Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305-5847 and Department of Radiation Oncology, University of California Davis School of Medicine, Sacramento, California 95817.
Med Phys. 2013 Oct;40(10):101907. doi: 10.1118/1.4820538.
Four-dimensional (4D) computed tomography (CT) ventilation imaging is a novel promising technique for lung functional imaging. The current standard 4D CT technique using phase-based sorting frequently results in artifacts, which may deteriorate the accuracy of ventilation imaging. The purpose of this study was to quantify the variability of 4D CT ventilation imaging due to 4D CT sorting.
4D CT image sets from nine lung cancer patients were each sorted by the phase-based method and anatomic similarity-based method, designed to reduce artifacts, with corresponding ventilation images created for each method. Artifacts in the resulting 4D CT images were quantified with the artifact score which was defined based on the difference between the normalized cross correlation for CT slices within a CT data segment and that for CT slices bordering the interface between adjacent CT data segments. The ventilation variation was quantified using voxel-based Spearman rank correlation coefficients for all lung voxels, and Dice similarity coefficients (DSC) for the spatial overlap of low-functional lung volumes. Furthermore, the correlations with matching single-photon emission CT (SPECT) ventilation images (assumed ground truth) were evaluated for three patients to investigate which sorting method provides higher physiologic accuracy.
Anatomic similarity-based sorting reduced 4D CT artifacts compared to phase-based sorting (artifact score, 0.45 ± 0.14 vs 0.58 ± 0.24, p = 0.10 at peak-exhale; 0.63 ± 0.19 vs 0.71 ± 0.31, p = 0.25 at peak-inhale). The voxel-based correlation between the two ventilation images was 0.69 ± 0.26 on average, ranging from 0.03 to 0.85. The DSC was 0.71 ± 0.13 on average. Anatomic similarity-based sorting yielded significantly fewer lung voxels with paradoxical negative ventilation values than phase-based sorting (5.0 ± 2.6% vs 9.7 ± 8.4%, p = 0.05), and improved the correlation with SPECT ventilation regionally.
The variability of 4D CT ventilation imaging due to 4D CT sorting was moderate overall and substantial in some cases, suggesting that 4D CT artifacts are an important source of variations in 4D CT ventilation imaging. Reduction of 4D CT artifacts provided more physiologically convincing and accurate ventilation estimates. Further studies are needed to confirm this result.
四维（4D）计算机断层扫描（CT）通气成像技术是一种新兴的肺部功能成像技术。目前，基于相位排序的标准 4D CT 技术经常会产生伪影，这可能会降低通气成像的准确性。本研究旨在量化由于 4D CT 排序而导致的 4D CT 通气成像的可变性。
对 9 例肺癌患者的 4D CT 图像集分别采用基于相位的方法和基于解剖相似性的方法进行排序，以减少伪影，并为每种方法创建相应的通气图像。通过基于归一化互相关的差异来量化所得 4D CT 图像中的伪影评分，该差异是在 CT 数据段内的 CT 切片之间和相邻 CT 数据段之间的界面处的 CT 切片之间定义的。使用所有肺体素的基于体素的 Spearman 秩相关系数量化通气变化，并使用低功能肺体积的空间重叠的 Dice 相似系数（DSC）。此外，为了研究哪种排序方法提供更高的生理准确性，对 3 名患者的匹配单光子发射 CT（SPECT）通气图像（假定为真实值）进行了相关性评估。
与基于相位的排序相比，基于解剖相似性的排序减少了 4D CT 伪影（峰值呼气时的伪影评分分别为 0.45 ± 0.14 和 0.58 ± 0.24，p = 0.10；峰值吸气时的伪影评分分别为 0.63 ± 0.19 和 0.71 ± 0.31，p = 0.25）。两种通气图像之间的基于体素的相关性平均为 0.69 ± 0.26，范围为 0.03 至 0.85。DSC 平均为 0.71 ± 0.13。基于解剖相似性的排序产生的具有反常负通气值的肺体素明显少于基于相位的排序（分别为 5.0 ± 2.6%和 9.7 ± 8.4%，p = 0.05），并改善了与 SPECT 通气的局部相关性。
4D CT 排序引起的 4D CT 通气成像的可变性总体上是中度的，在某些情况下是较大的，这表明 4D CT 伪影是 4D CT 通气成像变化的重要来源。减少 4D CT 伪影可提供更具生理说服力和准确性的通气估计。需要进一步的研究来证实这一结果。
