Euromedim 2006 :1st European Conference on Molecular Imaging Technology

Multi-Energy Densitometry of Bone Tissues Using ZnSe-based Scintielectronic Detectors

11 May 2006, 14:00

1h

Palais du Pharo, Marseille

poster • Status of animal and clinical PET, SPECT and CT (biomedical and technical) Poster session : Imaging systems, Molecular Imaging

Speaker

Prof. Vladimir Ryzhikov (Institute for Scintillation Materials of STC "Institute for Single Crystals")

Description

Modern digital radiography and X-ray computer tomography (CT) are among the most efficient methods used for diagnostics of cancer and other diseases requiring surgery. Transition from traditional to dual-energy CT allows separate diagnostics of bone and muscle tissues, i.e., discern between biological materials that substantially differ in their density and atomic number Zeff (by 2-3 times). For early detection of tumors, atherosclerosis and osteoporosis, it is essential to detect, by a non-invasive method, deviations of these physical parameters by 15-20% from their normal values. In the particular case of hepatitis, it is required to determine with such accuracy the content of calcium in the bone tissue. A vital factor for solution of this problem is selective energy sensitivity of high- and low-energy detectors. In traditional dual-energy radiography [1], the low-energy detector is made of gadolinium oxysulfide, and the high-energy detector - CsI(Tl). For diagnostics of early stages of osteoporosis, we propose to use an original combined detector [Patent of Ukraine No. 44547 of 15.02.02], where we use, for detection of low energies, scintillator ZnSe doped with an isovalent admixture (Cd, Te, O). This exclusive material is produced solely by ISMA of STC "Institute for Single Crystals", Kharkov, Ukraine. Light output of ZnSe is 20-30% higher as compared with CsI(Tl), and the afterglow level is 10-2 % after 3 ms. Another peculiar feature of ZnSe is its relatively low Zeff = 33. Thus, it is nearly not sensitive to high energies (above 100 keV) when used in the form of thin detectors (0.3-0.5 mm), with its maximum detection efficiency corresponding to the low energy range (30-40 keV). As a result, energy selectivity of the dual-energy system to high energies is ensured automatically. Another advantage is that placing this crystal before the high-energy detector ensures efficient filtration from the low-energy part of X-ray spectrum. This lowers the requirements to the filter (usually made of copper with Zeff = 31) or makes its use unnecessary. The use of zinc selenide crystals in state-of-the art inspection equipment [2] has already led to excellent results in detection of explosives in X-ray inspection of loads and luggage. Our comparative experimental studies of animal bone tissue samples in-vitro for determination of calcium composition in combination with tomography have confirmed high sensitivity and detecting ability of the method. The accuracy of determination of the effective atomic number reaches 80-90%. The sensitivity to relative variations of bone tissue density is at the level of 1-2%. Moreover, quality of radiographic images is improved, which were obtained using a dual-energy tomographic scanner in sandwich design of detectors 0.5 mm ZnSe / 4 mm CsI(Tl) / Hamamatsu photodiode. The dynamic range is 10^6, the contrast to noise ratio is of the order of 2-4%. Further studies will be aimed at separate visualization of trabecular and cortical bone tissues and determination of changes in their chemical composition related to the effects of different stages of osteoporosis. In addition, further development of multi-energy tomography on the basis of modern bright and fast scintillation materials, including ZnSe compounds, is expected to enable detection of microcalcification of soft tissues and organs, as well as diagnostics of tumor formations by means of mammography, etc. [1] R.M. Harrison, NIM 310, 24 (1991). [2] Smiths Heimann Prospects, 2006, http://www.heimannsystems.com