Real-time monitoring and analysis of MR-guided laser ablation in an open-configuration MR-system
Our goal was to investigate whether an open 0.5T MR-system with integrated frameless stereotactic guidance tools can provide sufficient intraoperative monitoring of interstitial laser therapy (ILT). Temperature-sensitive T1-weighted Fast-Spin-Echo (FSE)- or Spoiled Gradient-Echo sequences (SPGR) were applied and various image processing techniques (pixel-subtraction, phase mapping, optical flow computation) developed in order to control the thermal energy deposition during ILT in patients with brain- and liver tumors. While images from T1-weighted FSE- or SPGR sequences were acquired within 5 - 13 seconds, ILT lasted 2 to 26 minutes. Pixel subtraction or optical flow computation of T1-weighted images was performed within less than 110 msec. Alternating, phase-mapping of real- and imaginary components of SPGR sequences was performed within 220 msec. Pixel subtraction of T1-weighted images identified thermal changes in liver and brain tumors but could not evaluate the temperature values as chemical-shift based imaging, which was however, more affected by susceptibility effects and motion. Optical flow computation displayed the predicted course of thermal changes and revealed that the rate of heat deposition can be anisotropic, which may be related to heterogeneous tumor structure and/or vascularization.