Alexander Jöhl


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Jöhl

First Name

Alexander

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0000-0001-7116-6540

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2016 - 20206
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Publications 1 - 6 of 6
  • Body motion during dynamic couch tracking with healthy volunteers
    Item type: Journal Article
    Jöhl, Alexander; Bogowicz, Marta; Ehrbar, Stefanie; et al. (2019)
    Physics in Medicine and Biology
  • The ideal couch tracking system - Requirements and evaluation of current systems
    Item type: Journal Article
    Jöhl, Alexander; Ehrbar, Stefanie; Guckenberger, Matthias; et al. (2019)
    Journal of Applied Clinical Medical Physics
    Introduction Intrafractional motion can cause substantial uncertainty in precision radiotherapy. Traditionally, the target volume is defined to be sufficiently large to cover the tumor in every position. With the robotic treatment couch, a real‐time motion compensation can improve tumor coverage and organ at risk sparing. However, this approach poses additional requirements, which are systematically developed and which allow the ideal robotic couch to be specified. Methods and materials Data of intrafractional tumor motion were collected and analyzed regarding motion range, frequency, speed, and acceleration. Using this data, ideal couch requirements were formulated. The four robotic couches Protura, Perfect Pitch, RoboCouch, and RPSbase were tested with respect to these requirements. Results The data collected resulted in maximum speed requirements of 60 mm/s in all directions and maximum accelerations of 80 mm/s2 in the longitudinal, 60 mm/s2 in the lateral, and 30 mm/s2 in the vertical direction. While the two robotic couches RoboCouch and RPSbase completely met the requirements, even these two showed a substantial residual motion (40% of input amplitude), arguably due to their time delays. Conclusion The requirements for the motion compensation by an ideal couch are formulated and found to be feasible for currently available robotic couches. However, the performance these couches can be improved further regarding the position control if the demanded speed and acceleration are taken into account as well.
  • ELPHA: Dynamically deformable liver phantom for real-time motion-adaptive radiotherapy treatments
    Item type: Journal Article
    Ehrbar, Stefanie; Jöhl, Alexander; Kühni, Michael; et al. (2019)
    Medical Physics
  • Patient Motion Compensation via Treatment Couch in Radiotherapy
    Item type: Doctoral Thesis
    Jöhl, Alexander (2018)
  • Modeling and Performance Evaluation of a Robotic Treatment Couch for Tumor Tracking
    Item type: Journal Article
    Jöhl, Alexander; Lang, Stephanie; Ehrbar, Stefanie; et al. (2016)
    Biomedical Engineering / Biomedizinische Technik
    Tumor motion during radiation therapy increases the irradiation of healthy tissue. However, this problem may be mitigated by moving the patient via the treatment couch such that the tumor motion relative to the beam is minimized. The treatment couch poses limitations to the potential mitigation, thus the performance of the Protura (CIVCO) treatment couch was characterized and numerically modeled. The unknown parameters were identified using chirp signals and verified with one-dimensional tumor tracking. The Protura tracked chirp signals well up to 0.2 Hz in both longitudinal and vertical directions. If only the vertical or only the longitudinal direction was tracked, the Protura tracked well up to 0.3 Hz. However, there was unintentional yet substantial lateral motion in the former case. And during vertical motion, the extension caused rotation of the Protura around the lateral axis. The numerical model matched the Protura up to 0.3 Hz. Even though the Protura was designed for static positioning, it was able to reduce the tumor motion by 69% (median). The correlation coefficient between the tumor motion reductions of the Protura and the model was 0.99. Therefore, the model allows tumor-tracking results of the Protura to be predicted.
  • Performance comparison of prediction filters for respiratory motion tracking in radiotherapy
    Item type: Journal Article
    Jöhl, Alexander; Ehrbar, Stefanie; Guckenberger, Matthias; et al. (2020)
    Medical Physics
    Purpose In precision radiotherapy, the intrafractional motion causes substantial uncertainty. Traditionally, the target volume is expanded to cover the tumor in all positions. Alternative approaches are gating and adaptive tracking, which require a time delay as small as possible between the actual tumor motion and the reaction to effectively compensate the motion. Current treatment machines often exhibit large time delays. Prediction filters offer a promising means to mitigate these time delays by predicting the future respiratory motion. Methods A total of 18 prediction filters were implemented and their hyperparameters optimized for various time delays and noise levels. A set of 93 traces were standardized to a sampling frequency of 25 Hz and smoothed using the Fourier transform with a 3 Hz cutoff frequency. The hyperparameter optimization was carried out with ten traces, and the optimal hyperparameters were evaluated on the remaining 83 traces. Results For smooth traces, the wavelet least mean squares prediction filter and the linear filter reached normalized root mean square errors of below 0.05 for time delays of 160 and 480 ms, respectively. For noisy signals, the performance of the prediction filters deteriorated and led to similar results. Conclusions Linear methods for prediction filters are sufficient for respiratory motion signals. Reducing the measurement noise generally improves the performance of the prediction filters investigated in this study, even during breathing irregularities. © 2019 American Association of Physicists in Medicine
Publications 1 - 6 of 6