High Resolution ESR Microscopy

NMR microscopy is routinely employed to observe magnetic parameters and transport phenomena in small scale structures such as in biomaterials. Despite extensive efforts, the resolution of this method is limited (>10 µm for short acquisition times), and thus cannot answer many key questions. In contrast to NMR, ESR microscopy is still not well developed. Most of the efforts with respect to ESR imaging are directed towards low resolution imaging of large biological objects to identify the radical and the oxygen concentration. Recently, we have shown, through theoretical predictions and initial experiments, that ESR microscopy, can improve upon the resolution limits of NMR, and successfully undertake the 1 µm resolution challenge. Backed by these predictions, we initiated an experimental effort to achieve these resolution goals. An example for our initial results is depicted in the figure below, which presents a high resolution (~10×10 µm) image of LiPc crystal. The image was acquired using CW X-band imaging techniques employing a newly developed miniature dielectric resonator and a gradient coil set. The resonator is based on a SrTiO3 single crystal, machined to the appropriate shape. The unique miniature gradient coil set achieves gradients of 2.5 T/(m·A) and facilitates the examination of an “optical microscope-like” thin sample on a glass slide. The imaging method is based on the CW-ESR modulated fields gradient method. The initial imaging results support our theoretical predictions and already improve upon the previously reported state-of-the-art for 2D ESR image resolution. We have successfully extended this resolution to millimolar solutions of the trityl radical, frequently used in ESR imaging.