Sawall Stefan, Baader Edith, Trapp Philip, Kachelrieß Marc
German Cancer Research Center (DKFZ), Heidelberg, Germany.
Medical Faculty, Heidelberg University, Heidelberg, Germany.
Med Phys. 2025 Apr;52(4):2167-2190. doi: 10.1002/mp.17604. Epub 2025 Jan 10.
With the widespread introduction of dual energy computed tomography (DECT), applications utilizing the spectral information to perform material decomposition became available. Among these, a popular application is to decompose contrast-enhanced CT images into virtual non-contrast (VNC) or virtual non-iodine images and into iodine maps. In 2021, photon-counting CT (PCCT) was introduced, which is another spectral CT modality. It allows for scans with more than two different detected spectra. With these systems, it becomes possible to distinguish more than two materials. It is frequently proposed to administer more than one contrast agent, perform a single PCCT scan, and then calculate the VNC images and the contrast agent maps. This may not be optimal because the patient is injected with a material, only to have it computationally extracted again immediately afterwards by spectral CT. It may be better to do an unenhanced scan followed by one or more contrast-enhanced scans. The main argument for the spectral material decomposition is patient motion, which poses a significant challenge for approaches involving two or more temporally separated scans. In this work, we assume that we can correct for patient motion and thus are free to scan the patient more than once. Our goal is then to quantify the penalty for performing a single contrast-enhanced scan rather than a clever series of unenhanced and enhanced scans. In particular, we consider the impact on patient dose and image quality.
We simulate CT scans of three differently sized phantoms containing various contrast agents. We do this for a variety of tube voltage settings, a variety of patient-specific prefilter (PSP) thicknesses and a variety of threshold settings of the photon-counting detector with up to four energy bins. The reconstructed bin images give the expectation values of soft tissue and of the contrast agents. Error propagation of projection noise into the images yields the image noise. Dose is quantified using the total CT dose index (CTDI) value of the scans. When combining multiple scans, we further consider all possible tube current (or dose) ratios between the scans. Material decomposition is done image-based in a statistical optimal way. Error propagation into the material-specific images yields the signal-to-noise ratio at unit dose (SNRD). The winning scan strategy is the one with the highest total SNRD, which is related to the SNRD of the material that has the lowest signal-to-noise ratio (SNR) among the materials to decompose into. We consider scan strategies with up to three scans and up to three materials (water W, contrast agent X and contrast agent Y).
In all cases, those scan strategies yield the best performance that combine differently enhanced scans, for example, W+WX, W+WXY, WX+WXY, W+WX+WY, with W denoting an unenhanced scan and WX, WY and WXY denoting X-, Y-, and X-Y-enhanced scans, respectively. The dose efficiency of scans with a single enhancement scheme, such as WX or WXY, is far lower. The dose penalty to pay for these single enhancement strategies is about two or greater. Our findings also apply to scans with a single energy bin and thus also to CT systems with conventional, energy-integrating detectors, that is, conventional DECT. Dual source CT (DSCT) scans are preferable over single source CT scans, also because one can use a PSP on the high Kilovolt spectrum to better separate the detected spectra. For the strategies and tasks considered here, it does not make sense to simultaneously scan with two different types of contrast agents. Iodine outperforms other high Z elements in nearly all cases.
Given the significant dose penalty when performing only one contrast-enhanced scan rather than a series of unenhanced and enhanced scans, one should consider avoiding the single-scan strategies. This requires to invest in the development of accurate registration algorithms that can compensate for patient and contrast agent motion between separate scans.
随着双能计算机断层扫描（DECT）的广泛应用，利用光谱信息进行物质分解的应用变得可行。其中，一种流行的应用是将增强CT图像分解为虚拟平扫（VNC）或虚拟去碘图像以及碘图。2021年，光子计数CT（PCCT）问世，它是另一种光谱CT模式。它允许进行具有两种以上不同检测光谱的扫描。利用这些系统，可以区分两种以上的物质。人们经常建议使用不止一种造影剂，进行一次PCCT扫描，然后计算VNC图像和造影剂图。这可能不是最佳选择，因为患者被注射了一种物质，随后却又要通过光谱CT立即将其计算提取出来。先进行平扫，然后进行一次或多次增强扫描可能会更好。光谱物质分解的主要论据是患者运动，这对涉及两次或更多次时间上分开的扫描的方法构成了重大挑战。在这项工作中，我们假设可以校正患者运动，因此可以对患者进行多次扫描。然后我们的目标是量化进行一次增强扫描而不是一系列巧妙的平扫和增强扫描所带来的代价。特别是，我们考虑对患者剂量和图像质量的影响。
我们模拟了包含各种造影剂的三种不同尺寸的体模的CT扫描。我们针对各种管电压设置、各种患者特定预滤波器（PSP）厚度以及具有多达四个能量 bins 的光子计数探测器的各种阈值设置进行了模拟。重建的 bin 图像给出了软组织和造影剂的期望值。投影噪声在图像中的误差传播产生图像噪声。使用扫描的总CT剂量指数（CTDI）值来量化剂量。当组合多次扫描时，我们进一步考虑扫描之间所有可能的管电流（或剂量）比。物质分解以基于图像的统计最优方式进行。误差传播到特定物质的图像中产生单位剂量信噪比（SNRD）。获胜的扫描策略是具有最高总SNRD的策略，这与要分解的物质中具有最低信噪比（SNR）的物质的SNRD相关。我们考虑了多达三次扫描和多达三种物质（水W、造影剂X和造影剂Y）的扫描策略。
在所有情况下，那些将不同增强扫描组合的扫描策略表现最佳，例如，W + WX、W + WXY、WX + WXY、W + WX + WY，其中W表示平扫，WX、WY和WXY分别表示X增强、Y增强和X - Y增强扫描。单一增强方案（如WX或WXY）的扫描剂量效率要低得多。为这些单一增强策略付出的剂量代价约为两倍或更高。我们的发现也适用于具有单个能量 bin 的扫描，因此也适用于具有传统能量积分探测器的CT系统，即传统DECT。双源CT（DSCT）扫描优于单源CT扫描，这也是因为可以在高千伏光谱上使用PSP来更好地分离检测光谱。对于这里考虑的策略和任务，同时使用两种不同类型的造影剂进行扫描没有意义。在几乎所有情况下，碘的表现都优于其他高Z元素。
鉴于仅进行一次增强扫描而不是一系列平扫和增强扫描时存在显著的剂量代价，应该考虑避免单扫描策略。这需要投入开发能够补偿单独扫描之间患者和造影剂运动的精确配准算法。
