AMICentre Safety Course Flashcards
(42 cards)
What is the magnet used in the AMICentre?
GE VH/I 3T
How is the magnetic field created? What does this mean?
By a superconductive magnet. It does not need electricity to work and is always on until the end of its life (5-7 years)
What are some examples of ferromagnetic materials?
Iron, nickel, and kobolt
How do ferromagnetic items act in a magnetic field?
1) They move towards higher field strength
2) They align along the magnetic flux lines (creates torque effect)
What are the two types of magnets (and magnet fields) based on force?
1) Weak magnetic field, changes slowly -> weak attractive force
2) Strong magnetic field, changes quickly -> strong attractive force. Also called “actively shielded magnets”.
How does the strong magnetic field function in practice?
Due to research, the field does not extend very far. However, the side effect is that the effect starts very rapidly, once it does start.
What is the torque effect that affects elongated objects in a magnetic field?
Elongated objects tend to orient themselves along the magnet’s flux lines. This effect also occurs in the static field.
Can people with implants be near the MRI device?
Yes, if the implants are compatible at 3T magnetic field. THIS MUST BE KNOWN. This means they must not:
1) Move
2) Turn
3) Heat up (RF)
During the MR scan.
What is deflection testing of materials?
The object is attached to a thin string, and a static magnet is put next to it. Then we see if the object is deflected from the magnet.
What precaution must be taken with people with implants or surgical history?
They should NOT be allowed to enter the shielded room without further evaluation.
A questionnaire must be filled prior to entering the room.
Is the magnetic field hazardous?
It is not known to be dangerous OR safe, but it has been extensively studied. A static magnetic field has not been found to have any cell-level effects on living animals.
Cell-level effects, if any, are assumed to be catalytic, not inducing.
What does the static magnetic field affect in the human body?
The vestibular organs. They contain a liquid that has ions. This can cause dizziness and nausea during long term stay in a strong magnetic field (>= 3T).
What is Lenz’s law?
Time-varying magnetic fields induce currents to a loop.
The current is also induced if a loop is moved in a static field.
Why are vestibular organs affected by magnetic fields, according to Lenz’s law?
Because the vestibular system has three loops that are perpendicular to each other and contain a liquid that has ions, and is conducting.
The liquid moves, and the hairs in the organs bend, which causes a sensation of motion.
When moving in a magnetic field, the movement of the liquid is different than what is perceived.
What does the electricity shutdown button do, why is it there?
It turns off the electricity in the lab (but not the magnet!). It is there due to safety regulations.
What does the emergency magnet shutdown switch do?
1) Removes the magnetic field in about 90 seconds.
2) Boils about 1000L of liquid helium (10€/L)
When should the emergency magnet shutdown be initiated? What should be done if this is activated?
Only if human life is at risk or the well-being of the victim can be improved through disabling it. Report to AMI and GE service immediately. Service time is about 2 weeks.
Tell a little about the helium inside the magnet.
1) There is about 3000L of it.
2) When boiling, the volume will increase by a factor of 760.
3) Gaseous helium replaces oxygen in air and suffocates.
4) Boiling helium creates a white cloud and freezes air.
5) Immediate evacuation needs to be done if it enters the room. However, it should not: There is an exhaust pipe.
6) Liquid oxygen is flammable.
What usually causes accidents in the lab? What should be done if they are encountered?
1) Static field causes most accidents.
2) Accidents and near-accidents should always be reported.
What is a gradient with magnets?
A gradient is a magnetic field’s (linear) variation.
They are loops of wire or thin conductive sheets on a cylindrical shell lying just inside the bore of an MR scanner.
How do gradients work with MRI scans?
1) Gradients are activated and deactivated tens of times during a scan.
2) Fast varying gradient fields have been found to cause neural excitation.
3) The gradient coils create a secondary magnetic field that slightly distorts the main magnetic field in a predictable pattern, which causes the resonance frequency of protons to vary in as a function of position.
4) The main job of the gradient coils is to allow spatial encoding of the MR signal.
How can neurons be activated?
By bringing a charged electrode close to it. The charge generated an electric field around the electrode, and the field disturbs the charge distribution around a nerve cell. The potential across the neuron cell membrane will change and cause an action potential.
How can gradients excite neurons?
Gradient field is magnetic by nature. Lenz’s Law: Time-varying magnetic fields can induce currents to a loop.
The gradient field is indeed time-varying during slopes.
What do gradient slopes cause?
They induce eddy currents to a body. These currents generate the necessary electric field to activate neurons.
Large loops cause large electric fields. The frequency direction can be right or left.
