Department of Oncology, Aarhus University Hospital, Aarhus, Denmark.
Acta Oncol. 2011 Aug;50(6):908-17. doi: 10.3109/0284186X.2011.590525.
To evaluate the image quality obtained in a standard QA phantom with both clinical and non-clinical cone-beam computed tomography (CBCT) acquisition modes for the head and neck (HN) region as a step towards CBCT-based treatment planning. The impact of deteriorated Hounsfield unit (HU) accuracy was investigated by comparing results from clinical CBCT image reconstructions to those obtained from a pre-clinical scatter correction algorithm.
Five different CBCT acquisition modes on a clinical system for kV CBCT-guided radiotherapy were investigated. Image reconstruction was performed in both standard clinical software and with an experimental reconstruction algorithm with improved beam hardening and scatter correction. Using the Catphan 504 phantom, quantitative measures of HU uniformity, HU verification and linearity, contrast-to-noise ratio (CNR), and spatial resolution using modulation transfer function (MTF) estimation were assessed. To benchmark the CBCT image properties, comparison to standard HN protocols on conventional CT scanners was performed by similar measures.
The HU uniformity within a water-equivalent homogeneous region was considerably improved using experimental vs. standard reconstruction, by factors of two for partial scans and four for full scans. Similarly, the amount of capping/cupping artifact was reduced by more than 1.5%. With mode and reconstruction specific HU calibration using seven inhomogeneity inserts comparable HU linearity was observed. CNR was on average 5% higher for experimental reconstruction (scaled with the square-root of dose between modes for both reconstruction methods).
Judged on parameters affecting the common diagnostic image properties, improved beam hardening and scatter correction diminishes the difference between CBCT and CT image quality considerably. In the pursuit of CBCT-based treatment adaptation, dedicated imaging protocols may be required.
评估临床和非临床锥形束 CT（CBCT）采集模式在头颈部（HN）区域标准 QA 体模中获得的图像质量，以期实现基于 CBCT 的治疗计划。通过比较临床 CBCT 图像重建的结果与预临床散射校正算法获得的结果，研究了体素值（HU）精度降低的影响。
研究了临床系统上用于千伏 CBCT 引导放疗的五种不同 CBCT 采集模式。在标准临床软件和具有改进的束硬化和散射校正的实验重建算法中进行图像重建。使用 Catphan 504 体模，评估 HU 均匀性、HU 验证和线性、对比噪声比（CNR）以及使用调制传递函数（MTF）估计的空间分辨率的定量测量。为了基准 CBCT 图像特性，通过类似的测量方法与传统 CT 扫描仪上的标准 HN 协议进行了比较。
使用实验与标准重建相比，在水等效均匀区域内的 HU 均匀性得到了显著改善，部分扫描的改善因子为 2，全扫描的改善因子为 4。同样，帽檐/杯状伪影的数量减少了 1.5 倍以上。使用七个不均匀性插件进行模式和重建特定的 HU 校准，观察到相似的 HU 线性度。对于实验重建，CNR 平均高出 5%（对于两种重建方法，使用模式之间的剂量平方根进行缩放）。
根据影响常见诊断图像特性的参数判断，改进的束硬化和散射校正大大减小了 CBCT 和 CT 图像质量之间的差异。在基于 CBCT 的治疗适应的追求中，可能需要专用的成像协议。
