Rapid-scan EPR imaging

Abstract

In rapid-scan EPR the magnetic field or frequency is repeatedly scanned through the spectrum at rates that are much faster than in conventional continuous wave EPR. The signal is directly-detected with a mixer at the source frequency. Rapid-scan EPR is particularly advantageous when the scan rate through resonance is fast relative to electron spin relaxation rates. In such scans, there may be oscillations on the trailing edge of the spectrum. These oscillations can be removed by mathematical deconvolution to recover the slow-scan absorption spectrum. In cases of inhomogeneous broadening, the oscillations may interfere destructively to the extent that they are not visible. The deconvolution can be used even when it is not required, so spectra can be obtained in which some portions of the spectrum are in the rapid-scan regime and some are not. The technology developed for rapid-scan EPR can be applied generally so long as spectra are obtained in the linear response region. The detection of the full spectrum in each scan, the ability to use higher microwave power without saturation, and the noise filtering inherent in coherent averaging results in substantial improvement in signal-to-noise relative to conventional continuous wave spectroscopy, which is particularly advantageous for low-frequency EPR imaging. This overview describes the principles of rapid-scan EPR and the hardware used to generate the spectra. Examples are provided of its application to imaging of nitroxide radicals, diradicals, and spin-trapped radicals at a Larmor frequency of ca. 250 MHz.

Development of a fast‐scan EPR imaging system for highly accelerated free radical imaging

“…Therefore, major efforts were devoted to accelerate the EPR image acquisition. On par with key advances in pulsed EPR imaging, 3,10,29 Overhauser effect-based MRI, 13,[30][31][32] and development of rapid scan approaches, 4,8,33,34 traditional CW with field modulation and phase-sensitive detection has undergone major improvements. Pulsed EPR image modality allows functional (oxygen map) images of legs of living mice with matrices of 31 × 31 pixels and submillimeter resolution to be obtained in 264 seconds, 3 or for 3D imaging in vivo, the scan time is under 10 minutes and resolution is 1.4 mm to 2 mm.…”

“…The high sensitivity of EPR spectral parameters to many physical and chemical properties such as pH, oxygenation, redox state, and thiol content makes EPR‐based imaging techniques a valuable tool for in vivo and ex vivo studies . These techniques have been widely applied to study normal physiology and disease in small animal models such as mice and rats .…”

“…Electron paramagnetic resonance–related imaging techniques that are capable of detecting paramagnetic substances in vivo have been developed and continue to be improved. These include methods that enable direct detection of paramagnetic compounds such as continuous wave (CW) EPR, time domain pulsed EPR and rapid scan EPR, as well as MRI‐based indirect techniques such as T 1 contrast imaging, which visualize a radical‐induced enhancement, and EPR/proton MRI hybrid methods that enable visualization of radical‐induced Overhauser enhancement …”

“…The high sensitivity of EPR spectral parameters to many physical and chemical properties such as pH, oxygenation, redox state, and thiol content makes EPR-based imaging techniques a valuable tool for in vivo and ex vivo studies. [1][2][3][4][5] These techniques have been widely applied to study normal physiology and disease in small animal models such as mice and rats. 6 Electron paramagnetic resonance spectroscopy and imaging can provide critical mechanistic information on tissue redox status and oxygenation and detect free radicals produced in pathological conditions.…”

“…and continue to be improved. These include methods that enable direct detection of paramagnetic compounds such as continuous wave (CW) EPR, 1,7 time domain pulsed EPR 3 and rapid scan EPR, 4,8 as well as MRI-based indirect techniques such as T 1 contrast imaging, 9,10 which visualize a radicalinduced enhancement, and EPR/proton MRI hybrid methods that enable visualization of radical-induced Overhauser enhancement. [11][12][13] In vivo or ex vivo CW EPR imaging (EPRI) has been established as a powerful technique for determining the spatial distribution of free radicals and other paramagnetic species in living organs and tissues.…”