Denoising Scientific Images: The NLM Image Noise reRuction Technique (Term Paper Sample)
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The NLM Image Noise reduction Technique
Image denoising is widely used in different industries and sectors such as remote-sensing images, medical diagnostic images, and non-destructive test images in the industrial sector . With the speedy development of contemporary medical services like nuclear magnetic resonance images and CT scan images, the quality of these images is crucial in diagnosing the patient's condition. As such, effective image denoising techniques in scientific research are significant. The NLM is an excellent technique for denoising magnetic resonance images to enhance diagnostic accuracy. It is popular since it has a simple algorithm and good noise reduction performance while maintaining the edges.
The non-local means technique uses non-local filters to remove different noise components by comparing the area with the noisy picture component with the surrounding region to get a weight and consequently filter through the use of an average value and a Gaussian kernel. In the NLM technique, areas with the exact size of the mask within the region of interest are set to help identify pixel similarities. If the degree of similitude is high, then a higher allotted weight is used in the processing of images.
The principle of the NLM model
The basic principle in this algorithm is images contain lots of unnecessary information, since there are several similar blocks distributed across the whole pixel. While bilateral filtering algorithm, Gaussian filtering, median or mean and average filtering algorithm, anisotropic diffusion, and other standard filtering algorithms can eliminate image noise, the image geometry is affected, and the image structure information is not retained. The NLM acknowledges that the image pixels are not the sole components; an image structure also includes a combination of other individual pixels in the neighborhood to form this structure.
Although they are in different positions, the greyscale information is quite similar. In this denoising method, the current noise is located and calculated, and then the weighted average is used in recovering the value of the image to be restored (Li & Suen 241). The basic function of the non-local mean technique is represented as:
NLM I(m) =nϵNMw(Nm, Nn)I(n) [w(Nm, Nn) =( 1Z(m)e ℎ ²)⁽−d⁾] ! ,
I(n) denotes the noise component’s structure in the nth pixel
Nm indicates the pixel’s neighborhood
w (Nm, Nn) is a function, established on the weighted similarity
Z(m) illustrates the normalization constant
d illustrates the Euclidean distance
Different weight operations tend to have varying impacts on the pixel noise reduction effect. However, the key concept in the NLM estimate is calculating in a point-wise manner by defining the weights and weighted mean as the proximity measure between the pixel patches used. The estimation can be simplified as the minimization of the weighted squared residual criterion (Li & Suen 241). For instance, if the vitiated image contains a gray value of h(i) at pixel i and a clear gray estimate of NL (h) (i). To obtain the filtered NL (h) (i) for any pixel i, will require a calculation of the weighted average of the pixels in the whole image with indistinguishable surroundings.
NL (ℎ) (i)=j∈i ⩊(i,j) ℎ (j)
In this case, i represents the whole image space; ⩊(i,j) refers to the weighting factor and represents the level of influence of pixel j on i. Because this technique requires comprehensive computation, averaging the pixels in a smaller scope is more recommended. Also, the content of the weight variable in the image and the degree of noise in the actual image are related. Hence, restricting the noise elimination relative to the application of fixed variables is recommended (Gupta et al., 7). Even though the original definition considers linking each pixel’s intensity to that of the entire image, practical computation considerations require that the pixels used in the average or mean can be restricted to the neighborhood search window that is centered at the pixel.
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