Background and objective: Cardiotocography (CTG) is the most popular prenatal diagnostic examination, which includes continuous monitoring of foetal heart rate (FHR, bpm) and uterine contraction (UC, mmHg) signals. Compared with CTG paper reports, digitized reports have better storage, transmission and retrieval capabilities, in addition to being able to assess foetal health. However, most of the existing digitization methods extract signals from paper reports with colour background grids, and they cannot extract signals completely from paper reports with binary background grids, which are widely used in clinical CTG monitoring. Moreover, the existing digitization algorithms often neglect the image distortion caused by the imaging equipment.

Methods: To overcome the above drawbacks, a digitization method for CTG paper reports with binary background grids taken by smartphones is proposed in this paper. In the stage of removing the grid background, a region merger based on super-pixels and an improved binary line mask removal are designed. Then, signal extraction is performed separately according to the different states of the image column. Through a projection map used to synchronize the signal, the distortion effect of the mobile phone is removed.

Results: The experimental results show that the average correlation coefficient (ρ) between the recovery signal obtained by the proposed method and the reference signal is 0.9855±0.0108 for FHR and 0.9866 ± 0.1020 for UC, and the root mean square errors (RMSE) of FHR and UC processed by the proposed method are 1.0366 ± 0.4953 and 2.0355 ± 1.0246, and the mean absolute errors (MAE) of FHR and UC processed by the proposed method are 0.8735 ± 0.0684 and 1.4991 ± 0.2837, which are higher than those of the existing digitization methods. Compared with clinical signals, no significant difference is found in the feature of digitization CTG.

Conclusions: The proposed digitization method is a promising useful tool to realize the electronization of CTG signal.

背景与目的：心动描记法（CTG）是最受欢迎的产前诊断检查，其中包括连续监测胎儿心率（FHR，bpm）和子宫收缩（UC，mmHg）信号。与CTG纸质报告相比，数字化报告除了能够评估胎儿健康外，还具有更好的存储，传输和检索功能。然而，大多数现有的数字化方法从带有彩色背景网格的纸质报告中提取信号，并且它们不能从带有二进制背景网格的纸质报告中完全提取信号，这在临床CTG监测中得到了广泛的应用。而且，现有的数字化算法经常忽略由成像设备引起的图像失真。

方法：针对上述缺点，提出了一种智能手机拍摄的带有二进制背景网格的CTG纸质报告数字化方法。在去除网格背景的阶段，设计了一种基于超像素的区域合并和改进的二进制线掩模去除方法。然后，根据图像列的不同状态分别进行信号提取。通过用于同步信号的投影图，可以消除手机的失真效果。

结果：实验结果表明，该方法获得的恢复信号与参考信号之间的平均相关系数（ρ）对于FHR为0.9855±0.0108，对于UC为0.9866±0.1020，并且均方根误差（RMSE）为该方法处理后的FHR和UC分别为1.0366±0.4953和2.0355±1.0246，而该方法处理后的FHR和UC的平均绝对误差（MAE）为0.8735±0.0684和1.4991±0.2837，高于FHR和UC的平均绝对误差（MAE）。现有的数字化方法。与临床信号相比，数字化CTG的功能没有显着差异。

结论：提出的数字化方法是实现CTG信号电子化的有前途的有用工具。