Automatic Conformal Anti-radial Ultrasound Scanning for Whole Breast Screening
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Anti-radial ultrasound scanning is one of the main scanning approaches used in ultrasound breast screening. It can be used for cross-sectional imaging of mammary ductal/lobular tissue and provide information about suspicious tissue. It has certain advantages compared to linear scanning, but automatic anti-radial scanning is not yet available. Our goal is to propose an ultrasound scanning system for whole breast anti-radial scanning.
We previously developed an automatic ultrasound scanning system for whole breast screening involving linear scanning. The present study builds on our previous work by incorporating (1) surface-reconstruction algorithms, (2) a rotatable holder design, and (3) a scan-path-smoothing algorithm to achieve conformal anti-radial scanning.
An improvement of approximately 40% in the normal-vector estimation is obtained with our new method, and the scan stability is improved by the scan-path-smoothing algorithm. 3D volume data of each scan are available.
We have successfully developed an automatic ultrasound scanning system for anti-radial breast scanning. This type of system, which has not been reported previously in the literature, can be an effective tool for fully automatic ultrasound breast screening.
KeywordsAnti-radial Scan Surface reconstruction Scan-path smoothing Ultrasound conformal scanning
Ultrasound is an important imaging modality for breast screening due to several advantages. For example, it is noninvasive, free of ionizing radiation, and can be performed in real time. In addition, it has good contrast compared to mammograms in dense breast [1, 2, 3, 4, 5, 6]. Clinically, three hand-held scanning approaches are commonly used: linear (also called raster), radial, and anti-radial scanning . The linear scan is the most common way to obtain the sagittal and transverse planes of the breast. Nonetheless, the cross-sectional plane of the mammary duct, which is clinically important, is also important and should be obtained by using anti-radial scanning [7, 8, 9, 10].
Since whole breast ultrasound screening is performed manually by radiologists and sonographers, the obtained results are highly dependent on the experience of the operators. To reduce the influence of human factors in ultrasound examinations, robot-assisted systems have been designed [11, 12, 13, 14]. Automated breast ultrasound screening (ABUS) and automated whole breast ultrasound (AWBU)  are robotic medical systems that are specified designed for breast screening. Based on the system designs, breast ultrasound can be performed either semiautomatically or guided by a moving structure. On contrast to ABUS and AWBU, the iVu system  was developed for breast screening using radial scanning. Despite the importance of fitting the body shape, none of the existing systems focus on an adaptive scan path: a predetermined curved scan pattern is chosen by ABUS and iVu, and the orientation of the transducer in AWBU is handled and guided by the operator. The functionality of the available commercialized systems for breast screening is limited by these predetermined scan trajectories.
1.1 Goals of This Study
2 Materials and Methods
A newly designed transducer holder was introduced, and the surface reconstruction and scan-path-smoothing algorithms were proposed for better approximating the target surface and to improve the scan stability, respectively. The new system allows the 3D volume of the scanned ultrasound images to be generated. The system performance was evaluated both using simulations and by scanning a breast phantom.
2.1 Rotatable Transducer Holder for Anti-radial Scanning
2.2 Breast Phantom
A breast phantom (CIRS Model 052A, Norfolk, VA, US) designed to mimic the shape of the breast and the properties of amorphous lesions was used to evaluate the performance of the proposed system both using simulations and by scanning a breast phantom.
In our previous work we employed temporal median/mean filters to reduce the noise level of the 3D camera. The corresponding normal vector (estimated by principal-components analysis) and linear scan path (based on the desired conditions) were generated based on the detected surface. However, the effects of noise could not be effectively reduced, and the estimated normal vector exhibited a standard deviation of ~ 1.8° to 6.4°, which resulted in unnecessary movement of the robotic arm during scanning.
Algorithm development in the present study focused on (1) improving the scanning stability by using a better surface smoothing/approximation method, (2) generating the anti-radial scan path adapted to the target surface, and (3) visualizing the scan result with 3D volume data.
3 Spatial Data Smoothing for Target Surface Reconstruction
In this study we used a bilateral filter to address the noise issue, and a 2D B-spline was used to estimate the pixel values that could not be precisely detected.
4 Principal Component Analysis for Normal-Vector Estimation
5 Generation of the Anti-radial Scan Path
6 Model-Based Scan-Path Smoothing
7 3D Volume Data Reconstruction
8 Performance Evaluation Results
8.1 Surface Reconstruction
MSE/standard-deviation values for different filter configurations
Flat surface (mm)
Spherical surface (mm)
Bilateral filter + 2D B-spline
8.2 Improvement in Normal-Vector Estimation
MSE/standard-deviation values for the estimations of normal vectors
Estimated error, degrees
Bilateral filter + 2D B-spline
8.3 Model-Based Scan-Path-Smoothing Algorithm
8.4 Anti-radial Scanning and 3D Volume Data Reconstruction
We have successfully performed anti-radial scanning, improved the scan stability, and visualized the scan results in 3D volume representations using our modified system. However, there are some limitations that cannot be overcome using with the current prototype.
9.1 Limitations of the Current System Setup for Anti-radial Scanning
An additional rotation, as allowed by the modified transducer holder, minimized the requirement for a posture change in the anti-radial scan. Nonetheless, additional 100-mm and 250-mm extensions along the x- and z-axes were unavoidable because of the complexity of the holder design. Also, the effective scan region is restricted to within a circular region with a radius of less than 50 mm.
Choosing the appropriate scanning speed is important to obtain the optimal trade-off between the spatial resolution and the posture changes of the robot arm. By scanning at 5 mm/s, a spatial resolution of 0.67 mm can be obtained, but results in a processing time of around 3.5 min. Increasing the scan speed will reduce the scan accuracy (spatial resolution) and scan stability (due to larger changes in the posture of the robot arm). For processing at a higher scan speed, an ultrasound system with a higher frame acquisition rate (e.g., 100 fps) is suggested. On the other hand, processing with a smaller transducer holder can reduce the required posture changes of the robot arm while scanning. In addition, the algorithm for determining the scan region in anti-radial scanning was not addressed in this study due to the large variation between examinees. To avoid any improper determinations, manually selecting the scan region is suggested.
9.2 Performance of the Proposed Algorithms
The accuracy of our proposed method was tested using ideal flat and spherical surfaces. The results show that the proposed algorithm can reduce the standard deviation of the estimated error and improve the smoothness of the estimated normal vector. The proposed filter configuration (bilateral filter + 2D B-spline) reduced the estimated errors by 50%, which is the key point for smoothing the reconstructed surface. Based on the smoothed reconstructed surface, the principle-components analysis method can provide the estimated normal vector with a lower angle standard deviation (reduced from 2.51° to 1.51°, as indicated in Table 2). The reconstruction results also show that the proposed system can smoothly move across the breast phantom surface without unnecessary disturbances.
9.3 Deformation of the Breast Phantom
Due to deformation of the breast phantom, differences in the geometry between the reconstructed surface and the actual images are unavoidable. This deformation issue was alleviated in our study in two ways. First, our general setup includes a contact force sensor to make sure that the ultrasound transducer establishes good contact with the skin while avoiding applying an excessive force as often seen during manual scanning. In other words, our use of a force sensor minimized the deformation. Second, the image object (i.e., breast) can be easily confined and fixed during the scanning by using a large membrane covering the entire breast. This is already used in ABUS by applying a large transducer holder and a scanning frame on the breast. A similar scanning frame can also be used in our setup to avoid the deformation problem. The data presented in Tables 1 and 2 indicate the good accuracy (submillimeter) that can be achieved with our proposed approach.
This study developed a prototype of an automatic breast ultrasound screening system for anti-radial scanning. Such an automatic system is clinically important, and this is the first report of such a system in the literature. A modified transducer holder consisting of a stepping motor and rotating structure was designed to adjust the orientation of the transducer. The surface-model spatial-data-smoothing algorithm provided a better approximation of the target surface. The standard deviation of the surface reconstruction was effectively reduced (by 50–80%) in the simulations of flat and spherical surfaces, while an improvement of 35–40% in angle accuracy was also achieved. Our model-based scan-path-smoothing algorithm further optimized the suggested contact angle based on the shape of the breast, and this was verified in breast phantom scans. Six-direction anti-radial scanning was performed within a circular region (with a radius of 50 mm) of the breast phantom, and the overall processing time was approximately 3.5 min. The 3D volume reconstruction of each scan path was obtained, and the quality of the reconstruction was verified in comparisons of actual ultrasound images obtained from the breast phantom.
This study was funded by Ministry of Science and Technology, Taiwan (MOST 103-2221-E-002-016-MY3).
Compliance with Ethical Standards
Conflict of interest
The authors declare that they have no conflict of interest.
All procedures performed in the human scanning study were in accordance with the ethical standards of the institution.
Informed consent was obtained from each participant included in the study.
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