FREE MRSO Magnetic and RF Shielding Concepts Questions and Answers

Question 1

What is the primary function of the Faraday cage constructed around an MRI scan room?

Accepted Answer

To prevent external radiofrequency interference from corrupting images.

Answer

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.

Question 2

How does active magnetic shielding reduce the fringe field of an MRI scanner?

Accepted Answer

By using a set of secondary superconducting coils with an opposing current.

Answer

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.

Question 3

A persistent 'zipper' artifact appearing in the center of images across multiple sequences and patients is most likely caused by what?

Accepted Answer

A breach in the radiofrequency (RF) shield.

Answer

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.

Question 4

What is the primary material used for passive magnetic shielding?

Accepted Answer

Low-carbon steel

Answer

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.

Question 5

The main goal of magnetic field shielding is to contain which specific field strength line within a controlled area?

Accepted Answer

5 Gauss (0.5 mT)

Answer

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.

MRSO - Magnetic Resonance Safety Officer Practice Test

MRSO - Magnetic Resonance Safety Officer Practice Test

FREE MRSO Magnetic and RF Shielding Concepts Questions and Answers