Abstract
In biomedical imaging using video microscopy, understanding large tissue structures at cellular and finer resolution poses many image acquisition challenges including limited field-of-view and tissue dynamics during imaging. Automated mosaicing or stitching of live tissue video microscopy enables the visualization and analysis of subtle morphological structures and large scale vessel network architecture in tissues like the mesentery. But mosacing can be challenging if there are deformable, motion-blurred, textureless, feature-poor frames. Feature-based methods perform poorly in such cases for the lack of distinctive keypoints. Standard single block correlation matching strategies might not provide robust registration due to deformable content. In addition, the panorama suffers if there is motion blur present in a sequence. To handle these challenges, we propose a novel algorithm, Deformable Normalized Cross Correlation (DNCC) image matching with RANSAC to establish robust registration. Besides, to produce seamless panorama from motion-blurred frames we present gradient blending method based on image edge information. The DNCC algorithm is applied on Frog Mesentery sequences. Our result is compared with PSS/AutoStitch [1, 2] to establish the efficiency and robustness of the proposed DNCC method.
Index Terms—: Biomedical, Mesentery, Cross Correlation, Registration, Mosaicing, Gradient blending
1. INTRODUCTION
Due to wide applications of biomedical imagery mosaicing, lots of research has been done on this field. Correlation based image registration and mosaicing has been explored for stitching of high-resolution microendoscope (HRME) [3], multispectral and hyperspectral data [4], chicken breast [5], etc. Some applications include Lukas Canady Tracker for feature selection for mosaicing of colon [6], bone, blood and lung [7]. In addition, structure propagation for mouse brain [8], phase correlation for breast tissue [9], SIFT for confocal microscopy in the oral cavity [10], etc. have also shown promising results. Lots of research has been reported on retinal images with different features such as covariance [11], Y-feature: where 3 vessels converge [12], mSIFT [13], UR-SIFT [14], etc.
In biomedical data, two main concerns are deformable tissues and moving cells. In addition, Mesentery images suffer from high motion blur, irregular motion, large illumination change, low contrast and low textured region. Thus, we avoid feature-based registration and choose to proceed with correlation based matching which performs comparatively better in poorly-featured images. Single block correlation matching based mosaicing has been explored in many mosaicing applications including biomedical and aerial imagery [3, 4, 5, 15, 16, 17, 18]. What makes our approach distinctive from others is that DNCC establishes 3×3 block matching between two images followed by RANSAC [19] to ensure robust registration. In addition, we propose image gradient or edge based blending algorithm for seamless blending of motion-blurred frames which is difficult to achieve from other traditional blending methods.
2. METHODS
Mesentery images were captured on an Olympus Inverted Microscope (IX70) using a 10x (numerical aperture 0.22) lens. Basically the image was projected on to a black and white CCD camera (Dage-MTI 72) projecting a field of view of 0.65mm × 0.78 mm. After observing the Mesentery sequences, we assume that the motion between any two consecutive frames is translation.
2.1. Correlation-Based Registration and Mosaicing
The block diagram for the proposed Deformable Normalized Cross Correlation (DNCC) based method is shown in Fig. 1 where two frames are registered by image correlation as shown in row C. For a block, Bx in Fi and search window, Bx+Δ in frame Fi−1, NCC is defined as below:
(1) |
where, is the local mean of Fi−1(Bx). The three main steps in our DNCC approach as seen in Fig. 1. The translation between two consecutive frames is estimated using NCC as illustrated in Row C of Fig. 1. Let the top-left corner of the template be (px, py) on frame, Fi. Similarity score or correlation coefficient matrix is computed between the template and frame, Fi−1 using normxcorr2 (Matlab function). The maximum value of correlation matrix corresponds for best matching or similarity between the frames. Let (Px, Py) be the position of maximum correlation score as shown in Fig. 1. Finally, the target translation parameters (tx, ty) are calculated using the equations shown in row, C.
We define the position (xi, yi) of ith frame, Fi on canvas as the coordinate of the top-left corner. Once translation (tx, ty) between the previous Fi−1 and current Fi frames is estimated, then using the previous frame translation, (xi−1, yi−1), the position of Fi on the canvas under a rigid translation model can be calculated as:
(2) |
2.2. Deformable Normalized Cross Correlation (DNCC)
Initially, we use a single template of size 150×100 from Fi and search for similarity in the whole image, Fi−1. The mosaic generated from this approach is greatly influenced by the templates. As template position, (px, py) is fixed along the sequence, the characteristics such as image contrast, deformability, illumination, motion blur, texture, etc., of the template is varied for each frame. Thus, the content of the template for some of the frames might not be informative enough for a robust NCC matching. This can be shown by the examples in Fig. 3 for two sets of templates, i.e., (px, py): (200, 100), and (100, 60). It is noticeable that for works well where for other sequences, (px, py) = (100, 60) provide accurate registration. This observation holds even if the size of template is varied.
Thus, to get robust matching, we propose to use 3×3 non-overlapping templates from frame, Fi as shown in Fig. 2. Each template block, Bk is matched within its corresponding search window in the previous frame, Fi−1 where k=1, 2, …, 9 for the nine blocks (3 × 3 group of blocks as shown in Fig. 2). Consequently, 9-sets of translation parameters, (tx(Bk), ty(Bk)) are returned. Some of (tx(Bk), ty(Bk)) pairs might be false matching due to deformability and motion of cells in Mesentery. Thus, RANSAC [19] is introduced to remove false matches. After the RANSAC outlier block elimination, K remaining pairs of translation parameters, (tx(Bk), ty(Bk)) are used to update the non-rigid local translations,
(3) |
where 〈·〉 is the averaging operator, tx is averaged over K blocks Bk. If K < 2, we do not consider it as a robust match and discard the frame, Fi and continue for the next iteration. Otherwise, the mean of tx(j) and ty(j) become the target translation values: and as demonstrated in Fig. 2. This method is defined as Deformable NCC (DNCC) as it can filter out incorrect matches due to deformable objects and motion blur with the application of RANSAC.
2.3. Gradient Blending of frame, Fi With Mosaic,
Though widely used, alpha blending (Eq. 4) does not perform well for Mesentery sequences. We have tested for different values of α such as α =1 (pixel filling), α =0 (replacement) and α =0.5 (averaging). None of these can overcome the blurriness present in the sequences. Thus, to obtain seamless blending in presence of highly motion-blurred frames, the gradient or edge information of two frames are compared as shown in Fig. 4. Gradient Response Ratio (GRR) is defined as the ratio of edge pixels in and Fi. For this purpose, we use canny edge which is computed from Matlab edge function. Finally, α in Eq. 4 becomes a function of GRR as presented in Eq. 5.
(4) |
(5) |
3. RESULTS
As no ground truth is known for Mesentery sequences, we generated 4 synthetic datasets (D1, D2, D3 and D4) each with 50 frames. To establish similar characteristics of Mesentery data, we added low, medium and high level of motion blur in 12 images of D2, D3 and D4. The performance of DNCC method is compared with Panoramic Stitching using SURF (PSS/AutoStitch) [1, 2]. We manually identified 10 strong keypoints in groundtruth mosaic and checked Distance Error (DE) in DNCC and PSS mosaic. DE is defined as the euclidean distance between a keypoint, gp(x, y) on groundtruth mosaic and corresponding keypoint on candidate mosaic cp(x, y). We found that DE always smaller in DNCC compared to PSS. Due to space limitation, DE values of 4 points are presented in Table 1. It is also notable that with the increase of motion blur, DE values increase exponentially in PSS where it remains linear (and really small) in DNCC which proves the robustness of our 3×3 block matching method with RANSAC. More importantly, for D4 dataset, PSS algorithm fails to detect and match feature for the lack of strong keypoints due to high motion blur where DNCC performs very well. Structural Similarity (SSIM) also proves the superiority of DNCC over PSS for the sythetic datasets. We compared Gradient blending with other blending methods: averaging, averaging with deblurring (ADB) and replacement or overlay. From Table 2, it can be concluded that Gradient blending outperforms others in comparing SSIM, Root Mean Square Error (RMSE) and Peak Signal to Noise Ratio (PSNR).
Table 1.
Data | Blur | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
DNCC | PSS | DNCC | PSS | DNCC | PSS | DNCC | PSS | DNCC | PSS | DNCC | PSS | ||
D1 | - | 1 | 0.96 | 0 | 3.5 | 0.22 | 0.42 | 0.56 | 0.47 | 0.56 | 46.8 | 0.27 | 0.35 |
D2 | low | 1 | 0.97 | 0.71 | 13.5 | 0.71 | 40.5 | 0.44 | 58.3 | 0.56 | 68.3 | 0.27 | 0.38 |
D3 | med. | 0.99 | 0.93 | 2.54 | 59.54 | 0.71 | 172 | 0.56 | 271 | 2.78 | 288 | 0.28 | 0.72 |
D4 | high | 0.99 | - | 2.54 | - | 1.58 | - | 0.57 | - | 2.78 | - | 0.28 | - |
Table 2.
Data | Blur | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Grad. | Avg. | ADB | Rep. | Grad. | Avg. | ADB | Rep. | Grad. | Avg. | ADB | Rep. | ||
D2 | low | 0.991 | 0.987 | 0.985 | 0.983 | 1.32 | 1.47 | 1.50 | 1.62 | 45.58 | 44.68 | 44.5 | 43.84 |
D3 | medium | 0.985 | 0.979 | 0.977 | 0.971 | 1.81 | 1.91 | 1.98 | 2.18 | 42.85 | 42.38 | 42.0 | 41.23 |
D4 | high | 0.971 | 0.960 | 0.953 | 0.931 | 2.46 | 2.89 | 3.07 | 3.84 | 40.14 | 38.73 | 38.2 | 36.25 |
Finally, our algorithm is tested on 3 sequences of Mesentery images. The stitching results is presented in Fig. 6. Average time per frame for these sequences are: 0.4029, 0.3526 and 0.3830 seconds. We tried to mosaic these sequences with PSS [1, 2]. PSS method fails (crashes) to match motion blurred frames at some point due to lack of strong keypoints as explained in Fig. 7. Additionally, we also implemented SURF-based Translation (ST) model which requires only 1 feature match between two frames. Though this method can mosaic a whole sequence, unfortunately, it comes with lots of misregistration as shown in Fig. 7. The summary of the experiments are presented in Table 3.
Table 3.
Data (#fr.) | |||||
---|---|---|---|---|---|
DNCC | ST | PSS | ST | PSS | |
(491) | 491 | 491 | 22 | matching | feature |
(770) | 770 | 770 | 81 | matching | feature |
(956) | 956 | 956 | 47 | matching | feature |
Fig. 5 compares Gradient blending with average blending with raw frames and deblurred frames. From this example, it is comprehensible that proposed Gradient blending outperforms alpha-blending even after the attempt of restoration of original image structure by deblurring [20, 21].
4. DISCUSSION
In this paper we present a mosaicing algorithm for stitching motion-blurred, low-textured and feature-poor Frog Mesentery sequences. The novelty of our algorithm is the introduction of 3×3 non-overlapping block matching with RANSAC for filtering out false correspondences. We also propose a gradient-based blending method that offers high quality blending in presence of motion-blurred frames.
Currently the algorithm uses fixed size search window for NCC which is unnecessary when motion is slow. For efficiency, we want to introduce adaptive search window according to pairwise image motion. In addition, we plan to introduce deep learning based correlation matching for deep registration. Beside, we would like to apply histogram matching for more color-uniform blending.
5. ACKNOWLEDGMENTS
We gratefully acknowledge the support of the U. S. National Institute of Neurological Disorders and Stroke award R01-NS110915, the U.S. Army Research Laboratory project W911NF-18-2-0285, the U. S. National Science Foundation grant MCB-1122130, and the Executive Womens Forum.
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