Characterization of a fluoroscopic imaging system for kV and MV radiography

An on-line kilovoltage (kV) imaging system has been implemented on a medica l linear accelerator to verify radiotherapy field placement. A kV x-ray tub e is mounted on the accelerator at 90 degrees to the megavoltage (MV) sourc e and shares the same isocenter. Nearly identical CCD-based fluoroscopic im agers are mounted opposite the two x-ray sources. These systems are being u sed in a clinical study of patient setup error that examines the advantage of kV imaging for on-line localization. In the investigation reported here, the imaging performance of the kV and MV systems are characterized to prov ide support to the conclusions of the studies of setup error. A spatial-fre quency-dependent linear systems model is used to predict the detective quan tum efficiencies (DQEs) of the two systems. Each is divided into a series o f gain and spreading stages. The parameters of each stage are either measur ed or obtained from the literature. The model predicts the system gain to w ithin 7% of the measured gain for the MV system and to within 10% for the k V system. The systems' noise power spectra (NPSs) and modulation transfer f unctions (MTFs) are measured to construct the measured DQEs. X-ray fluences are calculated using modeled polyenergetic spectra. Measured DQEs agree we ll with those predicted by the model. The model reveals that the MV system is well optimized, and is x-ray quantum noise limited at low spatial freque ncies. The kV system is suboptimal, but for purposes of patient positioning yields images superior to those produced by the MV system. This is attribu ted to the kV system's higher DQE and to the inherently higher contrasts pr esent at kV energies. (C) 2000 American Association of Physicists in Medici ne.