Optimization of Beam Spectrum and Dose for Lower-Cost CT

In many parts of the developing world, easy access to volumetric imaging is not available. A Lower-Cost CT setup was proposed and found feasible by Dobbins et al., but was not yet optimized to maximize image quality while minimizing radiation dose to the patient. A combination of spectrum modeling and Monte Carlo simulations were used to compare x-ray beam parameters to determine which combination was optimal. The beam parameters considered were filter type, filter thickness, and tube peak kilovoltage (kVp). The optimization was based on the differential signal-to-noise ratio (dSNR) and the dose, using a factor referred to as dSNR Efficiency. After the three different filter materials at three different thicknesses were compared across five different kVp values, it was determined that one half-value-layer (HVL) of copper was the best filter type and thickness to achieve maximum image quality for minimal patient dose.

In order to verify that a good dSNR efficiency using the spectrum modeling and Monte Carlo meant that the system would provide useable images, the extended cardiac-torso (XCAT) phantom was used to simulate CT images, using a ray tracer, and estimate dose, using a full LCCT Monte Carlo simulation. The ray-tracer produced x-ray projections of the XCAT phantom which were then run through a Feldkamp reconstruction algorithm to produce CT images. The full LCCT Monte Carlo simulation modeled the LCCT setup using the XCAT phantom to determine the dose to the patient. From the reconstructed CT images, it was determined that for image studies that favor air contrast higher kVp values, such as 140 kVp, are optimal. For studies that favor bone contrast, however, the lower kVp values, such as 60 or 80 kVp, are optimal. For 140 kVp images, the average effective dose, calculated using the ICRP 103 protocol, was mSv per mAs. The average effective dose for 60 kVp was mSv per mAs, and the average effective dose for 80 kVp was mSv per mAs. Further work is needed to determine optimal mAs values for different imaging studies. The LCCT setup can provide volumetric imaging to developing parts of the world that currently have no volumetric imaging, which would greatly improve the quality of readily available medical care.