Martin Eschelbach

Motion Correction and Field Monitoring

Patient or subject motion is a known source of quality degradation in MRI images. One means of tackling this problem is prospective motion correction. In this project we're applying NMR field probes [1] to assess subject head motion via position changes of the field probes. The positions of the Field probes are measured via applied gradients with known strength in every spatial direction.

The NMR field probes use Hexafluorobenzene (C6F6) as an NMR active sample. The probes are connected to a tuning matching board and are tuned to the operating frequency of 376.14 MHz at the 9.4 T human MRI scanner and matched to an impedance of 50 Ohm. The sample is excited via a custom build transmit/receive-chain based on microelectronic components assembled onto a PCB board [2]. The signals are demodulated on the PCB board and transmitted to and from the board via a shielded ethernet cable. The demodulated signal is filtered and then digitized at a sampling rate of 200 kS/s using a commercial ADC (NI PCIe-6363, National Instruments, Austin, TX, USA) and the PCBs are operated via LabVIEW (National Instruments). Due to the external hardware, the setup doesn’t block any channels of the MR Scanner.

The measurements were carried out at a 9.4 T human scanner (Siemens Magnetom). The position of each probe was determined via 3 block  gradients along each axis (5 mT/m, 1 ms). The phase ? of the signal is then used to determine the field strength and thus the positions of the field probes. Using at least three field probes and assuming rigid body motion, one can now calculate the rotations and translations of the attached objects.

[1] C. Barmet et al., MRM 2008; 60:187-197;

[2] J. Handwerker et al., IEEE Biomedical Circuits and Systems Conference (BiOCAS) 2013, Rotterdam, The Netherlands, ID 5027.