Department of Biomedical Engineering, University of Cincinnati, Cincinnati, OH, United States; Center for Pulmonary Imaging Research, Division of Pulmonary Medicine, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, United States.
Center for Pulmonary Imaging Research, Division of Pulmonary Medicine, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, United States.
J Magn Reson. 2024 Aug;365:107741. doi: 10.1016/j.jmr.2024.107741. Epub 2024 Jul 26.
Lung diseases are almost invariably heterogeneous and progressive, making it imperative to capture temporally and spatially explicit information to understand the disease initiation and progression. Imaging the lung with MRI-particularly in the preclinical setting-has historically been challenging because of relatively low lung tissue density, rapid cardiac and respiratory motion, and rapid transverse (T*) relaxation. These limitations can largely be mitigated using ultrashort-echo-time (UTE) sequences, which are intrinsically robust to motion and avoid significant T* decay. A significant disadvantage of common radial UTE sequences is that they require inefficient, center-out k-space sampling, resulting in long acquisition times relative to conventional Cartesian sequences. Therefore, pulmonary images acquired with radial UTE are often undersampled to reduce acquisition time. However, undersampling reduces image SNR, introduces image artifacts, and degrades true image resolution. The level of undersampling is further increased if offline gating techniques like retrospective gating are employed, because only a portion (∼40-50%) of the data is used in the final image reconstruction. Here, we explore the impact of undersampling on SNR and T* mapping in mouse lung imaging using simulation and in-vivo data. Increased scatter in both metrics was noticeable at around 50% sampling. Parenchymal apparent SNR only decreased slightly (average decrease ∼ 1.4) with as little as 10% sampling. Apparent T* remained similar across undersampling levels, but it became significantly increased (p < 0.05) below 80% sampling. These trends suggest that undersampling can generate quantifiable, but moderate changes in the apparent value of T*. Moreover, these approaches to assess the impact of undersampling are straightforward to implement and can readily be expanded to assess the quantitative impact of other MR acquisition and reconstruction parameters.
肺部疾病几乎总是异质和进行性的，因此必须捕获时间和空间上的明确信息，以了解疾病的发生和进展。由于肺组织密度相对较低、心脏和呼吸运动较快以及横向（T*）弛豫较快，使用 MRI 对肺部进行成像-特别是在临床前环境中-一直具有挑战性。这些限制可以通过使用超短回波时间（UTE）序列来大大缓解，该序列对运动具有固有鲁棒性并且避免了明显的 T衰减。常见的径向 UTE 序列的一个显著缺点是，它们需要低效的、从中心到边缘的 k 空间采样，导致与传统笛卡尔序列相比采集时间较长。因此，使用径向 UTE 采集的肺部图像通常会进行欠采样以减少采集时间。然而，欠采样会降低图像 SNR、引入图像伪影并降低真实图像分辨率。如果使用离线门控技术（如回顾性门控），则欠采样程度会进一步增加，因为最终图像重建中仅使用数据的一部分（约 40-50%）。在这里，我们通过模拟和体内数据来探索欠采样对小鼠肺部成像中 SNR 和 T映射的影响。在大约 50%的采样率下，这两个指标的散射都明显增加。实质表观 SNR 仅略有下降（平均下降约 1.4），采样率低至 10%。表观 T在整个欠采样水平上保持相似，但在采样率低于 80%时显着增加（p <0.05）。这些趋势表明，欠采样会导致 T的表观值产生可量化但适度的变化。此外，这些评估欠采样影响的方法易于实施，并且可以很容易地扩展到评估其他 MR 采集和重建参数的定量影响。
