A resonant circuit sensor formed as a resiliently collapsible coil structure, for obtaining measurements within a blood vessel, which, when positioned at a monitoring position within a blood vessel, moves with the blood vessel wall as it expands and contracts due to changes in fluid volume within the blood vessel, wherein the resiliently collapsible coil structure has a constant capacitance and a variable inductance which changes when the dimensions of the coil change with movement of the blood vessel wall, and comprises a frame wrapped in conductive wire, the frame having a zig-zag form comprising a plurality of straight strut sections joined at their ends by crown sections forming acute angles.
