Sokole E B, Heckenberg A, Bergmann H
Department of Nuclear Medicine, Academic Medical Centre, University of Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands.
Eur J Nucl Med. 1996 Apr;23(4):437-42. doi: 10.1007/BF01247373.
Cobalt-57 flood sources are often used for system flood-field uniformity checks of scintillation camera performance. Such sources are now available in large sizes and with high activities. Uniformity images using new 57Co sources have shown artefacts which are not present in uniformity images obtained from technetium-99m flood sources of the same activity. The high-energy photons emitted by 57Co and cobalt isotope impurities appear to influence the images obtained. This was investigated in three cameras using three 57Co sources of different age. The flood-field images obtained with the 57Co sources showed non-uniformity patterns that were dependent on the age of the cobalt source, the distance of the source to the collimator, and the specific camera type. Quantification of the uniformity images reflected these findings. Energy spectra of a new 57Co source, obtained with an external 1024-channel analyser connected to the camera, showed a broad tail of high-energy photons above the 122-keV photopeak, due to Compton scatter and collimator penetration. This tail diminished with older sources and with increased source to collimator distance, indicating that in both situations fewer high-energy photons were being measured by the camera system. This tail of high-energy photons contributes to the total count rate, but because this is not obvious from the counts observed in the photopeak window, the camera can unsuspectingly be operated at too high a total countrate where pile-up effects become significant. Caution is therefore advised when using 57Co sources. Problems may be minimized by purchasing low-activity sources (certainly no more than 370 MBq), by placing the source at a distance from the collimator, e.g. 50 cm, by allowing a new source to decay, and by manufacturers producing 57Co sources without detectable radioactive contaminants.
钴-57泛源常用于闪烁相机性能的系统泛场均匀性检查。现在有大尺寸且活度高的此类源。使用新的钴-57源获得的均匀性图像显示出一些伪影，而从相同活度的锝-99m泛源获得的均匀性图像中不存在这些伪影。钴-57发射的高能光子以及钴同位素杂质似乎会影响所获得的图像。在三台相机中使用三个不同年代的钴-57源对此进行了研究。用钴-57源获得的泛场图像显示出不均匀模式，这些模式取决于钴源的年代、源到准直器的距离以及特定的相机类型。均匀性图像的量化反映了这些发现。通过连接到相机的外部1024通道分析仪获得的新钴-57源的能谱显示，在122千电子伏特光峰上方有一条高能光子的宽尾巴，这是由于康普顿散射和准直器穿透造成的。随着源变旧以及源到准直器距离增加，这条尾巴会变小，这表明在这两种情况下相机系统测量到的高能光子都更少。这条高能光子尾巴对总计数率有贡献，但由于这在光峰窗口观察到的计数中不明显，相机可能会在不知不觉中以过高的总计数率运行，此时堆积效应会变得很显著。因此，在使用钴-57源时建议谨慎。通过购买低活度源（肯定不超过370兆贝可）、将源放置在离准直器一定距离处（例如50厘米）、让新源衰变以及制造商生产无可检测放射性污染物的钴-57源，问题可能会最小化。
