In MRI, pulse sequences begin as digital waveform specifications, and images are constructed from digitized data. The RF transmit and receive chains provide the link from the digital representation of RF waveforms to the physical RF signal power transported to and from MRI coils. In this tutorial, we review the system issues and core components that convey the low level NMR signals for analog to digital conversion (the receive chain). Emphasis will be placed on the functional limitations that may be unfamiliar to MRI users and programmers.

The receive chain must amplify the NMR signals with minimal added noise, protect preamps from the high power transmitter, and digitize the data subject to dynamic range and clock jitter constraints of analog-digital conversion. To construct a fully operational receive chain, the critical components include 1) Preamplifier, 2) Protection Circuits, 3) Analog to Digital Converters. Finally, options now exist for optical and wireless transfer of the digitized data.

Today, all MRI preamps use a form of HEMT (high electron mobility) Field Effect Transistor (FET), which are capable of noise figures below 0.5dB. The typical gain is 26-28dB or roughly a voltage gain of 20. The role of the preamplifier is to amplify the signal from the coil to drive a coax cable and ensure the fundamental noise floor is above that of the digitizing circuits. The preamplifier must also have sufficiently fast recovery following the inevitable transmit pulse and feed-through.

HEMT preamps employ a series shunt matching circuit, that combined with the high input impedance of the FET, create a low input impedance for preamplifier decoupling. However, Silicon SiGe bipolar junction transistors (BJT) and even common silicon BJTs may also work if biased at low current and if the base resistance is low. These devices offer 10x more gain per mA bias, but at a cost of lower device input resistance (poorer preamp decoupling) and more distortion.

During transmit, voltages at coil receive ports can exceed 100 V which would destroy a transistor. Protection circuits include active PIN diode switches, and passive Schottky diodes, and limiter diodes, both of which attempt to clamp the input signal level below about 1 V. Since diode turn-on time is finite, some level of voltage spike can often leak through. Newer approaches include GaN FET power transistors which can act as switches.MRI signals can exhibit very high dynamic range for 3D acquisitions, with signal peaks at the center of k-space, and noise-like signal levels at the outer edges of k-space. The ADC bit resolution ultimately determines the dynamic range that can be supported. A common equation is the signal to quantizing noise ratio SNQR = 6.02n+1.76 dB which predicts for n=16 bits SNQR=98dB. However, sample timing jitter can limit real SNR by SNRJ = -20log2πft where t is rms jitter and f is input frequency, eg 100MHz signal and 0.3ps jitter yields 74.5dB SNR. However this applies only at high level (k-space center) and may be minor in MRI. Finally, the ADC sample and hold capacitor has kT/C noise which can be comparable to or higher than quantizing noise. Many ADC architectures exist, (SAR, Σ−Δ, pipeline, flash) but today, pipeline ADCs offer the best combination of bit depth, bandwidth and low sampling jitter for MRI. Oversampling, decimation, and sampling at alternate Nyquist bands are now used to improve ADC dynamic range in MRI

Much like ultrasound, we can expect to see more integration of receiver channels on single ICs for MRI. Recent 8 and 16 channel ultrasound ADCs with integrated I/Q demodulation could almost meet MRI needs but with borderline dynamic range. Low power receiver options include passive mixers and continuous time sigma delta ADCs.One particularly interesting possibility is wearable receiver coils with optical or even wireless data streaming. Today, plastic optical fiber combined with resonant cavity LEDs allows data rates of 1Gbps. New wireless standards such 5GHz WiFi 802.11ac with 1Gbps potential, and 60GHz 802.11ad with 4-7 Gbps rates could easily meet the data transfer needs of MRI coils.