INTRODUCTION: Vertical root fracture (VRF) in root canal-treated (RCT) teeth is a common cause of pain, bone resorption, and tooth loss. VRF is also difficult to diagnose and measure. Magnetic resonance imaging (MRI) has the potential to identify VRF due to beneficial partial volume averaging, without using ionizing radiation. This investigation aimed to describe the narrowest VRFs detectable based on MRI, using micro-computed tomography (microCT) as the reference standard and proposes a method using profile integrals to measure the widths of small VRFs. METHODS: VRFs were induced in 62 RCT tooth root samples. All samples were imaged in a phantom using MRI and reference imaging was obtained using microCT. The stacks of 3-dimensional axial MRIs were assessed by 3 board-certified endodontists. Evaluators determined the most coronal slice within the stack that was discernible as the extent of the VRF. This slice was measured on correlated microCT sections to determine the minimum VRF width ( μm) detectable using a profile integral-based method to measure small fractures and negate the effects of the point spread function. RESULTS: Using profile integrals to measure VRF width was repeatable and resulted in estimates that were on average 1 μm smaller than known reference widths. Adjusted median VRF width detected using MRI was 45 μm (first quartile: 26 μm, third quartile: 64 μm). CONCLUSION: Using profile integrals is a valid way to estimate small VRF width. The MRI approach demonstrated ability to repeatedly detect VRFs as small as 26 μm.