Institute of Medical Physics (IMP), University of Erlangen-Nürnberg, Henkestr. 91, 91052 Erlangen, Germany.
Phys Med Biol. 2010 Jul 21;55(14):4107-17. doi: 10.1088/0031-9155/55/14/010. Epub 2010 Jul 5.
X-ray CT measures the attenuation of polychromatic x-rays through an object of interest. The CT data acquired are the negative logarithm of the relative x-ray intensity after absorption. These data must undergo water precorrection to linearize the measured data and convert them into line integrals through the patient that can be reconstructed to yield the final CT image. The function to linearize the measured projection data depends on the tube voltage U. In most circumstances, CT scans are carried out with a constant tube voltage. For those cases there are dozens of different techniques to carry out water precorrection. In our case the tube voltage is rather modulated as a function of the object. We propose an empirical cupping correction (ECCU) algorithm to correct for CT cupping artifacts that are induced by nonlinearities in the projection data. The method is rawdata based, empirical and requires neither knowledge of the x-ray spectrum nor of the attenuation coefficients. It aims at linearizing the attenuation data using a precorrection function of polynomial form in the polychromatic attenuation data q and in the tube voltage U. The coefficients of the polynomial are determined once using a calibration scan of a homogeneous phantom. The coefficients are computed in the image domain by fitting a series of basis images to a template image. The template image is obtained directly from the uncorrected phantom image and no assumptions on the phantom size or of its positioning are made. Rawdata are precorrected by passing them through the once-determined polynomial. Numerical examples are shown to demonstrate the quality of the precorrection. ECCU is achieved to remove the cupping artifacts and to obtain well-calibrated CT values. A combination of ECCU with analytical techniques yielding a hybrid cupping correction method is possible and allows for channel-dependent correction functions.
X 射线 CT 通过测量穿过感兴趣物体的多色 X 射线的衰减来进行。获得的 CT 数据是吸收后相对 X 射线强度的负对数。这些数据必须经过水预校正，以使测量数据线性化，并将其转换为穿过患者的线积分，然后可以对这些线积分进行重建以生成最终的 CT 图像。线性化测量投影数据的函数取决于管电压 U。在大多数情况下，CT 扫描使用恒定的管电压进行。对于那些情况，有数十种不同的技术可以进行水预校正。在我们的案例中，管电压是作为物体的函数而被调制的。我们提出了一种经验杯形校正（ECCU）算法，用于校正由于投影数据中的非线性引起的 CT 杯形伪影。该方法基于原始数据，是经验性的，既不需要了解 X 射线光谱，也不需要了解衰减系数。它旨在使用多色衰减数据 q 和管电压 U 的多项式形式的预校正函数来线性化衰减数据。通过对均匀体模进行校准扫描，可以一次确定多项式的系数。通过将一系列基图像拟合到模板图像来在图像域中计算系数。模板图像直接从未校正的体模图像中获得，并且对体模的大小或位置没有任何假设。通过将原始数据通过一次确定的多项式来进行预校正。数值示例用于演示预校正的质量。ECCU 可以去除杯形伪影并获得校准良好的 CT 值。ECCU 与产生混合杯形校正方法的分析技术的组合是可能的，并且允许具有通道依赖性的校正函数。
