How can new biomedical imaging techniques help us understand strokes?


How can new biomedical imaging techniques help us understand strokes?

When they are working as they should, our brains are nothing short of miraculous. Within the gooey, pink lump that sits inside your skull, billions of neurons are communicating via tiny sparks of electricity, keeping you alive and allowing you to interact with the world around you. 

Unfortunately, a healthy brain is not a privilege shared by everyone. Just like cars and computers, as our brains get older, they become more susceptible to the wear and tear of daily use. The older we get, the more likely we are to suffer from health conditions such as stroke and dementia. 

One in three people over the age of 60 will have a stroke, develop dementia or suffer from both during their lifetime. “A stroke happens when insufficient blood is reaching brain tissue,” says Dr Shawn Whitehead from Western University. “This means that the oxygen, glucose and other nutrients that the brain needs do not arrive on time.” This can cause a loss of neurons, damage to brain blood vessels and increased inflammation and swelling of the brain. “Depending on the severity and location of the stroke, outcomes can include loss of motor function, loss of sensation and even cognitive impairment and dementia,” says Shawn. 

Shawn and his colleague Dr Jonathan Thiessen are investigating exactly how strokes affect the brain, and how this links to the likelihood of a patient developing dementia at a later stage. To do this, they are using two biomedical imaging technologies, combining the images produced by both to understand the complex interactions happening inside the brain. 

PET and MRI 

Two biomedical imaging technologies stand out as the most useful for examining the living brain’s functions and structure. “Positron emission tomography (PET) can measure tiny traces of molecules, drugs or cells that have been labelled with a radioactive atom that emits particles called positrons,” says Jonathan. “When a positron meets an electron, they create a burst of energy that we can detect with PET.” 

The labelled molecules are called PET tracers, and they have no effect on the body’s functions because they are only used in tiny amounts. “PET tracers can measure blood flow, metabolism and types of brain cells,” says Jonathan. “These measurements can provide unique insights into how the brain’s functions change after a stroke.” 

Magnetic resonance imaging (MRI) works in a different way. “Hydrogen atoms in water molecules have a property called ‘spin’ that aligns with the direction of a magnetic field,” explains Jonathan. “MRI uses a powerful magnet to align the spins of hydrogen atoms in the body, before using pulses of radio energy to knock the hydrogen atoms out of equilibrium.” After the pulse passes, the atoms return to equilibrium and can be measured and imaged. Because so much of the body is made up of water, the resultant images can highlight differences between healthy and diseased tissues, including in the brain. “MRI can detect early signs of stroke, assess stroke severity and type, and assess levels of recovery,” says Jonathan. 

Combining PET and MRI 

PET and MRI provide different insights about the body, but it is when they are used together that they are most powerful. “It’s possible to acquire PET and MRI images at exactly the same time,” says Jonathan. “This means that the resulting images are perfectly aligned in space and time.” 

While MRI images lend insights into brain tissue structure and chemistry, PET images tell us about levels of inflammation in the brain, the presence of abnormal proteins associated with dementia and the density of connections between neurons. “The sum result of PET/MRI systems is truly greater than its parts,” says Jonathan. “This is why we are developing PET/MRI techniques that detect changes in the brain after a stroke.” This is important because there is evidence that leaving such changes unaddressed, even if they do not appear to have any immediate effect on a person’s abilities, can lead to the early onset of dementia. 

From research to practice 

Shawn and Jonathan hope that their findings will lead to new therapeutic practices that could be used in hospitals and clinics to help recovering stroke patients. Research suggests that strokes can increase the number of clumps of abnormal proteins, known as amyloid plaques, found in the brain. Such plaques can be a precursor to dementia and bring about the degeneration of neuronal pathways and overall cognitive decline. Clinicians use the presence of these proteins in the blood as a biomarker – a biological sign that suggests dementia is present or likely to develop. 

Biomarkers alone, however, do not tell researchers how the brain is functioning after a stroke. “The benefit of PET/MRI imaging is that we can get a sense of how the brain looks and functions prior to, and at different stages after, a stroke,” explains Shawn. “In combination with bloodbased biomarkers, we can develop an approach to determine who is at risk for developing dementia after a stroke.” Once clinicians have this information, they can recommend therapeutic approaches for patients to help their recovery and minimise the chances of dementia developing. 

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