Investigation of physical image quality indices of a bone densitometry system

Radiographic texture analysis of densitometric calcaneal images: relationship to clinical characteristics and to bone fragility

Osteoporotic fractures are related not only to bone mineral density (BMD) but also to bone structure or microarchitecture, which is not assessed routinely with currently available methods. We have developed radiographic texture analysis (RTA) for calcaneal images from a peripheral densitometer as an easy, noninvasive method for assessing bone structure. We conducted a cross-sectional study of the relationship between RTA and prevalent vertebral fractures (n = 148) among 900 subjects (ages 19 to 99 years, 94 males) referred for bone densitometry as part of their routine medical care. RTA features were derived from Fourier-based image analysis of the radiographic texture pattern (including root mean square, first moment, and power spectral analyses). RTA features were associated with age, weight, gender, and race, as well as glucocorticoid use. When controlling for clinical risk factors and BMD (or a summary measure calculated using FRAX algorithms), RTA features were significantly different for subjects with and without prevalent vertebral fractures [adjusted odds ratio (OR) = 1.5 per 1 standard deviation (SD) decrease in RTA feature beta, 95% confidence interval (CI) 1.2-1.8, p = .001]. Gender and use of pharmacologic therapy for osteoporosis did not significantly affect this association, suggesting that RTA can be applied to a wide range of densitometry patients. We conclude that RTA obtained using a portable instrument has a potential as a noninvasive method to enhance identification of patients at increased risk of osteoporotic fractures.

Data acquisition analysis for femoral morphometric X-ray absorptiometry

Bone mineral density (BMD) is a contributing factor of hip fracture risk. Other factors, such as lifestyle, the propensity for falls, and femoral geometry may influence the risk of hip fracture. The DMS Lexxos densitometer, a dual-energy X-ray densitometer can produce either a single-energy X-ray or a BMD image. The purpose of this study was to evaluate which of these 2 images enables the best detection to make femoral morphometry measurements. Spatial resolution, contrast, and noise were evaluated separately. A contrast-detail phantom was also used for the purpose of overall visual analysis. The spatial resolution was the same in the 2 images. The contrast was better with the BMD image, but the noise was higher. Using the contrast-detail phantom, the single-energy X-ray image allowed globally a better detection of the objects, but results were in the same range with high contrast values. Hip volunteers' morphometric measurements and the Singh Index were evaluated 3 times for each image by 3 observers, and the intra-, inter-, and global reproducibility was computed. The reproducibility of the measurements seems to be better with the single-energy X-ray image but results were not statistically significantly different. These results suggest that even if the image-quality indices were different, the single-energy X-ray image and BMD image are closely useful for clinical morphometric femoral evaluation.

Reproducibility and sources of variability in radiographic texture analysis of densitometric calcaneal images

Radiographic texture analysis (RTA) is a computerized analysis of the spatial pattern of radiographic images used as a way of evaluating bone structure. We have shown that RTA performed on high-resolution heel images obtained using a portable densitometer differentiates subjects with and without osteoporotic fractures. In the present study, short-term precision of RTA was examined on densitometric heel images obtained from 33 subjects scanned 8 times each, with 3 observers placing a region of interest (ROI) 3 times on each image. The long-term precision was examined on images obtained from 10 subjects 3 times on each of 3 days separated by 1 week, with 2 observers placing an ROI on each image. The RTA features examined included the root mean square (RMS) variation, a measure of the contrast between the light and dark areas of the image, the first moment of the power spectrum, a measure of the spatial frequency of the trabecular pattern, and Minkowski fractal (MINK), a measure of roughness/smoothness of the trabecular pattern. The precision of the RTA features expressed as coefficient of variation ranged between the lowest of 0.5-0.7% for MINK and the highest of 14-16% for RMS. The short- and long-term precision was similar, and was not significantly influenced by repositioning and rescanning, or by ROI placement by the same or different observers. Significant sources of variability of RTA were the between-subject differences and differences between regions of the heel, but not differences due to repositioning, rescanning in the same position, or ROI placement by the same or different observers. We conclude that technical aspects of image acquisition and processing are adequate to allow further development of RTA of the densitometric images for clinical application as a method for noninvasive assessment of bone structure.

Reproducibility of pixel values for two photostimulable phosphor plates in consecutive standardized scannings

The objective of the present study was to determine the reproducibility of the pixel values obtained with the Digora system (Soredex, Finland). Exposures were standardized, with variation in exposure and scanning time of two photostimulable phosphor plates containing a stepwedge image. The smallest variation in pixel values ranged from 50 to 75%, with the widest variations being observed in less dense steps. A significant difference in pixel values was observed in terms of X-ray exposure and scanning times and between the two plates themselves (ANOVA, p < 0.01). Using the present methodology, the reproducibility of pixel values was not satisfactory for the tested white photostimulable plates. This wide variation in digitalization might be influenced by the amount of X-rays that sensitized the plates. It may be important to establish the reproducibility of the pixel values in quantitative studies using digital image.

Radiographic texture analysis of densitometer-generated calcaneus images differentiates postmenopausal women with and without fractures

INTRODUCTION

Bone fragility is determined by bone mass, measured as bone mineral density (BMD), and by trabecular structure, which cannot be easily measured using currently available noninvasive methods. In previous studies, radiographic texture analysis (RTA) performed on the radiographic images of the spine, proximal femur, and os calcis differentiated subjects with and without osteoporotic fractures. The present cross-sectional study was undertaken to determine whether such differentiation could also be made using high-resolution os calcis images obtained on a peripheral densitometer.

METHODS

In 170 postmenopausal women (42 with and 128 without prevalent vertebral fractures) who had no secondary causes of osteoporosis and were not receiving treatment for osteoporosis, BMD of the lumbar spine, proximal femur, and os calcis was measured using dual energy x-ray absorptiometry. Vertebral fractures were diagnosed on densitometric spine images. RTA, including Fourier-based and fractal analyses, was performed on densitometric images of os calcis.

RESULTS

BMD at all three sites and all texture features was significantly different in subjects with and without fractures, with the most significant differences observed for the femoral neck and total hip measurements and for the RTA feature Minkowski fractal (p<0.001). In univariate logistic regression analysis, Minkowski fractal predicted the presence of vertebral fractures as well as femoral neck BMD (p<0.001). In multivariate logistic regression analysis, both femoral neck BMD and Minkowski fractal yielded significant predictive effects (p=0.001), and when age was added to the model, the effect of RTA remained significant (p=0.002), suggesting that RTA reflects an aspect of bone fragility that is not captured by age or BMD. Finally, when RTA was compared in 42 fracture patients and 42 nonfracture patients matched for age and BMD, the RTA features were significantly different between the groups (p=0.003 to p=0.04), although BMD and age were not.

CONCLUSION

This study suggests that RTA of densitometer-generated calcaneus images provides an estimate of bone fragility independent of and complementary to BMD measurement and age.

Performance evaluation of a computed radiography imaging device using a typical “front side” and novel “dual side” readout storage phosphors

The Fourier-space modulation transfer function (MTF), normalized noise power spectrum (nNPS), and detective quantum efficiency (DQE) of a computed radiography (CR) x-ray imaging device were measured. Two different storage phosphor screens were used in conjunction with a single CR reader (Fuji, Clearview, CS). One of the storage phosphor plates (ST-BD) had a clear backing material which allowed "dual side read" of the latent image from both the "front" and "back" sides of the phosphor. The other phosphor plate had a light occluding backing material, limiting the readout to front side only (ST-VI). The standard RQA-5 beam quality was used. The MTF was measured using a 1 mm thick tungsten edge device. Compared to the ST-55BD phosphor, the ST-VI phosphor was found to have modestly higher MTF at all spatial frequencies. The nNPS(f) and DQE(f) were measured for nominal incident exposure levels ranging from 0.1 to 10 mR. The dual side read phosphor demonstrated superior DQE, especially at low spatial frequencies. At the frequency 0.5 cycles/mm, the DQE values for the 1 mR exposure were 0.36 and 0.21 for the ST-55BD and ST-VI phosphor plates, respectively. The differences between the spatial-frequency dependent DQE of the two plates can be attributed to the increased signal collection efficiency of the dual side read plates and differences in storage phosphor structure noise.

Comparison of radiographic texture analysis from computed radiography and bone densitometry systems

Osteoporosis is a disease that results in an increased risk of bone fracture due to a loss of bone mass and deterioration of bone structure. Bone mineral density (BMD) provides a measure of bone mass and is frequently measured by bone densitometry systems to diagnose osteoporosis. In addition, computerized radiographic texture analysis (RTA) is currently being investigated as a measure of bone structure and as an additional diagnostic predictor of osteoporosis. In this study, we assessed the ability of a peripheral bone densitometry (PD) system to yield images useful for RTA. The benefit of such a system is that it measures BMD by dual-energy x-ray absorptiometry and therefore provides high- and low-energy digital radiographic images. The bone densitometry system investigated was the GE/Lunar PIXI, which provides 512 x 512 digital images of the heel or forearm (0.2 mm pixels). We compared texture features of heel images obtained with this PD system to those obtained on a Fuji computed radiography (CR) system (0.1 mm pixels). Fourier and fractal-based texture features of images from 24 subjects who had both CR and BMD exams were calculated, and correlation between the two systems was analyzed. Fourier-based texture features characterize the magnitude, frequency content, and orientation of the trabecular bone pattern. Good correlation was found between the two modalities for the first moment (FMP) with r=0.71 (p value<0.0001) and for minimum FMP with r=0.52 (p value=0.008). Root-mean-square (RMS) did not correlate with r=0.31 (p value>0.05), while the standard deviation of the RMS did correlate with r=0.79 (p value<0.0001). Good correlation was also found between the two modalities for the fractal-based texture features with r=0.79 (p value<0.0001) for the global Minkowski dimension and r=0.63 (p value=0.0007) for the fractal dimension from a box counting method. The PD system therefore may have the potential for yielding heel images suitable for RTA.