Analysis of quantum noise-reducing filters on chest X-ray images: A review
Introduction
Automated medical image analysis is one of the challenging tasks due to inherent noise in medical imaging modalities, complex and overlapping anatomical structures, and vague appearance of disease manifestations. X-ray images are low-cost, globally accepted medical imaging modality used as a primary clinical adjunct for disease interpretation and accurate pathological decision making. Recent advancement in medical imaging technologies generates millimeter-scale resolution images to gain the insight of internal anatomical structures suitable for texture analysis used in computer-aided diagnosis (CAD) based image-guided therapy [1]. However, these images are predominantly corrupted from inherent Quantum noise, referred to as Photon noise, Poisson noise, or Shot noise [2]. The quantum noise creates random abrasive artifacts (shot noise) similar to normal micro-textural structures and disease manifestations, which significantly degrades the discriminative attributes in radiography images [3]. The signal-dependent characteristic of quantum noise has different influences at varying radiation levels (photon concentration). It is negatively correlated with the square root of the radiation exposure of the receptor plate and thus, quantum noise can be reduced by increasing the X-ray exposure time and radiation dose [4]. However, the prolonged exposure to high-dose X-ray radiation is hazardous to patients that may cause serious health issues, especially in the case of pediatric radiology [5]. Low dose X-ray imaging is a recent advancement in radiological imaging technique to prevent the hazards from exposure to high radiation level [5], but this technique suffers from amplified quantum noise and poor image quality due to the low photon flux density. Besides quantum noise, the electrical components and the structure of the films, intensifiers, or digital receptor plates generate a small amount of Gaussian or structural noise, respectively. However, with the advancement in medical imaging modalities, the effect of these noise has reduced [6], [7].
Image de-noising techniques are often preferred over using high dose X-ray radiation to subside the effect of quantum noise and to reduce health issues [8]. Quantum noise reduction in low-dose X-ray imaging has been widely researched, and various image restoration approaches in spatial as well as in frequency domain have been proposed over the past few years [2], [3], [9]. However, there is a trade-off between noise reduction and information loss (due to smoothing), which necessitates the selection of an efficient filtering algorithm. This paper aims to investigate the various benchmark filters to select an appropriate filter that effectively reduces the quantum noise while preserving important texture information. The quantitative and statistical evaluation of benchmark filters and its de-noising performance are investigated and compared based on disease classification accuracy using de-noised chest X-ray (CXR) images. This study ranks the filtering algorithm based on classification accuracy. Furthermore, the ability of the filters to preserve image quality without sacrificing the noise suppression is analyzed by two expert radiologists and based on traditional image quality metrics described in Table 4. The effectiveness of de-noising filters in terms of classification accuracy is validated with the ground truth data.
The next section presents an overview of techniques and the inclusion of signal-dependent quantum noise during the image acquisition process.
Quantum noise is signal-dependent and is predominant (since inherited from capturing device) in low contrast X-ray imaging modality, which gets added during image acquisition [10]. X-ray images are generated by passing the collimated X-ray beams through the patient body area to be imaged, and the attenuated beams are captured by a receptor plate on the other side, creating bone and soft-tissue shadow image as shown in Fig. 1(a). The random impinge (striking) of attenuated X-ray photons to the receptor plate for a given time interval causing uneven distribution of photons over the receptor’s surface (Fig. 1(b)) leads to irregular scene artifacts resulting in quantum noise.
The amount of quantum noise is inversely proportional to radiation exposure (photon concentration) and can be approximately expressed as the receptor. The noise decreases as the number of photons increases and if the number of photons used is quadrupled, the noise in the resultant image gets halved [11]. The random temporal scattering of photons over the receptor plate can be modeled by the standard Poisson distribution process [4], [12]. The probability distribution of photons captured on the receptor plate over a given time interval is described in Eq. (1).where, is the probability distribution of photons, λ is the expected number of photons, is the measured number of photons and is the given time interval.
The quantum noise follows the Poisson process where the expected photon count is equal to the variance of photon count over given timeinterval as described in Eq. (2) which represents the signal dependent nature of noise. Moreover, the noise grows proportionately with the square root of the number of photons captured by the receptor plate, as described in Eq. (3).
Fig. 2(a) represents a normal CXR image without noise and Fig. 2(b) represents a CXR image after the inclusion of synthetically generated Poisson noise using library function from MATLAB 2016a1. Since the quantum noise is not an additive or multiplicative type of noise, therefore the amount of noise included is entirely dependent on the input image pixel intensity. From the analysis of zoomed patch 1 and 2, as shown in Fig. 2(c) and 2(d), respectively, it is observed that the Poisson noise (or shot noise) creates a coarse appearance overlapped with the normal anatomical structure, which is challenging to analyze the micro-textural features on the CXR image accurately.
The significant contribution of this study are summarized as follows:
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Analyzed the performance of various benchmark filters for reducing quantum noise in CXR images.
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Implemented segmented lung ROI confined feature extraction technique on CXR images. Further classification accuracy is used as a measure to rank the performance of the de-noising filters.
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Quantitative, qualitative, subjective, and statistical analysis of different de-noising filters.
The rest of the paper is organized as follows. Section 2 presents a review of existing approaches for the elimination of quantum noise. In Section 3, the materials and methods used in this study are elaborated. Section 4 presents the quantitative, qualitative, and statistical analysis and their comparisons with benchmarked filters. The paper is concluded with some future directions in Section 5.
Section snippets
Review of quantum noise filtering approaches
This section elaborates on the quantum noise removal techniques used in this study. Most of the medical imaging modality suffers from noise [11], whose sources and characteristics are different. Filtering radiographic images reduces noise, restore corrupted texture information, and help to improve the expert’s diagnostic accuracy and robustness of automated CAD systems.
A filter is a squared kernel or mask used for smoothing noise by convolving the kernel window centering to each pixel
Materials and methods
In this section, we present the methodological design for experimentation and comparison of benchmark techniques. Fig. 3 depicts the overall roadmap of the methodology used in this study. All the experiments were implemented in MATLAB 2016a software. This study uses three evaluation methods (quantitative, qualitative, and statistical) to come up with an efficient quantum noise-reducing filter, which suppresses the noise while preserving the micro-textural pattern. The qualitative assessment was
Experimental results analysis and discussion
This section presents the comparative analysis of the various set of experimentation and results obtained from edge-aware quantum noise filtering techniques. The detailed discussion of the observed results is presented in the following subsections.
Conclusion
This study presented an extensive study of different benchmark filters, which are pervasively used for de-noising medical images. The objective was to introduce an efficient de-noising filter, which can preserve the diagnostic texture patterns of disease responses while effectively reducing the quantum noise in chest X-ray images. The tradeoff between texture preserving and de-noising performance of the filters were evaluated using quantitative, qualitative, subjective, and statistical
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank Dr. Satyabhuwan Singh Netam, Head, Department of Radiodiagnosis and Dr. Deepak Jain, Radiologist, Department of Radiodiagnosis, Pt. JawaharLal Nehru Memorial Medical College, Raipur (C.G.), India for their valuable guidance and suggestions.
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