Assessment of portal image resolution improvement using an external aluminum target and polystyrene electron filter

Abstract

Background In this study, an external 8 mm thick aluminum target was installed on the upper accessory tray mount of a medical linear accelerator head. The purpose of this study was to determine the effects of the external aluminum target beam (Al-target beam) on the portal image quality by analyzing the spatial and contrast resolutions. In addition, the image resolutions with the Al-target beams were compared with those of conventional 6 megavoltage (MV) images. Methods The optimized Al-target beam was calculated using Monte Carlo simulations. To validate the simulations, the percentage depth dose and lateral profiles were measured and compared with the modeled dose distributions. A PTW resolution phantom was used for imaging to assess the image resolution. The spatial resolution was quantified by determining the modulation transfer function. The contrast resolution was determined by a fine contrast difference between the 27 measurement areas. The spatial and contrast resolutions were compared with the those of conventional portal images. Results The measured and calculated percentage depth dose of the Al-target beam were consistent within 1.6%. The correspondence of measured and modelled profiles was evaluated by gamma analysis (3%, 3 mm) and all gamma values inside the field were less than one. The critical spatial frequencies ( f 50 ) of the images obtained with the Al-target beam and conventional imaging beam were 0.745 lp/mm and 0.451 lp/mm, respectively. The limiting spatial frequencies ( f 10 ) for the Al-target beam image and the conventional portal image were 2.39 lp/mm and 1.82 lp/mm, respectively. The Al-target beam resolved the smaller and lower contrast objects better than that of the MV photon beam. Conclusion The Al-target beams generated by the simple target installation method provided better spatial and contrast resolutions than those of the conventional 6 MV imaging beam.

A phantom based evaluation of the clinical imaging performance of electronic portal imaging devices

“…MTF is regularly employed to assess a system's spatial resolution properties as well as the signal transfer accuracy through the imaging chain of a system. In the present study, MTF was expressed by the Coltman's formula [ 5 , 10 , 18 , 21 , 27 , 28 , [30] , [31] , [32] ]: where CTF is the Contrast Transfer Function defined by Refs. [ 18 , 21 , 28 ]: …”

“…Studies, e.g. investigating various issues such as the effects of external aluminum target beam of the LINAC on EPID [ 10 ], the use of wavelet analysis to detect and isolate defective pixels and artifacts for quantitative quality assurance (QA) of EPIDs, characterizing the image noise, have been published [ 6 ]. In addition, a review is referred to the properties of dosimetry and imaging of radiation detectors, such as the physical principles of operation of calorimeters ionization, charge detectors, semiconductor, luminescent, chemical detectors, and radiochromic films [ 11 ].…”

Investigation of image quality of MV and kV CBCT with low‐Z beams and high DQE detector

“…Experimentally, Baek et al . showed that a gantry mounted aluminum target improved the limiting spatial frequency (f50) from 0.451 lp/mm to 0.745 lp/mm for a tungsten target 6 MV beam 8 . Additionally, Parsons et al .…”

“…6,7 Experimentally, Baek et al showed that a gantry mounted aluminum target improved the limiting spatial frequency (f50) from 0.451 lp/mm to 0.745 lp/mm for a tungsten target 6 MV beam. 8 Additionally, Parsons et al showed an increase in the contrast to noise ratio (CNR) in projection images of cortical bone by factors ranging from 3.7 to 7.4 between carbon and aluminum targets with 2.35 and 1.9 MV photon beams and the default 6 MV tungsten imaging setup. 9 Furthermore, advances in MV EPID design have shown promise in improving MV image quality.…”