Original articleMultiharmonic electron paramagnetic resonance imaging as an innovative approach for in vivo studies
Graphical abstract
Introduction
In recent years, electron paramagnetic resonance [1,2] has become an increasingly used technique in biological and biomedical research. Since current EPR spectroscopy may be performed in 3D [3,4] and with the right choice of the spin probe, it is possible to obtain three-dimensional maps of unique tissue micro-environment parameters. For example, the broadening of the EPR linewidth of some spin probes is proportional to the local oxygen concentration [5,6]. The decay in amplitude or changes in the EPR spectrum shape of nitroxide spin probes reflects the tissue redox state [[7], [8], [9], [10], [11]] as well as shifts of their EPR line position, which allows for pH mapping [12]. In vivo EPR imaging (EPRI) became possible by the development of low-frequency EPR tomographs, with a frequency range between 250 MHz and 1 GHz. The use of the lower frequency allows for animal applications, but in vivo experiments distinctly limit the acquisition time and decrease the sensitivity by lowering the Q factor of the resonator. Therefore, one of the biggest challenges in EPR in vivo imaging is to obtain an EPR signal that is sufficient for reliable data analysis, especially when the concentration of paramagnetic centres is low and the acquisition time is limited.
The easiest approach to improve the S/N ratio involves averaging, but it considerably lengthens the acquisition time, which is not desirable in in vivo studies. Continuous wave (CW) EPRI is a conventional, commercially available and the most popular mode of this technique, in which the magnetic field is modulated, often at 100 kHz, and the first-derivative of the absorption spectrum is acquired by phase-sensitive detection at the modulation frequency at a series of magnetic fields. Several ways were presented to overcome the S/N ratio limitation. Increasing the magnetic field modulation is one way to improve S/N, however, at the expense of line broadening. In conventional spectroscopy amplitude modulation should not be greater than one-third of EPR peak – to – peak linewidth ; otherwise, the spectrum is broadened [13]. Another way to improve the resolution and sensitivity of the reconstructed spectral-spatial EPR images involves the application of different amplitude modulations for each projection, instead of a constant one. In this case, amplitude modulation was dependent on gradient strength, which increased with it [14]. By using direct detection of resonances without conventional low-frequency modulation and phase sensitive detection, it was possible to speed up measurements based on a rapid scan (RS) strategy [[15], [16], [17], [18]]. With a rapid scan, the measured data needs to be deconvolved to obtain the absorption spectra, where its derivative would result in similar data from the classic CW technique. It was shown that the S/N ratio from spectra obtained from rapid scans was around an order of magnitude higher than that obtained from the conventional CW technique [19].
To improve the S/N ratio in the CW method, the multiharmonic analysis was presented. In most EPR spectrometers and imagers, the first – harmonic spectrum is measured. For a modulation amplitude smaller than the linewidth, the S/N ratio of the n-th harmonic decreases rapidly with n, but by using an amplitude modulation much higher than the linewidth, the harmonics have a much higher S/N compared to a conventional non-overmodulated spectrum [20,21]. The study performed at X-band using 10 kHz of frequency modulation where the maximum of 45 harmonics were analysed [20,21] shown that by using multiharmonic analysis, it is possible to reconstruct the correct shape of the first - derivative spectra, however, the peak-to-peak linewidths are broadened by up to 10%.
In this work, we applied multiharmonic analysis in 600 MHz EPR tomograph, which is suitable for in vivo research, and 1 kHz frequency modulation was used. For the first time, the dependency between the shape of the EPR spectrum reconstructed using multiharmonics analysis for a range from 1 to 100 multiharmonics was explored. Such an analysis allows characterizing the behavior of the reconstructed signal shape depending on the number of used harmonics, which allows us to determine crucial parameters for MHA and obtaining spectrum without broadening. Additionally, we examine the correlation between the applied modulation amplitude and the number of multiharmonics used on the reconstructed line shape. Finally, we present a proof – of – concept of this technique for EPR imaging in in vivo applications and compare the results with those from conventional CW EPR imaging.
Section snippets
Phantoms
The first phantom (PH1) – a glass tube contained TCNQ salts with N-methyl-pyridine base as an electron donor [N–Me-Py-(TCNQ)2] with a molecular mass of 502.5 g/mol was used [22,23]. PH1 was filled with 0.32 mg of [N–Me-Py-(TCNQ)2] radical salt powder, which corresponds to around 1.2 × 1021 spins/gram. A water solution of 2 ml of 20 μM of nitroxide 3-Carbamoyl-PROXYL (3-Carbamoyl-2,2,5,5-tetramethyl-1-pyrrolidineoxy) (3-CP) purchased from Sigma-Aldrich (St Louis, MI; USA) was used as the second
Spectroscopy study
As a reference for the EPR linewidth and S/N ratio, traditional detection of EPR 1st derivatives was measured. For the [N–Me-Py-(TCNQ)2] sample, the modulation amplitude was 0.05 G, which is less than 25% of . For the 3-CP sample, the modulation amplitude was 0.4 G, which is about 1/3 of the linewidth. For the [N–Me-Py-(TCNQ)2] samples, modulation amplitudes were increased from 0.5 G up to 8 G. For the 3-CP samples, amplitude modulations up to 10 G were used. After the multiharmonic
Discussion
In vivo EPR imaging experiments are usually performed using conventional CW or pulse EPR tomographs. This work is the first report of the application of the EPR tomograph to perform imaging on a live animal using multiharmonic analysis, which so far was used only in the phantom study.
In a conventional CW EPR experiment, whether it is spectroscopy or imaging, only the 1 st derivative EPR signal is measured. In this study, the practical aspects of the application of multiharmonic analysis were
Conclusions
In conclusion, the MHA methodology developed by our team, allows the selection of a second modulation amplitude even 40 times higher than the natural linewidth of the probe without spectral distortion, which highly improves the sensitivity of the acquired signal. The spectroscopy study confirmed that MHA significantly improved the S/N ratio without distorting the signal shape. The improvement depends on the overmodulation factor. A greater effect in the improvement in the S/N of EPR spectra was
Funding
This research was funded by Polish National Science Centre, grant number: UMO-2014/15/B/NZ5/01488 and UMO-2014/15/B/ST4/04946 as well as National Centre for Research and Development - Smart Growth Operation Programme under project POIR.01.01.01-00-0025/15.
Declaration of competing interest
M.B. T.C., P.S. are all current employees of Novilet. Novilet is a spin-off company specializing in the discovery, development, manufacturing of EPR tomographs. M.B. T.C. and P.S. provided technical support to adapt the ERI TM600 scanner to the scope of this work and provided engineering expertise. This does not alter the authors’ adherence to all the policies on sharing data and materials.
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Both authors contributed equally to this manuscript.