Vandenberghe Stefaan
ELIS/MEDISIP Ugent, Belgium.
Nucl Med Commun. 2006 Mar;27(3):237-45. doi: 10.1097/01.mnm.0000199476.46525.2c.
Impure positron emitters have physical characteristics that degrade image quality compared to conventional positron emitters like 18F. Two impure positron emitters with potentially interesting applications are 124I and 86Y. The degradation in image quality due to the imperfection of these isotopes is quantified for a human three-dimensional (3-D) positron emission tomography (PET) system. An acquisition protocol to obtain similar image quality as for 18F imaging is determined by Monte Carlo simulations.
The effects of larger positron range, associated singles and the other decay modes on image quality are determined by extensive Monte Carlo simulations of the Allegro scanner. Spatial resolution was evaluated for both isotopes and compared to spatial resolution of 18F. The loss in sensitivity due to triple coincidences was determined as a function of the axial acceptance angle of the PET scanner. The performance of the scanner at low count rates was studied by determining the noise equivalent count (NEC) values for different upper energy thresholds. The image degrading effect of spurious coincidences is taken into account by adding another factor to the NEC calculation. This allowed the contribution of spurious coincidences to be minimized by using a setting for the appropriate energy window. For this optimal energy window the amount of spurious and scattered coincidences was quantified. Simulations of count rate performance were also done to determine the peak NEC and the activity at which the maximum occurred.
Spatial resolution degradation, compared to 18F, is about 0.5 mm for 86Y and 1 mm for 124I. Associated singles have a similar effect as scattered coincidences, as they also add a background to the image. The effect, however, is less important than the effect of scatter. The fraction of triple coincidences is quite small for a 3-D PET scanner used for humans as the axial acceptance angle is still moderate. For the Allegro with an energy resolution of 18% the optimal upper energy threshold was determined at 600 keV. For 124I this leads to 2.5% extra contamination that needs to be added to the scatter fraction. For 86Y this fraction is about 5.5%.
3-D PET images of 124I and 86Y have lower spatial resolution. For PET scanners used for humans the difference is not as important as for scanners used for animals. The limited positron decay fraction of both isotopes can be compensated by increasing the imaging time by a factor of 3-5 (same activity). A short coincidence window limits the contamination from other decay modes. Good energy resolution allows setting a selective upper energy threshold to limit the effect of spurious coincidences. With an appropriate setting of the energy window it should be possible to obtain good image quality in a relatively short time because of the high sensitivity of 3-D PET scanners.
与像18F这样的传统正电子发射体相比，不纯的正电子发射体具有会降低图像质量的物理特性。两种具有潜在有趣应用的不纯正电子发射体是124I和86Y。针对人体三维（3-D）正电子发射断层扫描（PET）系统，对由于这些同位素的不完美而导致的图像质量下降进行了量化。通过蒙特卡罗模拟确定了一种获取与18F成像相似图像质量的采集方案。
通过对Allegro扫描仪进行广泛的蒙特卡罗模拟，确定了较大的正电子射程、相关单能峰和其他衰变模式对图像质量的影响。评估了这两种同位素的空间分辨率，并与18F的空间分辨率进行了比较。确定了由于三重符合导致的灵敏度损失作为PET扫描仪轴向接受角的函数。通过确定不同高能阈值下的噪声等效计数（NEC）值，研究了扫描仪在低计数率下的性能。通过在NEC计算中添加另一个因素来考虑虚假符合对图像的降解作用。这使得通过设置合适的能量窗口可以将虚假符合的影响最小化。对于这个最佳能量窗口，对虚假符合和散射符合的数量进行了量化。还进行了计数率性能模拟，以确定峰值NEC以及出现最大值时的活度。
与18F相比，86Y的空间分辨率下降约0.5mm，124I约为1mm。相关单能峰与散射符合具有类似的效果，因为它们也会给图像增加背景。然而，这种影响不如散射的影响重要。对于用于人体的3-D PET扫描仪，由于轴向接受角仍然适中，三重符合的比例相当小。对于能量分辨率为18%的Allegro，确定最佳高能阈值为600keV。对于124I，这会导致需要添加到散射分数中的额外污染为2.5%。对于86Y，这个分数约为5.5%。
124I和86Y的3-D PET图像具有较低的空间分辨率。对于用于人体的PET扫描仪，这种差异不如用于动物的扫描仪那么重要。两种同位素有限的正电子衰变分数可以通过将成像时间增加3至5倍（活度相同）来补偿。短符合窗口限制了来自其他衰变模式的污染。良好的能量分辨率允许设置选择性高能阈值以限制虚假符合的影响。由于3-D PET扫描仪的高灵敏度，通过适当设置能量窗口，应该有可能在相对较短的时间内获得良好的图像质量。
