Purpose: Computerized radiographic texture analysis (RTA) is currently being investigated as a measure of bone structure and a diagnostic predictor of osteoporosis. In this study, we assess the physical image quality indices of a bone densitometry system as well as its ability to yield images useful for RTA.
Methods and Materials: The bone densitometry system investigated was the GE/Lunar PIXI, which uses dual-energy subtraction to measure bone mineral density (BMD) and provides 512 x 512 digital images of the heel or forearm (0.2 mm pixels). We compared the physical imaging properties (contrast, resolution and noise) of the PIXI to those of a Fuji computed radiography (CR) system (0.1 mm pixels). Contrast was measured using a series of uniform exposure images, presampling modulation transfer functions (MTF) were measured using an edge method, and noise power spectra (NPS) were calculated using a two-dimensional Fourier analysis method. We also analyzed texture features of heel images from 24 subjects who had both CR and PIXI exams taken. The Fourier-based texture features characterized the magnitude, frequency content and orientation of the trabecular bone pattern.
Results: The slope of the characteristic curve of the BMD system was found to be position dependent, across the image, although this dependence was substantially reduced by use of the sustem's clinical-settings corrections. An MTF value of 0.5 was found at 0.5 cycles/mm for the PIXI system compared to the same value at 1.4 cycles/mm for the CR system. Unlike the CR system, the NPS of PIXI was found not to be directionally dependent and did not drop off at higher frequencies. There was correlation for the first moment features between the two modalities with r=0.61 (p-value=0.001) for first moment of the power spectrum (FMP) and r=0.50 (p-value=0.01) for minimum FMP. Root-mean-square (RMS) showed little correlation with r=0.22 (p-value=0.3).
Conclusion: Initial results indicate that a bone densitometry system is a promising means of performing computerized RTA. A more extensive patient study is warranted to determine the suitability of RTA in osteoporosis assessment. (M.L.G., M.D.C. are shareholders in R2 Technology, Inc. (Sunnyvale, CA).)
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