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Questions and Answers

What is the main purpose of the RF-transmit chain?

  • To generate an oscillating magnetic field
  • To detect the MR signal
  • To generate a static main magnetic field
  • To shape the RF-pulse into a short burst of energy (correct)

    • What is the most efficient coil for MRI?

  • Linear polarized RF-coil
  • Single-loop RF-coil
  • Circularly polarized (CP) coil (correct)
  • Oscillating magnetic field

    • What is the most important factor for magnetic field homogeneity?

  • Temperature stability (correct)
  • Active shimming
  • Passive shimming
  • Ferromagnetic materials

    • What is the most effective way to control eddy currents?

    <p>Prevent them from being generated in the first place</p> Signup and view all the answers

    What type of shimming is used in magnets with fields of 3T or higher?

    <p>Superconducting shimming</p> Signup and view all the answers

    What is the simplest form of RF-coil?

    <p>Single-loop RF-coil</p> Signup and view all the answers

    What is the purpose of resistive shim coils?

    <p>To be dynamically changed to create different images</p> Signup and view all the answers

    What is the purpose of the RF body coil?

    <p>To transmit the B1 field</p> Signup and view all the answers

    What type of materials are typically used for passive shimming?

    <p>Steel</p> Signup and view all the answers

    What is the typical duration of B1 pulses?

    <p>A few milliseconds</p> Signup and view all the answers

    What is the purpose of the “magnet stop” button?

    <p>To initiate an emergency quench and turn off the power to most of the scanner</p> Signup and view all the answers

    What type of current is driven through a single-loop RF-coil?

    <p>Sinusoidal current</p> Signup and view all the answers

    What type of artifacts are commonly observed with poor homogeneity?

    <p>Curved slice profiles</p> Signup and view all the answers

    What does the Larmor frequency depend on?

    <p>The main magnetic field strength (Bo</p> Signup and view all the answers

    What are two undesired phenomena caused by eddy currents within the MR scanner?

    <p>Unwanted time-varying gradients and shifts in the main magnetic field</p> Signup and view all the answers

    What is the purpose of RF-coils?

    <p>To generate an oscillating magnetic field</p> Signup and view all the answers

    What is the purpose of active shimming?

    <p>To correct for individual variations in field</p> Signup and view all the answers

    What is the main disadvantage of using a linear polarized RF-coil?

    <p>It wastes power</p> Signup and view all the answers

    What is the purpose of the “emergency run down” button?

    <p>To initiate a controlled quench and turn off the power to the magnet</p> Signup and view all the answers

    What is active shielding used for?

    <p>To reduce the static fringe field generated by the main magnet</p> Signup and view all the answers

    What type of coil is used to create the magnetic field inside an MRI scanner?

    <p>Coil of wire</p> Signup and view all the answers

    What are the two types of coils that can be used to transmit or receive NMR signals?

    <p>Linear and circularly polarized (CP) coils</p> Signup and view all the answers

    What is the range of the Larmor frequency for clinical MRI?

    <p>Tens to hundreds of MHz</p> Signup and view all the answers

    What type of stability is important for spectral fat suppression and MRS?

    <p>Temporal</p> Signup and view all the answers

    What is active shimming used for?

    <p>To increase magnetic field homogeneity</p> Signup and view all the answers

    What are patient coils used for?

    <p>To detect the MR signal</p> Signup and view all the answers

    What type of problems can poor homogeneity cause?

    <p>Shading</p> Signup and view all the answers

    What type of shimming relies on the passage of current through coils located near the room-temperature inner bore of the scanner?

    <p>Resistive shimming</p> Signup and view all the answers

    What is the purpose of RF gain/attenuation?

    <p>To ensure the transmitter chain is working properly</p> Signup and view all the answers

    What is the purpose of CP coils?

    <p>To create a rotating B1 field with no wasted power</p> Signup and view all the answers

    What is used to reduce residual linear inhomogeneities in the main magnetic field?

    <p>Gradient offset shimming</p> Signup and view all the answers

    What type of energy is absorbed and emitted by nuclei in MRI?

    <p>Magnetic</p> Signup and view all the answers

    What type of field is produced by driving a sinusoidal alternating current through a single-loop RF-coil?

    <p>Perpendicular field</p> Signup and view all the answers

    What is the purpose of passive shimming?

    <p>To improve static homogeneity of the main magnetic field</p> Signup and view all the answers

    What is actively shielded gradients used for?

    <p>To dynamically cancel changing magnetic fields</p> Signup and view all the answers

    When are eddy currents generated?

    <p>Whenever a magnetic field is present</p> Signup and view all the answers

    What type of shimming uses currents directed through specialized coils to generate a "corrective" magnetic field?

    <p>Active shimming</p> Signup and view all the answers

    What is the purpose of the pulse modulator?

    <p>To shape the RF-pulse into a short burst of energy</p> Signup and view all the answers

    What type of materials are used for active shimming?

    <p>Copper</p> Signup and view all the answers

    What is the main advantage of circularly polarized (CP) coils?

    <p>It creates a rotating B1 field with no wasted power</p> Signup and view all the answers

    What type of stability is important for permanent magnets?

    <p>Temperature</p> Signup and view all the answers

    What is the purpose of active shielding?

    <p>To reduce the static fringe field generated by the main magnet</p> Signup and view all the answers

    What is the structure of resistive shim coils?

    <p>A series of individual wire windings</p> Signup and view all the answers

    What is the purpose of the frequency synthesizer?

    <p>To shape the RF-pulse into a short burst of energy</p> Signup and view all the answers

    What is the main purpose of using a linear polarized RF-coil?

    <p>To be used in small animal imaging and spectroscopy applications</p> Signup and view all the answers

    What is the purpose of the high power amplifier?

    <p>To amplify and send the RF-pulse into the patient</p> Signup and view all the answers

    What type of artifacts are commonly observed with poor homogeneity?

    <p>Blurring</p> Signup and view all the answers

    What type of power is wasted with a linear polarized RF-coil?

    <p>Transmitted power</p> Signup and view all the answers

    What is the purpose of the gradient of the field in an MRI scanner?

    <p>To create images of the inside of the body</p> Signup and view all the answers

    What is the purpose of active shimming?

    <p>To increase magnetic field homogeneity</p> Signup and view all the answers

    Study Notes

    • Magnetic field homogeneity is important for image quality and various artifacts.
    • Shading, spatial distortion, blurring, intensity loss, and curved slice profiles are all commonly observed problems with poor homogeneity.
    • Temporal stability is also important for spectral fat suppression and MRS.
    • Permanent magnets require careful attention to temperature stability.
    • Passive shimming involves the use of ferromagnetic materials, typically iron or steel, placed in a regular pattern at specific locations along the inner bore of the magnet.
    • This method is used to improve static homogeneity of the main magnetic field, but has limitations due to the temperature sensitivity of the shim material and the field distortions caused by diamagnetic susceptibility.
    • Active shimming provides a method to correct for these individual variations in field from patient to patient.

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    Description

    Test your knowledge of magnetic field homogeneity in MRI with this quiz. Learn about the importance of homogeneity for image quality, common problems associated with poor homogeneity, and methods for improving magnetic field homogeneity.

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