High-speed slot-scanning radiography using small-angle tomosynthesis: Investigation of spatial resolution.

PURPOSE This paper studies spatial resolution that is achievable with a fast slot-scanning tomosynthesis approach for orthopedic examinations. Hereby, we use parallel scanning motion implemented in a twin-robotic X-ray system. METHODS We have measured and analyzed the modulation transfer function (MTF) for various combinations of scanning speed, X-ray tube voltage, pulse length, the nominal focal spot size as well as source-to-object distances. Moreover, we present a theoretical model which describes the system in terms of the MTF. The system was equipped with newly developed linear trajectory prototypes for slot-scanning. The acquired images form the basis for a small-angle tomosynthesis reconstruction. In total three different scanning speeds (27 cm/s, 14 cm/s, 8 cm/s), pulse lengths (1 ms, 2 ms, 4 ms), tube voltages (80 kV, 100 kV, 120 kV), two nominal focal spot sizes (0.6, 1.0) and three source-to-object distances (950 mm, 1050 mm, 1150 mm) were investigated. To determine the resolution capabilities we measured the MTF for the given parameter space. The results were then used to design a filter that yields a desired resolution in the reconstructed image. In addition, we also measured the noise power spectrum (NPS) to show the influence of the aforementioned filters on the noise distribution. RESULTS We have shown that the presented model is in good agreement with the performed measurements. Scanning speed and pulse width have an impact on the MTF in the scanning direction. Up to a travel distance of 0.3 mm during an X-ray pulse, an isotropic resolution can be achieved. Longer pulse width or higher scanning speed cause anisotropic resolution. Moreover, it could be shown that none of the investigated parameters have an influence on the MTF perpendicular to the scanning direction (slot direction). The 10 % MTF value ranges between 9 and 18 lp/cm in the scanning direction and about 18 lp/cm in slot direction. Tube voltage, nominal focal spot size as well as the source-to-object distance showed no major impact on the system MTF. In terms of the anisotropic resolution capabilities, we have shown that limiting the resolution in the slot direction to obtain isotropic resolution is possible yet at the cost of an inhomogeneous noise pattern. On the other hand, maintaining the resolution in slot direction will provide a better edge response and a more homogeneous noise texture at the cost of an inhomogenous image resolution. CONCLUSION We have demonstrated the feasibility of the slot-scanning technique using a twin-robotic X-ray system. Even the fastest scanning mode (27 cm/s) yields image resolution on a level that is sufficient for typical orthopedic examinations in terms of musculoskeletal measurements. Moreover, it could be shown that the application of specifically designed target MTFs on 2-D X-ray images is feasible.

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