Improved measurement of brain deformation during mild head acceleration using a novel tagged MRI sequence

Abstract

In vivo measurements of human brain deformation during mild acceleration are needed to help validate computational models of traumatic brain injury and to understand the factors that govern the mechanical response of the brain. Tagged magnetic resonance imaging is a powerful, noninvasive technique to track tissue motion in vivo which has been used to quantify brain deformation in live human subjects. However, these prior studies required from 72 to 144 head rotations to generate deformation data for a single image slice, precluding its use to investigate the entire brain in a single subject. Here, a novel method is introduced that significantly reduces temporal variability in the acquisition and improves the accuracy of displacement estimates. Optimization of the acquisition parameters in a gelatin phantom and three human subjects leads to a reduction in the number of rotations from 72 to 144 to as few as 8 for a single image slice. The ability to estimate accurate, well-resolved, fields of displacement and strain in far fewer repetitions will enable comprehensive studies of acceleration-induced deformation throughout the human brain in vivo.

Keywords: Acceleration; Deformation; Magnetic resonance imaging (MRI); Strain; Traumatic brain injury (TBI).

Figure 1

Photograph of the head rotation device and schematic of head at rest and stop positions :A mild angular acceleration was generated using an MRI-compatible head rotation device.(a,c) At rest, the subject lies in the supine position with his or her head facing straight ahead. The subject initiates the motion by releasing a latch, which allows for free rotation about the inferior(I)/superior(S) axis towards his or her left shoulder. (b,d) After a 32° rotation, the device encounters a stop, which generates a mild angular acceleration. The subject is instructed to pause for 1-2 seconds at the stop. The subject then rotates his or her head back to the rest position, and the motion is repeated when the subject is ready.

Figure 2

Angular position θ versus time for 30 rotations measured using an MRI-compatible angular position sensor. (a) In the gelatin phantom, the motion is highly repeatable whether the rotations are aligned at rest (0° rotation) or after a 29° rotation. (b) Rotations in live human subjects are more variable. Temporal variations can lead to image distortions. If the rotations are aligned at rest, then the standard deviation of the time to peak angular acceleration is 18.4 ms. Aligning the rotations after 29° rotation reduced the standard deviation at peak acceleration to 2.8 ms.

Figure 3

Schematic diagram comparing the timing for the standard tagging approach and proposed double trigger approach (one rotation is shown). The subject initiates the motion by releasing a latch on the head rotation device. Assisted by a counterweight, the subject rotates his or her head towards the left until the head support encounters a fixed stop at 32°. The sequence is triggered when the subject initiates the motion, and the tag lines are then applied. For the standard approach, the cine image acquisition begins immediately after the application of tag lines. For the proposed approach, the cine acquisition begins after the subject rotates through a preset threshold of 29°. An MRI-compatible angular position sensor generates the trigger when this threshold is reached.

Figure 4

(a) Tagged MR image from subject 1, slice 1 acquired using the standard approach. Breaks in the tag line (inside the red ellipse) can occur due to highly variable motion over time. When this occurs, motion tracking will not produce reliable results. (b) Tagged MRI acquired using the modified double trigger approach in subject 1, slice 1 at the time corresponding to peak deformation. The tag lines within the brain are continuous and can be used to accurately track motion and compute strain fields over time.