An Array of Active TX/RX 19F NMR Field Probes for Gradient Trajectory Mapping

Increased static field strengths of MR scanners in combination with advanced pulse sequences such as EPI or spiral MRI enable imaging with increased resolution in shorter imaging times. However, those advancements impose more stringent requirements on the gradient hardware which can currently not be met by existing instrumentation thus leading to image artifacts. To mitigate this problem, we present an active NMR sensor array which can be used for a real-time monitoring of the gradient trajectories during MR imaging experiments in order to perform a post-correction of the distorted image data. By using active sensor nodes, we eliminate the need for any high-frequency cable connection to the sensor nodes and due to the autonomous TX/RX operation the NMR sample excitation is independent of the scanner’s RF pulse. This both allows for an arbitrary pulse sequence selection and avoids blocking any channels of the MR scanner. Each miniaturized sensor node consists of a probe head with a hexafluorobenzene sample within a solenoid TX/RX coil, which is in turn encapsulated in an ellipsoidal susceptibility-matched epoxy casing. The tuned probe heads are directly connected to the transceiver electronics which are integrated into each sensor node. These electronics comprise a frequency synthesizer, which generates the local oscillator (LO) signals for the downconversion of the detected signals and the RF excitation pulse, a low-noise amplifier and a quadrature demodulator in the RX path and a power amplifier in the TX path. Due to the local generation of the excitation pulse and the downconversion of the NMR signal, only low frequency signals have to be transmitted to and from the sensor nodes, greatly reducing interferences between the trajectory mapping and the imaging experiment. The sensor nodes are connected via shielded twisted-pair cables to a signal-conditioning box which interfaces to a commercial ADC (NI PXIe-6368) with 2 MS/s and 16 bit per channel. In the current implementation of the sensor array, the transceiver electronics are realized as PCB-prototypes using commercially available IC components. These sensor nodes cover a frequency range of 175-660 MHz corresponding to field strengths of 4.4-16.4 T for 19F samples and have a noise figure of 2.7 dB for quadrature detection. They have been successfully characterized in a 9.4 T human scanner and an 11.7 T small-animal scanner where they displayed a frequency resolution better than 4 ppb. Furthermore, in these experiments, the sensors proved to be robust against gradient switching and recovered from the scanner RF pulse within a few microseconds. An array of four sensors has been successfully employed for the mapping of first-order gradient trajectories and is capable of measuring the unwrapped phase evolution for more than 100 ms with a single excitation. The number of sensor nodes can easily be scaled up to allow for a correction of higher-order spherical harmonics. Currently, we are testing an ASIC version of the transceiver electronics which has a size of only 1 mm2, reduces the power consumption by two orders of magnitude and increases the frequency range to 30-550 MHz