Ali Imad, Alsbou Nesreen, Ahmad Salahuddin
University of Oklahoma Health Sciences Center.
J Appl Clin Med Phys. 2015 Jan 8;16(1):5067. doi: 10.1120/jacmp.v16i1.5067.
The purpose of this study was to investigate quantitatively by measurement and modeling the variations in CT number distributions of mobile targets in cone-beam CT (CBCT) imaging. CBCT images were acquired for three targets manufactured from homogenous water-equivalent gel that was inserted into a commercial mobile thorax phantom. The phantom moved with a controlled cyclic motion in one-dimension along the superior-inferior direction to simulate patient respiratory motion. Profiles of the CT number distributions of the static and mobile targets were obtained using CBCT images. A mathematical model was developed that predicted the variations in CT number distributions and their dependence on the motion parameters of targets moving in one-dimension using CBCT imaging. The measured CT number distributions for the mobile targets varied considerably, depending on the motion parameters. The extension of the CT number distribution increased linearly with motion amplitude where maximum target elongation reached twice the motion amplitude. The CT number levels of the mobile targets were smeared over a longer distribution; for example, the CT number level for the 20 mm target dropped by nearly 30% at motion amplitude (A) equal to 20 mm in comparison with the CT number distribution of stationary targets. Frequency of motion played an important role in spatial and level variations of the CT number distributions. For example, the level of the CT number profile for the medium target (20 mm) decreased evenly by nearly 50% at A = 20 mm with high motion frequencies. Motion phase did not affect the CT number distributions for prolonged projection acquisition that included several respiratory cycles. The mathematical model of the CT number distributions of mobile targets in CBCT reproduced well the measured CT number distributions and predicted their dependence on the target size and phantom motion parameters such as speed, amplitude, frequency, and phase. The CT number distributions varied considerably with motion in CBCT. A motion model of CT number distribution for mobile targets has been developed in this work that predicted well the variations in the measured CT number profiles and their dependence on motion parameters. The model corrected the CT number distribution retrospective to CT image reconstruction where it used a first-order linear relationship between the number of projections collected in the imaging window of a mobile voxel to obtain the cumulative CT number. This model provides quantitative characterization of motion artifacts on CT number distributions in CBCT that is useful to determine the validity of CT numbers and the accuracy of localization and volume measurement of tumors in diagnostic imaging and interventional applications, such as radiotherapy.
本研究的目的是通过测量和建模，对锥束CT（CBCT）成像中移动目标的CT值分布变化进行定量研究。针对三个由均匀水等效凝胶制成的目标进行CBCT图像采集，这些目标被插入到商用移动胸部体模中。体模沿上下方向在一维空间中以可控的循环运动方式移动，以模拟患者的呼吸运动。利用CBCT图像获取静态和移动目标的CT值分布曲线。开发了一个数学模型，该模型可预测使用CBCT成像时，在一维空间中移动的目标的CT值分布变化及其对运动参数的依赖性。移动目标的实测CT值分布变化很大，这取决于运动参数。CT值分布的扩展随运动幅度呈线性增加，其中目标的最大伸长达到运动幅度的两倍。移动目标的CT值水平在更长的分布范围内被模糊；例如，与静止目标的CT值分布相比，在运动幅度（A）等于20 mm时，20 mm目标的CT值水平下降了近30%。运动频率在CT值分布的空间和水平变化中起着重要作用。例如，在运动幅度A = 20 mm且运动频率较高时，中等大小目标（20 mm）的CT值分布曲线水平均匀下降了近50%。对于包含多个呼吸周期的长时间投影采集，运动相位不影响CT值分布。CBCT中移动目标的CT值分布数学模型很好地再现了实测的CT值分布，并预测了其对目标大小和体模运动参数（如速度、幅度、频率和相位）的依赖性。在CBCT中，CT值分布随运动变化很大。在这项工作中开发了一个移动目标的CT值分布运动模型，该模型很好地预测了实测CT值分布曲线的变化及其对运动参数的依赖性。该模型在CT图像重建时对CT值分布进行校正，它利用移动体素成像窗口中采集的投影数量之间的一阶线性关系来获得累积CT值。该模型对CBCT中CT值分布上的运动伪影进行了定量表征，这对于确定CT值的有效性以及在诊断成像和介入应用（如放射治疗）中肿瘤定位和体积测量的准确性很有用。
