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...