We present a new method to measure sub-microcurie activities of photon-emitting radionuclides in organs and lesions of small animals in vivo. Our technique, named the collimator-less likelihood fit, combines a very high sensitivity collimatorless detector with a Monte Carlo-based likelihood fit in order to estimate the activities in previously segmented regions of interest along with their uncertainties. This is done directly from the photon projections in our collimatorless detector and from the region of interest segmentation provided by an x-ray computed tomography scan. We have extensively validated our approach with 225Ac experimentally in spherical phantoms and mouse phantoms, and also numerically with simulations of a realistic mouse anatomy. Our method yields statistically unbiased results with uncertainties smaller than 20% for activities as low as ~111Bq (3nCi) and for exposures under 30 minutes. We demonstrate that our method yields more robust recovery coefficients when compared to SPECT imaging with a commercial pre-clinical scanner, specially at very low activities. Thus, our technique is complementary to traditional SPECT/CT imaging since it provides a more accurate and precise organ and tumor dosimetry, with a more limited spatial information. Finally, our technique is specially significant in extremely low-activity scenarios when SPECT/CT imaging is simply not viable.

中文翻译：

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使用无准直仪探测器量化感兴趣区域中极低活性的技术


我们提出了一种新方法来测量小动物体内器官和病变中光子发射放射性核素的亚微居里活性。我们的技术称为无准直器似然拟合，它将非常高灵敏度的无准直器探测器与基于蒙特卡罗的似然拟合相结合，以估计先前分割的感兴趣区域中的活动及其不确定性。这是直接通过无准直仪探测器中的光子投影和 X 射线计算机断层扫描提供的感兴趣区域分割来完成的。我们使用 225Ac 在球形模型和小鼠模型中进行了实验，并通过对真实小鼠解剖结构进行数值模拟来广泛验证了我们的方法。我们的方法可产生统计上无偏差的结果，对于低至 ~111Bq (3nCi) 的活动和 30 分钟以下的暴露，不确定性小于 20%。我们证明，与使用商用临床前扫描仪进行 SPECT 成像相比，我们的方法可产生更稳健的恢复系数，特别是在活性非常低的情况下。因此，我们的技术是对传统 SPECT/CT 成像的补充，因为它提供了更准确和精确的器官和肿瘤剂量测定，且空间信息更有限。最后，当 SPECT/CT 成像根本不可行时，我们的技术在活动极低的情况下特别重要。