Abstract Book

S100

ESTRO 37

Results Object tracking accuracy can be found in Table 1, together with intra-observer variability. Non-optimized, the TLD framework could be applied simultaneously to three objects on a general desktop PC in real-time.

0.17Hz and 15mm amplitude. The absolute position was recorded on a workstation with an accuracy of approximately 10ms and served as the gold standard. After real-time reconstruction, the MR images were streamed directly to the same workstation and given a timestamp. After acquisition the center of mass of the high-contrast object was estimated on each image frame and fit to a sinusoidal model. The same model was fit to the gold standard. From the phase difference between the two fits the apparent latency was calculated and compared to simulations.

Conclusion The TLD framework seems promising for accurate (subpixel < 3 mm) and long-term MO tracking on clinical cine-MR images. A tracking component based on deformable registration is the next step to further improve the accuracy.

Results Partial Fourier only has an influence on the latency when a ‘reverse linear’ sampling pattern is used, showing a linear relationship (fig. 2). The latency can be reduced further by a ‘high-low’ sampling pattern, in which case it is consistently 0.17s. A ‘linear’ sampling pattern results in a consistently high latency of 0.55s. These results are in agreement with simulations (not shown) and the notion that most image content is contained in the center of k- space. Therefore the time from sampling k=0 until the end of the acquisition is the dominant factor for the overall imaging latency (fig. 1).

OC-0187 How the sampling strategy of 2D MRI affects imaging latencies in real-time MR-guided radiotherapy P. Borman 1 , H.N. Tijssen 1 , C. Bos 2 , C.T.W. Moonen 2 , B.W. Raaymakers 1 , M. Glitzner 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands 2 UMC Utrecht, Imaging Sciences Institute, Utrecht, The Netherlands Purpose or Objective The ultimate goal of MR guided radiotherapy is to adapt the treatment to anatomic changes due to e.g. respiratory motion, using a continuous stream of MR images. To react as quickly as possible, it is vital to minimize the latency between the occurrence of anatomic change and its appearance on the image. Since MR imaging is relatively slow with respect to other imaging modalities, it is necessary to use its acquisition flexibility to optimize latency and speed. In this work we present a systematic analysis of the latencies inherent to real-time MRI and show how the choice of sampling pattern and use of Partial Fourier (PF), influences the apparent imaging latency, i.e. the difference between actual and observed position after reconstruction. Material and Methods Partial Fourier is a common acceleration technique in MR, where only part k-space is acquired and missing parts are synthesized in reconstruction. As such it was used to accelerate the acquisition (full matrix: 256x256). In addition, ‘linear’, ‘reverse linear’ and ‘high-low’ sampling patterns were used to investigate the effects on apparent image latencies (fig. 1). The experiments were performed on the clinical prototype of the Elekta Unity system (1.5T high-field MR). Motion was generated using a 4D motion phantom (QUASAR MRI4D, modusQA). The phantom was set to perform a 1D sinusoidal trajectory of

Conclusion The MR acquisition can be a major contributor to the latency of the feedback chain, especially compared to the mechanical latency of the MLC. It is however controllable by choosing the sampling pattern in such a way that the central part of k-space is sampled latest. Although PF always increases the temporal resolution, it does not always decrease the imaging latency, as was observed for the ‘linear’ and ‘high-low’ sampling patterns. OC-0188 A ROI-based global motion model for MRI- guidance in radiation therapy: a phantom study N. Garau 1 , C. Paganelli 1 , G. Meschini 1 , R. Via 1 , M. Riboldi 2 , G. Baroni 1 1 Politecnico di Milano, Dipartimento di Elettronica- Informazione e Bioingegneria, Milano, Italy 2 Ludwig-Maximilians-Universität Münche n, Department of Medical Physics, Munchen, Germany