Slice‐selective images of free radicals in mice with modulated field gradient electron paramagnetic resonance (EPR) imaging

Abstract

Continuous wave (CW) electron paramagnetic resonance (EPR) imaging can be used to obtain slice-selective images of free radicals without measuring three-dimensional (3D) projection data. A method that incorporated a modulated magnetic field gradient (MFG) was combined with polar field gradients to select a slice in the subject noninvasively. The slice-selective in vivo EPR imaging of triarylmethyl radicals in the heads of live mice is reported. 3D surface-rendered images were successfully obtained from slice-selective images. In the experiment in mice, a slice thickness of 1. Reactive oxygen species (ROS) such as superoxides and hydroxyl radicals play a key role in pathophysiological processes and cell damage (1,2). To investigate the physiological functions of ROS in mice or rats noninvasively, an imaging method that could provide three-dimensional (3D) or multislice images of free radicals would be valuable. Electron paramagnetic resonance (EPR) techniques can be used for the noninvasive imaging of paramagnetic species such as free radicals (3-5). Usually, pixel-or voxelbased information is obtained by measuring 3D projection data. However, this process is time-consuming since n 2 projections are necessary for 3D images, instead of n projections for 2D images. However, there is no practical method that can provide 2D slice-selective EPR images in vivo. In this study, we demonstrate slice-selective in vivo EPR imaging in living mice while avoiding the need to collect n 2 projection data that are required for 3D imaging. Instead, a 3D image can be assembled by collecting m slices of n 2D projection data, which results in a considerably decreased imaging time. The modulated magnetic field gradient (MFG) method (6 -8) in combination with continuous wave (CW) EPR imaging can be used to obtain multislice images of free radicals in mice for the first time within a limited time that has not been achieved previously. This slice-selective imaging method should contribute to various biomedical studies of disease in animal models involving ROS. However, direct detection of ROS in vivo is a challenge in animal imaging not only with this method but also with other EPR imaging techniques. Therefore, the generation of ROS has been studied through reduction/oxidation reactions that can be measured from the decay of exogenously infused spin probes (9 -11).One of the most challenging problems for in vivo EPR imaging is to reduce the time needed to acquire the projection data that are necessary for image reconstruction. While pulsed EPR imaging can be used to acquire projection data rapidly (12), it is technically difficult to detect free radical molecules that have a short relaxation time. For example, it is challenging to measure nitroxide spin probes and iron-complex adducts of nitric oxide with pulsed imaging, whereas CW-EPR imaging can detect these species with adequate sensitivity (13). In this study, we adopt for the first time an approach used in EPR microscopy of paramagnetic crystals in the X-band (9 GHz) of slice-selec...

Brain Redox Imaging Using In Vivo Electron Paramagnetic Resonance Imaging and Nitroxide Imaging Probes

“…In 1996, Yokoyama et al [22] achieved a scanning rate of 1.4 s with an air-core Helmholtz coil pair, and in 2007, Samouilov et al [23] reported in vivo EPR imaging of mice with a scanning rate of 1.3 s. In the beginning of the 2010s, Hirata H. from Hokkaido University started developing an in vivo EPR imaging system with rapid field scanning capability. Using the developed EPR imager, Sato-Akaba et al [24] were able to take 3D EPR images of a phantom with an interval of 3.6 s, and Fujii et al [25] succeeded in obtaining a series of 3D EPR images of mouse heads every minute continuously. Figure 1A shows a schematic of a 750-MHz CW-EPR imager [26].…”

“…Therefore, to visualize the 3D-distribution of nitroxide probes in small rodents, faster EPR imagers are absolutely required. The first EPR image of mouse heads using BBB-permeable 4-hydroxy-2,2,6,6-tetramethylpperidine-d17-1-15N-1-oxyl was successfully obtained by Sato-Akaba et al [24], in which a set of projection data for 3D EPR imaging was acquired every 30 s with field scanning of 0.5 s and the number of projections set at 46. Their method enables a change in the number of averagings for measured spectra in post-processing of image reconstruction, since several datasets can be recorded during the time in which EPR spectra of nitroxide probes are detectable.…”

“…BBB-impermeableCOP was distributed outside the mouse brains. 2D slice images (128 × 128 pixels) were generated from the 3D image data (128 × 128 × 128 pixels)[24,25].…”

Electron paramagnetic resonance imaging

“…In EPR imaging, slice selection is not easy in the MRI sense where one excites a thin slice of the subject using a frequency selective pulse in the presence of a gradient followed by spatial encoding. However, by using a modulated gradient along one direction it is possible to select a 2-3-mm slice at the zerocrossing of this gradient, one can then perform 2D imaging of selected slices analogous to MRI [27]. For more details, please see [28].…”

Overmodulation of projections as signal‐to‐noise enhancement method in EPR imaging

“…The minimum time to measure a single projection is mainly determined by the gradient setting (about 50 ms). It has been reported that by decreasing the setup time of gradients and the sweeping time, the acquisition of a single projection may take from 78 to 500 ms and up to 4 s for 3D image acquisition …”