ASMIRT - MRI Accreditation Exam
Questions and Answers
Question 1
Mild sensory effects due to static magnetic fields include:
Correct Answer
• Vertigo
• Nausea
• Taste Sensations
Question 2
Sensory effects of the static magnetic field increase or decrease with field strength?
Correct Answer
Sensory effects are related to the strength of the magnetic field with few symptoms
at 1.5T and more appearing approaching a 4T field strength
Question 3
Explain what t-wave artefact is and why it occurs
Correct Answer
Any charged particle moving through a magnetic field will have a current induced
via this interaction. As blood, a conductive fluid flows through a magnetic field it
induces an electrical biopotential. This electrical biopotential can be seen on the
ECG waveform of patients in MRI. It is demonstrated by an increase in the T-wave
amplitude on ECG. This change in T-wave amplitude is directly proportional to the
strength of the Static Filed. It is important that MR radiographers are aware of this
as elevated T-waves are also associated with ischemia and myocardial infarction.
Page 1 of 78
,Question 4
What risks does the static magnetic field pose in terms of implants and objects?
Correct Answer
Static Fields also pose hazards through the displacement of ferro-magnetic implants
and objects. The static field imparts both a translation (attractive force) and a torque
(twisting force). The strong nature of the Static Field also has the potential to disrupt
the function of Cardiac Pacemakers. Anyone entering beyond the 5G line should be
screened for contraindications to strong Magnetic Fields. Also be aware of what is
being taken over the 5G line.
Question 5
What strategies can be employed to reduce the risks of the static magnetic field
impacting on implants and objects?
Correct Answer
• Ferromagnetic objects may become airborne - 40 km/hr terminal velocity possible
at 1.5T
• Test any metal objects with a hand held magnet before allowing them to enter the
room.
• Keep the general public behind the 5G line. •
Possible contra-indications include pacemakers, aneurysm clips, intra-vascular coils
and stents, heart valves, penile implants, cochlear implants, ocular implants (Fatio
eyelid springs), neuro stimulators, bone growth stimulators, drug infusion pumps.
Page 2 of 78
,Question 6
What effect do gradient magnetic fields have on patient safety?
Correct Answer
Faraday's Law dictates that changing magnetic fields will generate a voltage and a
current in a conductor. The switching of the gradients induces electrical currents in
conductive tissues. Voltage generation will occur during the rise and fall times - ie.
while the field is changing. The largest voltages will be generated at the periphery
where the gradient amplitudes are highest. The most common result of these
voltages is Peripheral Nerve Stimulation (PNS). Other side-effects are listed below;
• magnetophosphenes - electromagnetically induced flashes of light
• seizures
• tissue heating
• peripheral nerve stimulation
• muscle contractions
• cardiac arrhythmias
• some degree of patient heating has been shown to be a result of Gradient
Magnetic effects. However, this has been shown to be so low as to be negligible
Question 7
What strategies can be employed to keep gradient effects to a minimum?
Correct Answer
In order to keep these effects to a minimum the following is recommended.
• keep dB/dT less than those required to produce peripheral nerve stimulation.
• max limit for dB/dT of 6 Tesla / sec
Question 8
What is the cause of acoustic noise during an MRI scan?
Correct Answer
The characteristic noise that is heard during an MRI scan is the result of the
interaction of Lorentz forces generated by the Gradient Coil when a current is
pulsed through them in the presence of a Static Magnetic Field. The noise results as
these forces are so strong as to twist the coils on their mounts. In some cases this
can be in excess of 100dB particularly when running EPI sequences. Thus all patients
need to be provided hearing protection.
Page 3 of 78
, Question 9
What is the greatest concern in terms of MR safety?
Correct Answer
RF effects are of the greatest concern in terms of MR safety. The main effect is the
deposition of energy resulting in tissue heating. Of particular concern are heat
sensitive organs such as the eyes and testes. One of the major concerns is where a
patient has a metallic implant which results in greater heating.
Question 10
RF field safety concerns increase or decrease with increasing field strength>
Correct Answer
The amount of energy absorbed will increase with frequency of the RF field
delivered and is therefore greater at higher field strengths
Question 11
What is SAR?
Correct Answer
SAR= Specific Absorption Rate - the term used to describe energy dissipation
Question 12
SAR is expressed in:
Correct Answer
Watts / kg
Question 13
SAR will depend on factors including:
Correct Answer
• frequency (& therefore the field strength)
• RF pulse - where power deposited is proportional to (flip angle / 90)
• TR
• RF coil - transmit / receive or receive only
• volume of tissue within the coil
• conductivity of the tissue
Page 4 of 78
Questions and Answers
Question 1
Mild sensory effects due to static magnetic fields include:
Correct Answer
• Vertigo
• Nausea
• Taste Sensations
Question 2
Sensory effects of the static magnetic field increase or decrease with field strength?
Correct Answer
Sensory effects are related to the strength of the magnetic field with few symptoms
at 1.5T and more appearing approaching a 4T field strength
Question 3
Explain what t-wave artefact is and why it occurs
Correct Answer
Any charged particle moving through a magnetic field will have a current induced
via this interaction. As blood, a conductive fluid flows through a magnetic field it
induces an electrical biopotential. This electrical biopotential can be seen on the
ECG waveform of patients in MRI. It is demonstrated by an increase in the T-wave
amplitude on ECG. This change in T-wave amplitude is directly proportional to the
strength of the Static Filed. It is important that MR radiographers are aware of this
as elevated T-waves are also associated with ischemia and myocardial infarction.
Page 1 of 78
,Question 4
What risks does the static magnetic field pose in terms of implants and objects?
Correct Answer
Static Fields also pose hazards through the displacement of ferro-magnetic implants
and objects. The static field imparts both a translation (attractive force) and a torque
(twisting force). The strong nature of the Static Field also has the potential to disrupt
the function of Cardiac Pacemakers. Anyone entering beyond the 5G line should be
screened for contraindications to strong Magnetic Fields. Also be aware of what is
being taken over the 5G line.
Question 5
What strategies can be employed to reduce the risks of the static magnetic field
impacting on implants and objects?
Correct Answer
• Ferromagnetic objects may become airborne - 40 km/hr terminal velocity possible
at 1.5T
• Test any metal objects with a hand held magnet before allowing them to enter the
room.
• Keep the general public behind the 5G line. •
Possible contra-indications include pacemakers, aneurysm clips, intra-vascular coils
and stents, heart valves, penile implants, cochlear implants, ocular implants (Fatio
eyelid springs), neuro stimulators, bone growth stimulators, drug infusion pumps.
Page 2 of 78
,Question 6
What effect do gradient magnetic fields have on patient safety?
Correct Answer
Faraday's Law dictates that changing magnetic fields will generate a voltage and a
current in a conductor. The switching of the gradients induces electrical currents in
conductive tissues. Voltage generation will occur during the rise and fall times - ie.
while the field is changing. The largest voltages will be generated at the periphery
where the gradient amplitudes are highest. The most common result of these
voltages is Peripheral Nerve Stimulation (PNS). Other side-effects are listed below;
• magnetophosphenes - electromagnetically induced flashes of light
• seizures
• tissue heating
• peripheral nerve stimulation
• muscle contractions
• cardiac arrhythmias
• some degree of patient heating has been shown to be a result of Gradient
Magnetic effects. However, this has been shown to be so low as to be negligible
Question 7
What strategies can be employed to keep gradient effects to a minimum?
Correct Answer
In order to keep these effects to a minimum the following is recommended.
• keep dB/dT less than those required to produce peripheral nerve stimulation.
• max limit for dB/dT of 6 Tesla / sec
Question 8
What is the cause of acoustic noise during an MRI scan?
Correct Answer
The characteristic noise that is heard during an MRI scan is the result of the
interaction of Lorentz forces generated by the Gradient Coil when a current is
pulsed through them in the presence of a Static Magnetic Field. The noise results as
these forces are so strong as to twist the coils on their mounts. In some cases this
can be in excess of 100dB particularly when running EPI sequences. Thus all patients
need to be provided hearing protection.
Page 3 of 78
, Question 9
What is the greatest concern in terms of MR safety?
Correct Answer
RF effects are of the greatest concern in terms of MR safety. The main effect is the
deposition of energy resulting in tissue heating. Of particular concern are heat
sensitive organs such as the eyes and testes. One of the major concerns is where a
patient has a metallic implant which results in greater heating.
Question 10
RF field safety concerns increase or decrease with increasing field strength>
Correct Answer
The amount of energy absorbed will increase with frequency of the RF field
delivered and is therefore greater at higher field strengths
Question 11
What is SAR?
Correct Answer
SAR= Specific Absorption Rate - the term used to describe energy dissipation
Question 12
SAR is expressed in:
Correct Answer
Watts / kg
Question 13
SAR will depend on factors including:
Correct Answer
• frequency (& therefore the field strength)
• RF pulse - where power deposited is proportional to (flip angle / 90)
• TR
• RF coil - transmit / receive or receive only
• volume of tissue within the coil
• conductivity of the tissue
Page 4 of 78
