Wang Jing, Robar James, Guan Huaiqun
Department of Radiation Oncology, UT Southwestern Medical Center, Dallas, Texas 75235, USA.
Med Phys. 2012 Aug;39(8):5111-7. doi: 10.1118/1.4737116.
To improve the image contrast-to-noise (CNR) ratio for low-Z target megavoltage cone-beam CT (MV CBCT) using a statistical projection noise suppression algorithm based on the penalized weighted least-squares (PWLS) criterion.
Projection images of a contrast phantom, a CatPhan(®) 600 phantom and a head phantom were acquired by a Varian 2100EX LINAC with a low-Z (Al) target and low energy x-ray beam (2.5 MeV) at a low-dose level and at a high-dose level. The projections were then processed by minimizing the PWLS objective function. The weighted least square (WLS) term models the noise of measured projection and the penalty term enforces the smoothing constraints of the projection image. The variance of projection data was chosen as the weight for the PWLS objective function and it determined the contribution of each measurement. An anisotropic quadratic form penalty that incorporates the gradient information of projection image was used to preserve edges during noise reduction. Low-Z target MV CBCT images were reconstructed by the FDK algorithm after each projection was processed by the PWLS smoothing.
Noise in low-Z target MV CBCT images were greatly suppressed after the PWLS projection smoothing, without noticeable sacrifice of the spatial resolution. Depending on the choice of smoothing parameter, the CNR of selected regions of interest in the PWLS processed low-dose low-Z target MV CBCT image can be higher than the corresponding high-dose image.
The CNR of low-Z target MV CBCT images was substantially improved by using PWLS projection smoothing. The PWLS projection smoothing algorithm allows the reconstruction of high contrast low-Z target MV CBCT image with a total dose of as low as 2.3 cGy.
使用基于惩罚加权最小二乘（PWLS）准则的统计投影噪声抑制算法，提高低Z值靶材兆伏级锥形束CT（MV CBCT）的图像对比度噪声比（CNR）。
使用瓦里安2100EX直线加速器，采用低Z值（铝）靶材和低能X射线束（2.5 MeV），在低剂量水平和高剂量水平下，采集对比剂模体、CatPhan(®) 600模体和头部模体的投影图像。然后通过最小化PWLS目标函数来处理这些投影。加权最小二乘（WLS）项对测量投影的噪声进行建模，惩罚项强制执行投影图像的平滑约束。选择投影数据的方差作为PWLS目标函数的权重，它决定了每次测量的贡献。使用包含投影图像梯度信息的各向异性二次型惩罚来在降噪过程中保留边缘。在通过PWLS平滑处理每个投影后，使用FDK算法重建低Z值靶材MV CBCT图像。
经过PWLS投影平滑处理后，低Z值靶材MV CBCT图像中的噪声得到了极大抑制，且空间分辨率没有明显牺牲。根据平滑参数的选择，在PWLS处理的低剂量低Z值靶材MV CBCT图像中，选定感兴趣区域的CNR可能高于相应的高剂量图像。
使用PWLS投影平滑处理可显著提高低Z值靶材MV CBCT图像的CNR。PWLS投影平滑算法允许以低至2.3 cGy的总剂量重建高对比度低Z值靶材MV CBCT图像。
