This paper describes an approach to further improving the image quality of computed radiography (CR) by use of a both- side reading method and an Imaging Plate (IP) with a transparent support. The most important factor for determining the image quality in an X-ray imaging system is X-ray utilization efficiency. Accordingly, we have proposed an imaging plate that consists of a transparent support and a thicker photostimulable phosphor layer, and a reading method that permits detection of emissions from both sides of the IP, so that an improvement in image quality can be achieved by adding image data derived from both sides of the IP. We have also found that the optimal addition ratio is dependent upon spatial frequency, and proposed a spatial filter-based addition process method for achieving optimal addition over the entire spatial frequency. The system based on the proposed reading method and addition method provided 30 - 40 % increase in detective quantum efficiency (DQE), as compared to single- side reading method. Furthermore, it has been confirmed that the DQE of the addition image is dependent upon the correlation between the data of the two images detected from both sides of the imaging plate. The reading method proposed in this paper can provide increased DQE, so that enhancement in diagnostic performance can be anticipated.