Effect of slit scan imaging techniques on image quality in radiotherapy electronic portal imaging

This research effort focused on the application of the slit scan technique in improving image quality of teletherapy portal imaging for various imaging geometries. The primary result of this study was the validation that slit scan digital radiography improves the quality of a portal image by scatter reduction resulting from the patient and machine. This method provides the ability to maintain image quality independent of patient size and air gap.;The performance of the slit scan technique was evaluated by simulating a variety of imaging geometries to determine where it would have the greatest benefit on image quality when used clinically. The factors assessed that would demonstrate the effectiveness of this method on image quality were: (1) Energy response of the EPID. (2) Portal image field size. (3) Patient thickness. (4) Air gap. (5) Slit scan width.;The effect on the scatter-to-primary ratio relative to valves of the above physical properties was analyzed. The results revealed that the slit scan technique was successful in eliminating scatter radiation for various imaging geometries and are beneficial where large field sizes and short air gaps maybe clinically used.;Investigation into the energy response of the EPID detector confirmed published work that there is an increased sensitivity to lower energies. Due to 65 this physical characteristic the effect of scatter radiation on degrading image quality is amplified. Scatter contribution is relatively minor in small field sizes and large air gaps. Clinically though, it is desirable to visualize anatomical landmarks to determine proper patient position requiring larger field sizes. As demonstrated in this work, when a large field size is used scatter is increased. Decreasing the air gap is necessary for large patients in order to minimize magnification. The development of higher resolution detector systems may also lead to the use of smaller air gaps in order to minimize the loss of resolution due to geometric blurring of the focal spot. In situations where the air gap is decreased scatter radiation will have an adverse effect on image contrast. The use of slit scan imaging may be of particular benefit for the improvement of large field contrast applications where reduced air gap distances are advantageous.