Empirical Dual Energy Calibration (EDEC) for Cone-Beam Computed Tomography

Material-selective imaging using dual energy CT (DECT) heavily relies on well-calibrated material decomposition functions. These require the precise knowledge of the detected X-ray spectrum and even if this is exactly known the reliability of DECT will suffer from scattered radiation. We propose an empirical method to determine the proper decomposition function. In contrast to other decomposition algorithms our empirical dual energy calibration (EDEC) technique does neither require knowledge of the spectrum nor of the attenuation coefficients. The desired material-selective rawdata p₁ and p₂ are obtained as a function of the measured attenuation data q₁ and q₂ (one DECT scan = two rawdata sets) using a polynomial function whose coefficients are determined using a general least squares fit based on thresholded images of a calibration phantom. Assumptions on the calibration phantom size or of its positioning are not made. Once the decomposition coefficients are determined DECT rawdata can be decomposed by simply passing them through the polynomial. To demonstrate EDEC simulations of an oval CTDI phantom, a lung phantom and a thorax phantom were carried out and a physical phantom composed of water and calcium hydroxypatite was measured with a dedicated in vivo dual source micro-CT scanner (TomoScope 30s Duo, VAMP GmbH, Erlangen, Germany). The rawdata were decomposed into its components, reconstructed and the pixel values obtained were compared to the theoretical values. The determination of the calibration coefficients with EDEC is very robust and depends only slightly on the type of calibration phantom used. Images of the said test phantoms (simulations and measurements) show a nearly perfect agreement with the theoretical mu-values and density values. Since EDEC is an empirical technique it inherently compensates for scatter components, given that the calibration phantom is of similar size as the test objects. The empirical- - dual energy calibration technique is a pragmatic, simple and reliable calibration approach that produces highly quantitative DECT images.