Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 31

The purpose of the RF receiver coils is to detect the tiny (micro‐volt) MR signals. A parallel tuned circuit is used (Figure 1.20) to magnify the voltage prior to pre‐amplification and further processing. The receive coil requires protection circuitry to prevent the large transmit pulses from coupling into the coil. A simple means of achieving this is to use crossed diodes and a detuning capacitor. During the large Tx pulses the diodes conduct and so the total capacitance becomes the sum of both capacitors and the circuit is off‐resonance. During signal detection the diodes do not conduct, and C_d is “invisible.” A fault in this circuitry can lead to large induced currents in the coil and potential heating or burns for the patient.

Figure 1.20 Receive coil and pre‐amplifier: (a) the coil has inductance L and resistance R; C_d is a detuning capacitor; (b) response of the coil during signal reception (red line) and RF transmission (blue line).