Robust border enhancement and detection for measurement of fetal nuchal translucency in ultrasound images
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- Lee, Y., Kim, M. & Kim, M. Med Bio Eng Comput (2007) 45: 1143. doi:10.1007/s11517-007-0225-7
Ultrasonic measurement of nuchal translucency (NT) thickness in the first trimester of pregnancy has recently been proposed as the most useful marker in early screening for fetal chromosomal abnormalities. However, manual tracing of the two echogenic lines in the image, using on-screen calipers, is hampered by weak edges, together with noise and other artifacts, leading to variable results and inefficiency. Our semi-automatic method of fetal NT thickness measurement uses a coherence-enhancing diffusion filter to enhance the border and reduce noise, followed by detection of the NT by minimization of a cost function, that combines intensity, edge strength and continuity, using dynamic programming. This method has been validated by determining the correlation between manual and semi-automatic measurements.
KeywordsFetal nuchal translucencyEdge-enhancing filteringBorder detectionDynamic programmingSemi-automatic measurement
Ultrasonography is performed during early pregnancy for dating, determination of the number of fetuses, assessment of early complications, and increasingly for evaluation of the fetus, including measurement of the nuchal translucency (NT) thickness . Measurement of NT thickness in the first trimester of pregnancy has proved to be one of the most discriminating prenatal markers in screening for chromosomal abnormalities such as trisomies 13, 18 and 21 . Furthermore, an increased NT thickness (>2.5 mm) between 10 and 14 weeks has also been associated with an increased risk of congenital heart and genetic syndromes [9, 15, 19].
The NT thickness is measured in the sagittal section of the fetus by transabdominal or transvaginal ultrasound examination. To determine the NT thickness, most physicians trace the boundaries of the NT layer manually, but this is time-consuming, and the results vary according to the training, experience, and subjective judgment of the observer.
Ultrasound images frequently include weak echoes, echo dropouts, and speckle noise, which make feature extraction, analysis and quantitative measurement difficult and unreliable. Therefore, techniques for the suppression of noise and for feature enhancement have been developed to improve the accuracy and reliability of segmentation. Commonly used linear low-pass filters, such as the mean filter and Gaussian filter, are not suitable for reducing the speckle noise of ultrasound images since they eliminate the high frequencies and thus tend to smooth out edges in the image . The median filter is arguably the most popular nonlinear filter, and is well suited to the reduction of speckle-type structures, but it too has limited success in preserving structural features such as the weak, diffuse edges that characterize ultrasound images. Perona and Malik  have proposed anisotropic filtering, based on the diffusion theory, which is better able to distinguish regional structures, but can sometimes blur thin structures. It is possible to enhance thin edges by considering the local coherence of structures, and Weickert  has proposed a coherence-enhancing diffusion (CED) filter, which reduces speckle noise using a structure tensor, while improving the image contrast.
To avoid the problems inherent in manual measurements, many automated techniques have been proposed. Bernardino et al.  developed a semi-automatic measurement system, which uses the Sobel operator to detect the border of the NT layer. However, well-known edge-detection techniques such as the Sobel operator or the Canny’s method determine the location of an edge by local evaluation of a single image feature such as intensity or the intensity gradient. But no single image feature can provide reliable border measurement in fetal ultrasound images.
In their work on ultrasonic images of the carotid artery, Cheng et al.  suggested an automatic system for detecting the boundaries of the intima-media complex based on the snake. The snake is very sensitive to the initial contour. If the initial contour is placed far away from the boundary of interest then the snake will not be attracted. The method still has limitation that the starting and ending points of the initial contour must be defined by user interaction. Loizou et al.  proposed a snake segmentation method, which utilizes normalization and speckle reduction as preprocessing. This method proposed an initialization procedure for improving snake initialization. However, the procedure needs many steps such as ROI definition, filtering, Otsu’s thresholding, dilation, labeling connecting components, etc., a discussion about irregular boundaries is missing. A computerized method to automatically detect the boundaries of intima-media using dynamic programming (DP) was proposed [8, 10, 17], which takes multiple-image features into account. By considering multiple-image features, such as intensity, gradient and border continuity, this method has obtained accurate and reproducible automated ultrasonic measurements. However, the system did not consider speckle-reduction filtering for dealing with poor images, which have a high level of speckle noise and weak edges and also require some manual correction.
In this paper, we will present a semi-automatic detection procedure for measuring NT thickness based on DP [8, 10, 17] improved by a nonlinear anisotropic diffusion filtering. We start by preprocessing an image, defining a region of interest in order to reduce interference from the image boundary and to adapt to different fetal head positions and sizes of NT layer. At this stage we also apply coherence-enhancing diffusion filtering, which uses an understanding at the structural features of NT images to enhance the NT region. This filtering improves the difficulty of border segmentation due to the speckle noises and weak edges in ultrasound images of fetus NT. In the main segmentation step, we use a DP procedure to determine the location of the NT border by minimizing a cost function, which is a weighted sum of terms which expresses local measurements of the echo intensity and the intensity gradient of the image, and a geometrical constraint on the shape of the NT borders.
The rest of this paper is organized as follows: in Sect. 2, we describe our image-acquisition techniques, the characteristics of NT images and the preprocessing stage that defines the region of interest and show how we enhance the image using a CED filter. We also describe the segmentation procedure that we use to detect the border of the NT layer, and our measurement procedure. The results from this method are evaluated in Sect. 3, and we draw some discussions in Sect. 4.
2 Materials and methods
Fetal nuchal translucency images were acquired using an Accuvix XQ (Medison Co. Ltd, South Korea) ultrasound scanner with a convex 2–6 and 4–7 MHz transducer. They were saved in DICOM format using the SonoView software, which bundled with the scanner.
2.1.1 Image characteristics
2.1.2 The region of interest
Nuchal translucency images can differ widely because of fetal movement during a scan of several minutes. Therefore, we need to define a region of interest (ROI) which reduces interference by the image boundary and which can compensate for changes in the fetal head position and the size of the NT layer. Since our ultimate purpose is to measure the thickness of the widest part of the NT layer, the user creates a rectangular region of interest that is sure to include the appropriate part of the layer.
2.1.3 Coherence-enhancing diffusion
We now apply a CED filter, which emphasizes very thin structures within an NT image, even with a high level of noise, allowing us to detect the border accurately. CED filtering enhances the borders of the NT region while blurring the area inside the NT layer. CED filtering is based on the diffusion theory, which requires the solution of a partial differential equation (PDE). Locally coherent structures make the diffusion process more directional in both the gradient and the contour directions . This is why CED filtering is appropriate for enhancing thin structures such as blood vessels or the fetus NT.
2.2 Border detection
The accurate segmentation of the border is never going to be easy since almost all ultrasound images have a high level of speckle noise, other imaging artifacts and weak edges due to random scattering. We address this problem using a global optimization approach based on DP. We will now describe the segmentation method in detail.
2.2.1 Cost function
The terms in the cost function associated with a particular type of border must reflect the characteristics of the image features in its vertical neighborhood and the geometrical form of the border. These cost terms are therefore defined such that a stronger image feature at pi will yield a lower local cost. The desired border corresponds to the BN which minimizes the cost function C(BN).
We will now describe the cost terms used in the cost function for B2. The cost terms for B1 are similar to those for B2 except for the value of weighting factors.
The first term f1(pi) measures the average intensity of n (here n = 3) pixels below a pixel pi, with the aim of detecting a pixel that belongs to a line above a strong echo zone Z4. The second term f2(pi) measures the average intensity of m (here m = 2) pixels above a pixel pi, and favors a pixel that belongs to a line immediately below a dark NT space Z3. The third term f3(pi) measures the downward intensity gradient which is expected at the upper edge of an echo zone Z4. The intensity gradient is estimated as the vertical slope of intensity at the pixel pi using a vertical gradient operator [1 0 −1]T instead of a 5 × 5 neighborhood window used in [8, 10, 17]. The final cost term g(pi−1, pi) is proportional to the square of the difference in vertical distance between the border being estimated and a reference line at node pi; this term is designed to ensure border continuity. In the case of B2, g(pi−1, pi) is calculated with respect to a horizontal reference line. In estimating the cost term g(pi−1, pi) for B1, the reference line is the border B2 that has already been detected.
2.2.2 Dynamic programming
Dynamic programming, which is typically applied to optimization problems, can be used for border detection. The optimum border minimizes the cost function C(BN). The search for a solution must be performed globally in the region of interest so as to avoid local minima due to interference patterns such as speckles.
We will now describe the cost accumulation and back-tracking process in detail within an M × N ROI. A vertical window of height M is used to scan the border from the start to the end-point at N horizontal positions. The locations of the start and the end-point are the upper-left and upper-right pixels of the ROI, respectively, and correspond to the left and right arrows in Fig. 6b. At each column n, a candidate minimum cost function is found for each point, and its cost is accumulated. By scanning all the columns, the lowest accumulated cost is located. The position of the minimum-cost point becomes the end-point of the border. Then back-tracking is performed, following the pointers, until the first column in the ROI is again reached. This creates our estimate of the border. B2 can be identified relatively easily since the nearer echogenic zone Z4 is stronger than Z2. Therefore we find B2 first. We then search for B1 using a smoothed version of B2 as the reference line, which forms a lower limit in the search for B1.
The algorithm that we have described in this paper was implemented in Visual C++ on a Pentium IV with 1 GB of RAM.
Comparison between semi-automatic and manual methods
Semi-automatic measurement, μa ± σa (mm)
Manual measurement, μm ± σm (mm)
2.81 ± 0.79
2.79 ± 0.77
We have proposed a semi-automatic method of measuring NT in fetal ultrasonic scans, which combines a preprocessing step based on a nonlinear anisotropic diffusion filtering and DP procedure. Our method overcomes the speckle noise and weak edges in ultrasound image by applying a CED filter, which enhances the borders of the NT while blurring the NT layer itself. The proposed segmentation method then detects the borders of the NT by constructing a cost function that includes weighted cost terms to represent multiple image features such as intensity, gradient and border continuity. In addition, we display the locations where measurements of maximum and minimum NT thickness were obtained.
In our study, we have obtained good results without requiring manual correction in most cases. In particular, our method has overcome the problem of border continuity, which occurs when weak edges are disrupted by the scattering inevitably present in ultrasound images.
Our semi-automated measurements were compared with manual measurements, and were found to be equally accurate. In any case an automated approach naturally reduces the problems of operator variability, and hence reproducibility, that are always present with manual measurements. However, the process of measuring NT thickness basically has two limitations. The first limitation is to select the appropriate scanning plane. It is inherent in two-dimensional measurements in ultrasonography since the imaging plane does not always coincide with the required plane, that is, the true mid-sagittal section of the fetus. Both automatic and manual measurements may thus produce underestimates or overestimates of the NT thickness. The second limitation relates to the method itself due to fetal orientations. The proposed method may be only applied when fetal position is as horizontal as possible. However, the limitation can be explained by one of the standards for measuring NT , which a mid-sagittal section of the fetus should be obtained with the fetus horizontal on the screen.
For overcoming the limitations, we will investigate techniques to select the optimum NT view from a 3D data set. In addition, for clinical purposes some interactive tools may still be needed in order to correct some residual detection errors in extremely poor images.
This work is financially supported by the Ministry of Education and Human Resources Development (MOE), the Ministry of Commerce, Industry and Energy (MOCIE) and the Ministry of Labor (MOLAB) through the fostering project of the Lab of Excellency, and by the Korea Institute of Science and Technology Evaluation and Planning (KISTEP), under the Real Time Molecular Imaging program. We would like to thank the Medison Co. Ltd. for providing ultrasound image datasets for our experiments.