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MRI Physics > chapter 10 > Flashcards

chapter 10 Flashcards

1
Q
A
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2
Q

What is the primary reason for MRI screening forms?

A

To identify potential contraindications to MRI.

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3
Q

Which of the following is considered a contraindication for MRI?

A

Cochlear implants.

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4
Q

What is the fringe field in an MRI system?

A

The area outside the magnet where the magnetic field is still detectable.

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5
Q

What is the primary danger of ferromagnetic objects near an MRI scanner?

A

They can become projectiles.

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6
Q

Which zone in the MRI suite is considered the most restricted?

A

Zone IV.

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7
Q

The term ‘quench’ in MRI refers to:

A

The release of cryogens from the magnet.

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8
Q

What is the most common cause of burns during an MRI scan?

A

Skin-to-skin contact or loops of wires touching the skin.

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9
Q

Gadolinium-based contrast agents are contraindicated in patients with:

A

Severe renal impairment.

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10
Q

What is the FDA limit for the specific absorption rate (SAR) in whole-body MRI scans?

A

4 W/kg.

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11
Q

What should be done if a patient becomes claustrophobic during an MRI scan?

A

Provide reassurance and potentially offer sedation.

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12
Q

All MRI-compatible implants are safe for scanning at any field strength.

A

False.

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13
Q

Patients with pacemakers should never undergo an MRI scan.

A

False (conditional MRI-safe pacemakers exist).

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14
Q

A patient’s clothing should always be screened for metallic fibers before entering the scanner room.

A

True.

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15
Q

The static magnetic field is always on in an MRI scanner, even when the machine is not in use.

A

True.

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16
Q

Pregnant patients should avoid MRI scans entirely.

A

False (MRI may be used if clinically justified, especially after the first trimester).

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17
Q

What is the static magnetic field?

A

Always on, the source of the main magnetic field.

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18
Q

What are Radiofrequency (RF) Coils?

A

Produces images by transmitting and receiving signals.

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19
Q

What is Zone III?

A

Area requiring strict access control; adjacent to Zone IV.

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20
Q

What is Cryogen?

A

Liquid helium used to cool the superconducting magnet.

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21
Q

What is a Quench Pipe?

A

Vents cryogens safely outside in case of a quench.

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22
Q

During an MR examination, what is responsible for PNS?

A

TVMF (Time Varying Magnetic Fields).

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23
Q

The three levels of personnel in MRI are: Non-MRI Personnel, Level 1 Personnel, and Level 2 Personnel.

A

True.

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24
Q

Who is responsible for MR safety screening?

A

Level 2 trained MR technologist.

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25
Q

Who is responsible for training the custodial staff about MR safety?

A

All MRI technologists on staff.

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26
Q

A designated ‘Zone 4’ in an MRI facility includes:

A

Suitable for screened patients under direct supervision of MRI staff.

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27
Q

What is the most important safety consideration associated with a quench?

A

Displacement of oxygen.

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28
Q

What is the rotational force that causes alignment on an object to the lines of magnetic flux?

A

Torque effect.

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29
Q

Which of the following is always on in an MRI scanner?

A

B₀ (Static Magnetic Field).

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30
Q

Vertigo, headaches, nausea, and metallic taste are examples of:

A

Short-term effects of exposure to MR imaging.

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31
Q

Which MR scanning mode should a technologist consider a patient’s dissipation of heat before using?

A

First-level and second-level controlled operating modes.

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32
Q

What must be done for a patient who has taken oral sedation at home before their MRI scan?

A

At minimum, pulse oximetry.

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33
Q

Which MRI zones should all persons be supervised in?

A

Both Zone 3 & Zone 4.

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34
Q

Where should the warning sign with ‘Magnet is On’ be located?

A

Zone 3.

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35
Q

An object that has strong magnetic attraction is known to have:

A

Ferromagnetism.

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36
Q

The ideal location for the patient dressing room should be in:

A

Zone 2.

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37
Q

Which personnel level has passed minimal MRI safety education to ensure their own safety?

A

Level 1.

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38
Q

The ACR (American College of Radiology) prohibits anyone except the patient from being in Zone 4 during scanning.

A

False (Screened individuals may accompany anxious patients).

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39
Q

According to the ACR recommendations, what degree of training should the MRI technologist have?

A

Level 2.

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40
Q

What is a ‘quench’ in MRI?

A

Sudden loss of liquid helium and magnetic field.

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41
Q

How many colors of MR safety identifier labels are used?

A

3.

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42
Q

SAR (Specific Absorption Rate) is measured in:

A

W/kg.

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43
Q

SAR stands for:

A

Specific Absorption Rate.

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44
Q

Which force is responsible for ferromagnetic materials becoming attracted to the static magnetic field?

A

Translational force.

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45
Q

Who must be screened before entering the MR area?

A

All of the above (Patients, Visitors, Ancillary personnel).

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46
Q

A designated ‘Zone 1’ in an MRI facility includes:

A

All areas that are freely accessible to the general public.

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47
Q

The most commonly reported effect of exposure to RF fields is:

A

Increase in body temperature.

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48
Q

What is the best method for controlled access to the MR environment?

A

MR level 2 safety trained person.

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49
Q

Under what circumstances should a superconducting magnet be quenched?

A

In case of an emergency when life or limb is at risk.

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50
Q

How many zones are recommended for the MR area?

A

4.

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51
Q

What is the leading cause of accidents in the MRI environment?

A

Personnel not adhering to established safety guidelines.

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52
Q

‘MR Conditional’ means:

A

The item poses no known hazards in a specified MR environment with specific conditions of use.

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53
Q

What is the 5 Gauss line?

A

The magnetic field strength and distance that is safe for a person with any device.

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54
Q

If an implanted device is tested safe at 3T, then it is assumed safe at all other lower field strengths.

A

False.

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55
Q

Items that are ‘unsafe’ to go into the MRI scan room have a label that:

A

Is a red circle with a diagonal line and ‘MR’ inside of it.

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56
Q

What physical barriers should be used for the MR environment?

A

Controlled access to Zone 3.

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57
Q

What is PNS?

A

Peripheral Nerve Stimulation.

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58
Q

What is the main cause of acoustic noise in the scan room?

A

Time Varying Magnetic Fields (TVMF).

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59
Q

Which psychological effect is NOT associated with MRI-related anxiety?

A

Excitement.

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60
Q

What is the temperature of liquid helium in Fahrenheit?

A

-452°F.

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61
Q

The five factors impacting patient safety in MRI include:

A
  1. Psychological effects
  2. Spatially varying static magnetic fields
  3. Time varying gradient fields
  4. Radiofrequency (RF) effects
  5. Acoustic noise
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62
Q

All of the following are examples of projectiles EXCEPT:

A

Transmit coils.

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63
Q

The main biological effect of RF in MRI is:

A

Tissue heating.

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64
Q

What are the major factors used to calculate SAR?

A

All of the above (RF duration, patient weight, pulse sequence type).

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65
Q

What determines the amount of ferromagnetic reaction/attraction?

A

All of the above (Size, shape, material type).

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66
Q

What is the FDA SAR limit for the whole body in MRI?

A

4 W/kg.

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67
Q

What is the antenna effect in MRI?

A

Causes heating in conductive wires of critical length.

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68
Q

The greatest hazard of the main magnetic field is:

A

Ferromagnetic projectiles.

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69
Q

B₀, TVMF, and RF are examples of:

A

None of the above (They are not forms of ionizing radiation).

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70
Q

Why is MRI safety complex?

A

MRI safety is complex due to varying field strengths, machine configurations, rapidly switching gradients, diverse patient implants, and body jewelry.

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71
Q

Who is primarily responsible for MRI safety decisions?

A

The referring physician, radiology lead, and MRI practitioner are responsible for assessing MRI safety risks.

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72
Q

Why must MRI practitioners stay updated on safety?

A

New implanted devices and safety research emerge regularly, requiring updated knowledge on safety protocols.

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73
Q

What is the role of medical physicists in MRI safety?

A

They provide expert advice on device interactions, safety risks, and field strength compatibility.

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74
Q

What is Zone I in an MRI facility?

A

Zone I is freely accessible to the public and outside the controlled MRI environment.

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75
Q

What is Zone II in an MRI facility?

A

Zone II serves as a transition area where patient screening, history-taking, and supervision occur.

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76
Q

What is Zone III in an MRI facility?

A

Zone III is a restricted area where unscreened personnel and ferromagnetic objects pose serious risks.

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77
Q

What is Zone IV in an MRI facility?

A

Zone IV is the MRI scanner room itself, with a high static magnetic field that must be clearly marked as hazardous.

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78
Q

Who are Non-MRI Personnel?

A

Patients, visitors, and hospital staff who haven’t received MRI safety training in the past 12 months.

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79
Q

Who are Level 1 MRI Personnel?

A

Office staff and patient aides who have minimal MRI safety training to ensure their own safety in Zone III.

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80
Q

Who are Level 2 MRI Personnel?

A

MRI technologists, radiologists, and nurses trained extensively in thermal loading, burns, and neuromuscular risks.

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81
Q

What does MR Safe mean?

A

The item is nonmagnetic, nonmetallic, and nonconducting, posing no known hazards in any MRI environment.

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82
Q

What does MR Conditional mean?

A

The item is safe only under specific MRI conditions such as field strength limits, SAR restrictions, or lead routing.

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83
Q

What does MR Unsafe mean?

A

The item poses hazards in all MRI environments, such as ferromagnetic scissors or non-MRI-compatible implants.

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84
Q

What percentage of MRI patients require sedation due to anxiety?

A

14.3% of patients require sedation or anesthesia due to claustrophobia and anxiety.

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85
Q

Why are brain MRI scans more likely to cause anxiety?

A

Because the head is positioned at isocenter, with the bore and head coil creating a confined space.

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86
Q

What are common psychological concerns in MRI?

A

Claustrophobia, fear of suffocation, fear of fainting, panic attacks.

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87
Q

How do failed MRI scans affect hospital operations?

A

Delays diagnosis, wastes scanner time, increases costs and administrative workload.

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88
Q

How can scanner design improvements reduce anxiety?

A

Shorter, wider bore scanners with fans, lighting, and immersive video environments help patients relax.

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89
Q

What biological effects can occur at high magnetic fields (3T–7T)?

A

Metallic taste, vertigo and dizziness, optical phosphenes (flashing lights).

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90
Q

Why does vertigo occur at ultra-high-field MRI?

A

The Lorentz force affects ions in the inner ear’s vestibular system, leading to dizziness.

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91
Q

What is Faraday’s Law of Induction in MRI safety?

A

Moving a conductor (optic nerve) through a magnetic field can induce electrical currents, causing flashing lights (phosphenes).

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92
Q

What is the main safety concern for patients experiencing MRI-induced vertigo?

A

Patients may need to avoid driving or operating machinery for 15+ minutes post-scan.

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93
Q

What creates projectile hazards in MRI?

A

The spatially varying static field gradient exerts a strong force on ferromagnetic objects.

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94
Q

What types of objects are common MRI projectiles?

A

Oxygen tanks, wheelchairs, scissors, pens, coins, keys.

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95
Q

Why are modern actively shielded MRI scanners a double-edged sword?

A

They contain the fringe field, but projectiles accelerate suddenly near the bore.

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96
Q

What should be done immediately after a large projectile incident?

A

If life-threatening, quench the magnet to remove the object.

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97
Q

What is a quench, and why is it sometimes necessary?

A

A quench rapidly shuts down the magnet by releasing cryogens, but may damage the scanner.

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98
Q

What are the consequences of metallic foreign bodies in the scanner bore?

A

They can cause image distortion or damage scanner components.

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99
Q

How can MRI departments reduce projectile risks?

A

Strict screening protocols, ferromagnetic detectors, secured, locked magnet rooms.

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100
Q

What is the most common cause of projectile incidents in MRI?

A

Unscreened hospital staff or visitors entering the magnet room with ferromagnetic objects.

101
Q

Why are oxygen tanks particularly dangerous in MRI?

A

They are heavy, metallic, and can accelerate toward the magnet, causing serious injury or damage.

102
Q

How often should staff receive MRI safety training?

A

At least annually, per ACR safety recommendations.

103
Q

What happened in reported projectile injuries involving hospital staff?

A

Staff have suffered fractured forearms, facial fractures, and pelvic injuries from oxygen tank incidents.

104
Q

What safety measure prevents unauthorized access to MRI rooms?

A

Locked doors, card key access, and controlled entry points.

105
Q

What is torque in MRI?

A

The rotational force exerted on ferromagnetic implants or objects in the magnetic field.

106
Q

What types of implants can experience torque in MRI?

A
  • Pacemakers
  • Aneurysm clips
  • Deep brain stimulators
107
Q

Why is torque dangerous for implanted medical devices?

A

It can cause movement, heating, or malfunction, potentially leading to serious injury or death.

108
Q

How can MRI technologists determine if an implant is MRI safe?

A

By consulting device documentation, manufacturer safety guidelines, and MRI safety databases.

109
Q

What should be done before scanning a patient with an implant?

A
  • Check implant safety conditions
  • Use lowest necessary field strength
  • Monitor for adverse reactions
110
Q

Why are metallic foreign bodies a risk in MRI?

A

They can be dislodged, migrate, or heat up, causing serious injury.

111
Q

What type of patients require extra screening for metallic foreign bodies?

A

Patients with a history of metallic injuries, welding, or metal shavings in the eyes.

112
Q

How should suspected metallic foreign bodies in the eye be evaluated?

A

A screening X-ray should be performed before MRI.

113
Q

Why should some tattoos be screened before MRI?

A

Tattoos with metallic ink can cause burns or irritation.

114
Q

What is the risk of dermal piercings in MRI?

A

They can heat up, cause burns, or be pulled toward the magnet.

115
Q

What are cryogens used for in MRI?

A

To supercool superconducting magnets, maintaining magnetic field stability.

116
Q

What is a quench?

A

A sudden loss of cryogens, rapidly turning off the magnetic field.

117
Q

Why is an uncontrolled quench dangerous?

A

It can cause asphyxiation (oxygen displacement) and frostbite due to rapid gas expansion.

118
Q

When should a quench be manually activated?

A

Only in life-threatening emergencies, such as a trapped patient or projectile incident.

119
Q

What safety measures prevent quench-related injuries?

A
  • Proper room ventilation
  • Emergency escape plans
  • Regular maintenance of quench pipes
120
Q

What is torque in MRI?

A

Torque is a rotational force that aligns ferromagnetic objects with the magnetic field lines.

121
Q

Why is torque dangerous for implanted devices?

A

It can cause movement or twisting, potentially damaging soft tissues or blood vessels.

122
Q

Which implants are most at risk of torque in MRI?

A
  • Aneurysm clips
  • Neurostimulators
  • Pacemakers
  • Cardiac defibrillators
123
Q

How does implant shape affect torque risk?

A

Long, narrow implants (e.g., surgical clips) experience more torque than compact shapes.

124
Q

What are the safest types of implants regarding torque?

A

Implants anchored to bone (e.g., hip replacements, orthopedic nails) are less affected.

125
Q

What factors reduce torque risks in implants over time?

A
  • Fibrosis (scar tissue forming around the implant)
  • Proper anchoring in bone
126
Q

What MRI precaution should be taken for unknown aneurysm clips?

A

Cancel or postpone the scan until implant safety is confirmed.

127
Q

What fatal incident has been linked to torque effects?

A

A patient with an aneurysm clip suffered fatal vessel rupture due to MRI-induced movement.

128
Q

Why are some newer cardiac implants MRI-conditional?

A

They contain less ferromagnetic material and can be programmed into an MRI-safe mode.

129
Q

What safety measures are required for scanning patients with MRI-conditional pacemakers?

A
  • Device must be set to MRI-safe mode
  • MRI must be done under cardiology supervision
  • Strict monitoring during the scan
130
Q

What risks do ferromagnetic foreign bodies pose in MRI?

A

They can move, rotate, or heat up, causing serious injury.

131
Q

Why are ocular foreign bodies particularly dangerous?

A

Movement can cause retinal damage, hemorrhage, or blindness.

132
Q

What are two documented cases of MRI-induced blindness?

A
  • One patient suffered irreversible vitreous hemorrhage.
  • Another patient developed a lenticular cataract.
133
Q

Why are metal workers at high risk for MRI-related eye injuries?

A

They may have small, unnoticed metal fragments in their eyes from past work.

134
Q

What screening test is used for ocular metal foreign bodies?

A

A plain radiograph (X-ray) of the orbits.

135
Q

Should verbal history alone be used to rule out metallic foreign bodies?

A

No. Radiographic confirmation is required if there’s a history of eye trauma.

136
Q

What type of radiation does MRI use?

A

Non-ionizing electromagnetic radiation (RF waves).

137
Q

What is the difference between ionizing and non-ionizing radiation?

A
  • Ionizing radiation (e.g., X-rays) can damage DNA and cause cancer.
  • Non-ionizing radiation (MRI RF waves) does not break chemical bonds.
138
Q

What are the three major safety concerns with RF in MRI?

A
  1. Patient heating and burns
  2. Antenna effect (induced currents)
  3. Implant malfunctions due to RF currents
139
Q

Why does RF cause heating in MRI?

A

RF waves induce currents in tissues, and resistance causes heat generation.

140
Q

What does SAR stand for in MRI?

A

Specific Absorption Rate – the rate of RF energy absorption in the body.

141
Q

What are the SAR limits for normal MRI operation?

A
  • Whole body: 2 W/kg
  • Head: 3.2 W/kg
  • Extremities: 10 W/kg
142
Q

What patient groups are more susceptible to RF heating?

A
  • Elderly & neonates
  • Obese patients
  • Patients with fever or impaired thermoregulation
143
Q

What is the maximum core temperature increase allowed in MRI?

A
  • Normal mode: ≤0.5°C
  • First-level controlled mode: ≤1°C
  • Second-level mode: >1°C (research only)
144
Q

What strategies reduce SAR and heating risks?

A
  • Increase TR
  • Lower flip angle
  • Use a different RF coil
  • Reduce the number of slices
145
Q

What is the antenna effect in MRI?

A

A conductor (e.g., wires) resonates at RF frequencies, causing localized heating.

146
Q

What medical devices are at risk of antenna effect burns?

A
  • Pacemaker leads
  • ECG electrodes
  • Long metal implants
147
Q

What is the temperature risk for pacemaker leads in MRI?

A

The lead tip temperature can reach 80°C, causing serious burns.

148
Q

What patient positioning can cause heating loops?

A
  • Hands touching thighs
  • Heels resting together
  • Knees pressed against each other
149
Q

How can patient heating from the antenna effect be prevented?

A
  • Proper padding between body parts
  • Avoiding unnecessary conductive materials
150
Q

What is the primary cause of RF burns in MRI?

A

RF energy induces currents in conductive materials, leading to localized heating and burns.

151
Q

Where do RF burns typically occur?

A
  • Skin-to-skin contact points (e.g., hands touching thighs, knees pressed together)
  • Conductive materials in contact with the skin (e.g., ECG leads, jewelry)
152
Q

Why do loose loops of ECG leads increase the risk of RF burns?

A

Loops act as an antenna, absorbing RF energy and generating excessive heat.

153
Q

What precaution should be taken for ECG leads in MRI?

A

Position them straight and away from the body, avoiding loops.

154
Q

What can patients wear to reduce RF burns?

A

MRI-safe clothing without metallic fibers.

155
Q

Why is padding important in preventing RF burns?

A

Padding eliminates conductive loops and insulates skin-to-skin contact points.

156
Q

What common household items pose RF burn risks in MRI?

A
  • Tattoo ink with metallic particles
  • Makeup containing metallic pigments
  • Certain wound dressings with metallic coatings
157
Q

Why are tattoos a potential RF safety risk?

A

Some inks contain iron oxide, which can heat up and cause burns or irritation.

158
Q

What should be done if a patient has a large tattoo in the MRI scan area?

A

Monitor the patient closely for discomfort or heating during the scan.

159
Q

How do thermal burns from RF energy differ from traditional burns?

A

They occur without an external heat source, often with no immediate pain, but result in delayed redness, blistering, or tissue damage.

160
Q

What are time-varying gradient fields in MRI?

A

Rapidly changing magnetic fields used for spatial encoding and slice selection.

161
Q

What is peripheral nerve stimulation (PNS) in MRI?

A

PNS occurs when time-varying gradients induce small electric currents, causing muscle twitching or tingling.

162
Q

Why does PNS occur during MRI?

A

The rapid switching of gradient fields stimulates nerves and muscles.

163
Q

Which MRI sequences increase the risk of PNS?

A
  • Echo planar imaging (EPI)
  • Diffusion-weighted imaging (DWI)
  • Fast gradient-echo sequences
164
Q

How can PNS be minimized in MRI?

A
  • Lower gradient amplitude
  • Increase rise time of gradients
  • Use patient positioning techniques
165
Q

Is PNS dangerous in MRI?

A

PNS is generally harmless but may cause patient discomfort.

166
Q

What patient factors increase susceptibility to PNS?

A
  • Low body fat (less insulation)
  • Longer limbs (larger conductive loops)
167
Q

What sensation do patients experience during PNS?

A
  • Mild tingling
  • Involuntary muscle twitching
168
Q

What is the purpose of cryogens in MRI?

A

To keep superconducting magnets cold (typically using liquid helium).

169
Q

What happens during an MRI quench?

A

The cryogen rapidly boils off, shutting down the superconducting magnet.

170
Q

What is the primary safety risk during a quench?

A

Asphyxiation due to helium displacing oxygen.

171
Q

Why must the MRI quench pipe be checked regularly?

A

To ensure proper ventilation of helium outside the building.

172
Q

What happens if a quench pipe fails?

A

Helium disperses into the MRI room, creating an oxygen-deficient environment.

173
Q

What should staff do during a quench?

A

Evacuate the MRI room immediately and ensure proper ventilation.

174
Q

When is an emergency quench necessary?

A
  • A life-threatening projectile incident
  • Fire inside the MRI scanner
  • Trapped patient or staff member
175
Q

Why is MRI so loud?

A

The rapid switching of gradient coils generates loud vibrations.

176
Q

What is the average noise level inside an MRI scanner?

A

Between 80-110 dB, comparable to a rock concert.

177
Q

How can MRI noise be reduced for patients?

A
  • Earplugs
  • Noise-canceling headphones
  • MRI-compatible music systems
178
Q

What are long-term risks of MRI noise exposure?

A

Potential hearing damage in repeated exposures.

179
Q

Which MRI sequences generate the loudest noise?

A
  • Echo planar imaging (EPI)
  • Fast gradient-echo sequences
180
Q

Why should pregnant technologists avoid staying inside MRI rooms?

A

Uncertainty about the effects of prolonged RF exposure on fetal development.

181
Q

Can pregnant patients safely undergo MRI?

A

Yes, but only if medically necessary, avoiding gadolinium contrast.

182
Q

Why should patients with conductive implants be monitored closely?

A

Conductors can induce currents, heat up, or cause burns.

183
Q

What happens if a patient has a seizure inside the MRI?

A
  • Stop the scan immediately
  • Remove the patient from the bore
  • Provide medical assistance
184
Q

Why are implanted devices a major MRI safety concern?

A

MRI’s strong magnetic field can move, heat, or interfere with electronic implants, causing potential injury or malfunction.

185
Q

What does MRI Conditional mean for implanted devices?

A

The device is safe under specific MRI conditions, such as magnetic field strength, SAR limits, and gradient exposure.

186
Q

Which implanted devices are most at risk in MRI?

A
  • Aneurysm clips
  • Cardiac pacemakers & defibrillators
  • Cochlear implants
  • Deep brain stimulators
  • Neurostimulators with leads
187
Q

What should be done before scanning a patient with an implant?

A

Obtain manufacturer documentation and check the MRI safety database to determine scan conditions.

188
Q

Why are ferromagnetic aneurysm clips dangerous in MRI?

A

They may experience torque, causing vessel tearing, stroke, or hemorrhage.

189
Q

What does Lenz’s law explain about metallic implants in MRI?

A

Nonferrous metals can still experience forces due to eddy currents, potentially affecting function.

190
Q

How can pacemakers malfunction in an MRI scanner?

A

RF pulses and magnetic fields may cause inappropriate pacing, power loss, or lead heating.

191
Q

Why are retained bullet fragments a concern for MRI safety?

A

They can be ferromagnetic, causing displacement, heating, or distortion.

192
Q

Why is an MRI screening form alone insufficient?

A

Some patients may forget implants or not know their implant’s MRI compatibility.

193
Q

Why should patients with orbital foreign bodies be screened carefully?

A

Metal fragments in the eye may move during MRI, causing serious injury or blindness.

194
Q

What is the primary safety concern with RF fields in MRI?

A

RF pulses cause tissue heating, which can result in burns.

195
Q

Why is skin-to-skin contact a risk for RF burns?

A

It creates a conductive loop, concentrating RF energy in a small area.

196
Q

How can ECG leads cause burns in MRI?

A

Loops in the leads can act as antennas, absorbing RF energy and overheating.

197
Q

What is the best way to position conductive wires to prevent burns?

A

Keep wires straight, not looped, and insulated from the skin.

198
Q

Why are neonates and anesthetized patients at higher risk for RF burns?

A

They cannot report pain if heating occurs during the scan.

199
Q

How can SAR levels be reduced to prevent RF burns?

A
  • Lower flip angle
  • Increase TR
  • Use fewer slices
  • Avoid fast spin-echo sequences
200
Q

How does RF frequency change with MRI field strength?

A

Higher field strengths require higher RF frequencies, increasing SAR and heating risks.

201
Q

What causes peripheral nerve stimulation (PNS) in MRI?

A

Rapid switching of gradient magnetic fields induces currents in tissues.

202
Q

What sensations do patients feel during PNS?

A
  • Tingling
  • Muscle twitching
  • Throbbing
203
Q

Can MRI stimulate cardiac muscles?

A

No, diagnostic MRI does not reach the cardiac stimulation threshold.

204
Q

What sequence types are most likely to cause PNS?

A
  • Echo-planar imaging (EPI)
  • Diffusion-weighted imaging (DWI)
  • Fast gradient-echo sequences
205
Q

What is the primary safety concern of cryogens in MRI?

A

Helium can cause asphyxiation, burns, or pressure buildup.

206
Q

What happens during a quench?

A

The magnet loses superconductivity, rapidly boiling off helium.

207
Q

What is the expansion ratio of liquid helium to gas?

A

1 liter of liquid helium expands to 754 liters of gas.

208
Q

What should be done if helium leaks into the MRI room?

A

Evacuate immediately due to the risk of asphyxiation.

209
Q

Why might a quench prevent MRI room door access?

A

Rapid helium release can cause a pressure differential, sealing the door shut.

210
Q

Why is MRI so loud?

A

Rapid switching of gradients generates Lorentz forces, causing vibrations.

211
Q

What is the average dB level inside an MRI scanner?

A

Typically 80–115 dB, similar to a rock concert.

212
Q

What type of hearing protection should be used in MRI?

A
  • Earplugs
  • Noise-canceling headphones
  • MRI-compatible music systems
213
Q

Why are children and elderly patients more sensitive to MRI noise?

A

Their hearing is more sensitive, making noise-induced discomfort more likely.

214
Q

Why should non-MRI staff be screened like patients?

A

They may unknowingly bring ferromagnetic objects into Zone III.

215
Q

What is a common theme in projectile accidents?

A

Unauthorized personnel entering the MRI room with ferromagnetic objects.

216
Q

What should never be assumed about an implant?

A

That it is MRI-safe without official documentation.

217
Q

What is the purpose of MRI safety zones?

A

To restrict access and prevent unauthorized entry into hazardous areas near the MRI scanner.

218
Q

What are the four MRI safety zones?

A

1️⃣ Zone I: General public area
2️⃣ Zone II: Patient prep & screening
3️⃣ Zone III: Restricted access (MRI personnel only)
4️⃣ Zone IV: MRI scanner room

219
Q

Who qualifies as Level 2 MRI personnel?

A

MRI technologists, radiologists, and trained nurses who understand MRI hazards.

220
Q

What should be clearly marked in Zone IV?

A

A red light & sign stating ‘The Magnet is On’ at all times.

221
Q

What should MRI screening forms include?

A
  • Implants & medical devices
  • Past surgeries
  • Metallic foreign bodies
  • Claustrophobia history
222
Q

What causes projectile accidents in MRI?

A

Ferromagnetic objects are rapidly pulled into the magnet due to its high spatial field gradient.

223
Q

What common hospital equipment is a known projectile hazard?

A
  • Oxygen tanks
  • Scissors & clamps
  • Wheelchairs & stretchers
224
Q

What was the most famous MRI projectile accident?

A

A 6-year-old boy was fatally struck by an oxygen tank in 2001.

225
Q

Why is magnetic shielding important in MRI?

A

It limits the fringe field, reducing accidental projectile risks.

226
Q

What non-metallic materials should be used for MRI safety equipment?

A

Brass, titanium, plastic, or carbon fiber.

227
Q

Why does MRI cause heating?

A

RF pulses cause eddy currents, leading to tissue heating.

228
Q

What is SAR (Specific Absorption Rate)?

A

A measure of RF energy absorbed by the body per kilogram.

229
Q

What are safe SAR limits in MRI?

A

Whole-body SAR should not exceed 4 W/kg in first-level controlled mode.

230
Q

What patient positioning errors increase burn risks?

A
  • Hands touching thighs
  • Feet touching
  • Looped cables & wires
231
Q

How can burn risks be reduced in MRI?

A

Use insulating pads to break skin-to-skin contact.

232
Q

What causes peripheral nerve stimulation in MRI?

A

Rapid gradient switching induces electric currents in nerves.

233
Q

What does PNS feel like for patients?

A
  • Tingling
  • Muscle twitching
  • Mild discomfort
234
Q

How is PNS minimized in MRI?

A

Using slower gradient switching and adjusting pulse sequences.

235
Q

Why is MRI so loud?

A

Gradient coils create Lorentz forces, causing vibrations.

236
Q

What are MRI noise levels in dB?

A

Ranges from 80–115 dB, similar to a rock concert.

237
Q

What type of hearing protection is recommended for MRI?

A
  • Earplugs
  • MRI-safe headphones
  • Noise-canceling systems
238
Q

What happens during an MRI quench?

A

Liquid helium rapidly boils off, releasing gas into the room.

239
Q

What is the main danger of a quench?

A

Helium displaces oxygen, causing asphyxiation.

240
Q

How should a quench pipe failure be handled?

A

Evacuate immediately due to risk of suffocation.

241
Q

What does fMRI measure?

A

Changes in blood oxygen levels (BOLD contrast).

242
Q

What is the primary use of fMRI?

A

To map brain activity and localize functions.

243
Q

What is perfusion imaging used for?

A

To assess blood flow in tissues, often for stroke evaluation.

244
Q

What contrast agent is commonly used in perfusion MRI?

A

Gadolinium-based contrast agents (GBCA).

245
Q

What is the most common artifact in MRI?

A

Motion artifact caused by patient movement.

246
Q

How can motion artifacts be reduced?

A
  • Patient immobilization
  • Breath-hold sequences
  • Navigator echoes
247
Q

What is a chemical shift artifact?

A

A misalignment of fat & water signals due to different Larmor frequencies.

248
Q

How can aliasing (wrap-around) artifact be prevented?

A

Increase the FOV (field of view) or use anti-aliasing filters.

MRI Physics (6 decks)

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