In the assessment of vertebral bone mineral density (BMD) with quantitative computed tomography, accuracy is limited by systematic errors caused by beam-hardening effects and unknown marrow and fat concentrations in trabecular bone. Reproducibility depends on precise repositioning of the patient, selection of the region of interest (ROI), and apparatus stability. In an integrated approach, these error sources were taken into account. Dual-energy methods are incorporated and tested to avoid systematic errors. Contour-detection algorithms for automated ROI determination that work reliably and offer improved reproducibility and efficiency are presented. A new design of reference phantom with solid tissue-equivalent materials provides calibration in BMD units and checks apparatus stability. Dual-energy methods are necessary to achieve accuracy of 5% or better. Automated ROI definition and reference phantom methods are mandatory for reproducibility of 2% or better.
Polyethylene-based water-and bone-equivalent plastics for Calibration phantoms in quantitative computed tomography (QCT) were optimized by calculation and experimental verification A composition of 91.3 % polyethylene, 5.5 % CaCO 3 , and 3.2 % MgO (by weight) was accepted äs being water-equivalent with respect to absolute value and energy dependence of the X-ray attenuation coeff icient; the material was compared to other commercially available solids. Boneequivalent samples containing 200 mg of hydroxyapatite per ml were obtained by adding the mineral powder to the water-equivalent material. A procedure for industrial mass production of both materials was developed. A first application of these materials is given by a new QCT Calibration phantom. Schlüsselwörter: Hydroxylapatit, Kalibrierung, Knochen, Mineralsalzbestimmung, Polyäthylen, Phantom, quantitative Computertomographie Durch Berechnung und experimentelle Untersuchungen wurde ein Eichphantom für die quantitative Computertomographie optimiert. Es handelt sich um einen festen Körper, dessen Grundmaterial Polyäthylen ist, aus welchem ein wasser-und knochenäquivalentes Material hergestellt wurde. Eine Mischung aus 91,3% (Gewichtsprozent) Polyäthylen, 5,5% CaCO 3 und 3,2 % MgO wurde für das wasseräquivalente Material verwendet. Die Übereinstimmung mit Wasser bezieht sich sowohl auf den absoluten Schwächungswert als auch auf dessen Abhängigkeit von der Energie der Röntgenstrahlen. Das Material wurde mit anderen kommerziell erhältlichen verglichen. Für die Knochenäquivalenz wurde dem Grundmaterial 200 mg Hydroxylapatit pro ml beigemischt. Zur industriellen Herstellung wurde ein Produktionsprozeß entwickelt, der hohe Homogenität garantiert. Das Material findet Verwendung in einem neuen QCT-Kalibrierphantom. IndroductionWater-and tissue-equivalent materials were first proposed and developed for radiation dosimetry purposes [11]. Equivalence of a synthetic material to a tissue or material of interest is assumed if the attenuation or energy absorption characteristics of the two respective materials are the same for a defined type of radiation. In the context of this paper, we will consider the equivalence of materials with respect to the linear X-ray attenuation coeff icient// over the energy ränge of 30 to 150 keV which is relevant to diagnostic X-ray procedures. The aim was to obtain calibration materials for QCT. Calibration phantom techniques for QCT, äs introduced by Genant and Cann, serve to control apparatus stability and to provide a calibration surve for conversion of CT numbers, measured in Hounsfield units (HU) f into bone mineral density (BMD) values, specified in mg/cm 3 [1]. The Cann/Gcnant phantom, developed for verlebral bone mineral measurement, consists of a perspex body containing bore holes filled with aqueous Solutions of ^HPO-t. Concern about the long-term stability of these Solutions gave rise to other groups' investigations [7,8,9] and to our efforts regarding the development of solid materials.The particular requirements for our work were t...
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