Ay Mohammad Reza, Zaidi Habib
Division of Nuclear Medicine, Geneva University Hospital 1211 Geneva 4, Switzerland.
Eur J Nucl Med Mol Imaging. 2006 Nov;33(11):1301-13. doi: 10.1007/s00259-006-0086-6. Epub 2006 Apr 19.
Quantitative image reconstruction in positron emission tomography (PET) requires an accurate attenuation map of the object under study for the purpose of attenuation correction. Current dual-modality PET/CT systems offer significant advantages over stand-alone PET, including decreased overall scanning time and increased accuracy in lesion localisation and detectability. However, the contamination of CT data with scattered radiation and misclassification of contrast medium with high-density bone in CT-based attenuation correction (CTAC) are known to generate artefacts in the attenuation map and thus the resulting PET images. The purpose of this work was to quantitatively measure the impact of scattered radiation and contrast medium on the accuracy of CTAC.
Our recently developed MCNP4C-based Monte Carlo X-ray CT simulator for modelling both fan- and cone-beam CT scanners and the Eidolon dedicated 3D PET Monte Carlo simulator were used to generate realigned PET/CT data sets. The impact of X-ray scattered radiation on the accuracy of CTAC was investigated through simulation of a uniform cylindrical water phantom for both a commercial fan-beam multi-slice and a prototype cone-beam flat panel detector-based CT scanner. The influence of contrast medium was studied by simulation of a cylindrical phantom containing different concentrations of contrast medium. Moreover, an experimental study using an anthropomorphic striatal phantom was conducted for quantitative evaluation of errors arising from the presence of contrast medium by calculating the apparent recovery coefficient (ARC) in the presence of different concentrations of contrast medium.
The analysis of attenuation correction factors (ACFs) for the simulated cylindrical water phantom in both fan- and cone-beam CT scanners showed that the contamination of CT data with scattered radiation in the absence of scatter removal causes underestimation of the true ACFs, namely by 7.3% and 28.2% in the centre for the two geometries, respectively. The ARC was 190.7% for a cylindrical volume of interest located in the main chamber of the striatal phantom containing contrast medium corresponding to 2,000 Hounsfield units, whereas the ARC was overestimated by less than 5% for the main chamber and by approximately 2% for the left/right putamen and caudate nucleus compared with the absence of contrast medium.
Without X-ray scatter compensation, the visual artefacts and quantitative errors in flat panel detector-based cone-beam geometry are substantial and propagate cupping artefacts to PET images during CTAC. Likewise, contrast-enhanced CT images may create considerable artefacts during CTAC in regions containing high concentrations of contrast medium.
正电子发射断层扫描（PET）中的定量图像重建需要针对研究对象生成准确的衰减图以进行衰减校正。当前的双模态PET/CT系统相较于单独的PET具有显著优势，包括整体扫描时间缩短以及病变定位和可检测性的准确性提高。然而，已知在基于CT的衰减校正（CTAC）中，CT数据受散射辐射污染以及造影剂与高密度骨骼的误分类会在衰减图中产生伪影，进而影响最终的PET图像。本研究的目的是定量测量散射辐射和造影剂对CTAC准确性的影响。
我们最近开发的基于MCNP4C的蒙特卡罗X射线CT模拟器用于模拟扇束和锥束CT扫描仪，以及Eidolon专用3D PET蒙特卡罗模拟器用于生成重新对齐的PET/CT数据集。通过模拟均匀圆柱形水体模，研究了X射线散射辐射对商业扇束多层CT扫描仪和基于原型锥束平板探测器的CT扫描仪CTAC准确性的影响。通过模拟包含不同浓度造影剂的圆柱形体模，研究了造影剂的影响。此外，使用拟人化纹状体模进行了实验研究，通过计算不同浓度造影剂存在时的表观恢复系数（ARC），对造影剂存在导致的误差进行定量评估。
对扇束和锥束CT扫描仪中模拟的圆柱形水体模的衰减校正因子（ACF）分析表明，在未去除散射的情况下，CT数据受散射辐射污染会导致真实ACF被低估，在两种几何结构中，中心处分别低估7.3%和28.2%。对于位于纹状体模主腔中含有对应2000亨氏单位造影剂的圆柱形感兴趣体积，ARC为190.7%，而与无造影剂相比，主腔的ARC高估不到5%，左/右壳核和尾状核的ARC高估约2%。
在没有X射线散射补偿的情况下，基于平板探测器的锥束几何结构中的视觉伪影和定量误差很大，并且在CTAC期间会将杯状伪影传播到PET图像中。同样，在含有高浓度造影剂的区域，增强CT图像在CTAC期间可能会产生相当大的伪影。
