Flashcards in Module 8 Deck (44)
What are the Canadian exposure limits to RF, static magnetic field and time varying magnetic field?
Time varying magnetic field - 3T/s
Static magnetic field- 2T
RF- core body temp can't be raised more than 0.5*C and any body part more than 1*C
What are the guidelines for exposure for MRI operators?
Operators should not be continuously exposed to magnetic flux of more than 0.01T during the day
True or False : MRI safe means minimal hazard in the MRI scan room ?
False - MRI safe means NO hazard in the MRI scan room
True or False: if an item is safe at 3T it should definitely be safe at 1.5T
What is the risk of an item that is MRI unsafe 2?
Risk is the object heating resulting in burns
True or false : MRI unsafe 1 the main risk is that the object may move or dislodge
Fill in the blanks
Time varying field oscillate depending on the pulse sequence. Sequences with ____(1)_____ flip angles and ____(2)_____ RF pulses are of greater safety concern
1. Greater flip angles
2. more RF pulses
What are the units of SAR?
What are 2 areas that have an impaired ability to dissipate heat?
Eyes and testicles
What 3 factors are related to RF heating ?
frequency, field strength and patient size
Why is it mandatory to enter patient's weight on MRI scanners?
So SAR can be calculated and monitored
What steps can be taken to reduce risk of burns ?
- thermal insulation between patient and conductive material
- cold compress on tattoos or other conductors that can't be removed
Which of the following is not a primary effect of gradient magnetic field
a.. Peripheral nerve stimulation
d. acoustic noise
What determines the effects of time varying magnetic fields ?
- wave shape
- repetition pattern
- distribution of current
Why is the induced current greater in the peripheral tissues ?
amplitude of the gradient is higher farther from iso
Which of the following is not true about peripheral nerve stimulation?
a. Sensations such as itching and crawling are possible
b. Involuntary muscle contractions happen
c. Sequences with perfusion, diffusion and EPI are more likely to cause PNS
d. At lower amplitude and gradient these effects increase
d. is not true
- higher amplitude and gradients
What causes noise in MRI ?
as current passes through gradient coils they vibrate and cause noise
True or false - patients are required to wear hearing protection because the noise level is too high.
False - noise level falls within safety guidelines but recommended hearing protection
What are magnetophosphenes?
Visual disturbances caused by retinal phosphates being stimulated by induction from the gradient field
What is the static magnetic field responsible for?
Aligning the nuclei
What are some safety issues associated with a static magnetic field?
malfunctioning of implants
What causes the temporary t wave elevation on ECG when static magnetic field?
The magneto-hydrodynamic effect - when conductive fluid moves across a magnetic field
Which of the following is not an effect of static field?
-hypotension is an effect
What happens when a magnet is quenched ?
Quenching a magnet rapidly ramps down magnetic field in a life threatening emergenct
Why is there an alarm for oxygen levels?
Helium can back up into the scan room or venting equipment can fail
True or false: due to limited experience and testing at high field strengths patients with implants and pregnant patients should be imaged at lower field strengths
What types of shielding are used to contain a fringe field within the 5 Gauss line?
Active and passive shielding
What force is a concern for a patient that has an aneurysm clip?
Where is a ferromagnetic object attracted to in the MRI scanner?
The isocentre where a patient would be while being scanned
True or false : level 1 personnel do not need supervision in zone 4
false - they need supervision
What is an example of a level one MRI personnel?
Receptionist, cleaning staff, emergency team
What two types of screening should a patient undergo before a scan?
2. screening form
True or false: A quick verbal screening is adequate for regular staff in MRI.
True or false : metal detectors can replace screening.
False always need screening
What factors determine the size of an artefact?
-type of metal
- pulse sequence
- imaging parameters
True or false : most coils, filters and clips are imbedded in vessel wall and are unlikely to be dislodged
True or False: all heart valves are safe to scan
False - most are though need to be screened
True or false: Otologic and ocular implants are usually safe to scan.
False - usually unsafe due to the fact they may have magnetic components that could be attracted to or interfered with
What extra screening should patients who may have metal in his or her eye undergo?
A plain x-ray
What are the main considerations with bullets and shrapnel?
1. Where (different compositions) and 2. What (near vital organs)
What are some risks of halo vests in MRI?
-electric current induction
What criteria need to be met when scanning a patient with a contraindicated pacemaker ?
- need a cardiologist and radiologist nearby
- need a rep from the pacemaker company
- patient is fit and not dependent on pacemaker
What are some considerations when scanning pregnant patients ?
- no gadolinium contrast
- avoid imaging in 1st trimester
- weigh risk vs benefit