An Investigation of Calibration Phantoms for CT Scanners with Tube Voltage Modulation

The effects of calibration phantoms on the correction results of the empirical artifacts correction method (ECCU) for the case of tube modulation were investigated. To improve the validity of the ECCU method, the effect of the geometry parameter of a typical single-material calibration phantom (water calibration phantom) on the ECCU algorithm was investigated. Dual-material calibration phantoms (such as water-bone calibration phantom), geometry arrangement, and the area-ratio of dual-material calibration phantoms were also studied. Preliminary results implied that, to assure the effectiveness of the ECCU algorithm, the polychromatic projections of calibration phantoms must cover the polychromatic projection data of the scanning object. However, the projection range of a water calibration phantom is limited by the scan field of view (SFOV), thus leading to methodological limitations. A dual-material phantom of a proper size and material can overcome the limitations of a single-material phantom and achieve good correction effects. 1. Introduction In conventional computed tomography (CT) tube voltages are fixed (constant) while CT scans are carried out. However, tube voltage modulation is helpful for at least two reasons. First, tube voltage modulation has the potential for further dose reduction [1]. In addition, tube voltage modulation can obtain satisfactory image quality avoiding detector pixel saturation. When the thickness of the scanned object varies greatly with the view angle, the constant power of the X-ray generated by traditional CT may lead to pixel saturation or at least strike a balance of quantum noise among the different views. Dynamically adjusting the X-ray tube voltage in synchrony with the CT scanning can avoid this problem. Tube voltage modulation is very helpful in CT scanning, but when the voltage is modulated at different views during a CT scan, images reconstructed through commonly used reconstruction methods may contain various artifacts leading to inaccuracy and degradation of image quality. Ideally, monochromatic X-ray beams are required in CT scanning. However, conventionally used X-ray beams in CT are polychromatic with a moderately broad energy spectrum. In this paper, different polychromatic X-ray beams are used through tube voltage modulating at different scanning views. As we know, X-ray attenuation processes in matter are energy dependent [2]. However, the reconstruction algorithm is based on the assumption of the monochromatic property of the X-ray beam that just computed the average attenuation coefficient and thus