Spatial Resolution Evaluation of a Photon Counting Detector-Based X-Ray Scatter Imaging System for Real-Time Lung Tumor Tracking
Abstract
Purpose
To evaluate the spatial resolution of an X-ray scatter imaging system utilizing a Photon Counting Detector (PCD) for real-time lung tumor tracking. Unlike transmission imaging, scatter imaging offers direct 3D tumor localization. This study quantifies the system's spatial resolution through measurement of Line Spread Function (LSF) and Modulation Transfer Function (MTF) under varying geometric and energy configurations.
Methods
A prototype system was constructed using a digital radiography X-ray source and a PCD with a custom tungsten collimator. An acrylic rod phantom (13 mm diameter) was imaged in two orientations: (1) Vertical (X-ray beam along the rod axis) to assess Z-direction resolution via the flat edge, and (2) Horizontal (X-ray beam perpendicular to the rod) to assess XY-direction resolution via the flat edge. We varied tube voltage (80/120 kV), Source-to-Object Distance (SOD: 68.5/83.5 cm), and Object-to-Detector Distance (ODD: 22.2/32.2 cm). Resolution was quantified by calculating the FWHM of the LSF and MTF derived from the edge images.
Results
Spatial resolution exhibited strong dependence on acquisition geometry and beam energy. In the Z-direction, FWHM ranged from 10.07 mm to 18.24 mm. Lower tube voltage (80 kV) improved resolution, attributed to reduced collimator septal penetration. Horizontal (XY) resolution has a minimum FWHM of 8.23 mm (MTF50 = 0.54 lp/cm). Reducing the ODD significantly improved spatial resolution.
Conclusion
This study establishes baseline spatial resolution metrics for PCD-based scatter imaging. While inherent scatter LSF is broader than transmission imaging, the results confirm that resolution is controllable via geometric and energy optimization. The superior XY resolution supports the system’s viability for lateral tumor tracking in SBRT. Future work will focus on system integration, including the addition of a motion stage for automated data acquisition, and comprehensive optimization studies to determine the geometric configuration that yields the optimal scatter image quality.