THICK PHOSPHOR SCREENS FOR ONLINE PORTAL IMAGING

Gd2O2S phosphor screens between 250 and 1000 mg/cm(2) thick were evalu ated for use in megavoltage imaging systems. The phosphor layers were placed on brass plates ranging from 1 to 5 mm thick, each with and wit hout an optical back reflector (white paint). Light output and spatial resolution were measured at 6- and 23-MV x-ray energies. Light output was found to increase linearly with phosphor thickness up to 500 mg/c m(2), reaching a plateau at 1000 mg/cm(2). Spatial resolution [modulat ion transfer function (MTF)] decreased exponentially with phosphor thi ckness up to 750 mg/cm(2), where a minimum was reached. The variation of MTF with phosphor thickness was found to obey a simple empirical re lation.