Department of Radiotherapy, University Medical Center Utrecht, 3508 GA Utrecht, The Netherlands.
Med Phys. 2011 Oct;38(10):5321-9. doi: 10.1118/1.3633906.
In MRI-guided needle interventions, such as biopsies and brachytherapy, tissue deformation caused by needle movement may result in localization errors and thus hamper the outcome of the procedure. Monitoring the local tissue deformation provides the ability to compensate for it, e.g., by increasing the needle insertion depth. Fast MR scans are useful to track the needle, but cannot be used to quantify local tissue deformation, in case anatomical landmarks are missing. Artificial landmarks can be created by MR tagging. This method provides a spatial saturation pattern (tag) in the tissue. Deformation of this pattern reflects the tissue motion between tag creation and tag imaging. As the needle movement is nonperiodic, k-space cannot be acquired with a multishot approach, like is usually done for cardiac imaging. Hence, a single shot MR tagging sequence is needed, which entails tag creation, needle movement and tag acquisition. In this study, the feasibility of single shot MR tagging for MRI-guided needle interventions in phantom and volunteer experiments is shown.
Four different experiments were performed on a 1.5 T MR scanner: the first to quantify translations, the second to quantify rotations, the third to mimic a needle intervention, and the fourth to investigate the tag persistence in a volunteer. The tag pattern is created by a 1331 composite pulse. A balanced steady state free precession sequence is used for imaging. To minimize undesired changes in contrast or sharpness of the tag pattern, we chose a relatively small flip angle and a short imaging time in all experiments. In the volunteer experiments, we modified the sequence to also be able to inspect the influence of the used k-space sampling profile and the flip angle on the temporal persistence of image contrast and tag pattern. In all scans, head or surface coils were used for signal reception.
In all experiments, the tag pattern was clearly visible and could be used to quantify the local tissue deformation caused by (needle) movement. Strong correlations between the actual and measured (angular) phantom motions were obtained. In the needle intervention experiment, the tag lines were perfectly horizontal when there was no needle movement. With needle movement, local tissue displacements up to 5 mm were observed. Volunteer's anatomy could be discriminated, despite the tag pattern. The tag pattern in the prostate, for example, could still be read in all tagging images acquired 2 s after creating the tag pattern. With optimized scan parameters the tag persistence was even longer. The best image tag contrast was obtained using a large flip angle and the profile order low-high, although the image was slightly blurred.
This study demonstrates that single shot MR tagging can be used to quantify tissue deformation caused by needle movement. The in-vivo tag persistence is sufficient to enable the application of the tagging sequence during MRI-guided needle interventions in patients.
在 MRI 引导的针介入治疗中，如活检和近距离放射治疗，由于针的移动引起的组织变形可能导致定位误差，从而影响手术的结果。监测局部组织变形的能力可以通过增加针的插入深度来进行补偿。快速 MR 扫描可用于跟踪针，但在缺少解剖学标志的情况下，无法用于定量局部组织变形。人工标志可以通过 MR 标记创建。该方法在组织中提供了空间饱和模式（标记）。该模式的变形反映了标记创建和标记成像之间的组织运动。由于针的运动是非周期性的，因此不能像通常用于心脏成像那样使用多shot 方法采集 k 空间。因此，需要使用单次 shot MR 标记序列，该序列需要标记创建、针移动和标记采集。本研究在体模和志愿者实验中展示了单次 shot MR 标记在 MRI 引导的针介入治疗中的可行性。
在 1.5T MR 扫描仪上进行了四项不同的实验：第一项是量化平移，第二项是量化旋转，第三项是模拟针介入，第四项是在志愿者中研究标记的持久性。通过 1331 复合脉冲创建标记模式。使用平衡稳态自由进动序列进行成像。为了最小化对比度或标记模式清晰度的不期望变化，我们在所有实验中选择了相对较小的翻转角和较短的成像时间。在志愿者实验中，我们修改了序列，以便还能够检查所使用的 k 空间采样轮廓和翻转角对图像对比度和标记模式的时间持久性的影响。在所有扫描中，头部或表面线圈用于信号接收。
在所有实验中，标记模式都清晰可见，可以用于量化（针）运动引起的局部组织变形。实际和测量（角度）的 phantom 运动之间得到了很强的相关性。在针介入实验中，当没有针运动时，标记线是完全水平的。随着针的移动，观察到局部组织位移达 5mm。尽管存在标记模式，但仍可以区分志愿者的解剖结构。例如，在创建标记模式 2 秒后仍可以在所有标记图像中读取前列腺中的标记模式。使用优化的扫描参数，标记的持久性更长。尽管图像略有模糊，但使用大翻转角和低高轮廓顺序获得了最佳的图像标记对比度。
本研究表明，单次 shot MR 标记可用于量化针运动引起的组织变形。在体内，标记的持久性足以在患者的 MRI 引导的针介入治疗中应用标记序列。
