[论文解读] Complicated relationships between tissue T2 relaxation time and in vivo tissue diffusion measures, depending on the ranges of T2 value

Yì Wáng|arXiv (Cornell University)|Jun 19, 2023

MRI in cancer diagnosisMedicine被引用 3

一句话总结

本文指出，磁共振成像（MRI）中的表观扩散系数（ADC）值显著受T2弛豫时间影响，尤其在T2较长的组织中，导致扩散测量结果产生误导。研究证明，高T2值会主导b=0和高b值图像的信号强度，使ADC图反映T2效应而非真实扩散，特别是在囊性或黏液样变性肿瘤中。

ABSTRACT

Apparent diffusion coefficient (ADC) is a measure of the magnitude of diffusion of water molecules within tissues. We argue that ADC value contains information of both diffusion and T2 relaxation. In this letter, we list literature evidence to support our argument. Firstly, we list uterine myometrium tumors as examples. Myometrium has a T2 relaxation time of 79 ms at 3T. Literature shows, when the myometrium tumors are T2 weighted signal intensity hypertensive (relative to myometrium), these tumors likely show diffusion restriction. This is similar to that, while the spleen is T2 weighted signal intensity hypertensive relative to the liver, the spleen demonstrates diffusion restriction relative to the liver. On the other hand, when the myometrium tumors are T2 weighted signal intensity hypotensive (relative to myometrium), these tumors likely do not show diffusion restriction. However, when the myometrium tumors are very highly hypertensive such as the cases of leiomyoma cystic degeneration and myxoid degeneration, the relationship between T2 weighted signal intensity and diffusion is similar to that of a normal gallbladder, i.e., T2 weighted signal highly hypertensive without diffusion restriction. For most of the tumors, much longer T2 may suggest a tumor is waterier. On b=0 images, the signal of tumor (relative to other tissues) can be dominated by T2 effect. On a high b-value images such as b=1000 images, it is possible that tumor regions are dominated by noises. In the end, the ADCs, which are determined by the slope between the signal at b=0 and the signal at b=1000, are also dominated by the T2 effect, with longer T2 being associated with higher ADC measure. In fact, since there is no diffusion gradient with b=0 images, and there are only noises on b=1000 (or b=800), ADC map is devoid of diffusion information.

研究动机与目标

• 研究T2弛豫时间如何影响活体组织中表观扩散系数（ADC）的测量。
• 阐明T2效应在扩散加权成像（DWI）中的混淆作用，特别是在T2值异常的肿瘤中。
• 挑战ADC仅反映水分子扩散的假设，表明T2可能主导信号行为。
• 解释在不同T2弛豫时间的组织间比较时，扩散受限评估结果不一致的原因。
• 强调在解释具有高或极高T2的肿瘤（如平滑肌瘤囊性变性）的DWI时的临床意义。

提出的方法

• 回顾关于特定组织（包括子宫平滑肌和脾脏）中T2弛豫时间与扩散测量的现有文献。
• 分析b=0和高b值（如b=1000）图像上信号强度模式与T2及ADC值的关系。
• 利用b=0与高b值图像间信号强度关系，推断因T2效应导致的ADC计算偏差。
• 比较T2增高型与T2降低型肿瘤的临床病例，评估扩散受限模式。
• 评估高b值图像上的噪声主导现象，特别是在T2较长的组织中，影响ADC定量准确性。
• 应用ADC由b=0与高b值信号间斜率推导的原理，指出当b=0受T2主导且高b值受噪声影响时，T2效应可能扭曲该斜率。

实验结果

研究问题

• RQ1 T2弛豫时间如何影响具有不同T2值组织中的表观扩散系数（ADC）？
• RQ2 为何一些T2增高型肿瘤尽管在T2加权像上信号强度高，却未显示扩散受限？
• RQ3 如何解释在T2极高的肿瘤（如囊性或黏液样变性肿瘤）中扩散受限的矛盾性缺失？
• RQ4 T2弛豫在多大程度上主导b=0和高b值图像的信号强度，从而影响ADC图的解读？
• RQ5 高b值图像上的噪声（如b=1000）在T2较长的组织中如何影响ADC测量的准确性？

主要发现

• 在T2较长的组织中，如伴有囊性或黏液样变性的子宫平滑肌瘤，高T2可导致T2加权信号高信号，但无相应扩散受限。
• ADC值显著受T2弛豫影响，即使无真实扩散变化，较长的T2值也与较高的ADC测量值相关。
• 在b=0图像上，信号强度主要受T2效应支配，导致肿瘤信号无论扩散状态如何均显得增高。
• 在高b值图像上（如b=1000），信号常受噪声主导，尤其在T2较长的组织中，这会降低ADC计算的可靠性。
• 由b=0与高b值信号间斜率推导出的ADC图可能更多反映T2效应而非真实扩散，尤其当b=0受T2主导且高b值受噪声影响时。
• 在正常胆囊或伴有囊性变性的平滑肌瘤等病例中，高T2导致T2加权像上高信号但无扩散受限，表明T2高信号并不意味着扩散受限。