Ceberg C, Larsson I, Strand S E
Department of Radiation Physics, Lund, Sweden.
Eur J Nucl Med. 1991;18(12):959-63. doi: 10.1007/BF00180416.
Characterization of the count-rate performance of scintillation cameras should include not only the specification of count losses. At high count rates, there is also an image distortion due to the mispositioning of pile-up events. In this paper a simple and clinically relevant procedure to quantify this distortion is presented. The images of a square uniform technetium-99m phantom at high and low count rates are used. The fraction of the total counts being correctly positioned is determined as the peripheral count density divided by the total average count density. This ratio, corrected for the camera non-uniformity at low count rates, is called the 'positioning ability'. According to the National Electrical Manufacturers' Association (NEMA), the 'system count rate performance with scatter' should be reported as the measured count rate giving 20% count losses. In this paper it is suggested that this measure be complemented by a measure of the fraction correct positioned events at this count rate. This fraction, the 'high count rate positioning ability', can be easily and accurately measured using our method. The method has been tested on two different scintillation cameras. For one of them the high count rate positioning ability was determined as 91% at a measured count rate of 30,000 s-1 with 20% count losses. For the other camera, the corresponding figures were 88% at 59,000 s-1 and close to 100% at 38,000 s-1, before and after the installation of a new pile-up rejection circuit, respectively.
闪烁相机计数率性能的表征不仅应包括计数损失的说明。在高计数率下，由于堆积事件的错误定位还会出现图像失真。本文提出了一种简单且与临床相关的方法来量化这种失真。使用了方形均匀锝 - 99m 模体在高计数率和低计数率下的图像。正确定位的总计数的分数被确定为外周计数密度除以总平均计数密度。这个比率，针对低计数率下相机的不均匀性进行校正后，被称为“定位能力”。根据美国国家电气制造商协会（NEMA）的规定，“带散射的系统计数率性能”应报告为产生 20%计数损失的测量计数率。本文建议用该计数率下正确定位事件的分数来补充这一测量指标。这个分数，即“高计数率定位能力”，可以使用我们的方法轻松准确地测量。该方法已在两台不同的闪烁相机上进行了测试。对于其中一台相机，在测量计数率为 30,000 s⁻¹ 且计数损失为 20%时，高计数率定位能力被确定为 91%。对于另一台相机，在安装新的堆积拒绝电路之前和之后，相应的数字分别为在 59,000 s⁻¹ 时为 88%，在 38,000 s⁻¹ 时接近 100%。
