The brain exhibits cardiac-synchronized pulsatile motion, where brain motion is caused by transmission of arterial pulse into the cerebrovascular system and cerebrospinal fluid (CSF)^1-4. Displacement encoding with stimulated echoes (DENSE) is a motion imaging technique that was originally developed for assessing myocardial mechanics. DENSE encodes pixel-wise tissue displacements into the phase of the stimulated echoes^5,6. Compared to other techniques like phase contrast^1,3 and tagging⁷, DENSE offers many advantages including high motion sensitivity and high spatial resolution, that enable it measure brain motion as low as 0.01 mm^8,9.

Multiple factors may influence brain motion, including subject orientation, i.e. in the supine or prone position. The frontal lobe is in the anterior (upward) position when the subject is in the supine position, while the occipital lobe is in the posterior (upward) position when in the prone position. Considering the effects of gravity, the regions in the upward position may have more freedom to move in response to CSF pulsation and therefore undergo larger motion. To our knowledge, no previous studies investigated this effect. The purpose of this study was to use high motion sensitivity DENSE to investigate the influence of subject orientation (prone versus supine) on regional brain motion, and we hypothesized that the brain regions that were in the upward position would have increased motion.

Nine healthy volunteers (33.7 ± 11.0 years, two females) were scanned on a 3T scanner with the head and neck coils (Tim Trio, Siemens, Erlangen, Germany) after obtaining informed consent in accordance with protocols approved by our IRB.

Each subject was first positioned supine in the head coil using simple immobilization buffers around the head. After the localization, a mid-sagittal slice through the cervical cord and brain stem was imaged with a peripheral pulse-gated, segmented EPI, cine DENSE sequence. Image parameters included displacement encoding frequency k_e = 1.5 cycle/mm, through-plane dephasing frequency k_d = 0.08 cycle/mm, TE = 8.9-10.4 ms, TR = 55-59 ms, EPI factor = 8, segments = 16, pixel size = 1.2 × 1.2 mm², slice thickness = 7 mm, averages = 4, frames = 13-16 (depending on the pulse duration). Images were acquired to measure motion in two directions, foot-to-head and anterior-to-posterior. The subject was then removed from the scanner, positioned prone and scanned using the same protocol. The same imaging slice location was prescribed carefully.

The DENSE images were reconstructed inline¹⁰, and then exported to offline to process using ImageJ (National Institute of Health, Bethesda, MD, USA). Briefly, the phase-reconstructed images were divided by 2πke and both displacement direction measurements were used to create 2D displacement maps. Two regions of interest (ROIs) were placed in the frontal and occipital lobes, respectively. The ROIs on the supine and prone data were manually registered to ensure they were measuring the same locations. The mean 2D displacement values in the ROIs over each frame were recorded for all 9 volunteers, and the displacement-versus-time data was fitted using a 3rd order polynomial in Matlab (The Mathworks, Natick, MA, USA) to compensate for varying temporal resolutions in the acquired data. An analysis of variance (ANOVA) was performed using R v3.2 (R Core Team, Vienna, Austria) to determine the effects on the peak displacement. The model included both the orientation (supine versus prone) and the brain region (frontal versus occipital) as main effects, as well as their interaction.

Example DENSE magnitude-reconstructed images with the ROIs, and 2D displacement maps in the supine and prone positions are shown in Fig. 1. The different displacement levels of the frontal and occipital lobes in the supine and prone positions can be observed.

The displacement-versus-time curves from 9 volunteers are shown in Fig. 2. The peak displacement averaged across the 9 volunteers for each ROI is shown in Fig. 3. The ANOVA found that the overall model was significant (p = 0.045). Interaction between position (prone/supine) and region (frontal/occipital) was significant (p = 0.0113), indicating that being in the upward position led to a significant increase in displacement. In other words, displacements were significantly increased from prone to supine in the frontal lobe, and from supine to prone in the occipital lobe, respectively. These are consistent with our observations seen in both Fig. 2 and Fig. 3.

The DENSE technique was utilized to investigate the influence of subject orientation on regional brain motion. Preliminary results in 9 volunteers demonstrated that brain regions (frontal and occipital lobes) had a significant increase in displacement in the upward position. DENSE enables us to investigate brain motion to a level of detail that has not been previously possible.