Investigation of physical image quality indices of a bone densitometry system

Abstract

Osteoporosis is a disease characterized by a loss of bone mass and a deterioration of bone structure. Bone mineral density (BMD) measures bone mass and is currently the method used to diagnose osteoporosis, while computerized radiographic texture analysis (RTA) is being investigated as a measure of bone structure. The GE/Lunar PIXI peripheral bone densitometer (PD) system, which uses dual-energy subtraction to measure BMD, also provides a digital image of the heel or forearm. The goal of our current research was to evaluate the physical imaging properties of the PIXI system (pixel size of 0.2 mm) compared to a Fuji computed radiography (CR) system (pixel size of 0.1 mm) to determine its suitability for texture analysis from image data. Contrast was measured using a series of uniform images covering the useful clinical exposure range. Spatial resolution was characterized by the presampling modulation transfer function (MTF) determined by an edge method. Noise power spectra (NPS) for different exposures were calculated using a two-dimensional Fourier analysis method. The expectation modulation transfer function was measured and combined with the NPS data to calculate the noise-equivalent number of quanta. The slope of the characteristic curve of the peripheral densitometer (PD) system was found to be position dependent across the image, although this dependence was substantially reduced by use of the system's clinical-settings corrections. An MTF value of 0.5 was found at 0.5 cycles/mm for the densitometry system compared to the same value at 1.6 cycles/mm for the CR system. Unlike the CR system, the NPS of the densitometry system was found not to be directionally dependent and did not drop off at higher spatial frequencies.

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

“…The studies evaluating images over time, for example, bone healing 8,9,14 , digital subtraction 5 and 20 , all based on differences in the pixel values of images obtained at different times, may be affected by the variability of the digitalization process. A digitalization error (causing lack of reproducibility) of pixel values would be deleterious in these situations because it would attribute pixel values that differ from those corresponding to a true change that occurred in the patient.…”

“…Direct digital dental radiological systems offer the potential to radically change the way dentists diagnose and treat dentomaxillofacial diseases 5,8,9,14,20 .…”

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

“…A possible reason for this result is related to the differences in the noise power spectra ͑NPS͒ of the two systems. As previously described, 16 the NPS for the PD system is flat, indicating uncorrelated noise, while that for the CR system decreases with increasing spatial frequency. When equal pixel sizes are being examined, this high-frequency falloff in the NPS for the CR system contributes to the reduction in the FMP of the overall texture pattern.…”

“…It should be noted that for the PD and CR systems a linear relationship exists between exposure and pixel value, 16 and thus correction with a system's calibration curve is not necessary. Therefore, ␥ log 10 e was taken as a constant value of 1 for all RMS calculations.…”

“…16 Briefly, we found the characteristic curve of the PD system to be linear over a wide exposure range much like a typical CR system. A comparison of the presampling modulation transfer functions ͑MTFs͒ for the two systems indicated that the resolution of the CR system is superior to that of the PD system at all spatial frequencies.…”