Rapid scan EPR: Automated digital resonator control for low-latency data acquisition

Abstract

Automation has become an essential component of modern scientific instruments which often capture large amounts of complex dynamic data. Algorithms are developed to read multiple sensors in parallel with data acquisition and to adjust instrumental parameters on the fly. Decisions are made on a time scale unattainable to the human operator. In addition to speed, automation reduces human error, improves the reproducibility of experiments, and improves the reliability of acquired data. An automatic digital control (ADiC) was developed to reliably sustain critical coupling of a resonator over a wide range of time-varying loading conditions. The ADiC uses the computational power of a microcontroller that directly communicates with all system components independent of a personal computer (PC). The PC initiates resonator tuning and coupling by sending a command to MC via serial port. After receiving the command, ADiC establishes critical coupling conditions within approximately 5 ms. A printed circuit board resonator was designed to permit digital control. The performance of the resonator together with the ADiC was evaluated by varying the resonator loading from empty to heavily loaded. For the loading, samples containing aqueous sodium chloride that strongly absorb electromagnetic waves were used. A previously reported rapid scan (RS) electron paramagnetic resonance (EPR) imaging instrument was upgraded by the incorporation of ADiC. RS spectra and an in vivo image of oxygen in a mouse tumor model have been acquired using the upgraded system. ADiC robustly sustained critical coupling of the resonator to the transmission line during these measurements. The design implemented in this study can be used in slow-scan and pulsed EPR with modifications.

Keywords: Automated tuning and coupling; EPR imaging; Magnetic resonance; Rapid Scan EPR.

Fig. 1.

Function diagram of a digitally tunable capacitor. The net capacitance between points A and B depends on the state (on/off) of the five switches permitting 2⁵ = 32 net discrete capacitance values.

Fig. 2.

Fully functional printed circuit board resonator (PCBR). Photos of the top and bottom of the PCBR (left part of the figure) show essential resonator parts soldered on the PCB. The schematic diagram of the resonator is presented on the right part of the figure. Digital inputs to the PCB include clock, data, power, ground, and chip enable lines following Serial Peripheral Interface (SPI).

Fig. 3.

Block diagram of major components used for ADiC. The PC communicates with MC via serial protocol by sending short commands. MC communicates in real-time (there are no interruptions) with all units involved in ADiC. The microcontroller sends commands to AWG, DFS, the switch, and DTCs, and reads reflection levels from RP. A pair of DTC values are sent to RES via SPI. A frequency sweep waveform (pre-loaded into AWG) is output upon receiving a trigger from MC. Immediately before this event, MC flips the digitally-controlled switch to pass the frequency sweep signal to the resonator. The signal reflected from the resonator is measured by RP and digitized by MC, which also computes the frequency corresponding to the lowest reflection. Depending on the result, this process is repeated for neighboring DTC values. The DTC values for CCC are set at the end of this procedure. The resonance frequency is established by sending a digital command via SPI to DFS. The digital switch is flipped back to enable CW data acquisition. To reduce overhead time and ensure the collection of relevant RS EPR signals, MC blocks digitizer (DIG) from receiving incoming triggers during the tuning process. The entire tuning/coupling procedure is executed in real-time without involving the PC.

Fig. 4.

Firmware algorithm executed by Teensy MC. The PC initiates the tuning procedure by sending a serial command. After receiving this command, MC performs several steps aimed to find a pair of DTC values and the frequency that minimizes reflection from the resonator. Immediately after initiating tuning, the PC sends a command to the digitizer (DIG in Fig. 3) to measure RS EPR data. However, MC blocks the acquisition until the critical coupling conditions are established. As a result, there is no delay between the end of the tuning cycle and data acquisition. The reflection threshold is selected to satisfy the critical coupling (≈ −40 dB reflection compared to the incident power).

Fig. 5.

ADiC test over widely varied resonator loading conditions. EPR spectra were measured with and without ADiC when a tube filled with NaCl solution was used to abruptly change the resonator impedance.

Fig. 6.

Long-term experiment showing critical resonator coupling as resonator coupling conditions change. a) Resonance frequency (blue) and DTC values (red and black) change over time as the resonator coupling conditions change. b) EPR spectral intensity remains optimal over time. A total of 3000 EPR spectra were successfully measured with a time interval of 30 seconds without a single failure.

Fig. 7.

In vivo EPR image quantifying oxygen partial pressure (pO₂) in a murine mammary tumor. (a) 2D cross-section of 3D oxygen map. (b) Histogram of pO₂ values obtained from 3D image.