Modeling the performance characteristics of computed radiography (CR) systems

Abstract

Computed radiography (CR) using storage phosphors is widely used in digital radiography and mammography. A cascaded linear systems approach wherein several parameter values were estimated using Monte Carlo methods was used to model the image formation process of a single-side read ;;flying spot'' CR system using a granular phosphor. Objective image quality metrics such as modulation transfer function and detective quantum efficiency were determined using this model and show good agreement with published empirical data. A model such as that addressed in this work could allow for improved understanding of the effect of storage phosphor physical properties and CR reader parameters on objective image quality metrics for existing and evolving CR systems.

Fig. 1

Model for the photostimulable phosphor plate. The properties of the storage phosphor are listed in Tables I & II.

Fig. 2

Schematic of the CR reader.

Fig. 3

Parallel cascades based linear systems model for signal and noise propagation through the CR based imaging system. Stages 0 through 2 correspond to x-ray image acquisition and activation of PSL-centers. Stage 3 represents the fading of the activated PSL-centers. Stages 4 through 6 represent the photostimulation and PSL emission within the PSP plate. Stages 7 through 9 represent the signal collection, temporal filtering along scan direction, and additive electronic noise arising from the CR reader.

Fig. 4

Normalized x-ray spectra for mammography (A; top) and radiography (B; bottom) used in this study.

Fig. 5

Red photon fluence, ϕ(r,z) within the phosphor normalized to a maximum value of 1 for mammography (A; top) and radiography (B; bottom).

Fig. 6

The depth-dependent excitation (red photon) flux, ϕ_exc(z), computed for mammography and radiography.

Fig. 7

The depth-dependent bleaching efficiency, g₅(z), and escape efficiency, g₆(z), computed for mammography (A; top) and radiography (B; bottom).

Fig. 8

The depth-weighted point spread function (PSF), ϕ(r), before (represented as “w/o Laser”) and after (represented as “with Laser”) convolution with the Gaussian Laser beam for mammography and radiography.

Fig. 9

Sources that contribute to the system MTF. Left (A & B): Mammography; Right (C & D): Radiography. Top (A & C): Characteristic transfer function of Kα₁, Kα₂ and Kβ₁ emission lines, weighted by their relative intensities to obtain T_K. Bottom (B & D): In addition to T_K, the depth-weighted PSL spread function, $T_{\mathit{PSL}}^G$, the spread function due to imaging plate translation along the subscan direction, represented as sinc(πν_ssτ_D), and the pixel aperture function representing the spacing between adjacent scan lines, T_pix are shown. The resultant system MTF, T_sys and the effect of the Gaussian Laser beam are also shown.

Fig. 10

System presampling MTF along the subscan direction, T_sys, for mammography (A; top) and radiography (B; bottom). Experimental data points are shown as filled-in symbols and model results are shown as lines.

Fig. 11

Components of the normalized NPS along the subscan direction, W₉(v)/[q₉(0)]² including the granular, x-ray structural, and electronic noise (shown in A, only), for mammography.

Fig. 12

Components of the normalized NPS along the subscan direction, W₉(v)/[q₉(0)]² including the granular, x-ray structural, and electronic noise (shown in A, only), for radiography.

Fig. 13

A: Plot of absolute deviation in DQE(0.5, X_min) vs. optical collection efficiency, $g_7^c$. Minimum values were found to occur at $g_7^c=0.36$ and $g_7^c=0.18$ for mammography and radiography, respectively. B: Plot of chi-squared statistic value, ${\chi }_v^2$ vs. grain size parameter, M. Minimum values of ${\chi }_v^2$ were found to occur at M = 1.27 and M = 1.6825 for mammography and radiography, respectively.

Fig. 14

System DQE along the subscan direction, DQE(v) for mammography (A; top) and radiography (B; bottom). Experimental data points are shown as filled-in symbols and model results are shown as lines with open symbols corresponding to experimental data. The legend is in the same order as the zero-frequency DQE.

Fig. 15

Root Mean Squared Difference (RMSD) in DQE(0.5) between model and experimental results plotted as a function of spatial frequency (top panels; A & C) and exposure (bottom panels; B & D) for mammography (left panels; A & B) and radiography (right panels; C & D).

Fig. 16

Percent (%) change in DQE(0.5) as a function of exposure over the range of gain values obtained by varying work function (125 eV to 360 eV) and optical collection efficiency (0.1 to 0.4), for mammography (A; top) and radiography (B; bottom).

Fig. 17

Effect of protective layer (PET) on excitation flux (top left; A) and point spread function (top right; C) for both mammography and radiography. The effect of protective layer on bleaching and escape efficiencies (bottom left; B), and system presampling MTF (bottom right; D) for mammography.