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Magnetic Resonance Elastography
MRI scanners can be used to image mechanical waves propagating through tissue such as the lungs. In this section, we will discuss how MRE relates to the Audible Human Project®.
Encode Wave Propagation
When the hydrogen protons of organic tissue are placed in a high magnetic field their nuclear magnetic spins align with the field. This produces a resonant state of oscillation about the main magnetic field of frequency f (Figure 1).
A radio frequency signal of the same frequency as spins in resonance is then used to tip the spin over (Figure 2). Tipping the spin over puts it into an unstable position, it wants to return to the same state as in Figure 1. Before this happens other magnetic fields called gradients can be used to change the frequency and the phase of the protons (Figure 3).
It is the difference in phase that allows a mechanical wave to be imaged. Notice in Figure 3 that the phase accumulated in each proton depends on its position in the magnetic field. So when a shear wave propagates through the material it displaces the protons in the direction of the magnetic field Figure 4.
MRE Ex Vivo Rat Lung Insonification
Airway insonification can be performed using the MRE technique mentioned above. The experimental setup uses an excised rat lung. The lung is intubated with a 24G catheter which is connected to a compression driver by plastic tubing (Figure 5). Using MRE has the added advantage of being able to measure all of the wave displacement motion inside of the lung and not just on the surface. Based on the wavelength and attenuation of the displacement .
Using MRE has the added advantage of being able to measure all of the wave displacement motion inside of the lung and not just on the surface (Figures 6 and 7). The shear modulus can be derived from the wavelength and attenuation of the particle displacement.
Figure 6 : Insonification of ex vivo rat lung at 500 Hz showing shear wave displacement as accumulated phase, measured in radians. From left to right are the 3 Cartesian displacement directions, with the green arrows showing the direction of displacement. For the z-direction the displacement is in and out of plane. Near A is the faint outline of the trachea this is also the source location of acoustic waves. Location B is where the compression waves undergo mode conversion to shear waves. The shear waves then follow the arrows to the left and right lobes. The dashed ellipse outlines the location of the heart.
Figure 7 : Three dimensional images from an isometric view point. On the left is the shear wave 3D image with particle displacement in the Z direction at 500 Hz. On the right is the 3D anatomy image with the main bronchial path highlighted by the dashed line.
MRE and the Audible Human Project®
Magnetic resonance elastography (MRE) adds another dimension to the Audible Human Project® (AHP) using subsurface measurements of vibratory (acoustic) waves. MRE can also see waves propagating in all three dimensions, which can enable a more accurate estimation of tissue stiffness, since the planer wave assumption no longer needs to be made. Rat animal models will be used to refine the airway insonification MRE technique and to understand the complex interactions of vibratory waves in the porous material of the lung. Healthy rat lung models can then be compared to pathological lungs to examine key differences. MRE will also help to validate AHP computer models in the future.