Amir Kabir University of Technology, Tehran, Iran.
Med Phys. 2012 Apr;39(4):2078-89. doi: 10.1118/1.3694108.
Dual-energy CT (DECT) is arguably the most accurate energy mapping technique in CT-based attenuation correction (CTAC) implemented on hybrid PET/CT systems. However, this approach is not attractive for clinical use owing to increased patient dose. The authors propose a novel energy mapping approach referred to as virtual DECT (VDECT) taking advantage of the DECT formulation but using CT data acquired at a single energy (kV(P)). For this purpose, the CT image acquired at one energy is used to generate the CT image at a second energy using calculated kV(P) conversion curves derived from phantom studies.
The attenuation map (μ-map) at 511 keV was generated for the XCAT phantom and clinical studies using the bilinear, DECT, and VDECT techniques. The generated μ-maps at 511 keV are compared to the reference derived from the XCAT phantom serving as ground truth. PET data generated from a predefined activity map for the XCAT phantom were then corrected for attenuation using μ-maps generated using the different energy mapping approaches. In addition, the generated μ-maps using the above described methods for a cylindrical polyethylene phantom containing different concentrations of K(2)HPO(4) in water were compared to actual attenuation coefficients. Likewise, CT images of five clinical whole-body studies were used to generate μ-maps using the various energy-mapping approaches were compared with μ-maps acquired at 511 keV using (68)Ge/(68)Ga rod sources for the clinical studies.
The results of phantom studies demonstrate that the proposed method is more accurate than the bilinear technique. All three μ-maps yielded almost similar results for soft and lung tissues whereas for bone tissues, the DECT and the VDECT methods produced a much smaller mean relative difference (3.0% and 2.8%, respectively) than the bilinear approach (11.8%). Likewise, the comparison of PET images corrected for attenuation using the various methods showed that the proposed method provides better accuracy (6.5%) than the bilinear method (13.4%). Clinical studies further demonstrated that, compared to the bilinear method, the VDECT approach has better agreement for bony structures with the DECT technique (1.5% versus 8.9%) and transmission scanning (8.8% versus 17.7%).
It was concluded that the proposed method outperforms the bilinear method especially in bony structures. Further evaluation using a large clinical PET/CT database is underway to evaluate the potential of the technique in a clinical setting.
在基于混合 PET/CT 系统的 CT 衰减校正（CTAC）中，双能 CT（DECT）可被认为是最精确的能量映射技术。然而，由于患者剂量增加，该方法不适合临床应用。作者提出了一种新的能量映射方法，称为虚拟 DECT（VDECT），利用 DECT 公式，但使用在单一能量（kV(P)）下采集的 CT 数据。为此，使用从体模研究中推导出来的计算出的 kV(P)转换曲线，从采集的单一能量 CT 图像生成第二能量的 CT 图像。
使用双线性、DECT 和 VDECT 技术为 XCAT 体模和临床研究生成 511keV 的衰减图（μ-map）。生成的 511keVμ-map 与作为基准的来自 XCAT 体模的参考值进行比较。然后，使用不同的能量映射方法生成的μ-map 对来自 XCAT 体模预定义活动图的 PET 数据进行衰减校正。此外，还比较了使用上述方法生成的包含不同浓度 K(2)HPO(4)在水中的圆柱形聚乙烯体模的实际衰减系数的μ-map。同样，还比较了使用各种能量映射方法生成的 5 个临床全身研究的 CT 图像的μ-map 与使用临床研究中的 (68)Ge/(68)Ga 棒源在 511keV 采集的μ-map。
体模研究结果表明，该方法比双线性技术更准确。对于软组织和肺部组织，所有三种μ-map 都产生了几乎相似的结果，而对于骨骼组织，DECT 和 VDECT 方法的平均相对差异（分别为 3.0%和 2.8%）明显小于双线性方法（11.8%）。同样，使用各种方法校正衰减后的 PET 图像比较表明，该方法提供了比双线性方法更高的准确性（6.5%）。临床研究进一步表明，与双线性方法相比，VDECT 方法在骨结构上与 DECT 技术（1.5%比 8.9%）和透射扫描（8.8%比 17.7%）具有更好的一致性。
结果表明，该方法在骨结构方面优于双线性方法。正在使用大型临床 PET/CT 数据库进行进一步评估，以评估该技术在临床环境中的潜力。
