Dizendorf Elena, Hany Thomas F, Buck Alfred, von Schulthess Gustav K, Burger Cyrill
Division of Nuclear Medicine, University Hospital of Zurich, Zurich, Switzerland.
J Nucl Med. 2003 May;44(5):732-8.
CT images represent essentially noiseless maps of photon attenuation at a range of 40-140 keV. Current dual-modality PET/CT scanners transform them into attenuation coefficients at 511 keV and use these for PET attenuation correction. The proportional scaling algorithms hereby used account for the different properties of soft tissue and bone but are not prepared to handle material with other attenuation characteristics, such as oral CT contrast agents. As a consequence, CT-based attenuation correction in the presence of an oral contrast agent results in erroneous PET standardized uptake values (SUVs). The present study assessed these errors with phantom measurements and patient data.
Two oral CT contrast agents were imaged at 3 different concentrations in dual-modality CT and PET transmission studies to investigate their attenuation properties. The SUV error due to the presence of contrast agent in CT-based attenuation correction was estimated in 10 patients with gastrointestinal tumors as follows. The PET data were attenuation corrected on the basis of the original contrast-enhanced CT images, resulting in PET images with distorted SUVs. A second reconstruction used modified CT images wherein the CT numbers representing contrast agent had been replaced by CT values producing approximately the right PET attenuation coefficients. These CT values had been derived from the data of 10 patients imaged without a CT contrast agent. The SUV error, defined as the difference between both sets of SUV images, was evaluated in regions with oral CT contrast agent, in tumor, and in reference tissue.
The oral CT contrast agents studied increased the attenuation for 511-keV photons minimally, even at the highest concentrations found in the patients. For a CT value of 500 Hounsfield units, the proportional scaling algorithm therefore overestimated the PET attenuation coefficient by 26.2%. The resulting SUV error in the patient studies was highest in regions containing CT contrast agent (4.4% +/- 2.8%; maximum, 11.3%), whereas 1.2% +/- 1.1% (maximum, 4.1%) was found in tumors, and 0.6% +/- 0.7% was found in the reference.
The use of oral contrast agents in CT has only a small effect on the SUV, and this small effect does not appear to be medically significant.
CT图像本质上是40 - 140 keV范围内光子衰减的无噪声图谱。当前的双模态PET/CT扫描仪将其转换为511 keV时的衰减系数，并将这些用于PET衰减校正。在此使用的比例缩放算法考虑了软组织和骨骼的不同特性，但未准备好处理具有其他衰减特征的物质，如口服CT造影剂。因此，在存在口服造影剂的情况下基于CT的衰减校正会导致错误的PET标准化摄取值（SUV）。本研究通过体模测量和患者数据评估了这些误差。
在双模态CT和PET透射研究中，对两种口服CT造影剂在3种不同浓度下进行成像，以研究其衰减特性。如下估计10例胃肠道肿瘤患者中基于CT的衰减校正中由于造影剂存在导致的SUV误差。PET数据基于原始增强CT图像进行衰减校正，从而得到SUV失真的PET图像。第二次重建使用了修改后的CT图像，其中代表造影剂的CT值已被产生近似正确PET衰减系数的CT值所取代。这些CT值来自10例未使用CT造影剂成像的患者的数据。在含有口服CT造影剂的区域、肿瘤和参考组织中评估定义为两组SUV图像之间差异的SUV误差。
所研究的口服CT造影剂即使在患者中发现的最高浓度下，对511 keV光子的衰减增加也极小。因此，对于500亨氏单位的CT值，比例缩放算法将PET衰减系数高估了26.2%。在患者研究中，含有CT造影剂的区域中产生的SUV误差最高（4.4%±2.8%；最大值为11.3%），而在肿瘤中为1.2%±1.1%（最大值为4.1%），在参考组织中为0.6%±0.7%。
CT中使用口服造影剂对SUV的影响很小，并且这种小影响似乎在医学上不具有显著意义。
