We present a fully integrated detector suitable for active tracking of interventional devices in MR-guided interventions. The single-chip microsystem consists of a detection coil, a tuning capacitor, an intermediate frequency downconversion receiver, and a phase-locked-loop-based frequency synthesizer. Thanks to the integrated mixer, the chip output stage delivers an analog frequency-downconverted NMR signal in the frequency range from 0 to 200 kHz. The microchip, realized in a standard complementary metal oxide semiconductor technology, has a size of 1 x 2 x 0.74 mm3 and operates at a frequency of 63 MHz (i.e., in 1.5 T clinical scanners). Tests in a standard clinical scanner demonstrate the compatibility of the complementary metal oxide semiconductor microchip with clinical MRI systems. Using a solid sample of cis-polyisoprene having a size of 1 x 1.9 x 0.8 mm3 as internal signal source, the detector achieves a three-dimensional isotropic spatial resolution of 0.15 mm in a measuring time of 100 ms. Magn Reson Med, 2011. (C) 2011 Wiley-Liss, Inc.