The systolic increase in blood pressure over the cardiac cycle causes the regular variations in blood flow into and throughout the brain. Because the brain is enclosed in a rigid container, the arterial pulsations are in turn transferred into brain tissue and CSF. The intracranial pulsatility plays an important role in various cerebral pathologies, which is usually assessed by the flow velocity curve of CSF using Transcranial Doppler Ultrasound (TCD) or Phase-Contrast MRI (PC-MRI). The biomechanical properties of brain tissue are often altered by the pathological changes such as traumatic brain injury (TBI) and brain tumor. Previous studies proposed displacement-encoded MRI methods to measure the pulsatile brain motion^1-3.However, there are some drawbacks including high sensitivity to B₀ inhomogeneities, long acquisition time, and low spatial resolution. In the current study, we propose a new MRI method to assess the mechanical behavior of the brain by acquiring a series of 3D high-resolution whole brain volumes across a cardiac cycle.

Image acquisition: An SSFP-FID sequence was introduced recently by applying a dephasing gradient following the readout gradient with a gradient area that results in 2π dephasing across a single pixel⁴. This technique, herein referred to as integrated-SSFP or iSSFP, allows for removing banding artifacts while maintaining the unique bSSFP tissue contrast as well as the high SNR efficiency by averaging the bSSFP signal profile (Figure 1). An ECG-gated cine 3D iSSFP sequence was used for data acquisition. The sequence scheme is illustrated in Figure 1. The imaging parameters included: FOV= 220x165mm², voxel size=1x1x2mm³, TR=4.8ms, TE=2.4ms, flip angle=35˚, acceleration factor=2, 80 slices with slice thickness of 2mm. 6 to 9 brain volumes were collected over cardiac cycle with a temporal resolution of 105ms, within a total scan time of approximate 6 to 8 min. A total of 5 healthy volunteers were scanned on Siemens Prisma 3T scanner.

Data analysis: To estimate the voxel-wise volumetric deformation in the brain, the acquired brain volumes across the cardiac cycle were warped onto the last one at late diastole for each subject using Advanced Normalization Tools (ANTs). The warping parameters were transformed to Jacobian determinants. A Jacobian determinant of 1 means no shape change. After subtracting 1, the negative Jacobian values reflect contraction and positive values expansion. The Jacobian map was created for each volume. The deformation during cardiac cycle was calculated by the difference between the maximal and minimal Jacobian determinant (Max-Min) across the cardiac cycle at each voxel, which indicated the dynamic range of the deformation per voxel during cardiac cycle. The deformation maps were normalized to MNI space, and the mean deformation was extracted from cortical grey matter, white matter, ventricular CSF, basal ganglia and brain stem.

Figure 2 displays the mean spatial distribution of the deformation in brain from the 5 subjects in coronal (a), sagittal (b) and axial (c) views. A MNI T1 template at the same image position is shown in Figure 1d as a reference. One can visualize greater deformation in the basal ganglia and brain stem compared to the rest brain regions. Figure 3 shows the mean deformation from the 5 subjects in different brain components. The deformation in basal ganglia and brain stem were significant higher than those in CSF, grey and white matter (p<0.05). The smallest deformation was found in grey matter. These findings suggest the major brain deformation occurs in basal ganglia and brain stem, which might be caused by the pulsatile arterial flow around the Circle of Willis and CSF bulk flow in ventricles. The mechanical pressure then propagates to surrounding white matter and finally reaches brain cortex. As shown in Figure 4, the deformation map is overlaid onto structural MRI through the cerebellum and brain stem of a representative subject. It can be clearly seen that high deformation occurs in internal carotid arteries and the 4th ventricles. To address the potential motion effects on Jacobian calculation, the Jacobian maps were calculated by choosing different volumes as reference image. Highly consistent patterns of Jacobian over time were found, suggesting our measurement of brain deformation across cardiac cycle is reliable.Using a novel time-resolved high-resolution volumetric imaging technique in conjunction with brain deformation analysis using Jacobian determinants, our study revealed that greater deformation occurs in the brain stem and basal ganglia during cardiac cycle, which attenuates toward the white matter and brain cortex. This finding is in good agreement with the previous study³. This technique may be applied to exam changes of brain’s mechanical properties in TBI, brain tumor and Alzheimer’s disease.