Podcast
Questions and Answers
What is the main purpose of the RF-transmit chain?
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?
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?
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?
What is the most effective way to control eddy currents?
What type of shimming is used in magnets with fields of 3T or higher?
What type of shimming is used in magnets with fields of 3T or higher?
What is the simplest form of RF-coil?
What is the simplest form of RF-coil?
What is the purpose of resistive shim coils?
What is the purpose of resistive shim coils?
What is the purpose of the RF body coil?
What is the purpose of the RF body coil?
What type of materials are typically used for passive shimming?
What type of materials are typically used for passive shimming?
What is the typical duration of B1 pulses?
What is the typical duration of B1 pulses?
What is the purpose of the “magnet stop” button?
What is the purpose of the “magnet stop” button?
What type of current is driven through a single-loop RF-coil?
What type of current is driven through a single-loop RF-coil?
What type of artifacts are commonly observed with poor homogeneity?
What type of artifacts are commonly observed with poor homogeneity?
What does the Larmor frequency depend on?
What does the Larmor frequency depend on?
What are two undesired phenomena caused by eddy currents within the MR scanner?
What are two undesired phenomena caused by eddy currents within the MR scanner?
What is the purpose of RF-coils?
What is the purpose of RF-coils?
What is the purpose of active shimming?
What is the purpose of active shimming?
What is the main disadvantage of using a linear polarized RF-coil?
What is the main disadvantage of using a linear polarized RF-coil?
What is the purpose of the “emergency run down” button?
What is the purpose of the “emergency run down” button?
What is active shielding used for?
What is active shielding used for?
What type of coil is used to create the magnetic field inside an MRI scanner?
What type of coil is used to create the magnetic field inside an MRI scanner?
What are the two types of coils that can be used to transmit or receive NMR signals?
What are the two types of coils that can be used to transmit or receive NMR signals?
What is the range of the Larmor frequency for clinical MRI?
What is the range of the Larmor frequency for clinical MRI?
What type of stability is important for spectral fat suppression and MRS?
What type of stability is important for spectral fat suppression and MRS?
What is active shimming used for?
What is active shimming used for?
What are patient coils used for?
What are patient coils used for?
What type of problems can poor homogeneity cause?
What type of problems can poor homogeneity cause?
What type of shimming relies on the passage of current through coils located near the room-temperature inner bore of the scanner?
What type of shimming relies on the passage of current through coils located near the room-temperature inner bore of the scanner?
What is the purpose of RF gain/attenuation?
What is the purpose of RF gain/attenuation?
What is the purpose of CP coils?
What is the purpose of CP coils?
What is used to reduce residual linear inhomogeneities in the main magnetic field?
What is used to reduce residual linear inhomogeneities in the main magnetic field?
What type of energy is absorbed and emitted by nuclei in MRI?
What type of energy is absorbed and emitted by nuclei in MRI?
What type of field is produced by driving a sinusoidal alternating current through a single-loop RF-coil?
What type of field is produced by driving a sinusoidal alternating current through a single-loop RF-coil?
What is the purpose of passive shimming?
What is the purpose of passive shimming?
What is actively shielded gradients used for?
What is actively shielded gradients used for?
When are eddy currents generated?
When are eddy currents generated?
What type of shimming uses currents directed through specialized coils to generate a "corrective" magnetic field?
What type of shimming uses currents directed through specialized coils to generate a "corrective" magnetic field?
What is the purpose of the pulse modulator?
What is the purpose of the pulse modulator?
What type of materials are used for active shimming?
What type of materials are used for active shimming?
What is the main advantage of circularly polarized (CP) coils?
What is the main advantage of circularly polarized (CP) coils?
What type of stability is important for permanent magnets?
What type of stability is important for permanent magnets?
What is the purpose of active shielding?
What is the purpose of active shielding?
What is the structure of resistive shim coils?
What is the structure of resistive shim coils?
What is the purpose of the frequency synthesizer?
What is the purpose of the frequency synthesizer?
What is the main purpose of using a linear polarized RF-coil?
What is the main purpose of using a linear polarized RF-coil?
What is the purpose of the high power amplifier?
What is the purpose of the high power amplifier?
What type of artifacts are commonly observed with poor homogeneity?
What type of artifacts are commonly observed with poor homogeneity?
What type of power is wasted with a linear polarized RF-coil?
What type of power is wasted with a linear polarized RF-coil?
What is the purpose of the gradient of the field in an MRI scanner?
What is the purpose of the gradient of the field in an MRI scanner?
What is the purpose of active shimming?
What is the purpose of active shimming?
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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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