Magnetism - Gradient Coils Flashcards
Which of the following has been documented to cause muscle contractions, cardiac arrhythmias, mild cutaneous sensations and visual light flashes?
Time varying magnetic fields (gradient)
Which of the following’s primary biological effect is tissue heating/deposition?
Oscillating magnetic fields (radiofrequency)
The orientation of the main magnetic field in a high field, superconducting, short bore magnet is:
Horizontal
Right Hand Grip rule – Current passes through electrically induced coil of wire
The ____________ is responsible for the knocking noise the system makes while the sequence is in progress.
Rapid change/switching of the magnetic field by the gradient coils
The rapid, successive switching of the gradient coils is the primary cause of the noise in the MR scan room.
If a sagittal slice plane has been selected, the __________ is enabled to alter the magnetic field around the patient’s body.
X gradient
If a coronal slice plane has been selected, the __________ is enabled to alter the magnetic field around the patient’s body.
Y gradient
Y gradient encodes coronal slice orientation
If a transverse/axial slice plane has been selected, the __________ is enabled to alter the magnetic field around the patient’s body.
Z gradient
Z gradient is utilized to acquire axial/transverse images on a patient lying supine/prone in the magnet.
The primary biological effect of radiofrequency fields is:
Tissue heating
The measurement of radiofrequency absorption is delineated as:
Watts per kilogram
The effects of time varying magnetic fields can include all of the following EXCEPT:
Warmth and/or increase in body temperature
The effects of time varying magnetic fields can include peripheral nerve stimulation, acoustic damage/hearing loss, magnetophosphenes (“seeing stars”).
Current FDA limits on time varying magnetic fields are:
Once the patient experiences peripheral nerve stimulation
Current FDA guidance limits the Time Rate of Change of Magnetic Field (dB/dt) to levels which do not result in painful peripheral nerve stimulation.
Which of the following sequences would be more at risk for the effects that time varying magnetic fields can cause?
EPI
EPI (Echo Planar Imaging) sequences can have more of a detrimental effect on patients with regards to time varying magnetic fields.
The strength of the gradient magnetic fields are measured in:
A and D
The gradient rise time is defined as the:
Time it takes for the gradient to reach its full amplitude
The gradient slew rate is the:
Rate of speed of ascent or descent of a gradient from zero to its maximum amplitude
The gradient slew rate is defined as the rate of speed of ascent or descent of a gradient from zero to its maximum amplitude
The duty cycle is defined as the:
Time the gradients are on during a TR period
The duty cycle is defined as the time the gradients are on during a TR period
RF (radiofrequency) energy is:
Low energy non-ionizing radiation
Magnetic field inhomogeneity is expressed in:
Parts per million
Magnetic field inhomogeneity is expressed in parts per million (ppm)
A technique utilized to reduce the MRI scan time which requires the use of an array coil is:
Parallel imaging
Utilization of surface coils yields all of the following EXCEPT:
Increased FOV capabilty
Cables from RF coils and gating/trigger leads should be:
Braided and positioned straight in the bore
Cables from RF coils and gating leads should be braided and positioned straight in the bore, NEVER in direct contact with the patient’s skin
RF heating is more of a concern in ________ sequences.
Fast spin echo
RF heating is more of a concern in fast spin echo sequences due to the multiple echo train lengths, leading to potential increases in tissue heating.
Applying two gradients simultaneously during slice selection would:
Produce an oblique slice
Applying two gradients simultaneously during slice selection would produce an oblique slice.
Multiple coil elements combined with multiple receiver channels make a:
Phased array coil
A phased array coil is comprised of multiple coil elements combined with multiple receiver channels
The strength of the gradient over a specific distance is known as the:
Slew rate
A coil that is not properly tuned to the correct magnetic field strength will result in:
Signal loss
A coil that is not properly tuned to the correct magnetic field strength will result in signal loss.
The coil construction in an MR system, from the innermost aspect to the outer:
Radiofrequency coils, gradient coils, shim coils, main magnet
The coil construction in an MR system, from the innermost aspect to the outer: Radiofrequency coils, gradient coils, shim coils, main magnet.
A 6 inch surface coil is able to receive signal:
3 inches deep into the patient
When going from a linear coil to a quadrature coil:
SNR is increased by 40%
When going from a linear coil to a quadrature coil, SNR is increased by 40%.
The ______________ is the speed rate of ascent or descent of a gradient from zero to its maximum amplitude, either positive or negative
Slew rate
