KEYWORDS: Sensors, Imaging systems, Image filtering, X-ray computed tomography, X-rays, Scintillators, Data modeling, Data acquisition, Systems modeling, Signal attenuation
The material specificity of computed tomography is quantified using an experimental benchtop imaging system
and a physics-based system model. The apparatus is operated with different detector and system configurations each
giving X-ray energy spectral information but with different overlap among the energy-bin weightings and noise
statistics. Multislice, computed tomography sinograms are acquired using dual kVp, sequential source filters or a
detector with two scintillator/photodiodes layers. Basis-material and atomic number images are created by first
applying a material decomposition algorithm followed by filtered backprojection. CT imaging of phantom materials
with known elemental composition and density were used for model validation. X-ray scatter levels are measured with a
beam-blocking technique and the impact to material accuracy is quantified. The image noise is related to the intensity
and spectral characteristics of the X-ray source. For optimal energy separation adequate image noise is required. The
system must be optimized to deliver the appropriate high mA at both energies. The dual kVp method supports the
opportunity to separately engineer the photon flux at low and high kvp. As a result, an optimized system can achieve
superior material specificity in a system with limited acquisition time or dose. In contrast, the dual-layer and sequential
acquisition modes rely on a material absorption mechanism that yields weaker energy separation and lower overall
performance.
The GE Senographe 2000D, the first full field digital mammography system based on amorphous silicon (a-Si) flat panel arrays and a cesium iodide (CsI) scintillator, has been in clinical use for over five years. One of the major advantages of this technology platform over competing platforms is the inherent flexibility of the design. Specifically, it is possible to optimize the x-ray conversion layer (scintillator) independently of the light conversion layer (panel) and vice versa. This is illustrated by a new detector utilizing the same amorphous silicon (a-Si) flat panel design, but an optimized scintillator layer, which provides up to 15% higher DQE than the existing detector. By utilizing the existing flat panel with an optimized scintillator layer, it is possible to significantly boost performance without changes to the panel design. Future enhancements to both the panel and scintillator will raise the DQE at zero frequency to more than 80%. The a-Si/CsI platform is especially well suited to advanced applications utilizing very low doses.
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