Borsdorf Anja, Raupach Rainer, Flohr Thomas, Hornegger Joachim
Friedrich-Alexander-University Erlangen-Nuremberg (FAU), Chair of Pattern Recognition, 91058 Erlangen, Germany.
IEEE Trans Med Imaging. 2008 Dec;27(12):1685-703. doi: 10.1109/TMI.2008.923983.
The projection data measured in computed tomography (CT) and, consequently, the slices reconstructed from these data are noisy. We present a new wavelet based structure-preserving method for noise reduction in CT-images that can be used in combination with different reconstruction methods. The approach is based on the assumption that data can be decomposed into information and temporally uncorrelated noise. In CT two spatially identical images can be generated by reconstructions from disjoint subsets of projections: using the latest generation dual source CT-scanners one image can be reconstructed from the projections acquired at the first, the other image from the projections acquired at the second detector. For standard CT-scanners the two images can be generated by splitting up the set of projections into even and odd numbered projections. The resulting images show the same information but differ with respect to image noise. The analysis of correlations between the wavelet representations of the input images allows separating information from noise down to a certain signal-to-noise level. Wavelet coefficients with small correlation are suppressed, while those with high correlations are assumed to represent structures and are preserved. The final noise-suppressed image is reconstructed from the averaged and weighted wavelet coefficients of the input images. The proposed method is robust, of low complexity and adapts itself to the noise in the images. The quantitative and qualitative evaluation based on phantom as well as real clinical data showed, that high noise reduction rates of around 40% can be achieved without noticeable loss of image resolution.
计算机断层扫描（CT）中测量的投影数据，以及由此重建的切片都存在噪声。我们提出了一种基于小波的结构保持方法，用于CT图像降噪，该方法可与不同的重建方法结合使用。该方法基于这样的假设：数据可以分解为信息和时间上不相关的噪声。在CT中，可以通过从投影的不相交子集进行重建来生成两个空间相同的图像：使用最新一代双源CT扫描仪，可以从在第一个探测器处采集的投影重建一幅图像，从在第二个探测器处采集的投影重建另一幅图像。对于标准CT扫描仪，可以通过将投影集分为偶数和奇数编号的投影来生成这两幅图像。所得图像显示相同的信息，但在图像噪声方面有所不同。对输入图像的小波表示之间的相关性进行分析，可以将信息与噪声分离到一定的信噪比水平。相关性小的小波系数被抑制，而相关性高的小波系数被假定代表结构并被保留。最终的降噪图像是根据输入图像的平均加权小波系数重建的。所提出的方法具有鲁棒性、低复杂度，并能适应图像中的噪声。基于体模以及真实临床数据的定量和定性评估表明，该方法可以实现约40%的高降噪率，且不会明显损失图像分辨率。
