Hansen David C, Petersen Jørgen Breede Baltzer, Bassler Niels, Sørensen Thomas Sangild
Experimental Clinical Oncology, Aarhus University, 8000 Aarhus C, Denmark.
Medical Physics, Aarhus University Hospital, 8000 Aarhus C, Denmark.
Med Phys. 2014 Mar;41(3):031904. doi: 10.1118/1.4864239.
Proton computed tomography (CT) is a promising image modality for improving the stopping power estimates and dose calculations for particle therapy. However, the finite range of about 33 cm of water of most commercial proton therapy systems limits the sites that can be scanned from a full 360° rotation. In this paper the authors propose a method to overcome the problem using a dual modality reconstruction (DMR) combining the proton data with a cone-beam x-ray prior.
A Catphan 600 phantom was scanned using a cone beam x-ray CT scanner. A digital replica of the phantom was created in the Monte Carlo code Geant4 and a 360° proton CT scan was simulated, storing the entrance and exit position and momentum vector of every proton. Proton CT images were reconstructed using a varying number of angles from the scan. The proton CT images were reconstructed using a constrained nonlinear conjugate gradient algorithm, minimizing total variation and the x-ray CT prior while remaining consistent with the proton projection data. The proton histories were reconstructed along curved cubic-spline paths.
The spatial resolution of the cone beam CT prior was retained for the fully sampled case and the 90° interval case, with the MTF = 0.5 (modulation transfer function) ranging from 5.22 to 5.65 linepairs/cm. In the 45° interval case, the MTF = 0.5 dropped to 3.91 linepairs/cm For the fully sampled DMR, the maximal root mean square (RMS) error was 0.006 in units of relative stopping power. For the limited angle cases the maximal RMS error was 0.18, an almost five-fold improvement over the cone beam CT estimate.
Dual modality reconstruction yields the high spatial resolution of cone beam x-ray CT while maintaining the improved stopping power estimation of proton CT. In the case of limited angles, the use of prior image proton CT greatly improves the resolution and stopping power estimate, but does not fully achieve the quality of a 360° proton CT scan.
质子计算机断层扫描（CT）是一种很有前景的成像方式，可用于改进粒子治疗的阻止本领估计和剂量计算。然而，大多数商业质子治疗系统约33厘米水的有限射程限制了从360°全旋转扫描的部位。在本文中，作者提出了一种使用双模态重建（DMR）将质子数据与锥束X射线先验信息相结合来克服该问题的方法。
使用锥束X射线CT扫描仪扫描Catphan 600体模。在蒙特卡罗代码Geant4中创建体模的数字副本，并模拟360°质子CT扫描，存储每个质子的入射和出射位置以及动量矢量。使用扫描中不同数量的角度重建质子CT图像。质子CT图像使用约束非线性共轭梯度算法重建，在最小化总变差和X射线CT先验信息的同时，保持与质子投影数据一致。质子历史沿弯曲的三次样条路径重建。
在全采样情况和90°间隔情况下，保留了锥束CT先验信息的空间分辨率，调制传递函数（MTF）=0.5时的线对数/厘米范围为5.22至5.65。在45°间隔情况下，MTF = 0.5降至3.91线对数/厘米。对于全采样的DMR，以相对阻止本领为单位的最大均方根（RMS）误差为0.006。对于有限角度情况，最大RMS误差为0.18，比锥束CT估计提高了近五倍。
双模态重建在保持质子CT改进的阻止本领估计的同时，产生了锥束X射线CT的高空间分辨率。在有限角度的情况下，使用先验图像质子CT大大提高了分辨率和阻止本领估计，但没有完全达到360°质子CT扫描的质量。
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