RESUMEN
Real-time tumor tracking in external radiotherapy can be achieved by diagnostic (kV) X-ray imaging with a dynamic flat-panel detector (FPD). It is important to keep the patient dose as low as possible while maintaining tracking accuracy. A simulation approach would be helpful to optimize the imaging conditions. This study was performed to develop a computer simulation platform based on a noise property of the imaging system for the evaluation of tracking accuracy at any noise level. Flat-field images were obtained using a direct-type dynamic FPD, and noise power spectrum (NPS) analysis was performed. The relationship between incident quantum number and pixel value was addressed, and a conversion function was created. The pixel values were converted into a map of quantum number using the conversion function, and the map was then input into the random number generator to simulate image noise. Simulation images were provided at different noise levels by changing the incident quantum numbers. Subsequently, an implanted marker was tracked automatically and the maximum tracking errors were calculated at different noise levels. The results indicated that the maximum tracking error increased with decreasing incident quantum number in flat-field images with an implanted marker. In addition, the range of errors increased with decreasing incident quantum number. The present method could be used to determine the relationship between image noise and tracking accuracy. The results indicated that the simulation approach would aid in determining exposure dose conditions according to the necessary tracking accuracy.
Asunto(s)
Modelos Biológicos , Neoplasias/diagnóstico por imagen , Neoplasias/radioterapia , Interpretación de Imagen Radiográfica Asistida por Computador/métodos , Planificación de la Radioterapia Asistida por Computador/métodos , Radioterapia Conformacional/métodos , Radioterapia Guiada por Imagen/métodos , Simulación por Computador , Humanos , Proyectos Piloto , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Pantallas Intensificadoras de Rayos XRESUMEN
Real-time tumor tracking in external radiotherapy can be achieved by diagnostic (kV) X-ray imaging with a dynamic flat-panel detector (FPD). The purpose of this study was to address image lag in target tracking and its influence on the accuracy of tumor tracking. Fluoroscopic images were obtained using a direct type of dynamic FPD. Image lag properties were measured without test devices according to IEC 62220-1. Modulation transfer function (MTF) and profile curves were measured on the edges of a moving tungsten plate at movement rate of 10 and 20 mm/s, covering lung tumor movement of normal breathing. A lung tumor and metal sphere with blurred edge due to image lag was simulated using the results and then superimposed on breathing chest radiographs of a patient. The moving target with and without image lag was traced using a template-matching technique. In the results, the image lag for the first frame after X-ray cutoff was 2.0% and decreased to less than 0.1% in the fifth frame. In the measurement of profile curves on the edges of static and moving tungsten material plates, the effect of image lag was seen as blurred edges of the plate. The blurred edges of a moving target were indicated as reduction of MTF. However, the target could be traced within an error of ± 5 mm. The results indicated that there was no effect of image lag on target tracking in usual breathing speed in a radiotherapy situation.
Asunto(s)
Fluoroscopía/instrumentación , Neoplasias/diagnóstico por imagen , Neoplasias/radioterapia , Sistemas de Computación , Humanos , Neoplasias Pulmonares/diagnóstico por imagen , Neoplasias Pulmonares/radioterapia , Movimiento , Fantasmas de Imagen , Interpretación de Imagen Radiográfica Asistida por ComputadorRESUMEN
Although there are remarkable differences in maximum luminance in cathode ray tube (CRT) and liquid crystal display (LCD) monitors and film/viewer systems, these differences cannot be recognized in our perception of them. To clarify the reason for this conflict, we analyzed the psychophysical gradient (delta), which is based on the minimum perceptible luminance difference (DeltaL(min)) and can express contrast visually recognized by observers. In this study, we first confirmed the compatibility of the psychophysical analysis to the CRT and the LCD monitors by using their threshold contrasts (C(t)s). Second, we calculated and compared the delta's of the above output devices. The C(t)s values of each device were in good agreement. Moreover, the Moon & Spencer model, which expressed the perceptibility of luminance change, was well suited to the measured C(t)s over the whole luminance range. The psychophysical analysis is therefore available not only for the film/viewer system, but also for the CRT and LCD systems. The difference of physical gradient G of the luminance characteristics curve among the output devices was larger than 20 times, whereas that of d was within 3 times. The display devices listed in the order of decreasing delta were film/viewer>LCD>CRT. These results corresponded to the visual contrast sensation and our clinical experience, which cannot recognize remarkable differences in perception. By using the psychophysical analysis, we clarified the reason for the conflict between the results of physical evaluation and the contrast visually recognized by observers.