Feasibility of interactive magnetic resonance imaging of moving anatomy for clinical practice.

BACKGROUND: Real-time magnetic resonance imaging (MRI) imaging with real-time reconstruction has been available for some time. The technique acquires and presents the MRI images to the operator the instant they are acquired. However, besides guiding purposes, like catheter tracking and placement of electrodes during neurosurgery, the diagnostic value of this method is relatively unexplored. PURPOSE: To test an interactive slice-positioning system with respect to real-time MRI reconstruction for imaging of moving anatomical structures on two different scanner brands by using inexpensive computer hardware. MATERIALS AND METHODS: The MRI data were sampled using two acquisition schemes: a Cartesian sampling scheme and a radial sampling scheme based on the golden ratio. Four anatomical targets, which exhibit non-periodic movement, were identified and imaged: movement of the gastric ventricle emptying, movement of the small bowels, the articulators of a professional singer and of a 20-week old fetus. RESULTS: Informative anatomical images were obtained in different settings of moving targets. The implemented real-time system acquired, reconstructed and displayed MRI images in real time with a high frame rate using inexpensive computer hardware on two standard 1.5 T clinical MRI scanners. CONCLUSION: Our approach verified that when imaging selected moving anatomical targets, with no a priori knowledge of the movement, interactive slice positioning using real-time reconstruction may be a feasible approach for finding the optimal slice position in cases in which a standard 3D volumetric scan is impeded by movement. Future studies are needed to explore its full potential.

Real-time 3D target tracking in MRI guided focused ultrasound ablations in moving tissues.

Magnetic resonance imaging-guided high intensity focused ultrasound is a promising method for the noninvasive ablation of pathological tissue in abdominal organs such as liver and kidney. Due to the high perfusion rates of these organs, sustained sonications are required to achieve a sufficiently high temperature elevation to induce necrosis. However, the constant displacement of the target due to the respiratory cycle render continuous ablations challenging, since dynamic repositioning of the focal point is required. This study demonstrates subsecond 3D high intensity focused ultrasound-beam steering under magnetic resonance-guidance for the real-time compensation of respiratory motion. The target is observed in 3D space by coupling rapid 2D magnetic resonance-imaging with prospective slice tracking based on pencil-beam navigator echoes. The magnetic resonance-data is processed in real-time by a computationally efficient reconstruction pipeline, which provides the position, the temperature and the thermal dose on-the-fly, and which feeds corrections into the high intensity focused ultrasound-ablator. The effect of the residual update latency is reduced by using a 3D Kalman-predictor for trajectory anticipation. The suggested method is characterized with phantom experiments and verified in vivo on porcine kidney. The results show that for update frequencies of more than 10 Hz and latencies of less then 114 msec, temperature elevations can be achieved, which are comparable to static experiments.