Dementia decoded: brain scans
19 June 2022
There are several different ways to generate detailed images of the brain in a living human, each of which requires a specialised type of brain scanner. As technology advances, machines are developed that can even combine multiple types of scan in one. Learn about the different brain scanners that exist and how they work in this blog post.
Because the science that underpins these machines is incredibly complex, they are described by equally complicated terminology. But despite their complexity, the use of brain scanners (and their names) is commonplace in everyday medicine – so it is important to understand what they mean.
This blog post will take us through the six most commonly used methods of scanning the brain in both the clinic and the lab. It is important to note that most of these scanners can take images of other parts of the body, but as DPUK’s focus is dementia research, we will focus on the brain. The scientific word for taking complex images of the brain is neuroimaging, and types of brain scan may be referred to as neuroimaging techniques.
Some neuroimaging techniques work using similar scientific principles, so share certain words in their names, such as ‘tomography’. Tomography is the process of firing any kind of electromagnetic wave (this just means a beam from the light spectrum, such as x-rays and radio waves) into the brain to take images of it in sections. Encephalography is the term for any technique that records the electrical activity of the brain to determine its structure or function.
Magnetic Resonance Imaging (MRI)
The first method of scanning a brain we will look at is MRI, which usually takes between 30 minutes and an hour. Structural MRI scanners provide a static image of the anatomy of the brain. MRI scanners are effectively giant magnetic rings that surround the patient. These rings house wire coils – a magnetic field is created by passing electric currents through these coils. The magnetic field causes the water molecules in the person’s brain to align with the magnetic field – like a magnet pulling the needle of a compass towards it. Then, a radiofrequency current is turned on, producing radio waves that push the molecules out of line. As the radiofrequency field is pulsed on and off, the water molecules ‘wiggle’. This movement of molecules is captured by the MRI scanner and interpreted by a computer. All this information is then translated into a black and white image of the brain. MRI scans are very good at distinguishing between dead and living cells, so are useful to demonstrate the loss of brain cells and brain shrinkage that occur in dementia.
Functional MRI (fMRI) is a type of MRI that examines brain activity by measuring the changes in blood flow that occur as a result of oxygen usage and requirement. Brain cells that are being used require energy, which itself requires oxygen, delivered by blood – so active brain cells have more blood flowing to them. During fMRI scans, the patient does tasks like moving certain body parts, reading, looking at images, or speaking. The fMRI scanner detects the fluctuations in blood flow because molecules in blood wiggle differently to those in other features of the brain. The connected computer then produces an image of coloured patches on a monochrome background that indicate which of the patient’s brain areas were in use while they did the tasks.
Positron Emission Tomography (PET)
PET scans can also display blood flow as well as certain other physiological features of the brain at a cellular level, such as its oxygen and sugar use. By measuring these things, researchers can see where problems in brain functioning are occurring, which can help diagnose specific forms of dementia. PET scanners use fluorescent dyes containing radioactive tracers to visualise this information, which are injected into the patient about an hour before the brain scan. Researchers trace the movement of these dyes around the brain to see how it is working. Different dyes have been developed that bind to different molecules – such as glucose, which is converted to energy, revealing which areas of the brain are using the most energy. This activity can be visualised either as a multicoloured video or still image. Areas of the brain are colour coded from deep blue to red, with dark red signifying the areas with the most activity.
The images generated by PET-MR scans are a combination of a structural MRI scan and a PET scan. Both types of scan have their merits and drawbacks: the images PET scanners produce are fairly low resolution, but give an insight into brain functioning. Meanwhile, structural MRI scanners yield high-resolution images, but of just one snapshot in time. In PET-MR images, the physiological responses from the PET scanner can be visualised over a clear MRI image, allowing scientists to see more clearly where biochemical effects are taking place in the brain. DPUK has funded a network of eight PET-MR scanners, which are helping dementia researchers to understand different aspects of dementia pathology. You can read more about these scanners in this blog post.
Computerised Tomography (CT)
Also known as CAT scans (computerised axial tomography), CT scans use x-rays to create an image of the brain. In conventional x-ray scans, a high-energy beam of radiation is used, whereas in a CT scan, multiple x-ray beams and detector plates rotate around the patient. CT scans can produce multiple two-dimensional images –slices through the brain – that can be combined to build up a three-dimensional image of the brain. CT scans produce similar images to structural MRI scans, except they are less detailed and quicker to perform. CT scans typically take just 10-20 minutes, so are especially useful for people with dementia, who could get confused or distressed during a long procedure.
CT Perfusion Imaging is a type of CT scan that provides detailed information on the blood supply to different regions of the brain. It can provide crucial information about stroke, which is intrinsically related to vascular dementia – find out more about this relationship in this blog post.
An EEG scanner is a net of electrodes that lie over the patient’s head while they undergo the scan. These electrodes measure the electrical activity of the brain, which is released in tiny bursts by brain cells as they communicate (commonly known as brain waves). These electrical signals – which have positive and negative electrical charges – are detected by the electrodes. The electrical charges are amplified to make them easier to interpret and are presented as a jagged line on graph paper or a computer screen. EEG scans can display both the resting brain waves and the electrical activity in response to stimuli, such as a certain sounds or sights. EEG scans produced in response to a stimulus are called evoked potential studies. As dementia can affect a person’s brain activity, EEG scans can be used to diagnose certain types of dementia, such as Alzheimer’s disease.
Similar to fMRI and PET scans, MEG scans can be used to show which areas of the brain are functioning – or those which may not be functioning correctly due to neurodegeneration. The electrical activity of brain cells produces a magnetic field, which is then detected by the MEG scanner. Unlike most neuroimaging techniques, which surround patients in a tunnel, MEG scanners look like a helmet or a hair drying hood in a salon. This can make them feel less claustrophobic and intense than those other types of brain scan.
You can read what it is like to undergo various types of brain scan in the Volunteer Stories series on the DPUK website. One volunteer who shared her experience is Sally Harbourne, who had a PET scan, an MRI scan, and a MEG scan as part of DPUK’s New Therapeutics in Alzheimer’s Disease study.