Investigating the limits of PET/CT imaging at very low true count rates and high random fractions in ion-beam therapy monitoring
Investigating the limits of PET/CT imaging at very low true count rates and high random fractions in ion-beam therapy monitoring
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DOI：
10.1118/1.4921995
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发表时间：
2015-07-01
期刊：
影响因子：
3.8
通讯作者：
Parodi, Katia
中科院分区：
文献类型：
--
作者：
Kurz, Christopher;Bauer, Julia;Parodi, Katia
目的：使用质子和较重离子的外射束放射治疗能够使所施加的剂量相对于光子更紧密地符合任意形状的肿瘤体积，但对放射治疗链中的不确定性更敏感。因此，非常需要对所应用的处理进行独立验证。为此，在照射后不久，通过安装在海德堡离子束治疗中心（HIT）治疗室旁边的商用全环正电子发射断层扫描/X射线计算机断层扫描（PET/CT）扫描仪检测患者体内的辐射诱导β（+）发射体分布。这种方法的一个主要挑战是检测到的巧合数量很少。本文旨在表征所用PET/CT设备的性能，并在基于PET的治疗监测的特定统计条件下识别性能最佳的重建算法。此外，这项研究解决了辐射背景的影响，从固有的放射性lutetium-oxyorthosilicate（LSO）为基础的探测器在低counts.Methods：作者已经获得了30个后续PET扫描的圆柱形幻影模仿patientlike活动模式，并跨越整个患者计数制度方面的真正的巧合和随机分数（RF）。针对各种重建算法和设置，研究了活动量化的准确度和精度、图像噪声和扫描仪的几何保真度，以确定基于PET的治疗验证的实用、非常适合的重建方案。截断的列表模式数据已被用于分离小的真实计数和高RF对重建图像的影响。相应的模拟研究能够将结果扩展到更广泛的计数统计范围，并进一步研究散射重合的影响。最后，建议的重建方案已被应用到示范postirradiation患者data-sets.Results：在调查重建选项，整体最佳的结果，在图像噪声，活动量化，准确的几何恢复，实现了使用有序子集期望最大化重建算法与飞行时间（TOF）和点扩散函数（PSF）的信息。对于该算法，在96%RF下，可以提供合理准确（优于5%）和精确（平均活性的不确定性低于10%）的成像，达到80000个真实符合。图像噪声和几何保真度通常因较少的迭代而得到改善。基于PET的治疗监测的主要局限性已被确定为真正的巧合数量很少，而不是高内在随机背景。将优化的重建方案应用于患者数据集，与核医学应用中采用的HIT以前使用的重建方案相比，在相当的放射性量化精度下，图像噪声降低了25%-50%，几何性能得到了改善。结论：在基于PET的治疗监测的统计条件较差的情况下，通过在图像重建期间考虑PSF和TOF信息并且通过应用比常规核医学成像中更少的迭代，可以实现改进的结果。几何保真度和图像噪声主要受限于低数量的真实符合，而不是与LSO相关的高随机背景。检索的结果也可能影响其他新兴的PET应用在低计数统计。(C)2015年美国医学物理学家协会。
Purpose: External beam radiotherapy with protons and heavier ions enables a tighter conformation of the applied dose to arbitrarily shaped tumor volumes with respect to photons, but is more sensitive to uncertainties in the radiotherapeutic treatment chain. Consequently, an independent verification of the applied treatment is highly desirable. For this purpose, the irradiation-induced beta(+)-emitter distribution within the patient is detected shortly after irradiation by a commercial full-ring positron emission tomography/x-ray computed tomography (PET/CT) scanner installed next to the treatment rooms at the Heidelberg Ion-Beam Therapy Center (HIT). A major challenge to this approach is posed by the small number of detected coincidences. This contribution aims at characterizing the performance of the used PET/CT device and identifying the best-performing reconstruction algorithm under the particular statistical conditions of PET-based treatment monitoring. Moreover, this study addresses the impact of radiation background from the intrinsically radioactive lutetium-oxyorthosilicate (LSO)-based detectors at low counts.Methods: The authors have acquired 30 subsequent PET scans of a cylindrical phantom emulating a patientlike activity pattern and spanning the entire patient counting regime in terms of true coincidences and random fractions (RFs). Accuracy and precision of activity quantification, image noise, and geometrical fidelity of the scanner have been investigated for various reconstruction algorithms and settings in order to identify a practical, well-suited reconstruction scheme for PET-based treatment verification. Truncated listmode data have been utilized for separating the effects of small true count numbers and high RFs on the reconstructed images. A corresponding simulation study enabled extending the results to an even wider range of counting statistics and to additionally investigate the impact of scatter coincidences. Eventually, the recommended reconstruction scheme has been applied to exemplary postirradiation patient data-sets.Results: Among the investigated reconstruction options, the overall best results in terms of image noise, activity quantification, and accurate geometrical recovery were achieved using the ordered subset expectation maximization reconstruction algorithm with time-of-flight (TOF) and point-spread function (PSF) information. For this algorithm, reasonably accurate (better than 5%) and precise (uncertainty of the mean activity below 10%) imaging can be provided down to 80 000 true coincidences at 96% RF. Image noise and geometrical fidelity are generally improved for fewer iterations. The main limitation for PET-based treatment monitoring has been identified in the small number of true coincidences, rather than the high intrinsic random background. Application of the optimized reconstruction scheme to patient data-sets results in a 25%-50% reduced image noise at a comparable activity quantification accuracy and an improved geometrical performance with respect to the formerly used reconstruction scheme at HIT, adopted from nuclear medicine applications.Conclusions: Under the poor statistical conditions in PET-based treatment monitoring, improved results can be achieved by considering PSF and TOF information during image reconstruction and by applying less iterations than in conventional nuclear medicine imaging. Geometrical fidelity and image noise are mainly limited by the low number of true coincidences, not the high LSO-related random background. The retrieved results might also impact other emerging PET applications at low counting statistics. (C) 2015 American Association of Physicists in Medicine.