University of Lyon, CREATIS, CNRS UMR5220, Inserm U1294, INSA-Lyon, Université Lyon 1, Centre Léon Bérard, France.
University of Lyon, Université Claude Bernard Lyon 1, CNRS/IN2P3, IP2I Lyon, UMR 5822, F-69622, Villeurbanne, France.
Phys Med Biol. 2022 Aug 5;67(16). doi: 10.1088/1361-6560/ac7191.
Proton computed tomography (CT) is similar to x-ray CT but relies on protons rather than photons to form an image. In its most common operation mode, the measured quantity is the amount of energy that a proton has lost while traversing the imaged object from which a relative stopping power map can be obtained via tomographic reconstruction. To this end, a calorimeter which measures the energy deposited by protons downstream of the scanned object has been studied or implemented as energy detector in several proton CT prototypes. An alternative method is to measure the proton's residual velocity and thus its kinetic energy via the time of flight (TOF) between at least two sensor planes. In this work, we study the RSP resolution, seen as image noise, which can be expected from TOF proton CT systems.We rely on physics models on the one hand and statistical models of the relevant uncertainties on the other to derive closed form expressions for the noise in projection images. The TOF measurement error scales with the distance between the TOF sensor planes and is reported as velocity error in ps/m. We use variance reconstruction to obtain noise maps of a water cylinder phantom given the scanner characteristics and additionally reconstruct noise maps for a calorimeter-based proton CT system as reference. We use Monte Carlo simulations to verify our model and to estimate the noise due to multiple Coulomb scattering inside the object. We also provide a comparison of TOF helium and proton CT.We find that TOF proton CT with 30 ps mvelocity error reaches similar image noise as a calorimeter-based proton CT system with 1% energy error (1 sigma error). A TOF proton CT system with a 50 ps mvelocity error produces slightly less noise than a 2% calorimeter system. Noise in a reconstructed TOF proton CT image is spatially inhomogeneous with a marked increase towards the object periphery. Our modelled noise was consistent with Monte Carlo simulated images. TOF helium CT offers lower RSP noise at equal fluence, but is less advantageous at equal imaging dose.This systematic study of image noise in TOF proton CT can serve as a guide for future developments of this alternative solution for estimating the residual energy of protons and helium ions after the scanned object.
质子计算机断层扫描（CT）类似于 X 射线 CT，但依赖于质子而不是光子来形成图像。在其最常见的操作模式下，测量的量是质子在穿过被成像物体时损失的能量，通过断层重建可以获得相对阻止本领图。为此，已经研究或实现了一种在扫描物体下游测量质子沉积能量的量热计作为能量探测器，用于几种质子 CT 原型。另一种方法是通过至少两个传感器平面之间的飞行时间（TOF）测量质子的剩余速度，从而测量其动能。在这项工作中，我们研究了 TOF 质子 CT 系统可以预期的 RSP 分辨率，这可以看作是图像噪声。我们一方面依赖于物理模型，另一方面依赖于相关不确定性的统计模型，推导出投影图像噪声的封闭形式表达式。TOF 测量误差与 TOF 传感器平面之间的距离成正比，并以 ps/m 的速度误差报告。我们使用方差重建给定扫描仪特性获得水圆柱体模型的噪声图，并作为参考重建基于量热计的质子 CT 系统的噪声图。我们使用蒙特卡罗模拟来验证我们的模型并估计物体内部多次库仑散射引起的噪声。我们还提供了 TOF 氦和质子 CT 的比较。我们发现，具有 30 ps m 速度误差的 TOF 质子 CT 达到了与具有 1%能量误差（1 个标准差误差）的基于量热计的质子 CT 系统相似的图像噪声。具有 50 ps m 速度误差的 TOF 质子 CT 系统产生的噪声略小于 2%的量热计系统。重建的 TOF 质子 CT 图像中的噪声具有空间不均匀性，在物体边缘附近明显增加。我们建模的噪声与蒙特卡罗模拟图像一致。TOF 氦 CT 在相同的剂量下提供更低的 RSP 噪声，但在相同的成像剂量下则不那么有利。这项关于 TOF 质子 CT 图像噪声的系统研究可以为这种替代方案的进一步发展提供指导，该方案用于估计扫描物体后质子和氦离子的剩余能量。
