MRI equipment utilizes a strong magnetic field to generate images. To obtain the optimal performance the equipment needs minimal RF noise in the surrounding environment. The RF noise is frequencies of radio and magnetic waves which are measured in Decibel (dB) units. The critical frequency range for most MRI equipment is from 50 MHz to 150 MHz. To determine the Shielding Effectiveness (SE) a series of test are performed at different locations throughout the enclosure. These tests utilize specialized equipment to transmit and receive RF noise at a specified frequency in accordance to set of standards specified by the equipment vendor. To simplify the explanation of this process we will omit the technical details and focus on the general ideas behind the testing procedure. The test starts by placing a Transmit Antenna (TX) outside the enclosure that transmits a specified amount of RF noise (dB) at a specific frequency. Then the technician takes the Receive Antenna (RX) inside the enclosure and closes the door. The Shielding Effectiveness (SE) is then determined by the difference in dB once the door is closed. This difference in value is typically referred to as the shielding attenuation. Below is a chart that illustrates the enclosure performance levels based on the attenuation value.
1. Enclosure is at optimum performance
2. Enclosure is at decent performance
3. Enclosure is substandard (Door should be evaluated for maintenance)
4. Enclosure is compromised (Should investigate for RF leaks)
5. Enclosure is severely compromised (Should investigate for RF leaks, possible shield replacement)