Room: Room 202
Purpose: To address the fundamental limitations with commercial 4DCT and propose a comprehensive replacement.
Methods: The fundamental limitation with commercial 4DCT techniques is time. These techniques use either low-pitch helical or cine protocols for image acquisition imaging any one part of the patient for approximately 8 seconds. Irregular breathing therefore causes breathing-cycle sampling errors that are impossible to overcome and that yield artifact-ridden images with little or no quantitative value. The proposed replacement uses repeated, low-dose free-breathing fast-helical CT scans, acquired with a breathing surrogate. The scans are nearly motion artifact-free and are readily registered to one another. The resulting deformation vector fields describe the relative positions of all lung tissues as functions of the surrogate signal. A breathing motion model is employed, and its parameters fit using the deformation vector fields and breathing surrogate. The registered images are averaged and that image is deformed using the breathing motion model to 8 phases that match the current commercial 4DCT protocol. The model accuracy is determined by reproducing the original helical CT scans and deformably registering the synthesized to the original scans. A summary of the error distribution is sent to treatment plannings.
Results: The workflow provided images without breathing sorting artifacts and extremely minor breathing motion artifacts (primarily blurring), and noise characteristics consistent with using all of the radiation fluence to create the images. The protocol accuracy was within an average of 1.7+/-0.8 mm for 22 tumors in 20 patients.
Conclusion: The model-based fast-helical CT based replacement for commercial 4DCT is straightforward to implement. The image acquisition protocols can be conducted on any modern CT scanner with comparable or less dose than commercial 4DCT. The motion model can also be used to improve other parts of the workflow, including treatment planning and on-table adaptive radiation therapy.