Gradient Magnetic Fields Flashcards by William Kerns

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%

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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.

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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

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Earplugs can decrease patient noise exposure by ________ when positioned correctly.

10-15 dB

15-20 dB

20-25 dB

30-35 dB

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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

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Some possible effects of TVMFs include all the following EXCEPT:

Nystagmus

Peripheral nerve stimulation

Magnetophosphenes

Acoustic noise

Induced voltages into patient tissues

Nystagmus

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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

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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

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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

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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

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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

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Gradient induction across the optic nerve results in:

Magnetophosphenes

Headaches

Seizures

Hearing loss

Magnetophosphenes

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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

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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

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Maximum gradient amplitude is the most direct measurement of a system’s PNS potential. True or False?

True

False

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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.

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

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

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