MR-compatible ultrasound research platform for motion tracking to reduce motion induced artifacts in MR imaging

摘要

High resolution or time consuming MR-imaging methods suffer from motion artifacts due to breathing or cardiac motion. This has an impact on high resolution morphological images and increasingly on highly sensitive functional MRI. MR-inherent compensation techniques like navigator echoes and methods like ECG or respiratory gating can reduce these artifacts but are indirect methods monitoring surrogates and not very reliable indicating the actual organ positions. To improve MR imaging of moving organs we propose diagnostic ultrasound (US) as additional imaging method for organ motion detection and compensation during MRI scanning. We developed an MRI compatible US platform to enable non-invasive hybrid MR-US-imaging and real-time US-motion compensation. The ultrasound system and the transducers have to be insensitive to the high electromagnetic fields like we find them outside and inside the tomograph and they may not interfere with the MR-system electromagnetically. In this work, our US research platform and special ultrasound transducers are presented that meet these requirements consisting of the beamformer unit, a special LCD display for use close to the MR-magnet, special ultrasound probes (T-shaped 2×64 element phased array transducers) and a separate PC to be used in the control room of the MRI scanner. The system can use multiple transducers in parallel mode to acquire up to 4 US images simultaneously. Tracking of moving structures in the ultrasound images is performed by a stochastic tracking algorithm, which fits a contour to the ultrasound data using affine transformations in real-time. Interferences of the US system with the MR scanner were measured and characterized. It was possible to make simultaneous measurements (US and MRI) next to and inside the MRT system to reconstruct US motion compensated MR volumes. The interferences in the ultrasound imaging induced by the MRT system can be neglected. The effects of EMC emissions by the US device on t- e MRT system are measured and characterized. Data acquisition was done in parallel to the MRI scan of organ movements inside the body. The setup was tested in a 1.5 T and a 3 T MRI systems and first ultrasound compensated MR volumes were acquired.