Stochastic excitation and Hadamard correlation spectroscopy with bandwidth extension in RF FT-EPR

Abstract

The application of correlation spectroscopy employing stochastic excitation and the Hadamard transform to time-domain Fourier transform electron paramagnetic resonance (FT-EPR) spectroscopy in the radiofrequency (RF) band is described. An existing, time-domain FT-EPR spectrometer system with a Larmor frequency (L(f)) of 300 MHz was used to develop this technique by incorporating a pseudo-random pulse sequence generator to output the maximum length binary sequence (MLBS, 10- and 11-bit). Software developed to control the EPR system setup, acquire the signals, and post process the data, is outlined. The software incorporates the Hadamard transform algorithm to perform the required cross-correlation of the acquired signal and the MLBS after stochastic excitation. To accommodate the EPR signals, bandwidth extension was accomplished by sampling at a rate many times faster than the RF pulse repetition rate, and subsequent digital signal processing of the data. The results of these experiments showed that there was a decrease in the total acquisition time, and an improved free induction decay (FID) signal-to-noise (S/N) ratio compared to the conventional coherent averaging approach. These techniques have the potential to reduce the RF pulse power to the levels used in continuous wave (CW) EPR while retaining the advantage of time-domain EPR methods. These methods have the potential to facilitate the progression to in vivo FT-EPR imaging of larger volumes.

Integration of digital signal processing technologies with pulsed electron paramagnetic resonance imaging

“…The output FID average waveform SNR was $31 dB. This SNR represents a $8 dB improvement compared to earlier work [15] using an IF stage in the FT-EPR system configuration with the sample subjected to similar conditions and processed to correct for phase and amplitude mismatches.…”

“…We aim to complete all signal and imaging processing on the DSP platform, providing real-time feedback for process monitoring and control, while the laboratory computer is used solely for a system GUI and image display. Other sequences will be implemented utilizing stochastic excitation [15] and single-point imaging [18]. We are optimistic that our recent work in DSP-based FT-EPR system design will facilitate the development of completely new system configurations and imaging methodologies.…”

“…Since our applications are focused on in vivo studies (i.e., low frequency EPR), we are able to use the current state of ADC technology to perform direct TLSS of the RF FID signal [14,15]. This eliminates the need for analog hardware that has been traditionally used to shift the FID signal down to an intermediate frequency (IF), or to quadrature baseband, before signal acquisition.…”

“…An existing 300 MHz FT-EPR spectrometer [7,15,16] was modified for these experiments. The current system configuration simplifies past transmit section designs by replacing most of the previous analog hardware with an AWG610 arbitrary waveform generator (AWG) (Tektronix, Richardson, TX).…”

Radio frequency continuous‐wave and time‐domain EPR imaging and Overhauser‐enhanced magnetic resonance imaging of small animals: instrumental developments and comparison of relative merits for functional imaging

“…Stochastic or pseudo-random pulsed excitation coupled with correlation techniques (such as Hadamard transform) have been developed for solid-state deuteron NMR which employ RF powers reduced by at least two orders of magnitude, and yet achieve SNR comparable to the high power single pulse mode of excitation. 116,124 We have already carried out EPR experiments at 300 MHz on phantom objects with pseudo random excitation and Hadamard transform, 125 and are able to reduce the power requirements by at least an order of magnitude without compromising the sensitivity. Yet this is still in the developmental stage.…”