Above: Image © Afiller, Wikimedia Commons
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MAGNETIC RESONANCE IMAGING TECHNOLOGY

One of the most important types of medical imaging technologies is Magnetic Resonance Imaging (MRI). MRI is one of the key ways we see details of tissues inside the body. Unlike Computed Tomography (CT) scans, MRIs can show both how living tissues look as well as how they function. Since the 1990s this technology has increased our understanding of how the brain works more than all the information we had collected in the previous 100 years!

HISTORY

MRI is based on a physics discovery from the 1930s called nuclear magnetic resonance (NMR). Felix Bloch, from Stanford University, and Edward Purcell, from Harvard University, discovered that the interaction between magnetic fields and radio waves caused atoms to give off tiny radio signals. These radio signals could then be detected to form an image. This discovery helped scientists to better understand internal structures of objects without having to take them apart and destroy them, known as non-destructive testing.

The first one-dimensional MRI image was made in 1952. Then, in 1974 Paul Lauterbur created the first sectional images of a mouse. Peter Mansfield in 1972 worked on the mathematics that allowed for clearer images and scanning that would take seconds rather than hours. In 2003, the Nobel Prize in Physiology or Medicine went to Lauterbur and Mansfield for their contributions to MRI technology 30 years earlier.

HOW IT WORKS

Colour-enhanced MRI image of the brain
Figure 1: Colour-enhanced MRI image of the brain, optic nerves and eyes. This patient has a tumour behind his/her right eye (orange colour). Source: Nevit Dilmen, Wikimedia Commons

MRI scanners use strong magnetic fields and radio waves to form images of the body. First, powerful donut-shaped magnets line up the nuclei of hydrogen atoms in the object being scanned. Next, short pulses of radio signals cause the nuclei to return to their original positions. When this happens, the nuclei emit weak radio signals that are detected by receiver coils and analyzed by computers. The computer then converts these signals into an image. MRI images capture a lot of detail and can be colour-enhanced so that the various parts stand out even more (see Figure 1).

USES

MRI scans are mainly used to detect structural problems, such as tumours and blood clots. They can also detect damaged areas, such as those caused by accidents or disease. One unique type of MRI is called a Functional MRI (fMRI). An fMRI measures changes in blood flow within the brain. This technique allows doctors to visualize the living brain and observe changes to the brain as it undergoes different functions, and has resulted in many of the major discoveries made during the past 100 years!

HOW TO READ AN MRI IMAGE

Being able to read the images generated from MRI scans is extremely important, but can be quite difficult to do. There are different types of images and choosing the right image to use depends on the reason for needing the scan. The images most commonly used are the ‘T1-weighted’ and ‘T2-weighted’ images.

Differences in image composition between T1-weighted and T2-weighted MRI images
Figure 2: Differences in image composition between T1-weighted and T2-weighted MRI images. A) T1- weighted image of healthy teenage brain. B) T2-weighted image of teenage brain showing damaged areas (red circles). This type of damage is often observed with Alzheimer’s disease. Sources: (A) (B)

T1-weighted images are usually used when looking at details of brain anatomy. This is because the white matter brain tissue stands out against the dark matter brain tissue and the dark background (see Figure 2A). T2-weighted images are often used when trying to identify disease and mental illness. This is because abnormal growths, damaged tissue, etc. tend to have a greater amount of fluid and will show up as white on the scan (see Figure 2B). However, it is also possible to identify irregularities in the brain using T1-weighted scans.

T1-weighted MRI scans of three brains
Figure 3: T1-weighted MRI scans of A) a healthy teenage brain B) a healthy adult brain, and C) a damaged (red circles) adult brain. Notice that there are structural differences between a teenage and adult brain, however the damage is quite obvious in the diseased brain. Sources: OASIS Brain database: (A) (B) (C)

Figure 3: T1-weighted MRI scans of A) a healthy teenage brain B) a healthy adult brain, and C) a damaged (red circles) adult brain. Notice that there are structural differences between a teenage and adult brain, however the damage is quite obvious in the diseased brain.

To diagnose unhealthy brain tissue it is first important to know what a healthy brain looks like (see Figure 3, A and B). Unhealthy brain tissue associated with disease and mental illness often can be observed as irregular features on an MRI brain scan (see Figure 3C).

Colour-enhanced MRI image of the brain
Figure 4: Image of an MRI machine. Source: Jan Ainali, Wikimedia Commons

BENEFITS & RISKS

The greatest benefits of MRI machines are that they do not use x-ray radiation, which can be damaging to tissues and cause cancer. MRI machines are also excellent at imaging soft tissue (better than CT machines). One drawback is the long time it takes to make an image (20-90 minutes). Another is the design of the machine, which makes it difficult for patients who are afraid of small, enclosed spaces (see Figure 4). MRI machines are also quite expensive to buy. As the price comes down over time, they will likely get used more often because they provide us so much useful information about what is going on inside the body.

REFERENCES

How MRIs work (Accessed Aug. 14, 2015)
Todd A. Gould and Molly Edmonds, HowStuffWorks

How MRIs work (Accessed Aug. 14, 2015)
Dr D. Bulte from Oxford University's Functional Magnetic Resonance Imaging of the Brain centre

What it’s like to have an MRI (Accessed Aug. 14, 2015)
Via Christi Health Centre

Magnetic Resonance Imaging (Accessed August 17, 2015)
Brainfacts.org

MRI the Magnetic Miracle Game (Accessed August 17, 2015)
NobelPrize.org

ACKNOWLEDGEMENTS

OASIS data was made available through the Open Access Structural Imaging Series (OASIS) database.

Bryan Jenkins

Bryan Jenkins works as a Fellow with the CurioCity team. He is an academically trained neuroscientist, and has interests that span across a wide range of scientific topics. His past research has examined the role that molecular and cellular systems have in learning, memory, and sensory abilities. The communication of scientific discoveries through outreach and education initiatives is something that he is very passionate about. In his spare time he likes to read, write, and play his guitar.







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