Department of Medical Physics, Faculty of Physics, Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748, Garching b. München, Germany.
Univ Lyon, INSA-Lyon, Université Claude Bernard Lyon 1, UJM-Saint Étienne, CNRS, Inserm, CREATIS UMR 5220, U1206, F-69373, Lyon, France.
Med Phys. 2020 Apr;47(4):1895-1906. doi: 10.1002/mp.14084. Epub 2020 Mar 3.
Fluence-modulated proton computed tomography (FMpCT) using pencil beam scanning aims at achieving task-specific image noise distributions by modulating the imaging proton fluence spot-by-spot based on an object-specific noise model. In this work, we present a method for fluence field optimization and investigate its performance in dose reduction for various phantoms and image variance targets.
The proposed method uses Monte Carlo simulations of a proton CT (pCT) prototype scanner to estimate expected variance levels at uniform fluence. Using an iterative approach, we calculate a stack of target variance projections that are required to achieve the prescribed image variance, assuming a reconstruction using filtered backprojection. By fitting a pencil beam model to the ratio of uniform fluence variance and target variance, relative weights for each pencil beam can be calculated. The quality of the resulting fluence modulations is evaluated by scoring imaging doses and comparing them to those at uniform fluence, as well as evaluating conformity of the achieved variance with the prescription. For three different phantoms, we prescribed constant image variance as well as two regions-of-interest (ROI) imaging tasks with inhomogeneous image variance. The shape of the ROIs followed typical beam profiles for proton therapy.
Prescription of constant image variance resulted in a dose reduction of 8.9% for a homogeneous water phantom compared to a uniform fluence scan at equal peak variance level. For a more heterogeneous head phantom, dose reduction increased to 16.0% for the same task. Prescribing two different ROIs resulted in dose reductions between 25.7% and 40.5% outside of the ROI at equal peak variance levels inside the ROI. Imaging doses inside the ROI were increased by 9.2% to 19.2% compared to the uniform fluence scan, but can be neglected assuming that the ROI agrees with the therapeutic dose region. Agreement of resulting variance maps with the prescriptions was satisfactory.
We developed a method for fluence field optimization based on a noise model for a real scanner used in pCT. We demonstrated that it can achieve prescribed image variance targets. A uniform fluence field was shown not to be dose optimal and dose reductions achievable with the proposed method for FMpCT were considerable, opening an interesting perspective for image guidance and adaptive therapy.
使用笔束扫描的调制质子计算机断层扫描（FMpCT）旨在通过根据特定于物体的噪声模型逐点调制成像质子通量来实现特定于任务的图像噪声分布。在这项工作中，我们提出了一种用于通量场优化的方法，并研究了其在各种体模和图像方差目标下降低剂量的性能。
所提出的方法使用质子 CT（pCT）原型扫描仪的蒙特卡罗模拟来估计均匀通量下的预期方差水平。使用迭代方法，我们计算了一组目标方差投影，这些投影需要实现规定的图像方差，假设使用滤波反投影进行重建。通过将铅笔束模型拟合到均匀通量方差与目标方差的比值，可以计算出每个铅笔束的相对权重。通过对成像剂量进行评分并将其与均匀通量下的剂量进行比较，以及评估实现的方差与规定的一致性，可以评估得到的通量调制的质量。对于三个不同的体模，我们规定了恒定的图像方差以及两个具有不均匀图像方差的感兴趣区域（ROI）成像任务。ROI 的形状遵循质子治疗的典型束轮廓。
与具有相同峰值方差水平的均匀通量扫描相比，规定恒定图像方差会导致均匀水模体的剂量减少 8.9%。对于更不均匀的头部体模，相同任务的剂量减少增加到 16.0%。对于两个不同的 ROI，在 ROI 外部的剂量减少在 ROI 内的峰值方差水平相等的情况下在 25.7%到 40.5%之间。与均匀通量扫描相比，ROI 内的成像剂量增加了 9.2%至 19.2%，但假设 ROI 与治疗剂量区域一致，则可以忽略不计。生成的方差图与规定的吻合度令人满意。
我们开发了一种基于实际用于 pCT 的扫描仪的噪声模型的通量场优化方法。我们证明它可以实现规定的图像方差目标。均匀通量场不是剂量最优的，并且可以使用所提出的方法为 FMpCT 实现可观的剂量减少，为图像引导和自适应治疗开辟了一个有趣的视角。
