Department of Oncology, University of Alberta, Edmonton, Alberta, Canada.
Med Phys. 2012 Oct;39(10):6297-308. doi: 10.1118/1.4754647.
To evaluate the performance of a model based image reconstruction method in reducing metal artifacts in the megavoltage computed tomography (MVCT) images of a phantom representing bilateral hip prostheses and to compare with the filtered-backprojection (FBP) technique.
An iterative maximum likelihood polychromatic algorithm for CT (IMPACT) is used with an additional model for the pair∕triplet production process and the energy dependent response of the detectors. The beam spectra for an in-house bench-top and TomoTherapy™ MVCTs are modeled for use in IMPACT. The empirical energy dependent response of detectors is calculated using a constrained optimization technique that predicts the measured attenuation of the beam by various thicknesses (0-24 cm) of solid water slabs. A cylindrical (19.1 cm diameter) plexiglass phantom containing various cylindrical inserts of relative electron densities 0.295-1.695 positioned between two steel rods (2.7 cm diameter) is scanned in the bench-top MVCT that utilizes the bremsstrahlung radiation from a 6 MeV electron beam passed through 4 cm solid water on the Varian Clinac 2300C and in the imaging beam of the TomoTherapy™ MVCT. The FBP technique in bench-top MVCT reconstructs images from raw signal normalized to air scan and corrected for beam hardening using a uniform plexiglass cylinder (20 cm diameter). The IMPACT starts with a FBP reconstructed seed image and reconstructs the final image in 150 iterations.
In both MVCTs, FBP produces visible dark shading in the image connecting the steel rods. In the IMPACT reconstructed images this shading is nearly removed and the uniform background is restored. The average attenuation coefficients of the inserts and the background are very close to the corresponding values in the absence of the steel inserts. In the FBP images of the bench-top MVCT, the shading causes 4%-9.5% underestimation of electron density at the central inserts with an average of (6.3 ± 1.8)% for the range of electron densities studied. In the uniform plexiglass background, the shadow creates 0.8%-4.7% underestimation of electron density with an average of (2.9 ± 1.2)%. In the corresponding IMPACT images, the underestimation in the shaded plexiglass background is 0.3%-1.8% with an average of (0.9 ± 0.5)% and 1.4%-6.8% with an average of (2.8 ± 2.7)% in the central insert region. In the FBP images of the TomoTherapy™ MVCT, this shading creates 2.6%-6.7% underestimation of electron density with an average of (3.7 ± 1.4)% at the central inserts and 5.9%-7.2% underestimation in the background with an average of (6.4 ± 0.5)%. In the IMPACT images, the uniform background between the steel rods is restored with 0.3%-1.0% underestimation of electron density with an average of (0.7 ± 0.3)%. The corresponding underestimation at the central inserts of the IMPACT images is -0.4%-0.1% with an average of (-0.1 ± 0.2)%.
The shading metal artifact has been nearly removed in MVCT images using the IMPACT algorithm with the accurate geometry of the system, proper modeling of energy dependent response of detectors, and all relevant photon interaction processes. This results less than 1% difference in electron density in the background plexiglass and less than 3% averaged over the range of electron densities investigated.
评估基于模型的图像重建方法在降低双侧髋关节假体模拟体模的兆伏级计算机断层扫描（MVCT）图像中金属伪影的性能，并与滤波反投影（FBP）技术进行比较。
使用用于 CT 的迭代最大似然多色算法（IMPACT），并添加了对成对/成组产生过程和探测器能量依赖性响应的模型。为了在 IMPACT 中使用，对室内台式和 TomoTherapy™MVCT 的束谱进行建模。探测器的经验能量依赖性响应是使用约束优化技术计算的，该技术预测了各种厚度（0-24cm）的实心水板对光束的测量衰减。一个直径为 19.1cm 的有机玻璃体模，其中包含各种相对电子密度为 0.295-1.695 的圆柱形插件，放置在两个直径为 2.7cm 的钢棒之间，在利用瓦里安 Clinac 2300C 中的电子束韧致辐射通过 4cm 实心水扫描的台式 MVCT 中以及在 TomoTherapy™MVCT 的成像束中进行扫描。台式 MVCT 中的 FBP 技术从原始信号重建图像，该信号归一化为空气扫描，并使用均匀的有机玻璃圆柱体（直径 20cm）校正束硬化。IMPACT 从 FBP 重建的种子图像开始，并在 150 次迭代中重建最终图像。
在这两种 MVCT 中，FBP 在连接钢棒的图像中产生明显的暗阴影。在 IMPACT 重建的图像中，这种阴影几乎被消除，恢复了均匀的背景。插件和背景的平均衰减系数非常接近没有钢插件时的相应值。在台式 MVCT 的 FBP 图像中，阴影导致中心插件的电子密度低估了 4%-9.5%，平均为研究的电子密度范围内的（6.3±1.8）%。在均匀的有机玻璃背景中，阴影导致电子密度低估了 0.8%-4.7%，平均为（2.9±1.2）%。在相应的 IMPACT 图像中，阴影有机玻璃背景中的低估幅度为 0.3%-1.8%，平均为（0.9±0.5）%，中心插件区域为 1.4%-6.8%，平均为（2.8±2.7）%。在 TomoTherapy™MVCT 的 FBP 图像中，这种阴影导致中心插件的电子密度低估了 2.6%-6.7%，平均为（3.7±1.4）%，背景低估了 5.9%-7.2%，平均为（6.4±0.5）%。在 IMPACT 图像中，使用准确的系统几何形状、探测器能量依赖性响应的适当建模以及所有相关的光子相互作用过程，在钢棒之间恢复了均匀的背景，电子密度低估了 0.3%-1.0%，平均为（0.7±0.3）%。IMPACT 图像中中心插件的相应低估幅度为-0.4%-0.1%，平均为（-0.1±0.2）%。
使用 IMPACT 算法，通过准确的系统几何形状、探测器能量依赖性响应的适当建模以及所有相关的光子相互作用过程，在 MVCT 图像中几乎消除了金属伪影。这导致背景有机玻璃中的电子密度差异小于 1%，在研究的电子密度范围内的平均差异小于 3%。
