Schieferecke Juliane, Hoffmann Aswin, Pawelke Jörg
OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany.
Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiooncology - OncoRay, Dresden, Germany.
Med Phys. 2025 Apr;52(4):2454-2464. doi: 10.1002/mp.17622. Epub 2025 Jan 28.
Previous studies have shown that in-beam magnetic resonance imaging (MRI) can be used to visualize a proton beam during the irradiation of liquid-filled phantoms. The beam energy- and current-dependent local image contrast observed in water was identified to be predominantly caused by beam-induced buoyant convection and associated flow effects. Besides this flow dependency, the MR signal change was found to be characterized by a change in the relaxation time of water, hinting at a radiochemical contribution, which was hypothesized to lie in oxygen depletion-evoked relaxation time lengthening. The elucidation of the underlying contrast mechanism is required to enable the further assessment of the application potential of MRI-based proton beam visualization in tissue.
The underlying radiochemical cause of the observed local beam-induced change in the relaxation time of water should be identified in beam visualization experiments testing the hypothesis of beam-induced oxygen depletion-evoked relaxation time lengthening.
Combined irradiation and imaging experiments were performed using static proton pencil beam irradiation, background-nulled inversion recovery (IR) MRI and a range of flow-restricted phantoms differing in initial oxygen concentration and homogeneity. The similarity of the irradiation-induced MRI contrast to the proton pencil beam dose distribution acquired on radiochromic film, the expected dose dependence and temporal stability, the TR dependence as well as the dependence on the initial oxygen concentration and the oxygen consumption rate were tested. Moreover, the feasibility of oxygen depletion-based beam visualization in tissue-mimicking phantoms was assessed. The levels of irradiation-induced oxygen depletion and relaxation time lengthening were estimated based on the measured temperatures and initial oxygen concentrations of the phantoms, the experimentally determined inversion times required for phantom background signal nulling and dosimetric measurements.
The proton irradiation-induced contrast in background-nulled IR images of well oxygenated phantoms was found similar to the proton pencil beam dose distribution and showed the characteristics expected for oxygen depletion-induced MRI contrast. No beam-induced contrast was observed in the poorly oxygenated, inhomogeneous tissue-mimicking phantoms.
Proton beam-induced radiochemical oxygen depletion can be visualized using relaxation time contrast-based IR MRI and represents the first identified flow-independent contrast mechanism in MRI-based proton beam visualization in real-time. Beam detection in tissue, however, will be complicated by the increased relaxation time inhomogeneity and the lowered levels of initial oxygen concentration compared to in liquids at atmospheric equilibrium and requires further assessment.
先前的研究表明，束内磁共振成像（MRI）可用于在液体填充体模的照射过程中可视化质子束。在水中观察到的与束能量和电流相关的局部图像对比度主要被确定为由束诱导的浮力对流和相关的流动效应引起。除了这种对流动的依赖性外，还发现MR信号变化的特征是水的弛豫时间发生变化，这暗示了一种放射化学贡献，据推测这是由于氧消耗引起的弛豫时间延长所致。需要阐明潜在的对比度机制，以便能够进一步评估基于MRI的质子束可视化在组织中的应用潜力。
在测试束诱导氧消耗引起弛豫时间延长这一假设的束可视化实验中，应确定观察到的局部束诱导水弛豫时间变化的潜在放射化学原因。
使用静态质子笔形束照射、背景归零反转恢复（IR）MRI以及一系列初始氧浓度和均匀性不同的流动受限体模进行联合照射和成像实验。测试了照射诱导的MRI对比度与在放射变色胶片上获得的质子笔形束剂量分布的相似性、预期的剂量依赖性和时间稳定性、TR依赖性以及对初始氧浓度和氧消耗率的依赖性。此外，评估了基于氧消耗的束可视化在组织模拟体模中的可行性。基于测量的体模温度和初始氧浓度、实验确定的体模背景信号归零所需的反转时间以及剂量测量，估计了照射诱导的氧消耗水平和弛豫时间延长。
在充分氧合的体模的背景归零IR图像中，质子照射诱导的对比度与质子笔形束剂量分布相似，并显示出氧消耗诱导的MRI对比度预期的特征。在氧合不良、不均匀的组织模拟体模中未观察到束诱导的对比度。
质子束诱导的放射化学氧消耗可以使用基于弛豫时间对比度的IR MRI进行可视化，并且代表了实时基于MRI的质子束可视化中首次确定的与流动无关的对比度机制。然而，与大气平衡状态下的液体相比，组织中增加的弛豫时间不均匀性和降低的初始氧浓度水平将使组织中的束检测变得复杂，需要进一步评估。
