Institute of Medical Physics, Friedrich-Alexander-University, Erlangen, Germany.
Med Phys. 2012 Feb;39(2):825-31. doi: 10.1118/1.3676180.
X-ray CT measures the attenuation of polychromatic x-rays through an object. The raw data acquired, which are the negative logarithm of the relative x-ray intensity behind the patient, must undergo water precorrection to linearize the measurement and to convert them into line integrals that are ready for reconstruction. The function to linearize the measured projection data depends on the detected spectrum of the ray. This spectrum may vary as a function of the detector position, e.g., in cases where the heel effect becomes relevant, where a bow-tie filter introduces channel-dependent beam hardening, or where a primary modulator is used to modulate the primary intensity of the spectrum.
The authors propose a new approach that allows to handle these effects in a highly convenient way. Their new empirical cupping correction for primary modulation (ECCP) corrects for artifacts, such as cupping artifacts or ring artifacts, which are induced by nonlinearities in the projection data due to spatially varying pre- or postfiltration of the x-rays. To do so, ECCP requires only a simple scan of a homogeneous phantom of nearly arbitrary shape. Based on this information, coefficients of a polynomial series are calculated and stored for later use.
Physical measurements demonstrate the quality of the precorrection that can be achieved using ECCP to remove the cupping artifacts and to obtain well-calibrated CT values even in cases of strong primary modulation. A combination of ECCP with analytical techniques yielding a hybrid cupping correction method is possible and allows for channel-dependent correction functions.
The proposed ECCP method is a very effective and easy to incorporate approach that compensates for even strong detector channel-dependent changes of the detected spectrum. © 2011 American Association of Physicists in Medicine.
X 射线 CT 通过对物体进行多色 X 射线衰减来测量。所获得的原始数据是患者背后的相对 X 射线强度的负对数，必须进行水预校正，以使测量线性化，并将其转换为准备重建的线积分。线性化测量投影数据的函数取决于射线的检测光谱。该光谱可能会随探测器位置而变化，例如，在脚跟效应变得相关的情况下，使用蝶形滤波器引入与通道相关的束硬化，或者使用主调制器来调制光谱的主强度。
作者提出了一种新的方法，可以非常方便地处理这些效果。他们新的用于主调制的杯形校正（ECCP）可以校正伪影，例如由于射线的空间预过滤或后过滤导致投影数据中的非线性引起的杯形伪影或环形伪影。为此，ECCP 仅需要对几乎任意形状的均匀体模进行简单扫描。基于此信息，计算并存储多项式级数的系数，以备后用。
物理测量证明了可以使用 ECCP 实现的预校正质量，该预校正可以去除杯形伪影，并获得即使在强主调制的情况下也能很好校准的 CT 值。ECCP 与产生混合杯形校正方法的分析技术的组合是可能的，并允许进行通道相关的校正函数。
所提出的 ECCP 方法是一种非常有效且易于结合的方法，即使对于强烈的探测器通道相关的检测光谱变化也能进行补偿。©2011 年美国物理学会。
