Hwang Andrew B, Franc Benjamin L, Gullberg Grant T, Hasegawa Bruce H
Joint Graduate Group in Bioengineering, University of California San Francisco and University of California Berkeley, CA, USA.
Phys Med Biol. 2008 May 7;53(9):2233-52. doi: 10.1088/0031-9155/53/9/002. Epub 2008 Apr 9.
Small animal SPECT imaging systems have multiple potential applications in biomedical research. Whereas SPECT data are commonly interpreted qualitatively in a clinical setting, the ability to accurately quantify measurements will increase the utility of the SPECT data for laboratory measurements involving small animals. In this work, we assess the effect of photon attenuation, scatter and partial volume errors on the quantitative accuracy of small animal SPECT measurements, first with Monte Carlo simulation and then confirmed with experimental measurements. The simulations modeled the imaging geometry of a commercially available small animal SPECT system. We simulated the imaging of a radioactive source within a cylinder of water, and reconstructed the projection data using iterative reconstruction algorithms. The size of the source and the size of the surrounding cylinder were varied to evaluate the effects of photon attenuation and scatter on quantitative accuracy. We found that photon attenuation can reduce the measured concentration of radioactivity in a volume of interest in the center of a rat-sized cylinder of water by up to 50% when imaging with iodine-125, and up to 25% when imaging with technetium-99m. When imaging with iodine-125, the scatter-to-primary ratio can reach up to approximately 30%, and can cause overestimation of the radioactivity concentration when reconstructing data with attenuation correction. We varied the size of the source to evaluate partial volume errors, which we found to be a strong function of the size of the volume of interest and the spatial resolution. These errors can result in large (>50%) changes in the measured amount of radioactivity. The simulation results were compared with and found to agree with experimental measurements. The inclusion of attenuation correction in the reconstruction algorithm improved quantitative accuracy. We also found that an improvement of the spatial resolution through the use of resolution recovery techniques (i.e. modeling the finite collimator spatial resolution in iterative reconstruction algorithms) can significantly reduce the partial volume errors.
小动物单光子发射计算机断层扫描（SPECT）成像系统在生物医学研究中有多种潜在应用。虽然在临床环境中，SPECT数据通常是定性解释的，但准确量化测量的能力将提高SPECT数据在涉及小动物的实验室测量中的效用。在这项工作中，我们首先通过蒙特卡罗模拟评估光子衰减、散射和部分容积误差对小动物SPECT测量定量准确性的影响，然后通过实验测量进行验证。模拟对市售小动物SPECT系统的成像几何结构进行了建模。我们模拟了水中圆柱体中放射性源的成像，并使用迭代重建算法重建投影数据。改变源的大小和周围圆柱体的大小，以评估光子衰减和散射对定量准确性的影响。我们发现，当使用碘-125成像时，光子衰减可使大鼠大小的水圆柱体中心感兴趣体积内的放射性测量浓度降低高达50%，当使用锝-99m成像时，降低高达25%。当使用碘-125成像时，散射与原发射线的比率可高达约30%，并且在使用衰减校正重建数据时会导致放射性浓度的高估。我们改变源的大小以评估部分容积误差，我们发现这是感兴趣体积大小和空间分辨率的强函数。这些误差可导致测量的放射性量发生大幅（大于50%）变化。将模拟结果与实验测量结果进行比较，发现两者一致。在重建算法中纳入衰减校正可提高定量准确性。我们还发现，通过使用分辨率恢复技术（即在迭代重建算法中对有限准直器空间分辨率进行建模）提高空间分辨率可显著降低部分容积误差。
