Nonstop gated CBCT for respiratory gating lung SBRT: A feasibility study

Free-breathing gated cone-beam computed tomography (gCBCT), which captures a specific anatomy coinciding with a preset gating window in the breathing cycle, is routinely prescribed to gating lung SBRT patients for pretreatment setup verification. However, a half-fan gCBCT scan can take 2-8 min (for a typical gating duty cycle of 30%-60% and patient breathing period of 3-6 s) on a C-arm linear accelerator because the gantry movement is interrupted and resumed by the respiratory gating signal multiple times over the scan. The long scan time increases patient on-table time, leading to discomfort and a higher likelihood of patient movement. Meanwhile, extra kV projections are acquired while the gantry is accelerating for the gCBCT scan, resulting in a higher imaging dose compared to 3D CBCT.

To investigate the feasibility of a novel imaging paradigm named "nonstop gated CBCT (ngCBCT)" that improves upon current clinical gCBCT by substantially reducing the scan time and imaging dose while retaining high-quality images.

The ngCBCT is implemented by allowing the gantry to rotate continuously, with the kV x-ray beam activated only when the breathing signal falls within the preset gating window. Raw gCBCT projections of two gating lung SBRT patients were retrospectively retrieved and intentionally sampled based on each patient's breathing cycle to emulate the ngCBCT acquisitions. The datasets include both half-fan and full-fan acquisitions, representing the primary clinical scan geometries. Three reconstruction algorithms-Feldkamp-Davis-Kress (FDK), penalized likelihood iterative reconstruction (PL), and prior-image-based iterative reconstruction (PIBR)-were applied to these ngCBCT emulations to evaluate reconstruction performances on the non-uniform and under-sampled projections resulting from this acquisition strategy.

The FDK reconstructions of ngCBCT are degraded with streak artifacts and have insufficient quality for clinical use. While PL yields improved reconstructions over FDK, the PIBR method consistently delivers the best visual and quantitative results with the aid of patient-specific prior images.

The proposed ngCBCT technique addresses the key limitations of current clinical gCBCT by substantially reducing data acquisition time and imaging dose. The ngCBCT with PIBR achieves adequate image quality and offers a promising opportunity for pretreatment setup verification in gating lung SBRT.