Chapter 11 & 12; Read Chapter 9 & 10

Which of the following may present a hazard in the scanning of patients with passive implants?

All of the above.

The magnetic translational force for a strongly saturated ferromagnetic object depends upon:

The static fringe field spatial gradient dB/dz

Which of the following is true? The magnetic torque

Depends upon the square of B0

Which of the following is ferromagnetic?

Nickel

Which of the following will show the greatest resistance to movement in a field gradient due to the Lenz effect?

Copper

A patient with a ferromagnetic aneurysm clip is inadvertently introduced into the MR environment. The most dangerous effect is:

Magnetic torque within the bore of the scanner

A ferromagnetic implant that saturates at 1.5 T is introduced into a 3 T scanner by accident. Compared to a 1.5 T scanner with a similar magnitude of static field spatial gradient the attractive force will be approximately:

The same

A non‐ferromagnetic metal is introduced into a 3 T scanner by accident. Compared to a 1.5 T scanner with similar magnitude of static field spatial gradients the attractive force will be approximately:

Double

A patient with a non‐ferromagnetic implant is introduced into a 3 T scanner. Compared to a 1.5 T scanner with a lower magnitude of static field spatial gradients the magnetic torque will be approximately:

Quadruple

Which identical device made from the following will have the highest SAR?

Silver

A passive implant has undergone the ASTM deflection test with a deflection angle less than 45° for a maximum spatial gradient of 7.2 T m−1 on a 1.5 T magnet. This means that

The magnetic force is less than the force due to gravity

Which of the following is most likely to experience RF heating in MRI:

An external limb fixation frame.

Which of the following is true regarding active implants?

All active devices use electrical energy

Risks inherent in the MR scanning of active devices include:

All of the above.

Active implants can be safety scanned in MRI as long as:

You follow the appropriate conditions.

In which of the following is not a contraindication for MRI with an MR conditional pacemaker?

The capture threshold is less than 2 volts for a 0.4 ms pulse width

A neurostimulator has a B1+rms condition of 2 μT. Your next pulse sequence exceeds this. To comply with the condition, you could:

Increase TR

Use low SAR RF pulses

Reduce flip angle

In a horizontal bore MRI system, a deep brain stimulator extension lead is coiled under the scalp. Which lead configuration is likely to experience the least induced power from the RF?

Smaller loops but with more turns concentric with B0, lying in an axial plane

You attended an MR safety course where a presentation showing that conventional cardiac pacemakers have been safety scanned in MRI. How does this change your practice?

Scanning may be undertaken “off‐label” in specialist institutions only after a positive benefit v risk analysis under a strict protocol, appropriate clinical authorization and supervision, with patient consent.

Since 2012 what is the major cause of death from MRI‐related adverse events?

Drug overdose from implantable infusion pumps

Which of the following adverse events may occur when scanning patients with cochlear implants in MRI?

All of the above

The Fixed Parameter Option:

Limits the RF and gradient dB/dt values to predefined values

A passive implant has a MR condition of maximum spatial gradient of 7.2 T/m. Your scanner’s maximum spatial gradient is 11 T/m. What do you do?

Check your spatial gradient map to see where the implant may lie.

In the MR conditions RF heating is measured over:

15 minutes.

Which of the following is not part of the MR conditions?

Imaging gradient amplitude

Which of the following is most relevant for a paramagnetic or unsaturated weakly ferromagnetic object?

Static field‐spatial gradient product B.dB/dz

In a typical closed bore magnet

The maximum spatial gradient occurs near the bore entrance

An implant has a condition for the spatial gradient to be less than or equal to 120 mT/cm. This is equal to:

12 T/m

The scanner console informs you that the B1+ RMS is 5 μT. The MR condition states 4 μT. Which of the following parameter changes will ensure compliance with the condition?

Reducing the flip angle from 180° to 140°

The scanner console informs you that the whole‐body SAR is 2 W kg−1. The MR condition states 1 W kg−1. Which of the following parameter changes does not ensure compliances with the condition?

Reducing the flip angle from 180° to 140°

Regarding ferromagnetic detection systems. They

can act as final confirmatory check, provided they are situated in an environment where there is no moving ferromagnetic material

During a quench

all of the above.

Which of the follow are not essential safety features of an MRI scanner:

Ferromagnetic detection system

An MRI unit lacks a Zone 2. Which of the following are true?

Additional control measures may be required to ensure safety of staff, visitors and patients & It does not comply with accepted safety guidance.

Which of the following should be on an MR project group?

All of the above.

The MR controlled access area or Zone IV is usually defined by:

The extent of the 0.5 mT contour

What are the main potential hazards for patients with passive implants in MRI?

Potential hazards include device movement, torque, Lenz’s Law forces, RF or gradient heating, vibration, and induced electrical currents.
Unrelated Incorrect Statements:
Incorrect answer:
• Hazards include ionizing radiation burns.
• Hazards include chemical reactions with gadolinium.
• Hazards include permanent magnetization of all implants.

Why do MRI safety signs often prohibit implants even though passive implants are commonly scanned?

Signs indicate that risks exist, but in practice many passive implants can be scanned safely when properly evaluated.
Unrelated Incorrect Statements:
Incorrect answer:
• Signs indicate that all implants are always unsafe.
• Signs indicate that MRI is prohibited for orthopedic patients.
• Signs indicate that only active implants pose risks.

Which MRI field component is responsible for translational forces on implants?

Translational forces arise from the static magnetic field B0 and its spatial gradient dB/dz.
Unrelated Incorrect Statements:
Incorrect answer:
• Translational forces arise only from RF pulses.
• Translational forces arise only from patient motion.
• Translational forces arise from image reconstruction.

Which MRI field component is responsible for torque on implants?

Torque arises from the static magnetic field B0 acting on magnetically susceptible materials.
Unrelated Incorrect Statements:
Incorrect answer:
• Torque arises only from RF heating.
• Torque arises only from gradient switching.
• Torque arises from patient breathing.

Which MRI field components can cause heating of passive implants?

Heating can result from RF fields (B1) and time‑varying gradients that induce electrical currents.
Unrelated Incorrect Statements:
Incorrect answer:
• Heating occurs only from the static field B0.
• Heating occurs only when implants are ferromagnetic.
• Heating occurs only during patient positioning.

Which MRI field component is most associated with vibration of passive implants?

Vibration is associated with rapidly switching imaging gradients.
Unrelated Incorrect Statements:
Incorrect answer:
• Vibration is caused only by RF pulses.
• Vibration is caused by the static field B0.
• Vibration is caused by patient respiration.

Which MRI field components can induce electrical currents in implants?

Electrical currents can be induced by static field gradients, gradient dB/dt, and RF dB/dt.
Unrelated Incorrect Statements:
Incorrect answer:
• Electrical currents are induced only by patient movement.
• Electrical currents occur only in active implants.
• Electrical currents occur only at 7T.

Why is it useful to categorize risks by MRI field component?

Categorizing by field component clarifies which physical interactions contribute to each hazard and guides appropriate safety assessment.
Unrelated Incorrect Statements:
Incorrect answer:
• Categorizing risks is unnecessary because all hazards are identical.
• Categorizing risks applies only to active implants.
• Categorizing risks determines image contrast.

Why is it also valid to categorize risks by underlying physical interaction?

Many hazards arise from multiple field components acting together, so categorizing by physical interaction can better reflect real‑world behavior.
Unrelated Incorrect Statements:
Incorrect answer:
• Categorizing by interaction is irrelevant to MRI safety.
• Categorizing by interaction applies only to ferromagnetic implants.
• Categorizing by interaction replaces all device testing.

Which implant characteristics influence the strength of MRI interactions?

Material type, magnetic susceptibility, electrical conductivity, geometry, size, orientation, velocity, and anatomical location all influence interaction strength.
Unrelated Incorrect Statements:
Incorrect answer:
• Only implant color influences interaction strength.
• Only implant age determines MRI safety.
• Only patient height determines implant behavior.

Why can multiple physical causes contribute to a single hazardous effect?

Different field components may interact simultaneously with the implant, producing combined forces, heating, or currents.
Unrelated Incorrect Statements:
Incorrect answer:
• Only one field component can affect an implant at a time.
• Combined effects occur only at 7T.
• Combined effects occur only in pediatric patients.

What overall principle describes the risk landscape for passive implants in MRI?

Risks depend on the interaction between implant properties and MRI field components, requiring individualized assessment rather than blanket exclusion.
Unrelated Incorrect Statements:
Incorrect answer:
• All passive implants are universally unsafe.
• All passive implants are universally safe.
• MRI risk depends only on scanner brand.

How do diamagnetic materials behave in the MRI environment?

Diamagnetic materials have extremely low magnetic susceptibility and experience negligible translational force or torque in MRI.
Unrelated Incorrect Statements:
Incorrect answer:
• Diamagnetic materials are strongly attracted to the magnet.
• Diamagnetic materials always heat dangerously.
• Diamagnetic materials become permanently magnetized.

Why are plastics considered safe with respect to magnetic forces?

Plastics are inherently diamagnetic with very low susceptibility, so they do not experience meaningful magnetic attraction or torque.
Unrelated Incorrect Statements:
Incorrect answer:
• Plastics are ferromagnetic.
• Plastics generate strong eddy currents.
• Plastics amplify RF heating.

How do paramagnetic materials respond to the static magnetic field?

Paramagnetic materials experience small translational forces and torque proportional to B0 and dB/dz, but their low susceptibility limits risk.
Unrelated Incorrect Statements:
Incorrect answer:
• Paramagnetic materials behave like ferromagnets.
• Paramagnetic materials cannot be scanned safely.
• Paramagnetic materials always saturate at low fields.

Why is torque generally low for titanium implants?

Titanium has very low magnetic susceptibility, and torque depends on susceptibility squared, making the resulting torque extremely small.
Unrelated Incorrect Statements:
Incorrect answer:
• Titanium is ferromagnetic.
• Titanium torque increases exponentially with field strength.
• Titanium torque is determined only by implant size.

What distinguishes strongly ferromagnetic materials in MRI?

Strongly ferromagnetic materials have very high susceptibility and can experience large forces and torque, especially near the bore entrance.
Unrelated Incorrect Statements:
Incorrect answer:
• Strongly ferromagnetic materials behave like plastics.
• Strongly ferromagnetic materials are unaffected by MRI fields.
• Strongly ferromagnetic materials are safe at all field strengths.

Why does shape matter more than susceptibility for saturated ferromagnetic objects?

Once saturated, susceptibility no longer influences force, and the object’s geometry determines how it interacts with the fringe field gradient.
Unrelated Incorrect Statements:
Incorrect answer:
• Shape has no effect on magnetic forces.
• Saturation eliminates all magnetic forces.
• Geometry matters only for RF heating.

Why is translational force highest near the bore entrance?

Translational force depends on the spatial gradient dB/dz, which is largest near the bore entrance and approaches zero at isocenter.
Unrelated Incorrect Statements:
Incorrect answer:
• Translational force is highest at isocenter.
• Translational force depends only on implant temperature.
• Translational force is identical throughout the scanner.

Why is torque highest inside the bore?

Torque depends on the strength of B0, which is maximal inside the bore, causing the greatest twisting effect on susceptible materials.
Unrelated Incorrect Statements:
Incorrect answer:
• Torque is highest outside the scanner.
• Torque depends only on gradient fields.
• Torque is unrelated to magnetic susceptibility.

Why are austenitic stainless steels considered weakly ferromagnetic?

Their crystalline structure is mostly non‑ferromagnetic but may contain small martensitic regions that produce mild ferromagnetic behavior.
Unrelated Incorrect Statements:
Incorrect answer:
• Austenitic steels are strongly ferromagnetic.
• Austenitic steels contain no magnetic domains.
• Austenitic steels behave identically to pure iron.

Why can stainless steel implants vary in magnetic behavior?

Magnetic behavior varies due to differences in alloy composition, heat treatment, machining, and the amount of martensitic transformation.
Unrelated Incorrect Statements:
Incorrect answer:
• Stainless steel behavior is identical across all batches.
• Magnetic behavior depends only on implant age.
• Magnetic behavior is determined solely by implant size.

Why is 17‑7 PH stainless steel considered high‑risk?

Its susceptibility can be high enough to generate forces many times greater than gravity, depending on shape and field conditions.
Unrelated Incorrect Statements:
Incorrect answer:
• It is high‑risk because it melts under RF exposure.
• It is high‑risk because it contains gadolinium.
• It is high‑risk only at 1.5T.

Why do elongated objects sometimes experience greater magnetic force than spherical ones?

Elongated shapes can align with the field gradient in ways that increase net translational force compared to compact shapes.
Unrelated Incorrect Statements:
Incorrect answer:
• Elongated objects are immune to magnetic forces.
• Elongated shapes always eliminate torque.
• Elongated shapes reduce all MRI interactions.

Why do high‑conductivity materials experience stronger Lenz’s Law forces?

High conductivity increases induced current density, strengthening the opposing magnetic field generated by motion through dB/dz.
Unrelated Incorrect Statements:
Incorrect answer:
• Conductivity has no effect on Lenz’s Law.
• High‑conductivity materials become non‑magnetic.
• Conductivity determines only RF heating.

Which implant materials exhibit the greatest Lenz’s Law forces?

Materials such as aluminum, copper, and gold exhibit the greatest Lenz’s Law forces due to their high electrical conductivity.
Unrelated Incorrect Statements:
Incorrect answer:
• Titanium exhibits the greatest Lenz’s Law forces.
• Plastics exhibit the greatest Lenz’s Law forces.
• Nitinol exhibits the greatest Lenz’s Law forces.

What overall principle describes material‑dependent MRI risk?

MRI risk depends on magnetic susceptibility, conductivity, geometry, and saturation behavior, not simply on whether a material is “metal.”
Unrelated Incorrect Statements:
Incorrect answer:
• All metals behave identically in MRI.
• Only ferromagnetic materials pose risks.
• Material properties are irrelevant to MRI safety.

Why is translational force on an implant highest near the bore entrance?

Translational force depends on the spatial gradient dB/dz, which is largest near the bore entrance and approaches zero at isocenter.
Unrelated Incorrect Statements:
Incorrect answer:
• Translational force is strongest at isocenter.
• Translational force depends only on implant temperature.
• Translational force is identical throughout the scanner.

Why is translational force essentially zero at isocenter?

Translational force is zero at isocenter because the spatial gradient dB/dz is zero, eliminating net magnetic pull in any direction.
Unrelated Incorrect Statements:
Incorrect answer:
• Translational force is zero because B0 is weakest at isocenter.
• Translational force is zero only for titanium implants.
• Translational force is zero because gradients are turned off.

Why is torque on an implant greatest inside the bore?

Torque depends on the strength of B0, which is maximal inside the bore, producing the strongest twisting effect on magnetically susceptible materials.
Unrelated Incorrect Statements:
Incorrect answer:
• Torque is greatest outside the scanner.
• Torque depends only on gradient switching.
• Torque occurs only in ferromagnetic implants.

Why can torque occur even when translational force is low?

Torque depends on B0 strength, while translational force depends on dB/dz, so one can be high while the other is low.
Unrelated Incorrect Statements:
Incorrect answer:
• Torque and translation always occur together.
• Torque requires high electrical conductivity.
• Torque occurs only during RF transmission.

What physical principle explains Lenz’s Law forces on implants?

Lenz’s Law states that motion through a magnetic field gradient induces currents that generate an opposing magnetic field, resisting the motion.
Unrelated Incorrect Statements:
Incorrect answer:
• Lenz’s Law states that metals become permanently magnetized.
• Lenz’s Law applies only to ferromagnetic materials.
• Lenz’s Law eliminates all magnetic forces.

Which implant properties increase Lenz’s Law forces?

High electrical conductivity, larger size, higher velocity, and stronger spatial gradients increase Lenz’s Law forces.
Unrelated Incorrect Statements:
Incorrect answer:
• Only magnetic susceptibility determines Lenz’s Law forces.
• Lenz’s Law forces depend only on implant age.
• Lenz’s Law forces occur only in stainless steel.

Why are Lenz’s Law forces usually small for orthopedic implants?

Most orthopedic implants have low conductivity and move slowly relative to the field, producing minimal induced currents.
Unrelated Incorrect Statements:
Incorrect answer:
• Lenz’s Law forces are small because implants contain no metal.
• Lenz’s Law forces are small only at 1.5T.
• Lenz’s Law forces are small because gradients cancel them.

Why are Lenz’s Law forces generally negligible at normal MRI couch speeds?

Normal couch velocities are too low to generate significant induced currents, keeping opposing forces far below gravitational forces.
Unrelated Incorrect Statements:
Incorrect answer:
• Couch motion eliminates all magnetic forces.
• Lenz’s Law forces are negligible only for macrocyclic implants.
• Lenz’s Law forces are negligible because B0 is weak.

Why do high‑conductivity materials experience stronger Lenz’s Law forces?

High conductivity increases induced current density, strengthening the opposing magnetic field generated during motion.
Unrelated Incorrect Statements:
Incorrect answer:
• Conductivity has no effect on induced currents.
• Conductivity determines only RF heating.
• Conductivity eliminates torque.

Why are aluminum, copper, and gold more affected by Lenz’s Law than titanium?

Their much higher electrical conductivity produces stronger induced currents and therefore stronger opposing forces.
Unrelated Incorrect Statements:
Incorrect answer:
• Titanium is more affected because it is ferromagnetic.
• Aluminum, copper, and gold are unaffected by magnetic fields.
• Conductivity determines only translational force.

Why is torque more clinically relevant than Lenz’s Law forces for most implants?

Torque can be significant even at low conductivity, while Lenz’s Law forces require high conductivity and motion, making them usually smaller.
Unrelated Incorrect Statements:
Incorrect answer:
• Torque is irrelevant because implants cannot rotate.
• Lenz’s Law forces are always larger than torque.
• Torque occurs only in ferromagnetic implants.

Why do ASTM tests focus on fringe field gradients for translational force?

Fringe field gradients determine the maximum translational force an implant may experience before entering the bore.
Unrelated Incorrect Statements:
Incorrect answer:
• Translational force is highest at isocenter.
• ASTM tests measure only RF heating.
• ASTM tests apply only to active implants.

Why does torque often peak before the implant fully enters the bore?

Ferromagnetic materials may saturate before entering the bore, causing torque to reach its maximum outside or near the bore entrance.
Unrelated Incorrect Statements:
Incorrect answer:
• Torque peaks only after the scan begins.
• Torque peaks only at isocenter.
• Torque peaks because gradients are strongest outside the bore.

Why is translational force not a concern once the implant is fully inside the bore?

Inside the bore, dB/dz approaches zero, eliminating the spatial gradient needed to generate translational force.
Unrelated Incorrect Statements:
Incorrect answer:
• Translational force stops because B0 turns off.
• Translational force stops only for titanium implants.
• Translational force stops because RF pulses cancel it.

What overall principle describes translational force, torque, and Lenz’s Law interactions?

Translational force depends on field gradients, torque depends on B0 strength, and Lenz’s Law forces depend on conductivity and motion, making each hazard distinct.
Unrelated Incorrect Statements:
Incorrect answer:
• All three forces behave identically.
• Only ferromagnetic implants experience these forces.
• These forces occur only at high field strengths.

Why is ASTM translational‑force testing based on fringe‑field gradients?

Fringe‑field gradients represent the maximum spatial dB/dz an implant will encounter, making them the most relevant region for assessing worst‑case translational force.
Unrelated Incorrect Statements:
Incorrect answer:
• ASTM tests use fringe fields because torque is highest there.
• ASTM tests use fringe fields to measure RF heating.
• ASTM tests use fringe fields only for active implants.

Why does translational force become negligible once an implant is fully inside the bore?

Inside the bore the spatial gradient dB/dz approaches zero, eliminating the magnetic pull that produces translational force.
Unrelated Incorrect Statements:
Incorrect answer:
• Translational force stops because B0 turns off inside the bore.
• Translational force stops only for titanium implants.
• Translational force stops because gradients cancel it.

Why can torque peak before an implant enters the bore?

Ferromagnetic materials may saturate before entering the bore, causing torque to reach its maximum in the high‑field region outside the magnet opening.
Unrelated Incorrect Statements:
Incorrect answer:
• Torque peaks only after scanning begins.
• Torque peaks only at isocenter.
• Torque peaks because RF pulses are strongest outside the bore.

Why is torque often more clinically significant than translational force?

Torque depends on B0 strength rather than dB/dz, so it can remain substantial even when translational force is low or absent.
Unrelated Incorrect Statements:
Incorrect answer:
• Torque is insignificant because implants cannot rotate.
• Torque occurs only in strongly ferromagnetic implants.
• Torque is always smaller than translational force.

Why are Lenz’s Law forces generally negligible for large implants at normal couch speeds?

Normal couch velocities are too low to generate strong induced currents, making Lenz’s Law forces far smaller than gravitational forces.
Unrelated Incorrect Statements:
Incorrect answer:
• Lenz’s Law forces are negligible because implants contain no metal.
• Lenz’s Law forces are negligible only at 1.5T.
• Lenz’s Law forces are negligible because gradients are inactive.

Why do high‑conductivity materials experience stronger Lenz’s Law forces?

High conductivity increases induced current density, strengthening the opposing magnetic field generated during motion through dB/dz.
Unrelated Incorrect Statements:
Incorrect answer:
• Conductivity has no effect on induced currents.
• Conductivity determines only RF heating.
• Conductivity eliminates torque.

Why are aluminum, copper, and gold more affected by Lenz’s Law than titanium?

Their much higher electrical conductivity produces stronger induced currents and therefore stronger opposing forces.
Unrelated Incorrect Statements:
Incorrect answer:
• Titanium is more affected because it is ferromagnetic.
• Aluminum, copper, and gold are unaffected by magnetic fields.
• Conductivity determines only translational force.

Why is vibration a potential issue for some implants?

Rapid switching of imaging gradients can cause mechanical vibration in conductive or elongated implants.
Unrelated Incorrect Statements:
Incorrect answer:
• Vibration is caused only by RF pulses.
• Vibration occurs only in ferromagnetic implants.
• Vibration is caused by patient respiration.

Why can gradient fields induce electrical currents in implants?

Time‑varying gradients generate changing magnetic flux, inducing currents in conductive implants according to electromagnetic induction principles.
Unrelated Incorrect Statements:
Incorrect answer:
• Gradients induce currents only in active implants.
• Gradients induce currents only at 7T.
• Gradients induce currents because implants store charge.

Why can RF fields contribute to heating in passive implants?

RF fields induce currents in conductive implants, and resistive losses from these currents can produce localized heating.
Unrelated Incorrect Statements:
Incorrect answer:
• RF fields heat only ferromagnetic materials.
• RF fields heat implants by increasing magnetic susceptibility.
• RF fields heat implants only during patient motion.

Why is implant geometry important for MRI safety?

Geometry affects how forces, torque, induced currents, and heating distribute across the implant, influencing overall risk.
Unrelated Incorrect Statements:
Incorrect answer:
• Geometry has no effect on MRI interactions.
• Geometry matters only for cosmetic implants.
• Geometry determines only image artifact size.

Why can elongated implants experience greater translational force than spherical ones?

Elongated shapes can align with the field gradient in ways that increase net magnetic force compared to compact shapes.
Unrelated Incorrect Statements:
Incorrect answer:
• Elongated implants are immune to magnetic forces.
• Elongated shapes eliminate torque.
• Elongated shapes reduce all MRI interactions.

Why is stainless steel behavior highly variable between implants?

Variability in alloy composition, heat treatment, machining, and martensitic transformation leads to differing magnetic properties.
Unrelated Incorrect Statements:
Incorrect answer:
• Stainless steel behavior is identical across all batches.
• Magnetic behavior depends only on implant age.
• Magnetic behavior is determined solely by implant size.

Why is 17‑7 PH stainless steel considered high‑risk in MRI?

Its susceptibility can be high enough to generate forces many times greater than gravity, depending on shape and field conditions.
Unrelated Incorrect Statements:
Incorrect answer:
• It is high‑risk because it melts under RF exposure.
• It is high‑risk because it contains gadolinium.
• It is high‑risk only at 1.5T.

What overall principle guides practical MRI safety assessment for passive implants?

Safety assessment must consider material properties, geometry, field interactions, and testing data rather than assuming all metals behave the same.
Unrelated Incorrect Statements:
Incorrect answer:
• All passive implants are universally unsafe.
• All passive implants are universally safe.
• MRI risk depends only on scanner brand

What causes eddy currents when an implant moves through dB/dz?

Movement through the static field gradient induces circulating currents that can reach several amperes.
Unrelated Incorrect Statements:
Incorrect answer:
• Movement through dB/dz eliminates all magnetic forces.
• Movement through dB/dz only affects ferromagnetic implants.
• Movement through dB/dz produces only image artifacts.

Why are gradient‑induced eddy currents much larger than motion‑induced currents?

Gradient dB/dt values are more than two orders of magnitude higher than those produced by implant motion, generating far stronger induced currents.
Unrelated Incorrect Statements:
Incorrect answer:
• Gradient‑induced currents are smaller because gradients are static.
• Gradient‑induced currents occur only at high field strengths.
• Gradient‑induced currents require ferromagnetic materials.

What three consequences can arise from induced eddy currents?

Eddy currents can cause electrical excitation or shock, vibration, and heat generation.
Unrelated Incorrect Statements:
Incorrect answer:
• Eddy currents cause only cosmetic discoloration.
• Eddy currents cause only implant corrosion.
• Eddy currents cause only mechanical loosening.

Why are induced electric fields from gradients relatively small?

Gradient‑induced electric fields are small because the induced voltages follow directly from Maxwell’s equations and typical gradient dB/dt values are modest.
Unrelated Incorrect Statements:
Incorrect answer:
• Induced fields are small because implants block induction.
• Induced fields are small only at 1.5T.
• Induced fields are small because gradients do not change.

Why does the RF B1 field not stimulate nerves?

The nervous system is electrically insensitive to high‑frequency fields, so RF‑induced electric fields cause heating rather than stimulation.
Unrelated Incorrect Statements:
Incorrect answer:
• RF fields cannot induce electric fields.
• RF fields stimulate nerves only in ferromagnetic implants.
• RF fields stimulate nerves at low SAR.

Why do imaging gradients produce much stronger dB/dt than implant motion?

Imaging gradients switch rapidly, producing dB/dt values up to a hundred times larger than those generated by movement through the static fringe field.
Unrelated Incorrect Statements:
Incorrect answer:
• Imaging gradients produce stronger dB/dt because they heat the magnet.
• Imaging gradients produce stronger dB/dt only at 3T.
• Imaging gradients produce stronger dB/dt only in ferromagnetic materials.

Why do Lenz’s Law forces from gradients cause vibration rather than translation?

Self‑inductance and tissue damping limit translational motion, so induced forces manifest primarily as vibration.
Unrelated Incorrect Statements:
Incorrect answer:
• Vibration occurs because implants resonate with B0.
• Vibration occurs only in titanium implants.
• Vibration occurs only when gradients malfunction.