets with 195 college football impacts and 260 boxing impacts, the DRCA
model significantly outperformed the baseline model without domain adaptation
in MPS and MPSR estimation accuracy (p<0.001). The DRCA domain adaptation
reduces the MPS/MPSR estimation error to be well below TBI thresholds, enabling
accurate brain deformation estimation to detect TBI in future clinical
applications.
