Associated risks and effects

Question 1

What are the 3 components of the MRI machine and which ones are associated with risks?

Answer 1

• a strong, static magnetic field to generate a coherent signal from the proton spins
• a radiofrequency source to manipulate the proton signal such that it can be measured
• magnetic field gradients to allow for spatial localization of the signal within the body.

Each component is associated with safety risks

Question 2

What are the 4 risks associated with the Static magnetic field?

Answer 2

1. The static magnetic field pulls all ferromagnetic materials towards the scanner and therefore no ferromagnetic materials are allowed in the scanner room.
2. Ferromagnetic objects in the body (e.g., metal splinters) can lead to serious injury because of their tendency to align themselves to the magnetic field, which can cause serious internal injuries.
3. The static magnetic field induces a current in all conducting materials, which in turn causes a force opposing motion of the object into the magnetic field. This can be clearly felt when positioning e.g. an EEG amplifier at the scanner but could also affect large (non-ferrous) metal implants.
4. it disturbs electronic components unless these are explicitly designed to work in the magnetic field. This means that people with active implants such as pace makers and neurostimulators are never allowed to enter the scanner room.

Question 3

Examples of ferromagnetic objects

Answer 3

• Coins
• Pens
• Buckles
• Scissors
• Hair clips
• Staples
• Screws (e.g., in chairs)

Question 4

Two main effects of the gradient (/time-varying magnetic field)

Answer 4

1. Noise
   It creates a mechanical force on the scanner itself which, for most scans, produces a loud noise. For this reason, double hearing protection should be provided to the subjects using headphones and ear plugs.
2. PNS (Peripheral Nerve Stimulation?)
   It can cause currents to flow in nerves inside the subject’s body, which in turn can lead to muscle twitches. While the effect is harmless, it may cause discomfort to your subject. If you run a scan with increased risk of PNS the scanner will inform you, and you should in turn inform your subject of its potential occurrence.

Question 5

Main risk of the RF waves

Answer 5

During an MRI scan, radiofrequency (RF) waves are used to manipulate the signal from the proton spins. This causes energy to be deposited inside the subject’s body. While the scanner strictly regulates the maximum amount of power that can be delivered to the body, it is important to correctly position your subject and equipment to prevent burns resulting from local heating of the tissue. This means you should prevent contact between the subjects’ skin with the scanner bore itself, conducting wires and also other parts of the skin (e.g. arms and torso).

Question 7

How does the magnet quench work and what is the risk associated with it?

Answer 7

• The scanner magnet is superconducting = current is able to flow through the wires without resistance.
• Super conductance is attained by cooling the wires with liquid helium.
• A quench by – spontaneous or deliberate – local heating of the wires, which leads to loss of the superconducting properties and hence further heating due to increasing resistance, causing a cascade across the whole magnet.
• The heat that is produced in this process causes the helium in the magnet to evaporate through the quench pipe mounted on top of the magnet.
• There is a very small chance that the helium enters the scanner room. This will initially cause a white fog and leads to asphyxiation hazard due to the displacement of oxygen and frost bite. This is extremely dangerous: Never enter the room if this happens!