Real-time three-dimensional MRI for the assessment of dynamic carpal instability

Background
Carpal instability is defined as a condition where wrist motion and/or loading creates mechanical dysfunction, resulting in weakness, pain and decreased function. When conventional methods do not identify the instability patterns, yet clinical signs of instability exist, the diagnosis of dynamic instability is often suggested to describe carpal derangement manifested only during the wrist’s active motion or stress. We addressed the question: can advanced MRI techniques provide quantitative means to evaluate dynamic carpal instability and supplement standard static MRI acquisition? Our objectives were to (i) develop a real-time, three-dimensional MRI method to image the carpal joints during their active, uninterrupted motion; and (ii) demonstrate feasibility of the method for assessing metrics relevant to dynamic carpal instability, thus overcoming limitations of standard MRI.
Methods
Twenty wrists (bilateral wrists of ten healthy participants) were scanned during radial-ulnar deviation and clenched-fist maneuvers. Images resulting from two real-time MRI pulse sequences, four sparse data-acquisition schemes, and three constrained image reconstruction techniques were compared. Image quality was assessed via blinded scoring by three radiologists and quantitative imaging metrics.
Results
Real-time MRI data-acquisition employing sparse radial sampling with a gradient-recalled-echo acquisition and constrained iterative reconstruction appeared to provide a practical tradeoff between imaging speed (temporal resolution up to 135 ms per slice) and image quality. The method effectively reduced streaking artifacts arising from data undersampling and enabled the derivation of quantitative measures pertinent to evaluating dynamic carpal instability.
Conclusion
This study demonstrates that real-time, three-dimensional MRI of the moving wrist is feasible and may be useful for the evaluation of dynamic carpal instability.

背景
腕关节不稳（Carpal instability）被定义为腕部活动或负重引发机械功能异常，进而导致肌力减弱、疼痛及功能下降的病症。当常规方法无法明确不稳类型，但临床存在不稳体征时，通常会诊断为动态腕关节不稳，用以描述仅在腕部主动活动或应力负荷下才显现的腕关节紊乱。本研究旨在解答如下问题：先进磁共振成像（Magnetic Resonance Imaging, MRI）技术能否提供定量手段，以评估动态腕关节不稳，并弥补标准静态MRI采集的不足？本研究的目标为：(i) 开发一种实时三维MRI方法，用于对腕关节主动、连续运动过程中的腕关节间隙进行成像；(ii) 验证该方法用于评估与动态腕关节不稳相关指标的可行性，从而克服标准MRI的局限性。

方法
本研究纳入10名健康受试者的双侧腕关节共20个腕部，在尺桡偏运动及握拳动作期间进行扫描。对两种实时MRI脉冲序列、四种稀疏数据采集方案以及三种约束性图像重建技术所得到的图像进行了对比。由3名放射科医师采用盲法评分对图像质量进行评估，并结合定量成像指标进行分析。

结果
采用梯度回波（gradient-recalled-echo）采集结合稀疏径向采样与约束性迭代重建的实时MRI数据采集方案，似乎在成像速度（单层面时间分辨率最高可达135 ms）与图像质量之间实现了实用的平衡。该方法有效降低了数据欠采样引发的条纹伪影（streaking artifacts），并可推导得到与动态腕关节不稳评估相关的定量测量指标。

结论
本研究证实，对活动中的腕部进行实时三维MRI成像是可行的，且可用于动态腕关节不稳的评估。