ASMIRT - MRI LATEST
ACCREDITATION EXAM
What is magnetic shielding? - This is done to reduce the size of the
fringe field which may extend over a considerable area with a high field
superconductive system. This will minimise both the exclusion zone for
the general public as well as the area over which the homogeneity of the
magnetic field can be affected by metallic objects such as lifts and motor
vehicles. Passive shielding is done with steel plates either as part of the
magnet gantry or attached to the walls, ceiling and floor of the
examination room. Active shielding uses a second coil external to the
windings of the main magnetic field. The field generated by this
secondary coil is oriented to oppose the main field and reduce its spread.
What is RF shielding? - Magnetic shielding should not to be confused
with RF shielding which keeps unwanted RF from entering the
examination room and interfering with the detection of the MR signal.
RF shielding involves placing a Faraday Cage around the scan room.
This cage prevents any extraneous RF from entering or exiting the room
What is the gradient system? - The gradient system is designed to create
linear variations in the magnetic field to allow spatial localisation of the
MR signal. Three separate coils permit a gradient to be applied in the X,
Y or Z direction. Two gradients used in combination will permit oblique
slices to be acquired. Spatial resolution and speed depend on gradient
performance which is usually stated in terms of maximum amplitude and
rise time
END OF
PAGE
1
, ASMIRT - MRI LATEST
ACCREDITATION EXAM
What does the maximum amplitude of a gradient system mean? - This is
the highest gradient strength that the amplifier and coils can sustain.
Higher amplitudes will allow better resolution through smaller FOV's
and thinner slices. It is expressed in terms of mTesla per metre. (or
sometimes Gauss per cm) Typical values range from 10 to 25 mT/m
What does the rise time of a gradient system mean? - The rise time is the
period necessary for the gradient to reach its maximum amplitude - eg.
to go from zero to 23mT/m. The shorter this time the better - this will
allow shorter TE's, more slices per TR interval and shorter echo spacing
on FSE sequences. Rise times are expressed in mSec.
What does the slew rate of a gradient system mean? - The slew rate is
described as the strength of the gradient over distance as a function of
time. Typical gradient slew rates are in the order of 70mT/m/s. High
speed gradients are generally even stronger at 120mT/m/s. Comparisons
should also take into account the 'Duty Cycle' - which is the percentage
of time that the gradients can be switched on at maximum amplitude
during a TR interval
What is an eddy current and why are the problematic? - Gradient
switching will cause eddy currents to be induced in conductors including
END OF
PAGE
2
, ASMIRT - MRI LATEST
ACCREDITATION EXAM
the patient, the cables and most importantly, the scanner itself. Unless
compensated for, they will distort the gradient wave-form which will
impair the overall performance and create artefacts.
How can eddy currents be minimised? - • eddy current compensation or
pre-emphasis - whereby the gradient currents are deliberately altered to
compensate for the distortions created by eddy currents.
• shielded gradients which incorporate a second set of gradient windings
which are energised in the opposite direction to counteract the effect of
the imaging gradient.
Which sequences are the most likely to produce eddy currents? -
Gradient echo and particularly fast gradient echo sequences have the
greatest potential to produce eddy currents because of their intensive use
of rapid gradient switching.
What is the role of the RF coil? - RF coils are required to:
- Deliver the rotating B1 field for excitation and rephasing
- Receive the MR signal.
END OF
PAGE
3
, ASMIRT - MRI LATEST
ACCREDITATION EXAM
What is a linear coil? - The rotating magnetisation can be seen as having
one component moving left and right along the horizontal axis and a
second component moving up and down along the vertical axis. Linear
coils will receive only one of these components.
What is quadrature coil? - Quadrature coils use two receivers to detect
both components which are phase shifted by 90 degrees. When the
signal in the first coil is maximum, it will be minimal in the second coil
(and vice versa) The result is the detection of two signals, one at the
phase of the RF oscillator in the scanner and one phase shifted by 90
degrees. These are known as the 'real' and 'imaginary' components
respectively.
How do linear and quadrature coils differ in terms of SNR? - Compared
to linear design, using a quadrature coil results in a 1.4 (square root 2)
increase in the SNR of the signal received and a much more efficient
transfer of power to the patient in terms of the excitation and rephasing
pulses.
Explain how quadrature coil signal readout is achieved? - During
readout, both signals are detected and digitised so each k-space cell
contains a real and an imaginary number. 'Real' or 'imaginary' images
may be reconstructed by Fourier transformation of the relevant data, or
END OF
PAGE
4
ACCREDITATION EXAM
What is magnetic shielding? - This is done to reduce the size of the
fringe field which may extend over a considerable area with a high field
superconductive system. This will minimise both the exclusion zone for
the general public as well as the area over which the homogeneity of the
magnetic field can be affected by metallic objects such as lifts and motor
vehicles. Passive shielding is done with steel plates either as part of the
magnet gantry or attached to the walls, ceiling and floor of the
examination room. Active shielding uses a second coil external to the
windings of the main magnetic field. The field generated by this
secondary coil is oriented to oppose the main field and reduce its spread.
What is RF shielding? - Magnetic shielding should not to be confused
with RF shielding which keeps unwanted RF from entering the
examination room and interfering with the detection of the MR signal.
RF shielding involves placing a Faraday Cage around the scan room.
This cage prevents any extraneous RF from entering or exiting the room
What is the gradient system? - The gradient system is designed to create
linear variations in the magnetic field to allow spatial localisation of the
MR signal. Three separate coils permit a gradient to be applied in the X,
Y or Z direction. Two gradients used in combination will permit oblique
slices to be acquired. Spatial resolution and speed depend on gradient
performance which is usually stated in terms of maximum amplitude and
rise time
END OF
PAGE
1
, ASMIRT - MRI LATEST
ACCREDITATION EXAM
What does the maximum amplitude of a gradient system mean? - This is
the highest gradient strength that the amplifier and coils can sustain.
Higher amplitudes will allow better resolution through smaller FOV's
and thinner slices. It is expressed in terms of mTesla per metre. (or
sometimes Gauss per cm) Typical values range from 10 to 25 mT/m
What does the rise time of a gradient system mean? - The rise time is the
period necessary for the gradient to reach its maximum amplitude - eg.
to go from zero to 23mT/m. The shorter this time the better - this will
allow shorter TE's, more slices per TR interval and shorter echo spacing
on FSE sequences. Rise times are expressed in mSec.
What does the slew rate of a gradient system mean? - The slew rate is
described as the strength of the gradient over distance as a function of
time. Typical gradient slew rates are in the order of 70mT/m/s. High
speed gradients are generally even stronger at 120mT/m/s. Comparisons
should also take into account the 'Duty Cycle' - which is the percentage
of time that the gradients can be switched on at maximum amplitude
during a TR interval
What is an eddy current and why are the problematic? - Gradient
switching will cause eddy currents to be induced in conductors including
END OF
PAGE
2
, ASMIRT - MRI LATEST
ACCREDITATION EXAM
the patient, the cables and most importantly, the scanner itself. Unless
compensated for, they will distort the gradient wave-form which will
impair the overall performance and create artefacts.
How can eddy currents be minimised? - • eddy current compensation or
pre-emphasis - whereby the gradient currents are deliberately altered to
compensate for the distortions created by eddy currents.
• shielded gradients which incorporate a second set of gradient windings
which are energised in the opposite direction to counteract the effect of
the imaging gradient.
Which sequences are the most likely to produce eddy currents? -
Gradient echo and particularly fast gradient echo sequences have the
greatest potential to produce eddy currents because of their intensive use
of rapid gradient switching.
What is the role of the RF coil? - RF coils are required to:
- Deliver the rotating B1 field for excitation and rephasing
- Receive the MR signal.
END OF
PAGE
3
, ASMIRT - MRI LATEST
ACCREDITATION EXAM
What is a linear coil? - The rotating magnetisation can be seen as having
one component moving left and right along the horizontal axis and a
second component moving up and down along the vertical axis. Linear
coils will receive only one of these components.
What is quadrature coil? - Quadrature coils use two receivers to detect
both components which are phase shifted by 90 degrees. When the
signal in the first coil is maximum, it will be minimal in the second coil
(and vice versa) The result is the detection of two signals, one at the
phase of the RF oscillator in the scanner and one phase shifted by 90
degrees. These are known as the 'real' and 'imaginary' components
respectively.
How do linear and quadrature coils differ in terms of SNR? - Compared
to linear design, using a quadrature coil results in a 1.4 (square root 2)
increase in the SNR of the signal received and a much more efficient
transfer of power to the patient in terms of the excitation and rephasing
pulses.
Explain how quadrature coil signal readout is achieved? - During
readout, both signals are detected and digitised so each k-space cell
contains a real and an imaginary number. 'Real' or 'imaginary' images
may be reconstructed by Fourier transformation of the relevant data, or
END OF
PAGE
4
