Links to Junior Cycle
The science behind MRI scanners crosses over a lot of Junior Cycle topics and makes a good way to introduce a concept or to review several. It's also a great example of a real world application of physics.
MRI technology makes use of the fact that the various tissues in our bodies contain a lot of water. The machine is tuned to affect the hydrogen protons in these water molecules. By adding energy and recording how it is released the scanner can determine the type of tissue in a particular place.
MRI safety is all about different types of materials and there are many dramatic ways that things can go wrong when ferrous objects enter the magnetic field. MRI technicians have to constantly classify materials as magnetic or non-magnetic. Metals / non-metals is a simpler way to approach this if you don't want to address magentism.
Imagine you had to get an MRI scan right now, what metal objects do you have on you right now?
(Real world issues include hairclips, scissors, handguns)
Extension activity - which of the metal objects are magnetic?
An awareness of mixtures is also important, one particular problem is with iron based tattoo ink. A magnetic field will induce eddy currents in the tattoo and this creates heat. It's a problem with a big tattoo and during a scan the tattoo will need to be kept cool with a wet towel.
An MRI scanner has some fairly dramatic physical properties in terms of temperature, force, and resistance.
The magnet uses a superconducting electromagnet to generate the magnetic field. This means that it has no electrical resistance which makes it cheap to operate, there is no heat to dissipate and stronger magnetic fields can be generated because higher currents can be used.
The electrical resistance of superconducting materials drops to zero when they are cooled below a critical temperature. The technology used in current MRI scanners requires a critical temperature of 4 Kelvin which is achieved using liquid helium. This gives a nice chance to introduce the Kelvin scale and the concept of absolute zero. Cooling something to this temperature is difficult and expensive so there is a lot of research into materials with higher critical temperatures. If a material could use liquid nitrogen (77 K) that would make superconductors much more accessible.
If students were to explore this there are quite a lot of rare elements used and chemical bonding challenges.
Magnet levitating about a superconductor.
By Mai-Linh Doan - self photo, CC BY-SA 3.0.
Reading an MRI scan is essentially about finding patterns and anomalies and relating those patterns to biological structures. Using the MRI images on this website, students can compare scans of brains to identify differences and therefore diagnose diseases.
Watch Erik's brain scan. Can you identify any parts of the head?
(If students are stuck, start with the nose and eyes to orientate them)
Here are some key features that you can see in the MRI scan.
Click on the gallery to show the images full screen
Some key discoveries in physics that allowed the development of MRI scanners:
Discovery of rotating magnetic field
Quantum mechanics showed that protons can absorb or emit radio waves when exposed to a strong magnetic field. (Nuclear Magnetic Resonsance)
Advances in superconducting materials allowed far more powerful magnets to be used and provided more detailed images
(Note: the ability to create scans from the signals received also relies on some significant mathematical discoveries)
Conservation of energy is at the heart of the MRI imaging process. Essentially, the hydrogen atoms are lined up by the magnetic field, energy is added using a radio frequency pulse, the pulse is turned off and now the hydrogen atoms have more energy than they want so it needs to go somewhere. It's released as another radio-frequency pulse which can be detected by the MRI scanner. Since different tissues release this energy at different rates, we can determine the type of tissue in a particular place.
Hydrogen protons in your body line up with the magnetic field when you enter the scanner
Radio-frequency energy is absorbed by hydrogen protons and causes them to change their spin
Radio-frequency pulse is removed and the hydrogen protons return to their aligned state. The energy being released causes a radio signal that can be measured by the scanner.
A simpler explanation is provided on the main MRI page for students.
Science in Society Investigation
MRI scanners are a great potential topic for a Science in Society Investigation. They bring together developments across the different fields in science and computing to provide a crucial medical tool for doctors.
At a more basic level, students can explore magnetism, energy transfer, and biological structures.
At a more advanced level students can explore superconduction, energy levels in atoms and how we explore disease structures. A project could be focused on a particular aspect of the body such as the brain or sports injuries.
MRI allows a safe and hugely detailed view on the soft tissues of our internal biology. It is the only way to diagnose certain diseases such as Multiple Sclerosis and has the potential to improve early detection for a range of other diseases.