Ultrasonic Imaging of Traumatic Brain Injury (TBI)

The years 2010 to 2020 will be seen as the decade of scientific imaging. After all, chemistry and much of physics are about structure–activity relationships (SARs). Super-resolution and X-ray lasers have greatly increased the number of molecular images.

However, images can be difficult to obtain because of the nature of the sample, such as gelatinous mass inside a hard shell, or the brain inside the skull. Veterans, boxers, and football players all seem to have similar problems with postimpact damage to the brain. Hence the question: What precisely happens in the brain during impact events?

A paper by David Espindola and colleagues at the University of North Carolina, Chapel Hill (Espinodola, D.; Lee, S. et al. Shear shock waves are observed in the brain. Phys. Review Appl. 2017, 8, 044024), described rapid ultrasound imaging of porcine brains to measure the deformation of the brain related to traumatic brain injury (TBI). The skull obscures direct imaging; plus, it can reflect and focus shock waves. MRI is not fast enough to follow the physics of impact.

The authors describe a focused high-frame-rate ultrasound imager that can measure the near discontinuous velocity profile at shock front deep within the brain. This imager shows that a smooth excitation at the brain surface builds into a shock front as the pulse propagates. Gradients at the shock front increase acceleration, strain, and strain rate in soft tissue by as much as 8.5 times. The authors believe this is the primary mechanism for a broad range of TBIs.

The ultrasound source is coupled via gelatin to the brain specimen. The shaker produces an initial shock along the z-axis. Wave propagation is imaged with ultrasound (Figure 1) along the x-axis with an ultrasound scanner from Versasonics (Kirkland, WA). The effective frame rate is 6200 frames/sec, which are compiled into a movie showing the brain and movement induced by the impact. The authors point out that particle acceleration is more significant than particle velocity. Shear accelerations can be in the range of 8–9 times larger than the original impulse. This is sufficient to explain the small lesions (1–15 mm) that are observed deep in the brain following TBI.

Figure 1 – Apparatus for ultrasonic imaging TBI of porcine brains. The shaker produces the impact along the z-axis, which is delivered to the sample. The ultrasound imager is also on the arm. Gelatin provides the transparent interconnection between the target brain and the ultrasound transducer. The ultrasound image (Figure 2) is recorded at a frame rate as high as 6500/sec. Highest-resolution images are obtained at a focal point of 60 mm, which is deep within the specimen. A typical time slice is shown as the output in the figure.
Figure 2 – Image of the particle velocity in the x (shear) direction as a function in time after the z-axis impulse in Figure 1. The high-speed image shows the propagation of the impulse moving through the tissue. The waveforms are recorded on the right.

But what about other critical body organs such as the pancreas, liver, and glands? Could they also be affected by trauma? The authors are silent on this. The high-frame-rate ultrasonic image might be useful in characterizing trauma-induced diseases. The imager neglects the effect of the skull’s shape, although the simulations predict spherical geometry can focus shear shocks deep within the brain.

Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; email: [email protected]

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