Measurements of X-ray scatter signatures for some tissue-equivalent materials

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Abstract

In this work, we have measured the angular distribution of photons scattered (scattering signature) by five tissue-equivalent materials used in mammography and conventional radiology and four samples used in bone densitometry at the photon energy 17.44 keV using a X-ray tube equipped with a graphite monochromator, a goniometer system and a NaI detector. Results from the tissue-equivalent materials scattering signatures have been compared with previous results for human tissues (adipose, glandular, muscle and bone). The scattering profiles obtained with tissue-equivalent materials are different from those obtained with human tissue. These differences are due to intermolecular correlation on the elastic scattering. Some tissue-equivalent materials present some very sharp peaks, absent in real soft human tissue. These results suggest that a careful analysis of the scattering properties must be done in order to use tissue-equivalent material to simulate human tissues.

Introduction

Tissue-equivalent materials have been used to obtain information on image quality, dose and overall system performance. The use of tissue-equivalent materials is highly recommended for all radiographic diagnostic facilities (mammography, radiology and bone densitometry). These materials could be used for the identification of image quality problems, and to detect changes in the imaging system [1]. The main feature of these materials is to match the radiological properties of real human tissues. The most commonly used materials are plastic or epoxy resins. Since these materials present elemental composition and density similar to real human tissue, it could be thought that the scatter characteristics could also be similar. In the diagnostic X-ray energie range there are interference effect due to the coherence of elastic scattered photons, mainly at small-angles, which gives rise to a unique scattering signature, characteristic for each material. This scattering plays an important role in the choice of the tissue-equivalent materials or even in the development of new methods in diagnostic radiology [2], [3], [4], [5], [6]. Experimental data on the small-angle scatter distribution for tissue-equivalent materials are therefore desirable. Kozanetzky et al. [2] measured the scattering distributions of several plastics used in the AAPM CT phantoms. Other data are available for liquid water [7] and polymethylmethacrylate (PMMA) [8], [9]. Measurements of scattering signatures are presented in this paper for different tissue-equivalent materials used in mammography (BR12-RMI, BR12-CIRS and PMMA), conventional radiology (water and muscle-equivalent material) and four samples used in bone densitometry (two represent trabecular bone and two cortical ones). The scattering signature was measured with a molybdenum X-ray tube with 17.44 keV photons. These measurements were compared with previous results for breast tissues [6], muscle [2] and bone [10].

Section snippets

Experimental procedure

The experimental procedure along with the employed data handling was described in details elsewhere [11]. The experimental set-up is composed of a powder diffractometer operating in transmission mode. The photon source is an X-ray Mo tube (Z=42, Kα=17.44 keV, Kβ=19.6 keV) operating at 30 kVp. The X-ray generator is equipped with a graphite monochromator in the incident beam to select the fluorescence line (Kα), producing a reasonable monochromatic incident beam (E=17.44 keV with a FWHM=0.19

Comparison of breast-equivalent materials and breast tissues scatter characteristics

Fig. 1 presents the experimental scatter signature in terms of the linear differential scattering coefficient, μS, for breast-equivalent materials (PMMA, BR12-RMI, BR12-CIRS and water) together with previous results for breast tissues (adipose and glandular) [6]. They are clearly different in the small momentum transfer region (x<0.4 Å−1). Different shape, peak position and peak height are due to intermolecular correlation on the elastic scattering (changes in average intermolecular spacing).

Conclusions

The experimental results reported above show that there are large differences in X-ray scattering signatures for tissue-equivalent materials in the small momentum transfer region (x<0.4 Å−1) when compared to real human tissues. These differences are due to the effect of intermolecular correlation on small-angle scattering photons in these materials. The scattering properties of commercial breast-equivalent materials are very similar to adipose breast tissue whereas water is similar to glandular

Acknowledgments

This work was supported by the Brazilian agency Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP).

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