Development, calibration, and testing of 3D amplified MRI (aMRI) for the quantification of intrinsic brain motion

Abstract

Microvascular blood volume pulsations, combined with CSF circulation result in subtle deformation of the brain during each heartbeat. To visualize and quantify these small deformations, an image processing technique called amplified MRI (aMRI) was recently introduced. aMRI, however, is unable to visualize the 3-directional deformation of the human brain, which is caused by the physiological flow and its biomechanical coupling with the brain. Addressing this issue, we extended 2D aMRI to 3D, which allows visualization of the subtle motion in 3-directions. First, we validated 3D aMRI’s ability to measure out-of-plane motion while simultaneously increasing SNR in digital phantoms mimicking the brain’s deformation. We then applied 2D and 3D aMRI to 3D cine MRI of 6 healthy subjects and found approximately 80% higher temporal SNR in the 3D aMRI outputs with SNR $=26.8\pm 8.3$ compared to the 2D aMRI with SNR $=15.1\pm 2.6$ ($p<0.01$). 3D displacement maps and their dominant modeshapes were extracted, which demonstrated physiologically meaningful patterns of motion in response to heart pulsatility and CSF circulation. We observed the peak superior-inferior displacement near the pons and midbrain. Peak medial-lateral and anterior-posterior displacement were observed close to the $3{}^{rd}$ and lateral ventricles. Interestingly, the modeshapes showed an almost symmetrical expansion of the brain with $33\%\pm 4\%,$ $38\%\pm 4\%,$ and $29\%\pm 7\%$ of the deformation being predominantly towards superior-inferior, anterior-posterior, and medial-lateral, respectively ($p<0.01$). These preliminary results hint at 3D aMRI’s versatility and translatability for providing novel biomechanical imaging markers, which could simplify diagnostics and enable a deeper understanding of the biomechanics of a wide-range of pathophysiological conditions.

Statement of significance

The brain has very soft material properties and is under constant deformation as a result of physiological flow and its biomechanical coupling with the tissue. In this work, a novel image processing algorithm called 3D aMRI is introduced which allows visualization and quantification of this very subtle motion. After validation of the algorithm using digital phantom models, 3D aMRI was applied to in vivo 3D cine MRI data. This allowed measurement of the brain’s subtle deformation as a response of heart pulsatility and CSF circulation, which might hold essential information regarding the physiological state of the brain tissue. 3D aMRI is a post-processing algorithm carried out on standard imaging data. We believe that due to its versatility and translatability, it could simplify diagnostics and enable a deeper understanding of the biomechanics of a wide range of pathophysiological conditions.