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1. Field-domain rapid-scan EPR at 240 GHz for studies of protein functional dynamics at room temperature

   https://doi.org/10.1016/j.jmr.2024.107744  

   Price, Brad D; Sojka, Antonín; Maity, Shiny; Chavez, I Marcelo; Starck, Matthieu; Wilson, Maxwell Z; Han, Songi; Sherwin, Mark S (September 2024, Journal of Magnetic Resonance)

   We present field-domain rapid-scan (RS) electron paramagnetic resonance (EPR) at 8.6 T and 240 GHz. To enable this technique, we upgraded a home-built EPR spectrometer with an FPGA-enabled digitizer and real-time processing software. The software leverages the Hilbert transform to recover the in-phase (𝐼) and quadrature (𝑄) channels, and therefore the raw absorptive and dispersive signals, 𝜒′ and 𝜒′′, from their combined magnitude (√𝐼^2 + 𝑄^2). Averaging a magnitude is simpler than real-time coherent averaging and has the added benefit of permitting long-timescale signal averaging (up to at least 2.5 × 106 scans) because it eliminates the effects of source-receiver phase drift. Our rapid-scan (RS) EPR provides a signal-to-noise ratio that is approximately twice that of continuous wave (CW) EPR under the same experimental conditions, after scaling by the square root of acquisition time. We apply our RS EPR as an extension of the recently reported time-resolved Gd-Gd EPR (TiGGER) [Maity et al., 2023], which is able to monitor inter-residue distance changes during the photocycle of a photoresponsive protein through changes in the Gd-Gd dipolar couplings. RS, opposed to CW, returns field-swept spectra as a function of time with 10 ms time resolution, and thus, adds a second dimension to the static field transients recorded by TiGGER. We were able to use RS TiGGER to track time-dependent and temperature-dependent kinetics of AsLOV2, a light-activated phototropin domain found in oats. The results presented here combine the benefits of RS EPR with the improved spectral resolution and sensitivity of Gd chelates at high magnetic fields. In the future, field-domain RS EPR at high magnetic fields may enable studies of other real-time kinetic processes with time resolutions that are otherwise difficult to access in the solution state. 

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2. Multi-step phase-cycling in a free-electron laser-powered pulsed electron paramagnetic resonance spectrometer

   https://doi.org/10.1039/c8cp01876f  

   Wilson, C. Blake; Aronson, Samuel; Clayton, Jessica A.; Glaser, Steffen J.; Han, Songi; Sherwin, Mark S. (January 2018, Physical Chemistry Chemical Physics)

   Electron paramagnetic resonance (EPR) is a powerful tool for research in chemistry, biology, physics and materials science, which can benefit significantly from moving to frequencies above 100 GHz. In pulsed EPR spectrometers driven by powerful sub-THz oscillators, such as the free electron laser (FEL)-powered EPR spectrometer at UCSB, control of the duration, power and relative phases of the pulses in a sequence must be performed at the frequency and power level of the oscillator. Here we report on the implementation of an all-quasioptical four-step phase cycling procedure carried out directly at the kW power level of the 240 GHz pulses used in the FEL-powered EPR spectrometer. Phase shifts are introduced by modifying the optical path length of a 240 GHz pulse with precision-machined dielectric plates in a procedure we call phase cycling with optomechanical phase shifters (POPS), while numerical receiver phase cycling is implemented in post-processing. The POPS scheme was successfully used to reduce experimental dead times, enabling pulsed EPR of fast-relaxing spin systems such as gadolinium complexes at temperatures above 190 K. Coherence transfer pathway selection with POPS was used to perform spin echo relaxation experiments to measure the phase memory time of P1 centers in diamond in the presence of a strong unwanted FID signal in the background. The large excitation bandwidth of FEL-EPR, together with phase cycling, enabled the quantitative measurement of instantaneous electron spectral diffusion, from which the P1 center concentration was estimated to within 10%. Finally, phase cycling enabled saturation-recovery measurements of T 1 in a trityl-water solution at room temperature – the first FEL-EPR measurement of electron T 1 . 

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3. An Intensity Mapping Constraint on the CO-galaxy Cross-power Spectrum at Redshift ∼3

   https://doi.org/10.3847/1538-4357/ac4888  

   Keenan, Ryan P.; Keating, Garrett K.; Marrone, Daniel P. (March 2022, The Astrophysical Journal)

   Abstract The abundance of cold molecular gas plays a crucial role in models of galaxy evolution. While deep spectroscopic surveys of CO emission lines have been a primary tool for measuring this abundance, the difficulty of these observations has motivated alternative approaches to studying molecular gas content. One technique, line intensity mapping, seeks to constrain the average molecular gas properties of large samples of individually undetectable galaxies through the CO brightness power spectrum. Here we present constraints on the cross-power spectrum between CO intensity maps and optical galaxy catalogs. This cross-measurement allows us to check for systematic problems in CO intensity mapping data, and validate the data analysis used for the auto-power spectrum measurement of the CO Power Spectrum Survey. We place a 2σupper limit on the band-averaged CO-galaxy cross-power ofP_×< 540μK h⁻³Mpc³. Our measurement favors a nonzero 〈T_CO〉 at around 90% confidence and gives an upper limit on the mean molecular gas density atz∼ 2.6 of 7.7 × 10⁸M_⊙Mpc⁻³. We forecast the expected cross-power spectrum by applying a number of literature prescriptions for the CO luminosity–halo mass relation to a suite of mock light cones. Under the most optimistic forecasts, the cross-spectrum could be detected with only moderate extensions of the data used here, while more conservative models could be detected with a factor of 10 increase in sensitivity. Ongoing CO intensity mapping experiments will target fields allowing for extensive cross-correlation analysis and should reach the sensitivity required to detect the cross-spectrum signal. 

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4. Submillimeter lung MRI at 0.55 T using balanced steady‐state free precession with half‐radial dual‐echo readout (bSTAR)

   https://doi.org/10.1002/mrm.29757  

   Bauman, Grzegorz; Lee, Nam G; Tian, Ye; Bieri, Oliver; Nayak, Krishna S (November 2023, Magnetic Resonance in Medicine)

   PurposeTo demonstrate the feasibility of high‐resolution morphologic lung MRI at 0.55 T using a free‐breathing balanced steady‐state free precession half‐radial dual‐echo imaging technique (bSTAR). MethodsSelf‐gated free‐breathing bSTAR (TE₁/TE₂/TR of 0.13/1.93/2.14 ms) lung imaging in five healthy volunteers and a patient with granulomatous lung disease was performed using a 0.55 T MR‐scanner. A wobbling Archimedean spiral pole (WASP) trajectory was used to ensure a homogenous coverage of k‐space over multiple breathing cycles. WASP uses short‐duration interleaves randomly tilted by a small polar angle and rotated by a golden angle about the polar axis. Data were acquired continuously over 12:50 min. Respiratory‐resolved images were reconstructed off‐line using compressed sensing and retrospective self‐gating. Reconstructions were performed with a nominal resolution of 0.9 mm and a reduced isotropic resolution of 1.75 mm corresponding to shorter simulated scan times of 8:34 and 4:17 min, respectively. Analysis of apparent SNR was performed in all volunteers and reconstruction settings. ResultsThe technique provided artifact‐free morphologic lung images in all subjects. The short TR of bSTAR in conjunction with a field strength of 0.55 T resulted in a complete mitigation of off‐resonance artifacts in the chest. Mean SNR values in healthy lung parenchyma for the 12:50 min scan were 3.6 ± 0.8 and 24.9 ± 6.2 for 0.9 mm and 1.75 mm reconstructions, respectively. ConclusionThis study demonstrates the feasibility of morphologic lung MRI with a submillimeter isotropic spatial resolution in human subjects with bSTAR at 0.55 T. 

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5. Terahertz Polarimetric Imaging for Biomedical Applications

   Srinivasan, Nandhini; Caglayan, Cosan; Nahar, Niru K.; Sertel, Kubilay (July 2018, 2018 IEEE Antennas and Propagation Society Symposium on Antenna and Propagation and URSI CNC/USNC Joint Meeting)

   We present a reflection-mode, polarimetric imaging system that can achieve 45µm sample resolution and measure the co-polarized and cross-polarized reflected field components individually over the 750GHz1.1THz band. The system is based on a vector network analyzer (VNA) in conjunction with frequency extenders and illuminates the sample under test through a high-resistivity Silicon (HRSi) lens to achieve image resolution 3.42-times better than the free-space diffraction limit. The two ports of the VNA are used to capture the co-polarized and cross-polarized images of the same sample. A simple quasi-optical setup is used to isolate and direct the cross-polarized reflected signal without significantly degrading the copolarized signal. The utility of the proposed system is demonstrated using biomedical samples in form of formalin-fixed paraffin-embedded (FFPE) tissues. 

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