A focused ultrasound treatment system for moving targets (part I):generic system design and in-silico first-stage evaluation

Background Focused ultrasound (FUS) is entering clinical routine as a treatment option. Currently, no clinically available FUS treatment system features automated respiratory motion compensation. The required quality standards make developing such a system challenging. Methods A novel FUS treatment system with motion compensation is described, developed with the goal of clinical use. The system comprises a clinically available MR device and FUS transducer system. The controller is very generic and could use any suitable MR or FUS device. MR image sequences (echo planar imaging) are acquired for both motion observation and thermometry. Based on anatomical feature tracking, motion predictions are estimated to compensate for processing delays. FUS control parameters are computed repeatedly and sent to the hardware to steer the focus to the (estimated) target position. All involved calculations produce individually known errors, yet their impact on therapy outcome is unclear. This is solved by defining an intuitive quality measure that compares the achieved temperature to the static scenario, resulting in an overall efficiency with respect to temperature rise. To allow for extensive testing of the system over wide ranges of parameters and algorithmic choices, we replace the actual MR and FUS devices by a virtual system. It emulates the hardware and, using numerical simulations of FUS during motion, predicts the local temperature rise in the tissue resulting from the controls it receives. Results With a clinically available monitoring image rate of 6.67 Hz and 20 FUS control updates per second, normal respiratory motion is estimated to be compensable with an estimated efficiency of 80%. This reduces to about 70% for motion scaled by 1.5. Extensive testing (6347 simulated sonications) over wide ranges of parameters shows that the main source of error is the temporal motion prediction. A history-based motion prediction method performs better than a simple linear extrapolator. Conclusions The estimated efficiency of the new treatment system is already suited for clinical applications. The simulation-based in-silico testing as a first-stage validation reduces the efforts of real-world testing. Due to the extensible modular design, the described approach might lead to faster translations from research to clinical practice

Towards MR-guided high intensity focused ultrasound ablation of liver tumors

Magnetic Resonance-guided High Intensity Focused Ultrasound (MR-HIFU) is a promising technique which can be used for completely non-invasive tissue ablation. The converging ultrasound beam penetrates the skin and subcutaneous tissues with damage, while heating the tissue only in the focal point. The MRI guidance provides accurate treatment planning and real-time temperature feedback during the procedure (thermometry). The preclinical work in this PhD thesis investigates the possibilities for application of MR-HIFU ablation for the treatment of liver tumors. First, an overview is provided of the challenges which have to be overcome for clinical translation of this treatment technique. Subsequent chapters focus on each of these individual aspects. The next chapter describes the evolution of the ablation zone after MR-HIFU treatment. It is concluded that the optimal time point for imaging evaluation of the treatment is three to seven days after the procedure. Then, a technique is described which allows for MR-HIFU ablation in the liver dome, which is not possible under normal circumstances due to the lungs obstructing the HIFU beam. Next, methods for reducing and monitoring near-field heating are presented, which is a potential source of complications after MR-HIFU ablation. The results not only help in preventing complications but also help in decreasing the overall treatment time. The next study investigated the possibilities for MR-HIFU ablation of tumors which are located behind the rib cage. Is was demonstrated that intercostal ablation of superficially located tumors is feasible. The last study incorporates the previous techniques into one clinically-feasible treatment protocol, using respiratory-gated ablation and MR thermometry. It was demonstrated that it is feasible and safe to ablate a volume of liver tissue the size of a small metastasis. It is concluded that MR-HIFU for ablation in the liver is now ready for clinical translation. Since the expected impact of this treatment on quality of life is low, MR-HIFU may be a valuable treatment for patients who are not eligible for surgical treatment

Effectiveness of Reirradiation for Painful Bone Metastases: A Systematic Review and Meta-Analysis

Clinical Oncolog