Abstract: A medical instrument is disclosed that includes an elongated probe section having a length (L) between a proximal and a distal probe section end. The probe section includes a plurality of carbon fibers that is arrayed along the axis and embedded in a medical grade, biocompatible epoxy resin. The probe section is constructed to be rigid by controlling the density of carbon fibers such that the maximum predetermined off-axis deflection (dmax) is less than about 30% of the probe section length (dmax<0.3 L). A handle is affixed, and positioned to be coaxial with, the proximal end of the probe section. For the instrument, the probe section is constructed of materials that are impervious to an imaging modality (e.g. magnetic fields). For example, with this construction, the probe section can be immersed in a homogenous magnetic field that is established by an MRI system without distorting the resulting MRI image.