Estimation of Texture Variation in Malaria Diagnosis
Malaria parasite has been visually inspected from the Giemsa-stained blood smear image using light microscope. The trained technicians are needed to screen the malaria from the microscope; this manual inspection requires more time. To reduce the problems in manual inspection, nowadays pathologist moves to the digital image visual inspection. The computer-aided microscopic image examination will improve the consistency in detection, and even a semiskilled laboratory technician can be employed for diagnosis. Most of the computer-aided malaria parasite detection consists of four stages namely preprocessing of blood smear images, segmentation of infected erythrocyte, extracting the features, detection of parasite and classification of the parasites. Feature extraction is one of the vital stages to detect and classify the parasite. To carry out feature extraction, geometric, color, and texture-based features are extracted for identifying the infected erythrocyte. Among these clause of features, texture might be considered as a very fine feature, and it provides the characteristics of smoothness over the region of interest using the spatial distribution of intensity. The proposed work demonstrates the merit of the texture feature in digital pathology which is prone to vary with respect to change in image brightness. In microscope, brightness of the image could be altered by iris aperture diameter and illumination intensity control knob. However, the existing literature failed to mention the details about these illumination controlling parameters. So the obtained texture feature may not be considered as distinct feature. In this paper, we conducted an experiment to bring out the deviation of texture feature values by changing the brightness of the acquired image by varying the intensity control knob.
KeywordsMalaria diagnosis Image brightness Gray-level co-occurrence matrix Digital pathology Digital microscopy
1 Introduction and Background
1.1 Image Acquisition
It is the process of collecting malaria blood smear images from the digital camera mounted on the microscope. The stained smears are observed from microscope with the magnification of 100× objective lens. For computer-aided malaria parasite detection, thick and thin blood smear images are used. Images obtained from thin blood smear are used to identify the type of malaria and its severity stages , whereas thick blood smear is used for quantifying the infected erythrocytes . Though several dyes have been used to stain erythrocyte, popularly used dyes are Giemsa and Leishman .
1.2 Preprocessing of Blood Smear Images
It is the process of removing the unnecessary details present in the acquired malaria parasite image for better visualization and for further analysis. In the existing literature, image filters like median , geometric mean , Gaussian , Laplacian , wiener , low pass , SUSAN  were used to eliminate the noise. And to enhance the contrast adaptive or local histogram equalization , dark stretching , partial contrast stretching algorithm, and histogram matching  are used. Most of the literature suggested Gray World Assumption could be the ideal choice, because it is used to eliminate the variation in the brightness of the image .
1.3 Segmentation of Infected Erythrocyte
It is a process of isolating the red blood cell (erythrocyte) and eliminating the other details such as white blood cell, platelet, and other artifacts from the preprocessed image. The isolated segment may include infected and non-infected malaria parasites. Further, to discriminate the infected and non-infected erythrocytes, finer segmentation techniques have been used. From the previous studies, we listed the various segmentation techniques utilized for isolating erythrocytes. They are circle Hough transform , Otsu threshold , pulse-coupled neural network , rule-based algorithm , Chan-Vese algorithm , granulometry , edge detection algorithm , Marker-controlled watershed algorithm, and normalized cut algorithm [4, 5]. In most of the literature, watershed transformation algorithm was used to segment the erythrocytes. Similarly to discriminate the infected erythrocytes, few techniques are used such as histogram threshold, moving k-means clustering, fuzzy rule-based system, annular ring ratio method, Zack threshold, and N-cut algorithm.
1.4 Extracting the Features
It is the important step to gather the insights from the erythrocytes. And it is a set of process for deriving size, color, stippling, surface, and pigment features of the infected erythrocyte for subsequent human interpretation to detect the infected malaria parasite. From the segmented erythrocytes, we can extract the coarse-level features like size and color. The size of infected region is being inferred from the efficient image-based models and its area estimation technique [19, 20]. Area and color are first-level eliminating features to differentiate the species in the malaria parasite. It is a vital feature to differentiate the species in the malaria parasite. The existing literature refers some morphological features, which are retrieved using Hu moment , Chain code, Bending energy, Roundness ratio , Relative shape , Shape features [5, 6, 10, 14] and Area granulometry . From the previous literature, the features used to extract the intensity of the malaria images are histogram, color histogram, entropy, color channel intensity, and color autocorrelogram. Among all, texture is a finer feature used by most of research fraternity to discriminate infected and non-infected malaria. It provides the spatial distribution of intensity over the region of interest and gives the quantitative evaluation of the images. Some texture features like gray-level co-occurrence matrix (GLCM), gray-level run-length matrix (GLRLM), gray-level size-zone matrix (GLSZM), local binary pattern, flat texture, gradient texture, Laplacian texture, and wavelet feature  has been used in previous studies.
1.5 Detection and Classification of the Parasites
It is the process of extracting the information about infected parasite and categorizing its types. The various classification algorithms used in the literature are Bayesian classifier , feedforward backpropagation neural network [4, 10, 21], classification and regression tree , K-nearest neighbor classifier [9, 14, 16], logistic regression , AdaBoost algorithm , Naïve Bayes tree , support vector machine [3, 6, 7, 9, 17], and multilayer perception network [5, 9].
2 Motivation and Challenges
In microscope, image brightness variation occurs due to the change in the magnification, iris aperture diaphragm, and illumination intensity control knob. As per the WHO standard, the magnification of the malaria blood smear is fixed to 100× objective lens. Hence, the variation of the brightness can occur only because of the manual adjustments of the iris aperture diaphragm and illumination intensity control knob. Moreover, theory of the light microscope defines that brightness depends on the amount of light passing through the condenser controlled via the iris aperture. For low magnification factor (eg.10x), iris aperture is tuned to higher diameter, inversely for the higher magnification, iris aperture is small. It gathers optimum light so we will get bright images. But tuning the illumination intensity controller only makes the variation of brightness in the image. We tried to observe the impact of this brightness variation on the global texture feature.
Image texture provides information about the spatial arrangement of intensities of an image. These texture features are used in the digital pathology to detect or classify the malaria parasites. The characteristics of the texture features behave differently for infected and non-infected erythrocyte regions. So the texture descriptor plays an important role in identifying the malaria parasite. However, when extracting the infected region texture properties, the nearest region property also influences the texture feature . These artifacts influence the definite variation in the texture feature metric. We observed that especially in image-based malaria diagnosis, the previous computer-aided microscopic researches ignored to incorporate this variation in their formulations. From this proposed work, we tried to bring out the existence of non-uniformity in texture descriptor.
In this session, we describe the experiment which has been conducted by the authors. The purpose of this experiment is to identify the variation in texture values of malaria blood smear images related to its brightness. Texture is one of the important features to improve the sensitivity and specificity of image based malaria detection techniques. We considered the global features of the image rather than segmented erythrocyte. Here, we used Olympus CX21i bright field microscope to view the malaria blood smear images. And Canon EOS1200D digital camera was used to digitize the captured view of microscope. The blood smear is examined with the total magnification of 10× in ocular lens and 100× in objective lens. The procedure is divided into four steps namely smear collection, microscope setup, focusing, and image acquisition with varied brightness, which have been explained below.
3.1 Smear Collection
Giemsa staining primarily uses two solutions: eosin and methylene. Eosin helps to change the parasite nucleus into red, and methylene will change the cytoplasm into blue. WHO recommends Giemsa staining is the reliable method to detect the malaria parasite for early case detection of malaria disease. Therefore, we have collected few Giemsa-stained blood smears infected with P. falciparum- and P. vivax from Tagore Medical College & Hospital, Chennai, India. The pathologist works for Tagore Medical College & Hospital had labeled the infected regions in the acquired image and these labeled samples were used to evaluate the efficiency of the proposed experiment.
3.2 Microscope Setup
Before proceeding to the required microscopic adjustment, we need to properly clean the microscope. For cleaning the ocular lens, objective lens, and condenser lens, lens paper or clean cotton must be used. Raise the condenser knob to check the amount of light the condenser gathers into microscopic stage. Then, adjust the iris diaphragm to control the amount of light passing through condenser. In order to examine the slides with the magnification of 10× in ocular lens and 100× in objective lens, we need to rotate the nose piece and fix it.
To focus the specimen, we have to place the smear on the microscopic stage. Then, add a drop of liquid paraffin on the stained region of the smear to increase the resolving power of microscope. Now, move the microscopic stage in the upward direction so as to make a contact on the liquid paraffin with the objective lens. Ensure that the appropriate light has to pass through the stained region of the smear. Adjust the coarse adjustment knob, and look through the ocular eyepiece until the isolated erythrocyte gets visibility. Thereafter, tune the fine adjustment knob to focus the clear details of the erythrocyte image.
3.4 Image Acquisition with Varied Brightness
4 Performance Analysis
In this analysis, we try to figure out the variation of texture descriptor value for the same image with its six classes of brightness level as described in experiment section. We derived GLCM for every image and estimated the global texture feature like energy, contrast, and homogeneity from the constructed GLCM. The obtained texture feature value indicates the measure of closeness of the distribution of GLCM, the local intensity variation of GLCM, and uniformity of the image illumination distribution.
Though we have detected the variation of texture feature by varying the image brightness, we are in the process of categorizing the texture features which are sensitive and insensitive for malaria diagnosis. To normalize the variation of sensitive texture features, we are also in the process of formulating a model to nullify the texture variance.
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