OPTIMAL PHOSPHOR THICKNESS FOR PORTAL IMAGING

A theoretical approach known as quantum accounting diagram (QAD) analy sis has been used to calculate the spatial-frequency-dependent detecti ve quantum efficiency (DQE) of two portal imag ing systems: one based on a video camera and another based on an amorphous silicon array. The spatial frequency-dependent DQEs have then been used to determine ind ices of displayed and perceived image quality. These indices are figur es of merit that can be used to optimize the design of linear imaging systems. We have used this approach to determine which of eight phosph or screen thicknesses (ranging between 67 and 947 mg/cm(2)) is optimal for the two designs of portal imaging systems, The physical character istics (i.e., detection efficiencies, gains, and MTFs) of each of the eight x-ray detectors have been measured and combined with the physica l characteristics of the remaining components to calculate the theoret ical DQEs. In turn, the DQEs have been used to calculate theoretical i ndices of displayed and perceived image quality for two types of objec ts: a pelvis object and a pointlike object. The maximal indices of dis played and perceived image quality were obtained with screen thickness es ranging between 358 and 947 mg/cm(2), depending upon the imaging sy stem design and the object being imaged. Importantly, the results show ed that there is no single optimal screen thickness. The optimal thick ness depended upon imaging task (e.g., detecting large, low-contrast s tructures, or detecting edges and small structures). Nevertheless, the results showed that there were only modest improvements in the indice s of image quality for phosphor screens thicker than 350-400 mg/cm(2). (C) 1997 American Association of Physicists in Medicine.