03-07 Radiofrequency (Faraday) Shielding

Table 02-01 shows that the resonance frequencies of all MR scanners overlap with commercial, military, and amateur radio and television frequencies. Elec­tric machines can also create electromagnetic waves. It can easily happen that the receiver of the MR imaging equipment picks up such radio signals from the outside world, which then interfer with the signals from the examined sample or patient. This leads to noisy images or, in the worst case, the complete loss of ima­ges.

Faraday shielding is used as a protection against electromagnetic interference. High-field systems require a complete Faraday cage (usually a copper cage with windows, including an electrically conducting screen), which has to be grounded. Connections from the inside of the cage to the outside have to be very carefully made and shielded (Figure 03-13).

Figure 03-13: Simple Faraday cage. Copper covers floor, walls and ceiling of the entire room, and windows, the door and all cables connecting the room with the outside are also shielded to maintain elec­tri­cal isolation. Usually the shielding is co­ve­red with plaster and wallpapered, hiding the copper cage.

03-08 Data Acquisition System and Computer

The analog signal emanating from the spins must be converted into a digital (nu­me­ri­cal) form suitable for storing and processing on a computer. This di­gi­ti­za­tion of the signal is achieved by using an analog-to-digital converter (ADC). The output of the ADC is a digital version of the FID for each data point. After the recording is complete, the digitized FID is stored on a magnetic disk or an­other storage medium.

A specialized computer (the image processor) is used to pre-process and process the raw data into images. The Fourier transform of the raw data can be greatly accelerated by array processors or dedicated hardware.

The host computer is responsible for controlling and monitoring the entire MR system. Additional consoles can be connected to the host computer for such functions as patient management, display of image data and calculation results, image post-processing, and documentation and archiving.