How do brain scans work John Borghi and Elizabeth Waters
As far as we know,
there’s only one thing in our solar system
sophisticated enough to study itself:
the human brain.
But this self-investigation
is incredibly challenging;
a living brain is shielded
by a thick skull,
swaddled in layers of protective tissue,
and made up of billions of tiny,
connected cells.
That’s why it’s so difficult to isolate,
observe, and understand diseases
like Alzheimer’s.
So how do we study living brains
without harming their owners?
We can use a trio of techniques
called EEG,
fMRI,
and PET.
Each measures something different
and has its own strengths and weaknesses,
and we’ll look at each in turn.
First is EEG, or electroencephalography,
which measures electrical
activity in your brain.
As brain cells communicate,
they produce waves of electricity.
Electrodes placed on the skull pick
up these waves,
and differences in the signals detected
between electrodes
provide information
about what’s happening.
This technique was invented
almost 100 years ago,
and it’s still used to diagnose conditions
like epilepsy and sleep disorders.
It’s also used to investigate what areas
of the brain are active
during learning or paying attention.
EEG is non-invasive,
relatively inexpensive,
and fast:
it can measure changes that occur
in just milliseconds.
Unfortunately, it’s hard to determine
exactly where any particular
pattern originates.
Electrical signals are generated
constantly all over the brain
and they interact with each other to
produce complex patterns.
Using more electrodes or sophisticated
data-processing algorithms can help.
But in the end, while EEG can tell you
precisely when certain activity occurs,
it can’t tell you precisely where.
To do that, you’d need another technique,
such as functional magnetic
resonance imaging, or fMRI.
fMRI measures how quickly oxygen
is consumed by brain cells.
Active areas of the brain use
oxygen more quickly.
So watching an fMRI scan while a person
completes cognitive or behavioral tasks
can provide information about which
regions of the brain might be involved.
That allows us to study everything
from how we see faces
to how we understand what we’re feeling.
fMRI can pinpoint differences in brain
activity to within a few millimeters,
but it’s thousands
of times slower than EEG.
Using the two techniques together
can help show when,
and where, neural activity is occurring.
The third, even more precise, technique
is called positron emission tomography
and it measures radioactive elements
introduced into the brain.
That sounds much scarier
than it actually is;
PET scans, like fMRI and EEG,
are completely safe.
During a PET scan, a small amount
of radioactive material called a tracer
is injected into the bloodstream,
and doctors monitor its
circulation through the brain.
By modifying the tracer
to bind to specific molecules,
researchers can use PET to study
the complex chemistry in our brains.
It’s useful for studying
how drugs affect the brain
and detecting diseases like Alzheimer’s.
But this technique has
the lowest time resolution of all
because it takes minutes for the tracer
to circulate and changes to show up.
These techniques collectively
help doctors and scientists
connect what happens in the
brain with our behavior.
But they’re also limited
by how much we still don’t know.
For example, let’s say researchers are
interested in studying how memory works.
After asking 50 participants to memorize
a series of images while in MRI scanners,
the researchers might analyze the results
and discover a number
of active brain regions.
Making a link between memory
and specific parts of the brain
is an important step forward.
But future research would be necessary
to better understand
what’s happening in each region,
how they work together,
and whether the activity is because
of their involvement in memory
or another process
occurring simultaneously.
More advanced imaging
and analysis technology
might one day provide
more accurate results
and even distinguish
the activity of individual neurons.
Until then, our brains will
keep measuring, analyzing, and innovating
in pursuit of that quest to understand
one of the most remarkable things
we’ve ever encountered.