Static Magnetic Fields Flashcards by William Kerns

If an object is magnetically saturated, it means that:

The object is wet and should not be used in the magnetic field

An increase in the applied external magnetic field cannot increase the magnetization of the object further

The object is no longer considered magnetic

Condensation has formed on the object while it was in the static magnetic field

An increase in the applied external magnetic field cannot increase the magnetization of the object further

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Within the static magnetic field, where is the rotational force/torque exerted on a metallic object the greatest?

Mouth of the bore

Wall of the bore

Entrance to Zone 4

Isocenter

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An object deflects 45 degrees during deflection testing. What force(s) has the most effect on the device?

The external magnetic field

Gravity

Scarring within the patient due to healing

Gravity and the external magnetic field are exerting equal force on the device

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The twisting force on a ferromagnetic object as it attempts to align parallel to the external magnetic field is called _____.

Missile effect

Translation

Torque

Magnitude

Torque

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Mechanical forces related to the static magnetic field include translational (missle), rotational (torque), and Lenz effects. True or False?

True

False

True

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1T converted to Gauss would be:

10 Gauss

100 Gauss

1,000 Gauss

10,000 Gauss

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rimary concerns with static magnetic fields are __________ .

Translation and rotation (torque) forces

Focal heating

Induced voltages

Retention

Translation and rotation (torque) forces

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If there is an area that reads 16T/m on a spatial gradient map, that means:

The strength of the magnetic field would change by 16 Tesla if that same localized rate of change in that small area were to occur over 1 meter of distance.

The strength of the magnetic field would change by 16 Tesla if an implant were to be spread out over 1 foot of distance.

The maximum field strength of that area within the magnet is 16T

The magnet will change from 1T to 16T across the length of the bore.

The strength of the magnetic field would change by 16 Tesla if that same localized rate of change in that small area were to occur over 1 meter of distance.

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Within the magnetic field, torque/rotational force will be the greatest at __________.

mouth of the bore

edge of gradient coil

isocenter

within transmit coil

isocenter

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What type of map is this?

Spatial Gradient Map

Gradient Map

RF Deposition Map

B0 map

B0 map

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Shielding of the magnet has what effect on the magnetic field?

Increases the size of the fringe field and decreases the MSG

Increases both the size of the fringe field and the MSG

Decreases both the size of the fringe field and the MSG

Decreases the size of the fringe field and increases the MSG

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The concentration of the orbiting magnetic particles in space within the magnetic field is termed:

Proton Density

Field Density

Torque Density

Flux Density

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If an object is magnetically saturated, increasing the external magnetic field will have no further effect on the object. True or False?

True

False

True

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Potential biologic effects of the static magnetic field include all the following except:

Magnetophosphenes

Peripheral nerve stimulation

Magnetohydrodynamic effect

EKG perturbances

Vertigo/dizziness

Peripheral nerve stimulation

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In viewing a spatial gradient map, there will be no translational forces exerted on an object when:

Behind the 5 G line

Below the patient table

The spatial gradient is at its maximum

There is no change in the strength of the main static magnetic field- Like at isocenter

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How does shielding within the magnet housing affect the system spatial gradient?

it doesn’t, a system’s SG is the same shielded or unshielded

it increases the system SG because it decreases the size of the fringe field, forcing lines of flux closer together

it decreases the SG because the fringe field is smaller

the shielding is in the walls, not in the magnet system

it increases the system SG because it decreases the size of the fringe field, forcing lines of flux closer together

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What is defined as “A visual representation of the change of the magnetic field strength over distance?”

Spatial gradient map

Gradient map

B0 map

Deposition map

Spatial gradient map

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What line is regarded as the point in which non-cleared implants and devices cannot cross safely?

5 mT

500 G

0.05 mT

5 G

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The static magnetic field has no bioeffects. True or False?

True

False

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What is the purpose of shielding within the magnet housing/bore?

it makes the magnet safer because the fringe field is smaller

it compresses the fringe field, allowing for a shorter distance to the 5 Gauss line

shielding within the magnet housing replaces shielding within the scan room walls

it increases the cost of the system

it compresses the fringe field, allowing for a shorter distance to the 5 Gauss line

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What phrase describes second level in regards to static magnetic fields?

No undo physiologic effects are associated with the static magnetic fields of these strengths

A potential for harm to a patient exists

There is definite potential for harm to a patient

It is safe to scan the patient at second level only

There is definite potential for harm to a patient

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Based on the MSG map above, what would be the maximum spatial gradient that the patient’s head might be exposed to in this position while being moved into the bore?

5 T/m

3 T/m

15 T/m

7 T/m

0.5 T/m

3 T/m

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A 3T system will always have higher magnetic attractive force than a 1T, regardless of magnet format. True or False?

True

False

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Does gravity or the static magnetic field have a greater effect on an implant that shows a deflection angle of 39 degrees?

Gravity

Magnetic Field

Both Equally

Gravity

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Bioeffects of the static magnetic field are temporary and disappear when the patient is removed from the field. True or False? True False

True

The maximum Tesla strength that is safe for infants younger than one month per the FDA is: 8T 1T 4T 1.5T

Spatial gradient map values are typically measured in _______ or ________. mT/Kg or J/cm W/Kg or J/cm G/Kg or W/cm T/m or G/cm

T/m or G/cm

Lenz forces are more likely to be experienced by electrically conductive material oriented _______ the static magnetic field. through parallel to perpendicular to diagonally through

perpendicular to

During MR safety testing, an implant demonstrates a deflection angle of 45 degrees. Based solely on this information, the implant would be labeled _____. MR Safe MRI Compatible MR Unsafe MR Conditional

MR Unsafe

Please convert 100 mT to Gauss 1 G 10 G 100 G 1000 G

1000 G

The maximum Tesla strength that is safe for adults, adolescents, and infants older than one month per the FDA is: 3 Tesla 7 Tesla 8 Tesla 10 Tesla

8 Tesla

Which of the following is NOT a possible bioeffect caused by the static magnetic field? Magnetophosphenes (flashes of light) Induced voltages within patient tissues Nystagmus (repetitive, uncontrolled eye movements) Vertigo (inner ear effects)

Nystagmus (repetitive, uncontrolled eye movements)

Which object would experience the most translational force when exposed to the static magnetic field? 1 kg spherical object 1 kg elongated object oriented parallel to the static magnetic field 1 kg elongated object oriented perpendicular to the static magnetic field 1 kg wire wound into a tight ball

1 kg elongated object oriented parallel to the static magnetic field

Translational (missile) force is affected only by the spatial gradient of the static magnetic field. True or False? True False

True

In horizontal bore scanners, where will the translational force exerted on a metallic object the greatest? Isocenter Entrance to Zone 4 Mouth of the bore Wall of the bore

Mouth of the bore

What possible effects can static magnetic fields have on active implants? Attractive force, focal heating, induced voltages Attractive force, affect implant functionality, displacement of implant PNS, rotational force, translational force False feedback, affect implant functionality, torque

Attractive force, affect implant functionality, displacement of implant

Fringe fields are typically either measured in ___________ or ___________, depending on the part of the world. Gauss / Joules MilliTesla / Joules Gauss / milliTesla Watts / decibels Gauss / Tesla

Gauss / milliTesla

Lenz's forces are exhibited when a large conductive object moves through the field while oriented perpendicular to the main magnetic field. True or False? True False

True

What static magnetic field scanning level would result in “No undo physiologic effects” to the patient? Normal Mode First Level Mode Second Level Mode MR Safe Mode

Normal Mode

Maximum spatial gradient is proportional to static magnetic field strength, in that higher field strength systems will always have higher MSG. True or False? True False

False

A patient complains of a pulling sensation in the area of their implant while they are being advanced into the bore. The sensation goes away when the table motion is stopped. The pulling sensation is due to: Translational forces of the static magnetic field Lenz forces caused by the implant moving through the lines of flux The patient's imagination It doesn't matter because the exam should be cancelled

Lenz forces caused by the implant moving through the lines of flux

The size and shape of a metallic object will affect its potential projectile effect within the magnetic field. True or False? True False

True

The spatial gradient of the static magnetic field is consistent throughout the entire fringe field. True or False? True False

False

Magnetophosphenes are more easily produced in higher static field strength systems. True or False? True False

True

All of the following are true for spatial gradient maps EXCEPT: Measured in T/m May vary in appearance depending on the manufacturer Visual representation of the change of the magnetic field strength over distance Visual way of telling the field strength of a static magnetic field

Visual way of telling the field strength of a static magnetic field

The potential biologic effects of the static magnetic field are thought to be harmful and irreversible. True or False? True False

False

The patient may be exposed to magnetic lines of flux upon entering zone 4. True or False? True False

True

t is possible for a vertical field open system to have a higher magnetic attractive force than a 3T system. True or False? True False

True

Pure aluminum and titanium do not have any interactions with the static magnetic field. True or False? True False

False

During MR safety testing, an implant demonstrates a deflection angle of 25 degrees. Based only on this information, the implant would be labeled _______. MR Safe MR Conditional MR Unsafe MRI Compatible

MR Conditional

Bioeffects of the static magnetic field are permanent. True or False? True False

False

What is 5 Gauss converted to milliTesla? 0.5 mT 0.05 mT 50 mT 500 mT 5000 mT

0.5 mT

A way to demonstrate Lenz's forces is: Walk toward the bore with a pen in your pocket Throw your keys toward the magnet from the scan room door Walk toward the magnet while holding a large aluminum object oriented perpendicular to the main magnetic field Quench the magnet and see what happens

Walk toward the magnet while holding a large aluminum object oriented perpendicular to the main magnetic field

Implants that have a ________ will try to align ____________ to the direction of the main static magnetic field. short axis / perpendicular long axis / perpendicular long axis / parallel

long axis / parallel

Which deflection angle(s) would constitute an MR Safe implant? 0 degrees 15 degrees 30 degrees 44 degrees

0 degrees

Conductive, nonmagnetic implants/objects will not experience Lenz forces as they move through the static magnetic field. True or False? True False

False

For the magnet diagram shown here, approximately how far from isocenter (in the Z-direction) lies the "pacemaker line" where there is the concern that the static magnetic field could potentially interfere with active implanted medical devices? 1 meter 2.5 meters 3.5 meters 4 meters

4 meters

If you are scanning on a 1.5T scanner, this means that a static magnetic field strength of 1.5T is the maximum a patient will encounter within zone 4. True or False? True False

False

Force product is the product of: The strength of the static magnetic field and the spatial gradient of the static field The RF field and the slope of the time varying gradients The weight of the magnet and the length of the bore The length of the body coil and the gradient slew rate

The strength of the static magnetic field and the spatial gradient of the static field

Which factor will NOT affect the strength of Lenz forces on an object? Speed of an objects entry into the bore Object's electrical conductivity Cross-sectional size of object Spatial gradient Deflection angle of object

Spatial gradient

The translational force exerted on an object in the magnetic field will be constant throughout the field. True or False? True False

False

A surgical clip with a long axis oriented in the head to foot direction will experience a __________ force and align ___________ to the magnetic lines of flux when placed into a vertical field / open MRI. rotational (torque) / parallel translational / parallel rotational / perpendicular translational / perpendicular

rotational (torque) / parallel

It is possible for magnetic lines of flux to extend through: Floors Walls Ceilings Floors and walls Ceilings and walls All of the above

Floors Walls Ceilings Floors and walls Ceilings and walls All of the above

Does the shape of an implant or device affect the translational force exerted on the implant/device by the static magnetic field? Yes No

Yes

For the electromagnetic fields used during MR imaging, which is of concern beyond the volume of the bore? Static magnetic field Gradient magnetic field Radiofrequency magnetic field

Static magnetic field

5 Gauss translates into _______ mT. 5 mT 0.5 mT 0.005 mT 50 mT 0.05 mT

0.5 mT

0.35 mT translates into _______ G. 35 G 3.5 G 0.035 G 350 G

3.5 G

The torque force exerted on an object is a function of: The magnetic spatial gradient The strength of the static magnetic field The gradient slew rate The length of time is has been exposed to the magnetic field

The strength of the static magnetic field

Which effects/forces would be exerted on a metallic object as it enters the bore of the magnet? Lenz, Translational, Induced Voltage Lenz, Rotational, Translational Translational, Induced Voltage, Rotational Torque, Heating, Rotational

Lenz, Rotational, Translational

Translational (missile) force is affected by which field? Strength of the static magnetic field Radiofrequency field Spatial gradient of the static magnetic field Slope of the time varying gradients

Spatial gradient of the static magnetic field

Ferromagnetic materials do not exhibit Lenz effects. True or False ? True False

False

Which deflection angles (without sufficient anchoring forces) would constitute an MR Conditional implant? 1-44 degrees 15-45 degrees 30-65 degrees 46-90 degrees

1-44 degrees

During MR safety testing, an implant demonstrates no deflection. Based solely on this information, the implant would be labeled _____. MR Conditional MR Safe MR Unsafe The implant cannot be labeled because we do not have enough information

The implant cannot be labeled because we do not have enough information

What static magnetic field level mode would a 9T MRI Scanner fall under? Normal Mode First Level Mode Second Level Mode MR Safe Mode

Second Level Mode

A patient is known to be free of metallic implants/devices. Possible interactions between the patient and the static magnetic field could be: Claustrophobia Nothing, The patient is metal-free and will therefore have no interaction with the static magnetic field Dizziness, nystagmus, altered EKG Hearing loss

Dizziness, nystagmus, altered EKG

It is impossible for non-magnetic metals to interact with the static magnetic field. True or False? True False

False

What factor does NOT directly contribute to Lenz effects? Conductivity Cross-sectional area Speed of motion Magnetic field strength Spatial gradient though which the object is moving

Magnetic field strength

Torque force exerted on an object in the magnetic field will be the greatest as the object enters the bore. True or False? True False

False

What is referred to as “stray lines of magnetic flux that extend outside the bore of the MRI scanner"? Magnetophosphenes Fringe field Flux density Shielding

Fringe field

What would be an example of first level in respect to static magnetic field strength? 0.2T 1.0 T 3.0 T 7.0 T 9.0 T

7.0 T

If the patient is feet first, supine, what spatial gradient will the head be exposed to at approximately 1.5 meters from isocenter? approx 5 T/m approx 3 T/m approx 1 T/m approx 0.7 T/m approx 0.5 T/m

approx 0.5 T/m

_______________ is when an object becomes airborne when exposed to a static magnetic field. Missile effect Translational force Flux effect Torque effect

Missile effect

Characteristics of an object that affect its potential projectile effect within a magnetic field include all of these EXCEPT: Ferromagnetic property/material Mass Age Size/shape

Age

Because the static magnetic field is homogeneous at isocenter, translational (missile) forces there are: Unable to be measured At their maximum Zero Unimportant

Zero

Within the magnetic field, translational force will be the greatest at ____________ and the least or zero at ___________. the location of maximum spatial gradient / isocenter isocenter / end of table close to the bore walls / edge of body coil

the location of maximum spatial gradient / isocenter

For most full body MR systems, the homogeneity of the static magnetic field around isocenter is an area the size of: The entire body coil A tennis ball Only isocenter A basketball

A basketball

During MR safety testing, a metallic implant demonstrates no deflection. Based on this information, the implant would be labeled _____. MR Safe MR Unsafe MR Conditional MR Compatible

MR Conditional

A paramagnetic object or substance will retain magnetism when removed from the external magnetic field. True or False? True False

False

The translational force exerted on an object is a function of: The strength of the static magnetic field The magnetic spatial gradient The gradient slew rate The length of time it has been exposed to the magnetic field

The magnetic spatial gradient

A ferromagnetic object or substance will retain magnetism when removed from the external magnetic field. True or False? True False

True

The ______ and ________ of an implant is very important in regards to rotational (torque) force. Size / Shape Composition / Position Shape / Position Size / Position Composition / Size

Deflection testing is performed by: Placing the object inside a patient then exposing it to the magnetic field Deciding how the object will behave in the magnetic field by studying its material content Suspending the object from a string and placing it in the magnetic field We don't know, it's a proprietary process

Suspending the object from a string and placing it in the magnetic field

Which of the following is NOT a tool / procedure that the technologist should use to avoid a static magnetic field injury? Ferrous metal detector 5mm gap between patient and bore wall Screening questionnaire Verbal interview

5mm gap between patient and bore wall

What types of materials are most likely to exhibit noticeable Lenz effects? PVC, Plastics Aluminum, Brass, Copper Surgical Steel, Cobalt, Nickel Gadolinium

Aluminum, Brass, Copper

Lenz's forces are harmful and there is no way to mitigate them. True or False? True False

False