Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University, Baltimore, MD, 21287, USA.
Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO, USA.
Med Phys. 2017 May;44(5):1707-1717. doi: 10.1002/mp.12164. Epub 2017 Mar 28.
We investigated the feasibility of using simpler methods than manual whole-organ volume-of-interest (VOI) definition to estimate the organ activity concentration in single photon emission computed tomography (SPECT) in cases where the activity in the organ can be assumed to be uniformly distributed on the scale of the voxel size. In particular, we investigated an anatomic region-of-interest (ROI) defined in a single transaxial slice, and a single sphere placed inside the organ boundaries.
The evaluation was carried out using Monte Carlo simulations based on patient indium In pentetreotide SPECT and computed tomography (CT) images. We modeled constant activity concentrations in each organ, validating this assumption by comparing the distribution of voxel values inside the organ VOIs of the simulated data with the patient data. We simulated projection data corresponding to 100, 50, and 25% of the clinical count level to study the effects of noise level due to shortened acquisition time. Images were reconstructed using a previously validated quantitative SPECT reconstruction method. The evaluation was performed in terms of the accuracy and precision of the activity concentration estimates.
The results demonstrated that the non-uniform image intensity observed in the reconstructed images in the organs with normal uptake was consistent with uniform activity concentration in the organs on the scale of the voxel size; observed non-uniformities in image intensity were due to a combination of partial-volume effects at the boundaries of the organ, artifacts in the reconstructed image due to collimator-detector response compensation, and noise. Using an ROI defined in a single transaxial slice produced similar biases compared to the three-dimensional (3D) whole-organ VOIs, provided that the transaxial slice was near the central plane of the organ and that the pixels from the organ boundaries were not included in the ROI. Although this slice method was sensitive to noise, biases were less than 10% for all the noise levels studied. The use of spherical VOIs was more sensitive to noise. The method was more accurate for larger spheres and larger organs such as the liver in comparison to the kidneys. Biases lower than 7% were found in the liver when using large enough spheres (radius ≥ 28 mm), regardless of the position, of the VOI inside the organ even with shortened acquisition times. The biases were more position-dependent for smaller organs.
Both of the simpler methods provided suitable surrogates in terms of accuracy and precision. The results suggested that a spherical VOI was more appropriate for estimating the activity concentration in larger organs such as the liver, regardless of the position of the sphere inside the organ. Larger spheres resulted in better estimates. A single-slice ROI was more suitable for activity estimation in smaller organs such as the kidneys, providing that the transaxial slice selected was near the central plane of the organ and that voxels from the organ boundaries were excluded. Under those conditions, activity concentrations with biases lower than 5% were observed for all the studied count levels and coefficients of variation were less than 9% and 5% for the 25% and 100% count levels, respectively.
我们研究了使用比手动全器官感兴趣区（VOI）定义更简单的方法来估计单光子发射计算机断层扫描（SPECT）中器官活性浓度的可行性，前提是器官内的活性可以假定在体素大小的范围内均匀分布。特别是，我们研究了在单个横轴切片中定义的解剖区域感兴趣区（ROI），以及放置在器官边界内的单个球体。
该评估是使用基于铟 111 标记喷曲肽 SPECT 和计算机断层扫描（CT）图像的蒙特卡罗模拟进行的。我们在每个器官中模拟了恒定的活性浓度，通过将模拟数据中器官 VOI 内体素值的分布与患者数据进行比较来验证此假设。我们模拟了对应于临床计数水平的 100%、50%和 25%的投影数据，以研究由于采集时间缩短导致的噪声水平的影响。使用先前验证的定量 SPECT 重建方法重建图像。评估是根据活性浓度估计的准确性和精度进行的。
结果表明，在具有正常摄取的器官中，在重建图像中观察到的不均匀图像强度与器官在体素大小范围内的均匀活性浓度一致；在图像强度中观察到的不均匀性是由于器官边界处的部分容积效应、由于准直器-探测器响应补偿而在重建图像中出现的伪影以及噪声的综合作用。使用在单个横轴切片中定义的 ROI 与三维（3D）全器官 VOI 产生了类似的偏差，前提是横轴切片靠近器官的中心平面，并且器官边界的像素不包含在 ROI 中。尽管该切片方法对噪声很敏感，但在研究的所有噪声水平下，偏差均小于 10%。球形 VOI 对噪声更敏感。与肾脏等较小的器官相比，对于较大的肝脏等器官，该方法使用较大的球体时更准确。使用足够大的球体（半径≥28mm）时，即使采集时间缩短，在肝脏中也能发现偏差低于 7%的结果，无论 VOI 在器官内的位置如何。对于较小的器官，偏差更依赖于位置。
两种较简单的方法在准确性和精度方面都提供了合适的替代方法。结果表明，对于较大的器官（如肝脏），球形 VOI 更适合估计活性浓度，而与球体在器官内的位置无关。较大的球体可产生更好的估计值。对于较小的器官（如肾脏），单一切片 ROI 更适合活动估算，前提是选择的横轴切片靠近器官的中心平面，并且排除器官边界的体素。在这些条件下，在所有研究的计数水平下，观察到偏差低于 5%的活性浓度，并且变异系数分别小于 9%和 5%，分别用于 25%和 100%的计数水平。
