Department of Clinical Radiology, University Hospitals-Grosshadern, Ludwig-Maximilians University, Munich, Germany.
Invest Radiol. 2010 Jun;45(6):347-53. doi: 10.1097/RLI.0b013e3181df901d.
New generation Dual Source computed tomography (CT) scanners offer different x-ray spectra for Dual Energy imaging. Yet, an objective, manufacturer independent verification of the dose required for the different spectral combinations is lacking. The aim of this study was to assess dose and image noise of 2 different Dual Energy CT settings with reference to a standard chest scan and to compare image noise and contrast to noise ratios (CNR). Also, exact effective dose length products (E/DLP) conversion factors were to be established based on the objectively measured dose.
An anthropomorphic Alderson phantom was assembled with thermoluminescent detectors (TLD) and its chest was scanned on a Dual Source CT (Siemens Somatom Definition) in dual energy mode at 140 and 80 kVp with 14 x 1.2 mm collimation. The same was performed on another Dual Source CT (Siemens Somatom Definition Flash) at 140 kVp with 0.8 mm tin filter (Sn) and 100 kVp at 128 x 0.6 mm collimation. Reference scans were obtained at 120 kVp with 64 x 0.6 mm collimation at equivalent CT dose index of 5.4 mGy*cm. Syringes filled with water and 17.5 mg iodine/mL were scanned with the same settings. Dose was calculated from the TLD measurements and the dose length products of the scanner. Image noise was measured in the phantom scans and CNR and spectral contrast were determined in the iodine and water samples. E/DLP conversion factors were calculated as ratio between the measured dose form the TLDs and the dose length product given in the patient protocol.
The effective dose measured with TLDs was 2.61, 2.69, and 2.70 mSv, respectively, for the 140/80 kVp, the 140 Sn/100 kVp, and the standard 120 kVp scans. Image noise measured in the average images of the phantom scans was 11.0, 10.7, and 9.9 HU (P > 0.05). The CNR of iodine with optimized image blending was 33.4 at 140/80 kVp, 30.7 at 140Sn/100 kVp and 14.6 at 120 kVp. E/DLP conversion factors were 0.0161 mSv/mGycm for the 140/80 kVp protocol, 0.0181 mSv/mGycm for the Sn140/100 kVp mode and 0.0180 mSv/mGy*cm for the 120 kVp examination.
Dual Energy CT is feasible without additional dose. There is no significant difference in image noise, while CNR can be doubled with optimized dual energy CT reconstructions. A restriction in collimation is required for dose-neutrality at 140/80 kVp, whereas this is not necessary at 140 Sn/100 kVp. Thus, CT can be performed routinely in Dual Energy mode without additional dose or compromises in image quality.
新一代双源计算机断层扫描（CT）扫描仪可为双能成像提供不同的 X 射线光谱。然而，缺乏针对不同光谱组合所需剂量的客观、制造商独立的验证。本研究的目的是评估具有参考标准胸部扫描的 2 种不同双能 CT 设置的剂量和图像噪声，并比较图像噪声和对比噪声比（CNR）。此外，还应基于客观测量的剂量建立准确的有效剂量长度乘积（E/DLP）转换系数。
采用热释光探测器（TLD）组装人体模体，并在双源 CT（西门子 Somatom Definition）上以 140 和 80 kVp 进行双能模式扫描，准直器为 14×1.2mm。在另一台双源 CT（西门子 Somatom Definition Flash）上，在 140 kVp 下使用 0.8mm 锡滤器（Sn）和 128×0.6mm 准直器进行 100 kVp 扫描。参考扫描在等效 CT 剂量指数为 5.4 mGy*cm 的情况下，在 120 kVp 下以 64×0.6mm 准直器进行。装有水和 17.5mg/ml 碘的注射器用相同的设置进行扫描。剂量通过 TLD 测量和扫描仪的剂量长度乘积计算得出。在体模扫描中测量图像噪声，并在碘和水样本中确定 CNR 和光谱对比度。E/DLP 转换因子的计算方法为 TLD 测量的剂量与患者方案中给出的剂量长度乘积之比。
TLD 测量的有效剂量分别为 140/80 kVp、140Sn/100 kVp 和标准 120 kVp 扫描的 2.61、2.69 和 2.70mSv。在体模扫描的平均图像中测量的图像噪声分别为 11.0、10.7 和 9.9HU（P＞0.05）。优化图像混合后的碘 CNR 分别为 140/80 kVp 时的 33.4、140Sn/100 kVp 时的 30.7 和 120 kVp 时的 14.6。140/80 kVp 协议的 E/DLP 转换因子为 0.0161 mSv/mGycm，140Sn/100 kVp 模式的 E/DLP 转换因子为 0.0181 mSv/mGycm，120 kVp 检查的 E/DLP 转换因子为 0.0180 mSv/mGy*cm。
双能 CT 可以在不增加剂量的情况下进行。图像噪声没有显著差异，而通过优化的双能 CT 重建可以将 CNR 提高一倍。在 140/80 kVp 时需要限制准直以实现剂量中性，而在 140Sn/100 kVp 时则不需要。因此，在不增加剂量或不影响图像质量的情况下，可以常规进行 CT 双能扫描。
