A Road Trip Down Memory Lane: Mapping the Brain’s Pathways With Diffusion MRI

What Do Neuroscientists Study?

Have you ever heard the expression “take a trip down memory lane”, referring to someone remembering something that happened in the past? Did you know that the human brain actually has something that could be called “memory lanes”? These are pathways between brain areas that are important for remembering things. Scientists who study the brain, called neuroscientists, use a technology that can create colorful images of these pathways! In this article, we will explain the machine used to make these images, how it works, and why this is important in medicine—for example, for studying people with memory problems.

What Are the Pathways of the Brain?

You can see a colorful representation of the paths within the brain in Figure 1. What are these pathways? The cells that do all the work in the brain (called neurons) are connected with each other through tiny wires, called axons. The cells send electrical signals to each other through these wires. When many axons come together, they form the bigger pathways that you can see in the figure. These pathways are very important for keeping the brain healthy and working properly. They keep growing from when a person is a baby all the way to adulthood. The pathways also change in different ways when people get certain diseases that make it difficult for the brain to do its complicated work. If scientists can see how a disease changes the brain’s pathways, they can understand the disease better and learn something about how to help people with brain diseases.

Figure 1 - An image of a brain made with an MRI machine. The colorful lines are the brain pathways that connect different areas so that they can communicate with each other. At each point in the image, a color shows the direction of pathways at this point: here, blue means the pathway is going top to bottom, green means left to right, and red means the pathway is moving out of the image.

In the past, scientists could only study these pathways in people who had died, because the brain needed to be opened to look inside with a microscope, for example. But now they can investigate the pathways in living people, without touching the brain. To do so, scientists and doctors can use a powerful technology to take pictures of the brain and to study people who have a brain disease or injury.

How Does an MRI Machine Work?

The technology that scientists use to make images of pathways in the brain is called magnetic resonance imaging, or MRI. You will see soon why this name makes sense. Perhaps you have seen an MRI machine before or even gotten an MRI scan yourself.

Looking from the outside, an MRI machine is made up of a bed and a tunnel (Figure 2). Patients lie comfortably on the bed, and when experts are ready to start looking at the patient’s brain, the bed slides inside the tunnel. What is hidden inside of the MRI machine? Mainly, a giant magnet that attracts metals and other magnets!

Figure 2 - An MRI machine contains a giant magnet that cannot be seen from the outside. The greenish arrow shows where the magnet is located. A patient lies on the bed, which slides into the tunnel of the machine. Then, experts take an image of the patient’s brain or other body parts.

But how is MRI useful if there are no metals or magnets in our bodies—or are there? Surprisingly, the answer is YES! Our bodies are made mainly of water, and water is made of particles that are so tiny we cannot see them with our eyes. These tiny particles behave like magnets. But do not worry, this does not mean that you will stick to the fridge or start attracting spoons! The particles in our bodies are too small to have any effect like that. But when people lie inside an MRI scanner and are surrounded by its giant magnet, the tiny magnets in their bodies react to it. This is called resonance. This does not hurt—you cannot feel it and it is not dangerous.

Inside the MRI machine, the tiny magnets in our bodies send out signals (in the form of electromagnetic waves) that tell the scanner exactly where they are. They also tell the scanner what kind of tissue they are located in, for example, blood, fat, or water. This is how an MRI image of a body part, like the brain, can be created. The image looks like a black and white picture. In short, an MRI machine uses magnets to create resonance in water particles inside our bodies, and the water particles help it create an image:

That is why the name is magnetic resonance imaging. It is an incredible way to take a picture of the brain without touching or seeing it!

But how do scientists get from a black-and-white brain image to a colorful one showing pathways, as in Figure 1? For this, you need to look at an example that will seem at first like it has nothing to do with brains or images.

How Does Diffusion MRI Work?

Imagine looking down from a window in your home: you can see the streets around your house, with bicycles rushing along them (Figure 3A). But if you look outside when it is dark, you will not be able to see the streets in the darkness. However, you can still see the bicycles because they have lights! If a street is really busy, seeing lots of lights dotting it can help you see where that street must be going, because you know that bicycles cannot go anywhere else and cannot move through the walls and into houses. So, wherever you see light, there must be a street (Figure 3B).

Figure 3 - (A) A street scene seen from above during the day. The blue dots are bicycles on the streets. There are no bicycles around the construction site, and many bicycles around the toy shop. (B) The same street seen by night. Now you cannot see the street, but you can see the lights from the bicycles, so you know where the street is.

A very busy street will have more light, and a street that is damaged and closed for construction will have no light. This example shows how you can learn something about an invisible structure by following something different that you can see. Scientists use a technique that works in a similar way to explore paths in the brain, just like bicycle lights helped you explore streets in the example. This technique is a special kind of MRI called diffusion MRI [1].

But there are no bicycles in the brain, so what do scientists follow instead? They follow the water particles discussed earlier! Water particles in our bodies are not still—they are in fact quite busy. They are constantly moving around on “random walks”. The random walks of a big group of water particles is called diffusion, and this is where the name diffusion MRI comes from.

However, just like a bicycle, a water particle in the brain can only move along available paths and cannot move through “walls”. So, if scientists follow the water particles in the brain and trace where they are going, they can see where the paths in the brain must be! Scientists use computer programmes to give the paths different colors depending on their direction, so they can see where in the brain the paths are coming from and where they are going. So, diffusion MRI allows scientists to see where these paths are located, which brain areas they are connecting, and how strong these connections are. The mathematical technique that allows scientists to trace the brain’s pathways is called tractography [2]. Diffusion MRI can also provide information about some properties of the pathways, for example whether there are many or few small wires in one bigger pathway [3].

How Does Diffusion MRI Help People With Brain Diseases?

How do scientists and doctors use diffusion MRI to study people with brain diseases [4]? Do you know any brain diseases? One example is stroke, which happens when a brain area suddenly stops receiving blood because a blood vessel becomes blocked. Lack of blood flow can hurt the brain and cause problems with thinking and function. It is important for doctors to understand what happened so they can help the patient. Diffusion MRI can show doctors the problem in the brain very precisely.

Doctors also use diffusion MRI to plan brain surgeries. It shows them where important pathways are located so they can be careful not to damage those pathways during surgery. In the past this was impossible, and sometimes patients lost certain abilities after brain surgery when some pathways were damaged. But luckily, diffusion MRI can help avoid this!

What is Next for Diffusion MRI?

As you have seen, there is a lot going on in the world of diffusion MRI [5]! Scientists are constantly working to make this technology better, faster, and more accurate. The aim is to make diffusion MRI more and more like a microscope, so that it can show the tiniest brain structures correctly and tell scientists and doctors what these structures are made of.

Scientists are also trying to make diffusion MRI cheaper and easier, so that it can be used in countries with little access to advanced technologies, and so that people all over the world can benefit from it. Moreover, scientists are using diffusion MRI to learn more about diseases that they do not understand yet, such as dementia, which is a problem with memory and thinking. Scientists take diffusion MRI scans of dementia patients in memory clinics and look closely at the “memory lanes” in the brain—the pathways involved in memory and thinking. They hope to learn how these memory lanes are different between patients with dementia and healthy people.

We hope that you have enjoyed our little road trip through the brain!

Glossary

Neuroscientists: ↑ The people who study the brain and its structure, function, development, and diseases. They have studied biology, medicine, physics, psychology, mathematics or similar subjects in university and work in laboratories.

Magnetic Resonance Imaging: ↑ A technology that is used in clinics and research to create detailed images of body parts. The MRI machine relies on the magnetic properties of the body and complex engineering.

Diffusion MRI: ↑ A type of MRI that creates images showing how water molecules move in the body: how freely and in which directions they move. This requires complex mathematical processing of images.

Diffusion: ↑ The random movement of water molecules, for example in the brain. It depends on the temperature and on the surroundings of the water molecules.

Tractography: ↑ A method of mathematically analyzing diffusion MRI images. It results in images showing the location of pathways in the brain. This can be used by doctors or scientists.

Dementia: ↑ A disease of the brain which occurs mostly in older people. It can lead to problems with memory, concentration, speaking and understanding speech, and other areas of life.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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[1] ↑ Le Bihan, D. 2003. Looking into the functional architecture of the brain with diffusion MRI. Nat. Rev. Neurosci. 4:469–80. doi: 10.1038/nrn1119

[2] ↑ Jeurissen, B., Descoteaux, M., Mori, S., and Leemans, A. 2019. Diffusion MRI fiber tractography of the brain. NMR Biomed. 32:e3785. doi: 10.1002/nbm.3785

[3] ↑ Le Bihan, D. 2013. Apparent diffusion coefficient and beyond: what diffusion MR imaging can tell us about tissue structure. Radiology 268:318–22. doi: 10.1148/radiol.13130420

[4] ↑ Schaefer, P. W. 2001. Applications of DWI in clinical neurology. J. Neurol. Sci. 186:S25–35. doi: 10.1016/s0022-510x(01)00488-9

[5] ↑ Bihan, D. L. 2024. From Brownian motion to virtual biopsy: a historical perspective from 40 years of diffusion MRI. Jpn. J. Radiol. 42:1357–71. doi: 10.1007/s11604-024-01642-z