Gradient Magnetic Fields Flashcards
Second level mode in regards to TVMFs is classified as when the maximum amplitude and rise times of gradients used in the image acquisition _______________of the mean detection threshold. As a result, there is a high possibility of the patient experiencing PNS.
Reaches 100%
Exceeds 100%
Exceeds 80%
Reaches 80%
Exceeds 100%
Duty Cycle refers to:
the percentage of time that the gradient is able to work at maximum amplitude during a pulse sequence.
maximum strength that the gradient can achieve
amount of time it takes for a gradient magnetic field to reach its maximum strength
how quickly in strength, distance and time that it takes for a gradient magnetic field to reach its maximum strength.
the percentage of time that the gradient is able to work at maximum amplitude during a pulse sequence.
Which of the following sequences would most likely cause hearing damage?
3DT1GRE at 85 dB over 5 minutes
Diffusion at 120 dB over 3 minutes
2D T2 FSE at 95 dB over 3 minutes
EPI Bold at 90 dB over 7 minutes
Diffusion at 120 dB over 3 minutes
Earplugs can decrease patient noise exposure by ________ when positioned correctly.
10-15 dB
15-20 dB
20-25 dB
30-35 dB
30-35 dB
Gradient switching on commercially available MR systems may cause peripheral nerve stimulation. This may cause the patient to experience which of the following?
Cardiac arrythmia
Stimulation of the nerves of the diapraghm, causing difficulty breathing
Tapping or feeling of ‘ants crawling’ on the bridge of the nose
Burns to anatomy in the center of the bore
Tapping or feeling of ‘ants crawling’ on the bridge of the nose
Some possible effects of TVMFs include all the following EXCEPT:
Nystagmus
Peripheral nerve stimulation
Magnetophosphenes
Acoustic noise
Induced voltages into patient tissues
Nystagmus
During which sequence will a patient have the greatest chance of experiencing peripheral nerve stimulation if all factors remain the same?
Spin echo
Fast spin echo
Echo planar imaging
Gradient echo
Echo planar imaging
With regards to patient safety, the primary concern for time varying gradients is:
Slope
Amplitude
Slew rate
What’s a time varying gradient?
Slew rate
What is described as “the amplitude and rise time values of gradients at which patients start experiencing some form of nerve stimulation.”
Mean jerk time
Rise threshold
Rise time
Duty cycle threshold
Mean detection threshold
Mean detection threshold
In regards to the time varying gradient field, where in the field will the patient have the least risk of peripheral nerve stimulation?
At the ends of the gradient coil and close to the walls of the bore
Just outside the transmit coil
Isocenter
The entire magnetic field
Isocenter
In regards to implants, what are we NOT concerned about in relation to time varying magnetic fields (gradients)?
Induced voltages
High amplitude sound waves
False feedback on monitoring devices
Triggering unneeded therapy
High amplitude sound waves
Gradient induction across the optic nerve results in:
Magnetophosphenes
Headaches
Seizures
Hearing loss
Magnetophosphenes
In general, we are primarily concerned with _________ when it comes to time varying gradients
Patient heating
Torque
Translational force
Induced voltages
Double helical breaks
Induced voltages
In relation to the X and Y gradients in conventional magnets, where within the bore will the greatest chance for PNS?
At isocenter
Near Isocenter
Closer to the bore walls
The opening of the magnet bore
Closer to the bore walls
Maximum gradient amplitude is the most direct measurement of a system’s PNS potential. True or False?
True
False
False
The greatest change in the time-varying gradient magnetic field occurs at what location during imaging?
Isocenter
Wherever we have centered our imaging volume
Close to the bore walls and at the ends of the gradient coil.
Close to the bore walls and at the ends of the gradient coil.
Rapidly changing magnetic fields may induce a voltage within the patient’s retina/optic nerves, causing the patient to experience ________.
Magnetophosphenes
PNS
Looping burns
Nystagmus
Tinnitis
Magnetophosphenes
First Level Mode in regards to TVMFs is classified as when the maximum amplitude and rise times of gradients used in the image acquisition _______________of the mean detection threshold. As a result, there could be a possibility of the patient experiencing PNS.
Reaches 100%
Exceeds 100%
Reaches 80%
Exceeds 80%
Reaches 100%
T/m can be used to describe aspects of both time varying gradient magnetic fields and static magnetic fields. True or False?
True
False
True
Headphones are an acceptable substitute for ear plugs for decreasing a comparable amount of acoustic noise to the patient. True of False?
True
False
False
Which of the following is NOT a tool / procedure that the technologist should use to avoid a potential injury caused by gradients / TVMFs?
Avoiding high spatial gradient areas
Minimizing slew rate
Increasing the TE
Giving the patient hearing protection
Minimizing slew rate
The International Electrotechnical Commission (IEC) has set a maximum permitted patient exposure of _______ before a patient experiences long-term hearing loss.
75 dB
89 dB
99 dB
100 dB
115 dB
99 dB
Hearing protection is required for all systems that exceed sound pressures of 99 dB. True or False?
True
False
True
Which of the following is a possible bioeffect caused by TVMFs?
Induced voltages within patient tissues
Looping Burns
Harmful Diffuse and/or focal heating
Significant Torque forces
Induced voltages within patient tissues
What hardware element is responsible for the acoustic noise that is experienced by the patient?
Transmitting RF coil
Gradients
Static magnetic field
Rx-only coils
Cryogen chamber
Gradients
For time varying gradients, there is also a spatial gradient component. True or False?
True
False
True
With regards to patient safety, we are concerned about the slew of the gradients, not the slope or maximum amplitude. True or False?
True
False
True
In general, if a gradient coil covers 60 cm of the bore in the Z direction of a solenoid electromagnet, what approximate area on either side of isocenter in the Z direction will most likely see the maximum spatial linearity of the gradient field?
25-30 cm from isocenter
40-50m cm from isocenter
10-20 cm from isocenter
50-60 cm from isocenter
25-30 cm from isocenter
Normal Mode in regards to TVMFs is classified as the maximum amplitude and rise times of gradients used in the image acquisition does not exceed _______________of the mean detection threshold.
60 %
70 %
80 %
90 %
80 %
Acoustic noise is measured in:
db/dt
W/Kg
dB
SNR
J/Kg
dB
In regards to the time varying gradient field, where in the field will the patient have the most risk of peripheral nerve stimulation?
Isocenter
At the end of the bore
At the ends of the gradient coil and close to the walls of the bore
The risk is the same throughout the entire magnetic field
At the ends of the gradient coil and close to the walls of the bore
Because different people experience gradients differently, their detection threshold for experiencing peripheral nerve stimulation may actually be higher or lower than the mean detection threshold. True or False?
True
False
True
Time varying gradient magnetic fields are the cause of which of the following?
Vertigo and inner ear symptoms
Heating of human tissue
Loud noises associated with MRI
Translation of ferrous materials
Loud noises associated with MRI
Everyone experiences PNS at the mean detection threshold. True or False?
True
False
False
Gradients/TVMFs are measured in:
mT/M
db/dt
W/Kg
J/Kg
T/cm
db/dt
Prolonged exposure of a patient to less than __________ can also cause long-term hearing loss.
99 dB
110 dB
75 dB
30 dB
99 dB
The amount of time it takes for a gradient magnetic field to reach its maximum strength is referred to as __________ and is measured in milliseconds (ms).
slew rate
duty cycle
ramp time
rise time
db/dt max
rise time
Both the time varying gradients and the static magnetic field have spatial gradient components. True or False?
True
False
True
What refers to “how quickly in strength, distance and time that it takes for a gradient magnetic field to reach its maximum strength”.
duty cycle
rise time
max slope
slew rate
slew rate
