This report presents a system model of the STATSCAN slit scanning full body radiography machine. A Cascaded Linear Systems model of the detector was developed and the theoretical DQE, MTF and NPS were compared to measured values for the RQA9 beam quality described in IEC 62220-1. The effect on detector DQE of various system parameters such as coupling efficiency, CCD noise and pixel binning was quantified. System performance for various thicknesses of Gd2O2S:Tb was analyzed. The notion of a “System DQE” has been suggested by several authors to facilitate the comparison of overall systems. An expression for the overall “System DQE” was developed by including the effects of scattered radiation, grid attenuation and focal spot unsharpness in the cascaded model. Scattered radiation was quantified as a function of system geometry parameters and was treated as an “additive noise stage”. A realistic model of the focal spot was used to calculate the MTF due to beam divergence in the scan direction and focal spot unsharpness in the slit direction. It was found that the “System DQE” is a valuable parameter for the purpose of comparing gridless slit scanning system performance to conventional geometry system performance.
Clinical trials performed for the FDA’s Section 510k compliance submission of the Statscan digital, full-body, linear slit scanning diagnostic radiography system revealed that comparable diagnostic results with a commercial full-field screen film device were obtained with the Statscan using much lower radiation doses. For certain imaging procedures the doses for Statscan were as much as twenty to thirty times lower. However the results varied by a large amount and in particular the results for chest radiographs were anomalous in that the Statscan dose was less reduced. Whilst it is well known that slit scanning radiography has considerably lower radiation exposure than full-field devices due to its much lower scatter to primary ratio and also that digital radiography has the potential for lower radiation dosages, it was thought that that this alone did not fully account for the dose differences. This paper suggests that these dose differences, including the anomaly mentioned above, can be explained by considering the unique way that slit scanning is undertaken by Statscan i.e. by scanning the tube, detector, slit and collimators together along a linear path. The effect on measured skin entrance doses is explained and the dosage differences as affected by digital technology, higher DQE, slit scanning (low scatter to primary ratio) and linear slit scanning methods are quantified. Furthermore it is explained how the Statscan geometry leads an improved “skin sparing” effect.