Fast marching over the 2D Gabor magnitude domain for tongue body segmentation
© Cui et al.; licensee Springer. 2013
Received: 4 June 2013
Accepted: 12 December 2013
Published: 26 December 2013
Tongue body segmentation is a prerequisite to tongue image analysis and has recently received considerable attention. The existing tongue body segmentation methods usually involve two key steps: edge detection and active contour model (ACM)-based segmentation. However, conventional edge detectors cannot faithfully detect the contour of the tongue body, and the initialization of ACM suffers from the edge discontinuity problem. To address these issues, we proposed a novel tongue body segmentation method, GaborFM, which initializes ACM by performing fast marching over the two-dimensional (2D) Gabor magnitude domain of the tongue images. For the enhancement of the contour of the tongue body, we used the 2D Gabor magnitude-based detector. To cope with the edge discontinuity problem, the fast marching method was utilized to connect the discontinuous contour segments, resulting in a closed and continuous tongue body contour for subsequent ACM-based segmentation. Qualitative and quantitative results showed that GaborFM is superior to the other methods for tongue body segmentation.
KeywordsImage segmentation Tongue diagnosis Fast marching 2D Gabor filter Active contour model
Medical image segmentation, analysis, and diagnosis have received much attention in image analysis and computer vision. Several methods of segmentation have been proposed for medical images such as magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound medical imagery. Yezzi et al. proposed the geometric active contour model for segmentation of medical imagery . Tsai et al. proposed a shape-based method using level sets to segment the medical images . For prostate segmentation in transrectal ultrasound (TRUS), Yan et al. proposed a discrete deformable model . Graph cuts , proposed by Boykov et al., have also been used for the segmentation of medical images. As one part of medical image analysis and diagnosis, because of its convenience and non-invasion nature, tongue image diagnosis has received considerable research interest.
Tongue diagnosis has played an important role in traditional Chinese medicine (TCM) for thousands of years [5, 6]. The TCM practitioner can analyze the physiological and pathological conditions of the patient by inspecting the color, shape, and texture of the tongue, making tongue diagnosis very promising for convenient and non-invasive diagnosis. However, traditional tongue diagnosis is a subjective skill which requires years of experience and practice. Moreover, for different practitioners, the diagnosis results may be inconsistent.
To address these problems, computational tongue diagnosis was studied for the digital acquisition and quantitative analysis of tongue images [7–9]. Generally, the computational tongue diagnosis system involves three major modules: tongue body segmentation, feature extraction, syndromes and disease analysis [10–12]. Several imaging systems have been developed for the acquisition of tongue images [13–15]. A number of feature extraction methods have been proposed for extracting features of tongue color [16, 17], coating, and texture [18–21], fissures [22–25], shape [26, 27], tooth marks , petechia spots , and the sublingual vein [30, 31]. Pattern recognition approaches such as the Bayesian network and support vector machine have been used for syndromes and disease analysis [9, 32–34].
Among the modules of computational tongue diagnosis systems, tongue body segmentation is a prerequisite for subsequent feature extraction and diagnostic analysis. Tongue body segmentation usually involves three steps: edge enhancement, contour initialization, and segmentation. For edge enhancement, conventional edge detectors, such as the Gaussian gradient , color gradient , and Canny detector , have been used. Considering the shape and gray-level characteristics of tongue contour, Zuo et al. [10, 11] proposed a polar edge detector to suppress adverse interference from the lip boundary and tongue fissures. For contour initialization, several heuristic , deformable model-based , and watershed-based  methods have been suggested. For segmentation, active contour models (ACM) [11, 13, 35, 37] and the gradient vector flow (GVF) snake  have been used for the final segmentation of the tongue body [14, 39, 40].
Despite the progresses in tongue body segmentation, the existing methods usually suffer from some limitations and cannot meet the performance requirements of a practical computational tongue diagnosis system. For example, in the edge enhancement step, conventional edge detectors usually neglect the characteristics of gray-level variation along the tongue body contour. In the contour initialization step, parts of the tongue body contour might exhibit very weak edges, and the detected contour would be discontinuous, making it hard to obtain a continuous contour for initialization. Moreover, the interference from lips and tongue fissures makes tongue body segmentation more challenging.
Edge enhancement is fundamental for any edge-based active contour and level set based segmentation method. Thus, the enhancement of the tongue body contour by the proposed 2D Gabor magnitude-based edge detection method was first studied. There are several region-based models, such as color-texture segmentations [38, 41] and the C-V model and its extensions , but, tongue texture is very complex and varies with images, so this study used the edge-based model and first investigated the edge enhancement problem.
In the contour initialization stage, the stable segment selection and the fast marching method for contour initialization were used. This initialization strategy allowed the incorporation the various tongue priors to obtain a satisfactory initial contour. For example, segments with less length were abandoned. Considering the symmetrical characteristics of the tongue body contour, we selected pairs of segments with better symmetry. Moreover, to avoid the interference of lips and to remove the shadow parts, two paths were found for any two points. If the difference in length L F of the two paths was low, the inner curve was chosen, otherwise the one with the lower length was chosen.
The contour initialization strategy allowed the employment of many types of tongue priors, such as color, shape, and symmetry, to obtain a satisfactory initial contour, and the GVF snake was able to obtain a local optimal and smooth segmentation results. Geodesic active contours [43, 44] can be used to address the limited capture range problem of the conventional snake model. Most globally optimal active contours [45–48] can obtain the global optimal solution based on the energy function, which generally is only a non-ideal separation of the tongue body from the other parts. Several globally optimal active contours [49, 50], are similar to the fast marching method used in GaborFM, and can obtain the global optimal path to connect the two points. Thus, we did not use the methods for global minimization of the active contour model.
By taking the characteristics of gray-level variation of the real tongue body contour into account, the 2D Gabor magnitude-based detector is more effective than the conventional edge detector for the enhancement of the tongue body contour and the suppression of interference from lip and tongue fissures. For this reason, the 2D Gabor magnitude-based edge detection method was used to address the edge enhancement problem.
To circumvent the edge discontinuity problem, we used fast marching over the 2D Gabor magnitude domain method to connect the discontinuous contour segments into a closed tongue body contour. First, several stable segments were selected based on the edge enhancement result. Then, the contour initialization problem was modeled as the minimal geodesic paths over the 2D Gabor magnitude domain. Finally, the fast marching algorithm was used to obtain a closed tongue body contour for initialization.
The remainder of the paper is organized as follows. Section 2 presents the 2D Gabor magnitude-based edge detector for the enhancement of the tongue body contour. Section 3 describes the scheme for contour initialization and tongue body segmentation. Section 4 provides the qualitative and quantitative results to evaluate the proposed GaborFM method. Finally, Section 5 offers the conclusion.
2. 2D Gabor magnitude-based edge detection
In this section, a 2D Gabor magnitude-based edge detector for the enhancement of the tongue body contour is proposed. It then utilizes the color characteristics of the tongue body to suppress interference from the tongue texture and fissures, and finally uses Otsu's method for edge thresholding.
2.1 2D Gabor magnitude-based detector
where x′ = (x − x 0)cos θ + (y − y 0)sin θ, y′ = (x − x 0)sin θ + (y − y 0)cos θ, (x 0 , y 0) is the center of the function, ω is the radial frequency in radians per unit length, and θ is the orientation of the Gabor functions in radians. The κ is defined by , where δ is the half-amplitude bandwidth of the frequency response, which is between 1 and 1.5 octaves according to neurophysiological findings . When ω and δ are fixed, σ can be derived from σ = κ/ω.
where ‘¯’ denotes the complex conjugate operator. Generally, any profiles in Figure 2b would result in local maxima in M max(x, y). Moreover, the promising performance of the Gabor filter on noise robustness and time-frequency tradeoff makes the proposed 2D Gabor magnitude-based detector suitable for the enhancement of the tongue boundary.
2.2 Edge thresholding
We used a two-step strategy for edge thresholding. First, based on the color characteristics of the tongue body and face, parts of the non-boundary pixels were identified. Then, the binarization of the edge image was obtained by defining the proper threshold.
Unlike the background, due to physiological and pathological factors, the color of the tongue body varies with tongue images. Fortunately, it is relatively easy to distinguish the color of tongue body from that of other facial parts: the pixel values of the tongue body component usually have higher Q value. Let T 1 denote the threshold obtained by Otsu's method. We set the threshold T Q = 2 T 1 and set the pixels with a Q value higher than T Q as tongue body pixels. Morphological operators, mentioned above, were then used to define the tongue body region. A subset of the background and tongue body components was then obtained. As shown in Figure 4b, by assigning these pixels as non-boundary pixels, a modified edge image can be derived for edge binarization.
After edge thresholding, the morphological operators were employed to obtain single-pixel edge curves, resulting in the final binarized edge image shown in Figure 4c.
3. Tongue body segmentation using fast marching and the GVF snake
In this section, first several stable segments from the binarized edge image are selected, and then the fast marching algorithm is used to obtain a closed contour for initialization. Finally, the GVF snake model is adopted for tongue body segmentation.
3.1 Selection of stable segments
Although the 2D Gabor magnitude-based detector is effective, false detection of the tongue body boundary cannot be completely avoided. Moreover, it is also difficult to detect the entire closed tongue body contour by only using the 2D Gabor magnitude-based detector. Fortunately, the location and shape of the tongue body are useful in distinguishing a true boundary from a false boundary. Thus, to suppress the adverse influence of false detection, we used the strategy of selecting stable segments of the tongue body contour.
By referring to the location and shape of the tongue body, the following approach for selecting several stable segments from the binarized edge image is suggested. First, the mean of the coordinate of all the edge pixels and the mean of the coordinate of all the tongue body pixels obtained in Section 2.2 were computed. Then, the center of the tongue body was estimated as the average of and .
3.2 Contour initialization using fast marching
where C is the set of curves with γ(0) = (x A , y A ) and γ(1) = (x C , y C ).
with T A (x A , y A ) = 0, where T A (x, y) denotes the shortest distance L F of (x A, y A) and (x, y). The fast marching methoda was used to solve the Eikonal equation to obtain T A (x, y). The computational complexity of fast marching is O(N logN), where N = mn is the size of the tongue image. Moreover, fast marching stops when the point (x C, y C) is reached, so T A (x, y) can be more efficiently obtained.
with the initial value X t = 0 = (x C , y C ). This ODE problem can be efficiently solved by using second-order Heun's method .
In practice, to avoid the interference of the lips, we found two paths for any two points. If the difference in length L F of the two paths was low, the inner curve was chosen, otherwise the one with the lower length was chosen. Figures 1e and 5c show two examples of the shortest paths constructed by the fast marching method. Satisfactory initialization of the tongue body contours can be obtained by using the fast marching method.
3.3 Gradient vector flow snake
where α and β are weighting parameters that control the snake's tension and rigidity, respectively, x ' (s) and x ' ' (s) denote the first and second derivatives of x(s) with respect to s, and E ext is the external energy.
where | ⋅ | denotes the l 2 norm with . For fast GVF computation, the augmented Lagrangian method (ALM)-based algorithm developed by our group was used .
For tongue body segmentation, μ = 0.1, α = 0.05, and β = 0.01 were chosen.
4. Experimental results
4.1 Database and evaluation criteria
We constructed a tongue image data set of 300 imagesc to evaluate the proposed method. Manual segmentation results were used as the ground truth. All the images were acquired by our tongue image acquisition device. The image size was 768 × 576. Since the images were captured in a semi-enclosed environment under stable lighting conditions, it was not necessary to use any pre-processing method for illumination normalization. All the experiments were executed on a PC with T2250 @1.73 GHz CPU and 2G memory.
4.2 Components of GaborFM
We compared the edge enhancement results obtained using the Sobel, DoG, and 2D Gabor magnitude-based edge detector, as shown in Figure 3. It can be seen that the proposed 2D Gabor magnitude-based detector can faithfully enhance the tongue body contour and is more robust against inference from tongue texture. Thus, compared with Sobel and DoG, the 2D Gabor magnitude-based method is more effective for the enhancement of the tongue body contour.
Gabor + FM: we only used the first two steps (Gabor magnitude-based edge detection and fast marching) for tongue body segmentation and used the contour after fast marching as the contour of the tongue body.
Canny + FM + GVF: we replaced the Gabor magnitude-based edge detection with the Canny detector.
Gabor + FM + GVF: we used all the three steps, GaborFM, for tongue body segmentation.
The HD, MD, and SD of Canny + FM + GVF, Gabor + FM, and GaborFM
Canny + FM + GVF
33.85 ± 39.44
6.83 ± 8.88
9.20 ± 13.69
Gabor + FM
19.15 ± 12.31
4.28 ± 2.18
4.20 ± 3.04
Gabor + FM + GVF
17.79 ± 11.87
4.26 ± 2.17
3.98 ± 2.99
4.3 Comparison with the other segmentation methods
Average CPU time of methods
Average CPU time (s)
Based on the evaluation results above, it can be seen that GaborFM is better than BEDC, Watershed, and PolarSnake for tongue body segmentation. From Table 3, the proposed GaborFM method can achieve the HD, MD distance, and SD of 17.79, 4.26, and 3.98 pixels, respectively. From Table 4, the proposed method can achieve the FPVF and FNVF of 0.96% and 5.44%, respectively. Moreover, the qualitative and quantitative evaluation results also show that GaborFM satisfies the practical performance requirements of tongue body segmentation and can be embedded into a real computational tongue diagnosis system.
In this paper, we proposed a novel method, GaborFM, for automated tongue body segmentation. First, a Gabor magnitude-based detector was developed for edge enhancement. Second, both the color characteristics and the edge enhancement result for the thresholding of the edge image were taken into account. Third, stable segments were selected from the binarized edge image and use the fast marching algorithm over 2D Gabor magnitude domain to obtain a continuous closed curve for contour initialization. Finally, the GVF snake was used for tongue body segmentation. Generally, the proposed method can well address the edge enhancement and the contour discontinuity problems. Experimental results also showed that the proposed method is more effective than other tongue body segmentation methods, i.e., BEDC , Watershed , and PolarSnake .
aTo solve the Eikonal equation, we used the Matlab Toolbox Fast Marching provided by Dr. Gabriel Peyre, which is available to the public at http://www.mathworks.com/matlabcentral/fileexchange/6110-toolbox-fast-marching.
bThe code of GVF snake was provided by the authors of , which is available to the public at http://www.iacl.ece.jhu.edu/static/gvf/. Our Matlab code on ALM-based GVF computation has been released at https://sites.google.com/site/cswmzuo/IALM-GVF_GGVF.rar.
cWe will soon make the tongue image dataset together with the ground truth segmentation results available to the public.
dWe will soon make the tongue image dataset together with the ground truth segmentation results available to the public.
The work is partially supported by the GRF fund from the HKSAR Government, the central fund from the Hong Kong Polytechnic University, the NSFC funds of China (grant nos. 61001037, 61071179, 61271093, and 61102037), and the Fundamental Research Funds for the Central Universities (grant no. HIT.NSRIF.2010051). The authors would like to thank the associate editor and the anonymous reviewers for their constructive suggestions. Thanks to Dr. Edward C. Mignot, Shandong University, for linguistic advice.
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