The present invention generally provides improved devices, systems, and methods for measuring materials with NMR and/or MRI. Exemplary embodiments provide a sensor array for NMR mapping of the material. For example tissue can be measured with the sensor array mounted on a probe body having a distal portion which can be inserted through a minimally invasive aperture. While many tissues can be measured and/or diagnosed, one exemplary embodiment includes a probe adapted for insertion into a lumen of a blood vessel. The sensor array can provide improved spatial resolution of tissue and/or tissue structures positioned near the sensor array to diagnose potentially life threatening diseases, for example a fibrous cap covering a vulnerable plaque. In specific embodiments, the sensors are attached to an expandable member, for example a balloon, which can be inflated to urge the probe sensors radially outward to position the sensors near the tissue structures.