Magnetic resonance microscopy is a promising tool for high resolution 3D imaging of ex vivo soft tissue specimens since it allows morphometric measurements and non-invasive imaging of multiple tissue contrasts. Achieving micron-scale resolutions remains a technical challenge however due to the rapid signal-to-noise degradation with increasing magnification. Special consideration in RF coil design is thus required. In this work we describe the construction of a remotely-tunable bridged loop-gap resonator RF coil optimized for scanning perfusion-fixed mouse brains at 15T. The optimized coil allowed acquisition of 20 μm³ isotropic voxels that, to our knowledge, represent the highest resolution yet achieved in MR imaging of the mouse brain.Coaxial cylinders made of plastic (outer cylinder) and quartz (inner cylinder), with dimensions as shown in Figure 1, were used as formers for the coupled resonator devices. The gap in the inner cylinder was bridged by a 24 mm long × 8 mm wide copper tape to improve the magnetic field homogeneity [1][2]. Remote tuning was achieved by rotating the cylinders with respect to each other and adjusting the height of the tuning loop [3]. All experiments were conducted on a Magnex (Agilent, Yarnton, Oxford, UK) 15 T 130 mm horizontal bore magnet equipped with a Resonance Research (Billerica, MA, USA) 60 mm ID gradient insert and 2370 mT/m maximum gradient interfaced to a Siemens Medical Systems (Erlangen, Germany) console. The coil’s B₁ map was experimentally measured on a water phantom using the manufacturer’s mapping sequence and compared to maps of a simulated coil model designed in HFSS (Ansys, Inc., Canonsburg, PA, USA). Mouse brain experiments were conducted, with the approval of the animal IRB committee, on freshly dissected mouse brains (Fisher Scientific, Inc., Waltham, MA, USA) placed in sodium acetate buffered 10% formalin for a week then transferred to a 4mM solution of Magnevist (Bayer Healthcare) in the formalin solution for another week to shorten relaxation times. The sample was immersed in liquid fluorocarbon (Fomblin) and degassed in a 50 Torr vacuum to remove tissue air bubbles. The brain was scanned using a 3D GRE sequence with the following parameters: flip angle=45°, FOV=12.4 mm³, matrix size=6203, TE=3.79ms, NEX=8 and TR=30ms which was the shortest TR achievable on this system without overheating the gradient coil. The relatively long TR resulted in a total scan time of 27 hours.Axial slices from the (a) simulated and (b) experimental B₁ maps are shown in Figure 2. The mean of the normalized experimental B₁ map was 0.91 ± 0.18, similar to the 0.93 ± 0.05 of the normalized simulated map. The acquired mouse brain data is shown in Figures 3a-c for the coronal, axial and sagittal orientations. A comparable slice from the Allen Developing Mouse Brain Atlas [4] is shown in Figure 3d for comparison and identification of the various brain structures visible in the images. Some of the visible structures include the corpus callosum, dentate gyrus and the stratum lacunosum-moleculare.The proposed coil benefits from a homogenous excitation, as shown by the simulated and experimental B₁ maps (Figure 2). Coil dimensions were expressly selected to maximize the filling factor of the mouse brain specimens which yielded high resolution images (Figure 3). Although it was first introduced nearly thirty years ago, to our knowledge this is the first application of an LGR based coil design at 15 T The high resolution and simple construction of our coil allows precise measurements of distinct brain regions which is important in studies of genetic or neurological pathologies. A limitation of this coil design is that it is not suitable for larger samples or in vivo imaging. The long scan time, however, is a result of limitations in our system, not the coil, and can easily be reduced with adequate gradient cooling.We have demonstrated the utility of an RF coil that is simple to construct and allows high resolution imaging of ex-vivo mouse brains at ultra high fields. The voxel volumes acquired with this coil represent the smallest yet achieved on a mouse brain.