From the Department of Radiology.
Department of Engineering and Natural Sciences, University of Applied Sciences Merseburg, Merseburg, Germany.
Invest Radiol. 2022 Aug 1;57(8):502-509. doi: 10.1097/RLI.0000000000000862. Epub 2022 Feb 23.
Tissue stiffness can guide medical diagnoses and is exploited as an imaging contrast in elastography. However, different elastography devices show different liver stiffness values in the same subject, hindering comparison of values and establishment of system-independent thresholds for disease detection. There is a need for standardized phantoms that specifically address the viscosity-related dispersion of stiffness over frequency. To improve standardization of clinical elastography across devices and platforms including ultrasound and magnetic resonance imaging (MRI), a comprehensively characterized phantom is introduced that mimics the dispersion of stiffness of the human liver and can be generated reproducibly.
The phantom was made of linear polymerized polyacrylamide (PAAm) calibrated to the viscoelastic properties of healthy human liver in vivo as reported in the literature. Stiffness dispersion was analyzed using the 2-parameter springpot model fitted to the dispersion of shear wave speed of PAAm, which was measured by shear rheometry, ultrasound-based time-harmonic elastography, clinical magnetic resonance elastography (MRE), and tabletop MRE in the frequency range of 5 to 3000 Hz. Imaging parameters for ultrasound and MRI, reproducibility, aging behavior, and temperature dependency were assessed. In addition, the frequency bandwidth of shear wave speed of clinical elastography methods (Aplio i900, Canon; Acuson Sequoia, Siemens; FibroScan, EchoSense) was characterized.
Within the entire frequency range analyzed in this study, the PAAm phantom reproduced well the stiffness dispersion of human liver in vivo despite its fluid properties under static loading (springpot stiffness parameter, 2.14 [95% confidence interval, 2.08-2.19] kPa; springpot powerlaw exponent, 0.367 [95% confidence interval, 0.362-0.373]). Imaging parameters were close to those of liver in vivo with only slight variability in stiffness values of 0.5% (0.4%, 0.6%), 4.1% (3.9%, 4.5%), and -0.63% (-0.67%, -0.58%), respectively, between batches, over a 6-month period, and per °C increase in temperature.
The liquid-liver phantom has useful properties for standardization and development of liver elastography. First, it can be used across clinical and experimental elastography devices in ultrasound and MRI. Second, being a liquid, it can easily be adapted in size and shape to specific technical requirements, and by adding inclusions and scatterers. Finally, because the phantom is based on noncrosslinked linear PAAm constituents, it is easy to produce, indicating potential widespread use among researchers and vendors to standardize liver stiffness measurements.
组织硬度可以指导医学诊断，并在弹性成像中用作成像对比。然而，同一受试者的不同弹性成像设备显示出不同的肝硬度值，这阻碍了对值的比较以及为疾病检测建立与系统无关的阈值。需要标准化的体模，专门针对与频率相关的硬度分散进行研究。为了提高包括超声和磁共振成像（MRI）在内的设备和平台上临床弹性成像的标准化程度，引入了一种经过全面表征的体模，该体模模拟了人体肝脏的硬度分散性，并且可以重复生成。
该体模由线性聚合的聚丙烯酰胺（PAAm）制成，其根据文献中报道的体内健康人肝的粘弹性特性进行了校准。使用 2-参数弹簧模型分析了刚度分散，该模型通过剪切流变仪、基于超声的时谐弹性成像、临床磁共振弹性成像（MRE）以及台式 MRE 在 5 至 3000 Hz 的频率范围内测量的 PAAm 的剪切波速度分散进行拟合。评估了超声和 MRI 的成像参数、可重复性、老化行为和温度依赖性。此外，还对临床弹性成像方法（Aplio i900，佳能；Acuson Sequoia，西门子；FibroScan，EchoSense）的剪切波速度带宽进行了表征。
在本研究分析的整个频率范围内，尽管 PAAm 体模在静态加载下具有流体特性（弹簧模型硬度参数，2.14 [95%置信区间，2.08-2.19] kPa；弹簧模型幂律指数，0.367 [95%置信区间，0.362-0.373]），但仍能很好地再现人体肝脏的刚度分散。成像参数与体内肝脏接近，仅在刚度值方面略有变化，批次之间分别为 0.5%（0.4%，0.6%）、4.1%（3.9%，4.5%）和-0.63%（-0.67%，-0.58%），6 个月期间和温度每升高 1°C。
该液体肝脏体模具有用于标准化和开发肝脏弹性成像的有用特性。首先，它可以在超声和 MRI 中的临床和实验弹性成像设备中使用。其次，由于它是液体，因此可以轻松地根据特定的技术要求调整大小和形状，并可以添加内含物和散射体。最后，由于该体模基于非交联线性 PAAm 成分，因此易于生产，这表明研究人员和供应商有可能广泛使用它来标准化肝硬度测量。
