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Phase-Selected 4d CBCT Acquisition Based On An Internal-External Motion Correlation Model

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M Fast

M F Fast*, E Wisotzky, U Oelfke, S Nill, German Cancer Research Center (DKFZ), Heidelberg, Germany

TU-G-141-4 Tuesday 4:30PM - 6:00PM Room: 141

X-ray projections acquired with linac-based scanners for 4d CBCT reconstruction are traditionally binned into breathing phases 'after-the-fact', i.e. after the image acquisition. This approach leads to an uneven spreading of projections over breathing phases, which compromises the image quality. We have therefore developed a novel approach based on actively triggered projections employing the forward-predicted position of the tumor at the time of image acquisition.

The prediction is based on a high-frequency external motion sensor and uses linear regression to compensate for the effective latency of the external motion sensor and the x-ray imaging chain (160 ms). An internal-to-external motion model is established from the first 40 images, by correlating the external chest-wall displacement (AP) with the internal motion (SI) derived from the images (using an 'Amsterdam shroud'). We used a lung phantom moving on a sinusoidal trajectory (15 mm SI amplitude) with a 3.5/5.0 s period. The AP motion (5 mm) was phase-shifted by 13°. The gantry rotation times were adapted depending on the breathing period during prediction training (first 30 s).

The first 40-50 projections were acquired at 6.25 Hz to cover 1-2 breathing cycles. During post-processing, these images were binned and used alongside the actively triggered frames. For the 4d CBCT mode, we were able to evenly spread the projections over all breathing phases. Motion-induced artifacts were effectively mitigated and the tumor-to-lung contrast became more evenly distributed over the respiratory phases. We also demonstrated a quasi-4d mode consisting of actively triggered peak-exhale/peak-inhale images only. By increasing the image frequency for the quasi-4d mode, it was possible to reduce the gantry rotation to 2.5-3.5 min without compromising the number of projections.

Our new approach to 4d CBCT imaging allows for active phase selection as opposed to the traditional retrospective phase binning.

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