Division of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany.
Faculty of Physics and Astronomy, University of Heidelberg, Heidelberg, Germany.
Magn Reson Med. 2020 Mar;83(3):920-934. doi: 10.1002/mrm.27983. Epub 2019 Sep 18.
The application of amide proton transfer (APT) CEST MRI for diagnosis of breast cancer is of emerging interest. However, APT imaging in the human breast is affected by the ubiquitous fat signal preventing a straightforward application of existing acquisition protocols. Although the spectral region of the APT signal does not coincide with fat resonances, the fat signal leads to an incorrect normalization of the Z-spectrum, and therefore to distorted APT effects. In this study, we propose a novel normalization for APT-CEST MRI that corrects for fat signal-induced artifacts in the postprocessing without the need for application of fat saturation schemes or water-fat separation approaches.
The novel normalization uses the residual signal at the spectral position of the direct water saturation to estimate the fat contribution. A comprehensive theoretical description of the normalization for an arbitrary phase relation of the water and fat signal is provided. Functionality and applicability of the proposed normalization was demonstrated by in vitro and in vivo experiments.
In vitro, an underestimation of the conventional APT contrast of approximately -1.2% per 1% fat fraction was observed. The novel normalization yielded an APT contrast independent of the fat contribution, which was also independent of the water-fat phase relation. This allowed APT imaging in patients with mamma carcinoma corrected for fat signal contribution, field inhomogeneities, spillover dilution, and water relaxation effects.
The proposed normalization increases the specificity of APT imaging in tissues with varying fat content and represents a time-efficient and specific absorption rate-efficient alternative to fat saturation and water-fat separation approaches.
酰胺质子转移（APT）化学交换饱和转移（CEST）MRI 应用于乳腺癌诊断正受到越来越多的关注。然而，人体乳房中的 APT 成像是受无处不在的脂肪信号影响的，这阻碍了现有采集方案的直接应用。尽管 APT 信号的谱区与脂肪共振不重合，但脂肪信号会导致 Z 谱的不正确归一化，从而导致扭曲的 APT 效应。在这项研究中，我们提出了一种新的 APT-CEST MRI 归一化方法，用于在不应用脂肪饱和方案或水脂分离方法的情况下，对后处理中的脂肪信号引起的伪影进行校正。
新的归一化方法使用直接水饱和时的剩余信号来估计脂肪的贡献。提供了一种任意水和脂肪信号相位关系的归一化的全面理论描述。通过体外和体内实验证明了所提出的归一化的功能和适用性。
在体外，观察到常规 APT 对比度约每 1%脂肪分数低估 -1.2%。所提出的归一化方法得到的 APT 对比度不受脂肪贡献的影响，也不受水脂相位关系的影响。这允许对患有乳腺癌的患者进行 APT 成像，同时校正脂肪信号贡献、场不均匀性、溢出稀释和水弛豫效应。
所提出的归一化方法提高了具有不同脂肪含量的组织中 APT 成像的特异性，是一种高效且特定吸收率的替代方法，可替代脂肪饱和和水脂分离方法。
