ASMIRT - MRI ACCREDITATION EXAM
QUESTIONS & ANSWERS VERIFIED 100%
CORRECT!!
Mild sensory effects due to static magnetic fields include: Answer - • Vertigo
• Nausea
• Taste Sensations
Sensory effects of the static magnetic field increase or decrease with field
strength? 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
Explain what t-wave artefact is and why it occurs 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.
What risks does the static magnetic field pose in terms of implants and
objects? 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.
What strategies can be employed to reduce the risks of the static magnetic
field impacting on implants and objects? 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.
What effect do gradient magnetic fields have on patient safety? 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
,What strategies can be employed to keep gradient effects to a minimum?
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
What is the cause of acoustic noise during an MRI scan? 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.
What is the greatest concern in terms of MR safety? 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.
RF field safety concerns increase or decrease with increasing field strength>
Answer - The amount of energy absorbed will increase with frequency of the
RF field delivered and is therefore greater at higher field strengths
What is SAR? Answer - SAR= Specific Absorption Rate - the term used to
describe energy dissipation
SAR is expressed in: Answer - Watts / kg
SAR will depend on factors including: 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
What determines the SAR calculation? Answer - The SAR is calculated for each
sequence on the basis of the sequence parameters and the patient weight
What are the FDA recommendations regarding SAR? Answer - • Maximum SAR
of 0.4 W/kg (whole body)
• 3.2 W/kg (head)
• 8.0 W/kg (in any one gram of tissue)
• RF exposure should be insufficient to produce a core temperature increase of
1 degree C
Why is patient heating a concern in MRI? Answer - • Heating will also vary with
the state of the patient's thermo-regulatory system, the ambient temperature,
the humidity and the air flow around the patient.
• The eye and the testes are particularly sensitive organs due to a low capacity
for heat dissipation.
• Excessive RF exposure and temperature increase can lead to an increase in
blood pressure and heart rate.
A SAR warning appears during a scan preventing you from continuing with an
examination unless factors are altered to a safe level, what can be changed in
order to achieve this? Answer - Factors which will reduce SAR if required
include:
• reducing the number of slices
QUESTIONS & ANSWERS VERIFIED 100%
CORRECT!!
Mild sensory effects due to static magnetic fields include: Answer - • Vertigo
• Nausea
• Taste Sensations
Sensory effects of the static magnetic field increase or decrease with field
strength? 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
Explain what t-wave artefact is and why it occurs 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.
What risks does the static magnetic field pose in terms of implants and
objects? 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.
What strategies can be employed to reduce the risks of the static magnetic
field impacting on implants and objects? 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.
What effect do gradient magnetic fields have on patient safety? 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
,What strategies can be employed to keep gradient effects to a minimum?
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
What is the cause of acoustic noise during an MRI scan? 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.
What is the greatest concern in terms of MR safety? 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.
RF field safety concerns increase or decrease with increasing field strength>
Answer - The amount of energy absorbed will increase with frequency of the
RF field delivered and is therefore greater at higher field strengths
What is SAR? Answer - SAR= Specific Absorption Rate - the term used to
describe energy dissipation
SAR is expressed in: Answer - Watts / kg
SAR will depend on factors including: 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
What determines the SAR calculation? Answer - The SAR is calculated for each
sequence on the basis of the sequence parameters and the patient weight
What are the FDA recommendations regarding SAR? Answer - • Maximum SAR
of 0.4 W/kg (whole body)
• 3.2 W/kg (head)
• 8.0 W/kg (in any one gram of tissue)
• RF exposure should be insufficient to produce a core temperature increase of
1 degree C
Why is patient heating a concern in MRI? Answer - • Heating will also vary with
the state of the patient's thermo-regulatory system, the ambient temperature,
the humidity and the air flow around the patient.
• The eye and the testes are particularly sensitive organs due to a low capacity
for heat dissipation.
• Excessive RF exposure and temperature increase can lead to an increase in
blood pressure and heart rate.
A SAR warning appears during a scan preventing you from continuing with an
examination unless factors are altered to a safe level, what can be changed in
order to achieve this? Answer - Factors which will reduce SAR if required
include:
• reducing the number of slices
