A Multi-Source CT Architecture with Three Imaging Modes for SBRT Image Guidance

Abstract

Stereotactic ablative radiotherapy of oligometastatic disease is an emerging paradigm. Current image guidance via CBCT is limited by respiratory motion, scatter, and the requirement of fiducials for some sites. We investigate an alternative to CBCT that can surmount these limitations. Our proposed CT architecture has three imaging modes. These modes are (1) localization of high-contrast lung tumors in a 10 x 2 cm FOV in 100 milliseconds; (2) visualization of low-contrast liver tumors in a 10 x 2 cm FOV in 1 second; (3) daily setup imaging of a 50 cm by 15 cm volume in two minutes. Our proposed design includes a ring detector, an array of compact x-ray sources on an adjacent ring, and a movable collimator that switches between narrow and wide FOV modes. Narrow and wide FOVs are used for motion tracking and daily alignment, respectively. The ring detector is 4 cm wide, with total active area comparable to a flat panel detector. In the narrow FOV mode, 15 sources activate simultaneously and illuminate non-overlapping arcs of the detector. In lung imaging, each source is activated only once. In liver imaging, sources are re-activated many times over 1 second as the source ring rotates to sample more projection angles. Finally, in wide FOV mode, the sources activate sequentially to reduce scatter and distribute heat. The ring is mounted adjacent to the MV beam to avoid collision, or could be mounted at an angle if imaging during treatment is desired. We assessed the feasibility of this design in terms of number of sources required, impact of truncation artifacts, compatibility with existing x-ray source technology, and radiation dose. Visibility was assessed qualitatively using two patient datasets in simulation. For sufficient visibility of lung tumors, at least 30 x-ray sources are needed. Dose for lung imaging can be 100x less than CBCT, and imaging would require only 2 detector readouts of 15 sources each (<70 milliseconds), enabling cine reconstruction of lung tumor motion without data rebinning. For a 25 mm liver tumor with 15 HU contrast, 300 mAs was needed for adequate visualization. These requirements are compatible with the heat loading limitations of conventional fixed-anode x-ray sources, which are also inexpensive and compact (3 pounds, 6.5 cm wide) and therefore a practical choice for this design. Radiation dose for liver imaging is 10 times less than CBCT because of the small FOV. Acquisition with a wide field of view (250 mAs, 14 cm in z) requires about 100 seconds but would have reduced scatter compared to CBCT. A multi-source CT architecture with three imaging modes could enable fast image guidance tailored towards lung and liver lesions, while still maintaining adequate performance for daily alignment. This architecture could make intrafraction motion monitoring practical and could eliminate fiducial implantation in abdominal SBRT for tumors is visible in noncontrast CT.