Development of a digital X-ray-imaging system at the National Accelerator Centre

[摘要] ENGLISH ABSTRACT: A digital portal X-ray imaging system was developed to replace the radiographic X-ray filmscurrently used for patient position verification at the National Accelerator Centre (NAC)proton therapy facility. The main advantage of a digital system is the short time in which theimage can be obtained. Other advantages include optimisation of the image display, effectivearchiving of the digital images, access from various locations through data networks, andlower operational costs. The digital system described in this thesis consists of a Gd202S:Tbscintillator screen for converting X-rays to visible light, a protected aluminum front silveredmirror to direct the light to a Charge Coupled Device (CCD) camera for capture and apersonal computer for data acquisition, processing and display. Compared with other digitalimaging systems, this is a simple, compact and affordable system.The properties of the various components were investigated. The Rarex G-130 (Gd202S:Tb)scintillation screen was chosen for its good spatial resolution, high emission efficiency andgood matching between the spectral emission wavelength peak and the quantum efficiency ofthe CCD camera. The spatial resolution measured for the system with a field of view (FOV)of 290 x 190 mnr' is 1.3 lp/mm, which can be improved by increasing the CCD chipresolution or decreasing the field of view, since the CCD camera limits the spatial resolution.Intrinsic detector noise determines the lower limit of the dynamic range of the detector and isreduced by cooling the CCD camera. A dark current exposure is subtracted from the image toremove the bias signal and background signal level mainly caused by thermal noise. Photonnoise, beam in-homogeneity and efficiency variations across the CCD chip are removed by aflat field correction. The digital images obtained with this system compare very well with thecurrently used radiographic film images and they are satisfactory for the purpose of patientposition verification. Using the digital system it is possible to reduce the patient dose by 19 %and still obtain satisfactory image quality.

[发布机构] Stellenbosch University