Physical measures of image quality in photostimulable phosphor radiographic systems

Ehsan Samei, Michael J. Flynn, and David A. Reimann

International Symposium on Medical Imaging 1997. 22-28 February 1997. Newport Beach Marriott Hotel & Tennis Club, Newport Beach, California USA.

Digital photostimulable phosphor (PSP) systems are now commonly used in radiographic applications. The wide dynamic range and image processing capabilities associated with PSP systems have particular significance. Systems are now available that support different screen thicknesses with various pixel sizes. We report in this paper the modulation transfer function (MTF) and the noise power spectra (NPS) of modern PSP systems in a variety of operating modes and pixel sizes using regular and high resolution screens.

To date, measurements have been made on four PSP systems: FCR-9000, FCR-9501, and FCR-9501-HQ of Fuji Medical systems, USA, and KESPR-400 of Eastman Kodak Company. Regular screens (Fuji, ST-V and Kodak, GP-25) and high resolution screens (Fuji, HR-V and Kodak, HD) were evaluated. The presampled MTF of the systems was measured using a sharp, attenuating edge-device consisting of a polished edge of 250-micron-thick lead foil laminated between two slabs of acrylic. The edge-device was placed on the screen and aligned to be perpendicular to the incident x-ray beam. The radiograph of the edge was processed to obtain the presampled MTF in the direction perpendicular to the edge. The NPS of the systems at different input exposures was determined by 2D Fourier analysis of uniformly exposed screens.

Using comparable screen and pixel sizes, the measured MTF of the systems from different manufacturers are similar. Maintaining the same pixel size, for a given PSP system, high resolution screens offer improved MTF response over a wide frequency range with an increase in the NPS due to their reduced sensitivity compared to regular screens. The performance of FCR-9501-HQ was found to be anomalous in that the small pixel size (100 microns) associated with the HQ mode resulted in degraded resolution with improved noise response relative to the Standard mode (pixel size, 200 microns).

Although PSP systems are being widely used in radiographic applications, their image quality in terms of quantitative measures of performance has not been fully assessed. In this paper, we provide the MTF and the NPS measures for modern PSP systems and explain the results is terms of opto-physical features of these systems.

We have developed tools to measure the MTF and the NPS of the digital radiographic systems in the field. In this paper these tools are applied to modern PSP systems. The results suggest that performance of these systems are mainly limited by the thickness, sensitivity, and optical properties of the screens and is influenced by their reading time.

Copyright © 1997, David A. Reimann. All rights reserved.