Medical Physics: Diffusion Tensor Imaging

Quantum mechanics and electrodynamics are used in more than just theoretical physics; applications for these theory-intense fields show up in, among other things, modern medical technology. The above picture is a tractographic reconstruction of neural connections in a human brain using a technique called diffusion tensor imaging, or DTI. Some vector analysis – as this blog has described before, here and here – allows calculation of the average flow of water molecules at various places in the brain. All of these calculations require knowledge about quantum electrodynamics to properly interpret the magnetic resonance information from an MRI scan. Using DTI also means solving the diffusion equation, an equation prevalent in many sciences.

This average flow is described by a diffusion tensor. If the diffusion tensor is highly isotropic – that is, if it has no preferred direction – then DTI doesn’t tell us much. But when water is constrained between folds and connections in the brain – especially in white matter – it has an anisotropic diffusion tensor, indicating that the water does have a preferred direction: namely, along the physical channels allowed by the neural structure. How is this useful? Some intelligent mapping can turn this information about water diffusion into a map of the brain’s channels and connections. Data showing how the water flows can be transformed into information about what the water is flowing around… and that information is what’s represented in the image above: neural connections in the brain’s white matter.

Once information about the neural connections in the brain has been calculated, it can be used to construct a 3D model that can be rotated, cut open, and drawn on using a computer program. This is done using technology similar to that of video games, but the information contained by the brain model can be very important to medical physicists, neuroscientists, and neurosurgeons. If a part of the brain needs to be removed (a tumor, for example), an MRI using DTI can provide critical information to the surgeon. Using the DTI-generated 3D brain model could show the doctor what parts of the brain use neurons near the tumor; knowing that surgery has a risk of damage to a language center, for instance, could drastically alter the doctor’s strategy for treatment. DTI can also be used for directly identifying white matter lesions that do not show up in most types of MRI, making it an important tool for physicians.

Refrigerated Electron Beam Ion Trap (REBIT)

An ion trap is an experimental physics device that captures ions for use in condensed matter experiments. A common design, pictured above, uses extremely strong magnetic fields (on the order of teslas) to accelerate a central electron beam. When a gas is released into the chamber, particles near the beam have their outer electrons ripped off by the magnetic field. The stronger the magnetic field, the more you can ionize your particles.

Refrigerated EBITs can allow the magnetic field to become absurdly strong due to superconducting effects in the magnetic coils. These devices are capable of producing highly charged ions (HCIs). A REBIT can take xenon gas, for example, and give back Xe34+. That’s xenon – a noble gas that loves its electrons – with 34 of its 54 usual electrons ripped off! Particles like this are extremely energetic, and surface physics experiments often investigate the interaction of these particles with metallic surfaces. These very angry HCIs can create microscopic craters in a previously clean surface.

Besides surface physics, HCIs are found in several astrophysical systems. This makes REBIT facilities one of the rare places where scientists can perform experimental astrophysics, by generating and experimenting with high charged plasmas in the lab. Highly charged ions can be found in powerful cosmic phenomena like stellar coronae and accretion disks in quasars.