Brix Lau, Ringgaard Steffen, Sørensen Thomas Sangild, Poulsen Per Rugaard
Department of Procurement and Clinical Engineering, Region Midt, Olof Palmes Allé 15, 8200 Aarhus N, Denmark and MR Research Centre, Aarhus University Hospital, Skejby, Brendstrupgaardsvej 100, 8200 Aarhus N, Denmark.
MR Research Centre, Aarhus University Hospital, Skejby, Brendstrupgaardsvej 100, 8200 Aarhus N, Denmark.
Med Phys. 2014 Apr;41(4):042302. doi: 10.1118/1.4867859.
Combined magnetic resonance imaging (MRI) systems and linear accelerators for radiotherapy (MR-Linacs) are currently under development. MRI is noninvasive and nonionizing and can produce images with high soft tissue contrast. However, new tracking methods are required to obtain fast real-time spatial target localization. This study develops and evaluates a method for tracking three-dimensional (3D) respiratory liver motion in two-dimensional (2D) real-time MRI image series with high temporal and spatial resolution.
The proposed method for 3D tracking in 2D real-time MRI series has three steps: (1) Recording of a 3D MRI scan and selection of a blood vessel (or tumor) structure to be tracked in subsequent 2D MRI series. (2) Generation of a library of 2D image templates oriented parallel to the 2D MRI image series by reslicing and resampling the 3D MRI scan. (3) 3D tracking of the selected structure in each real-time 2D image by finding the template and template position that yield the highest normalized cross correlation coefficient with the image. Since the tracked structure has a known 3D position relative to each template, the selection and 2D localization of a specific template translates into quantification of both the through-plane and in-plane position of the structure. As a proof of principle, 3D tracking of liver blood vessel structures was performed in five healthy volunteers in two 5.4 Hz axial, sagittal, and coronal real-time 2D MRI series of 30 s duration. In each 2D MRI series, the 3D localization was carried out twice, using nonoverlapping template libraries, which resulted in a total of 12 estimated 3D trajectories per volunteer. Validation tests carried out to support the tracking algorithm included quantification of the breathing induced 3D liver motion and liver motion directionality for the volunteers, and comparison of 2D MRI estimated positions of a structure in a watermelon with the actual positions.
Axial, sagittal, and coronal 2D MRI series yielded 3D respiratory motion curves for all volunteers. The motion directionality and amplitude were very similar when measured directly as in-plane motion or estimated indirectly as through-plane motion. The mean peak-to-peak breathing amplitude was 1.6 mm (left-right), 11.0 mm (craniocaudal), and 2.5 mm (anterior-posterior). The position of the watermelon structure was estimated in 2D MRI images with a root-mean-square error of 0.52 mm (in-plane) and 0.87 mm (through-plane).
A method for 3D tracking in 2D MRI series was developed and demonstrated for liver tracking in volunteers. The method would allow real-time 3D localization with integrated MR-Linac systems.
用于放射治疗的联合磁共振成像（MRI）系统和直线加速器（MR-Linacs）目前正在研发中。MRI是非侵入性且非电离的，能够产生具有高软组织对比度的图像。然而，需要新的跟踪方法来实现快速实时的空间目标定位。本研究开发并评估了一种在具有高时间和空间分辨率的二维（2D）实时MRI图像序列中跟踪三维（3D）呼吸肝脏运动的方法。
所提出的在2D实时MRI序列中进行3D跟踪的方法有三个步骤：（1）记录一次3D MRI扫描，并选择在后续2D MRI序列中要跟踪的血管（或肿瘤）结构。（2）通过对3D MRI扫描进行重切片和重采样，生成与2D MRI图像序列平行的2D图像模板库。（3）通过找到与图像产生最高归一化互相关系数的模板和模板位置，在每个实时2D图像中对选定结构进行3D跟踪。由于被跟踪结构相对于每个模板具有已知的3D位置，特定模板的选择和2D定位转化为对该结构的平面内和平面外位置的量化。作为原理验证，在五名健康志愿者中，对持续30秒、频率为5.4 Hz的两个轴向、矢状面和冠状面实时2D MRI序列中的肝脏血管结构进行了3D跟踪。在每个2D MRI序列中，使用不重叠的模板库进行两次3D定位，这导致每名志愿者总共得到12条估计的3D轨迹。为支持跟踪算法而进行的验证测试包括对志愿者呼吸引起的3D肝脏运动和肝脏运动方向性进行量化，以及将西瓜中一个结构在2D MRI中估计的位置与实际位置进行比较。
轴向、矢状面和冠状面的2D MRI序列为所有志愿者生成了3D呼吸运动曲线。当直接测量为平面内运动或间接估计为平面外运动时，运动方向性和幅度非常相似。平均峰-峰呼吸幅度为1.6毫米（左右）、11.0毫米（头脚方向）和2.5毫米（前后方向）。在2D MRI图像中估计的西瓜结构位置的均方根误差为0.52毫米（平面内）和0.87毫米（平面外）。
开发了一种在2D MRI序列中进行3D跟踪的方法，并在志愿者肝脏跟踪中得到了验证。该方法将允许在集成的MR-Linac系统中进行实时3D定位。
