Safety Flashcards
regulatory bodies
- HPA
- MHRA
- IEC
- ICNIRP
Static field
What type of hazard
How is fringe size reduced in moder scanner? What’s the downside of this?
How are objects in the room classified?
Permanent hazard, always on.
Reduced frindge size by having outer coil current flow in opposite direction to main coil. Acts to cancel the field outside the bore. Downsize is that rate of change of field is larger = greater force = projectile
- MR SAFE = no hazards
- MR CONDITIONAL = no hazards if used in the right way (e.g. at particular field strength, in particular RF and gradient)
- MR UNSAFE = Known to cause hazard.
Implants
What’s the issue for different types of metals?
Any other issues?
Ferromagnetic materials may be attracted to Bo and have force profuced on them to allign with main magnetic field.
Non-ferromagnetic metals, gradient coils can induce eddy currents causing torsion e.g. aneurysm clips
Alter function of electronics e.g. pacemakers
Modes of operation
what are they? why do you have them?
- Normal - patient should not experience vertigo, or nausua <3T
- Controlled - some patients get those effects 4-8T
- Research/experimental - exposure is unrestricted >8T
have them to prevent effects caused by motion-induced currents/
RF fields
Issue?
How do you quantify?
What does it depend on?
What can happen if you exceed limits?
MHRA limits
Small fraction of energy from RF is absorbed in the body as heat.
Specific Absorption Rate = total power absorbed (w) / mass of tissue (Kg).
Affected by RF power, duty cycle, field strength, tissue density.
Local field raised by induced currents in coils, implant cables, tissue loops (positioning of arms) which can result in RF burns
Whole body normal mode SAR (W/Kg) = 2
head normal mode SAR (W/Kg) = 3.2
Gradients
What’s the issues?
How do you help solve them?
Field gradients need high current, voltage and switching rate,
can be noisy, 120dB, need ear defenders
Switching of magnetic fields can induce currents in tissue that is electrically conductive, i.e. peripheral nerves. Can cause people to tingle, tap, pain. Scanner limits dB/dt to avoid sever discomfort
- Normal - PNS but not painful
- Controlled - Some might get pain
- Research - exposure is restricted to prevent cardiax stimulation
Contrast Agents
How do Ga contrasts work?
Potential issues
Which people are most at risk?
What can you do?
Ga is 3+ oxidation state, metal has seven unpaired electrons, causes water around it to relax quickly. Shortens T1 relax, makes them bright.
Issues:
- Potential for reaction (0.07-2% of patients) - majority are mild e.g. headache, itch, dizziness
- Minority (<0.7%) are allergic e.g. hives, rash
- Very rare (55 in 20 million): anaphalactic shock
Nephrogenic systemic fibrosis can occur days-months after contrast, can be fatal, people with poor kidney function e.g. dialysis, kidney function.
You have to check medical history for renal problems, and have people on standby in case of allergic reaction, oberve patient with contrast.
Magnet quench
what is it
what’s the issue?
How do you mitigate risk and protect magnet?
Failure of superconducting magnet which must be kept cool, eddy currents induced = heat, turns liquid helium into gas. Expelled from room. expensive and scanner down-time can be weeks. Or can press in emergency.
Hazard, .arge volume of gas can suffocate patient. Needs to go outside. Oxygen sensors in room.
Can try to protect magnet by having resistive loads that dump heat in the safe part of the magnet.
Safe Working Practice - MHRA
What 4 types of people should be listed in LR?
How do you control access?
Screening process
MR safety advisor, responsible person (clin scientist), authorised person (understand MR safety enough to work freely in MR environment, unathorised (member of public).
Controll access with doors, and demarkate 0.5mT lines (the min field streength at which some pacemakers might be affected). Area around scanner is controlled.
Screen patients and volunteers, checklist to identify any MR contraindications, e.g. implantibles, tattoos, piercings
