Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 21

The heart of the MRI system is the magnet, creating the static B₀ field that produces the tissue magnetization. The B₁ pulse that manipulates the magnetization is generated by a RF transmit coil (Tx) or coils fed by high‐powered broadband RF amplifiers. The pulse shapes are generated digitally and converted to analog waveforms prior to amplification. The gradient coils provide the pulses used for spatial encoding of the signal. Finally, the MR signal is detected by receiver coils (Rx). These are usually positioned close to the anatomy of interest to maximize the signal received. Most receive coils are array or matrix coils formed from numerous minimally interacting smaller elements. The advantages of array coil technology include improved signal‐to‐noise ratio (SNR) and the ability to utilize parallel imaging techniques. Once detected, the MR signal is demodulated, i.e. the RF “carrier” component (at f₀) is removed, as the spatial information is stored in the VLF signal region. With “direct digital” systems, demodulation and digitization are applied directly to the amplified RF signal, close to or at the coil. The ensuing signals are transmitted digitally or optically and stored for reconstruction. Figure 1.12 shows a schematic of a typical MR system. The gradient and RF amplifier systems, control and signal processing systems, and cooling equipment are situated in the equipment or technical room, external to the MR examination or magnet room. The console and host computer are located in the control room (Figure 1.13).

Figure 1.12 Schematic of principal MRI system components: DAC is digital‐to‐analog convertor; ADC is analog‐to‐digital convertor; Tx transmit; Rx receive; black lines denote digital signal; colored lines denote analogue signal. The magnet power supply is only required for the initial ramp‐up.

Figure 1.13 MRI operator’s console in the control room with observation window into the magnet room. Source: James Steidl/iStockphoto.