Magnetic and RF Shielding Concepts Flashcards
6 cards from real MRSO practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Magnetic and RF Shielding Concepts flashcards as text
What is the primary function of the Faraday cage constructed around an MRI scan room?
Answer: To prevent external radiofrequency interference from corrupting images.
A Faraday cage is an enclosure made of conductive material, such as copper or galvanized steel, that blocks external electromagnetic fields. Its purpose in MRI is to prevent external radiofrequency (RF) signals from sources like radio stations, cell phones, and other equipment from entering the room and causing artifacts on the images.
How does active magnetic shielding reduce the fringe field of an MRI scanner?
Answer: By using a set of secondary superconducting coils with an opposing current.
Active shielding involves a secondary set of superconducting coils located within the magnet cryostat, outside the main magnet windings. An electrical current is passed through these coils in the opposite direction to the main coils, creating an opposing magnetic field that actively cancels out and contains the magnetic field that would otherwise extend far from the scanner.
A persistent 'zipper' artifact appearing in the center of images across multiple sequences and patients is most likely caused by what?
Answer: A breach in the radiofrequency (RF) shield.
A zipper artifact is a classic sign of a breach in the RF shield. This breach allows external RF noise to 'leak' into the scan room and be picked up by the receiver coil, creating a distinct line or band of noise in the image. Common causes include a poorly sealed door, a compromised window, or a faulty penetration panel.
What is the primary material used for passive magnetic shielding?
Answer: Low-carbon steel
Passive magnetic shielding relies on surrounding the magnet room with a material that has high magnetic permeability to contain the fringe field. Low-carbon steel plates are the most common material used for this purpose, as they effectively attract and redirect the magnetic field lines, preventing them from extending into adjacent areas.
The main goal of magnetic field shielding is to contain which specific field strength line within a controlled area?
Answer: 5 Gauss (0.5 mT)
The 5 Gauss (0.5 mT) line is the critical threshold recognized by the FDA and ACR for the general public and individuals with biomedical implants like pacemakers and defibrillators. Magnetic shielding, both active and passive, is engineered to ensure this line does not extend into uncontrolled public spaces or critical patient care areas.
What is the principle behind using a waveguide for penetrations through an RF shield?
Answer: It acts as a high-pass filter, attenuating frequencies in the MRI operational range.
A waveguide is a hollow conductive pipe used for penetrations like IV lines or fiber optics. Its dimensions (length and diameter) are specifically calculated to act as a high-pass filter. It allows physical objects to pass through but severely attenuates (blocks) electromagnetic waves at or below a certain cutoff frequency, including the RF used by the MRI scanner, thus preserving the integrity of the Faraday cage.