Line detection in noisy image (and no detection when it is not present)

Question

I have tried to extract the dark line inside very noisy images without success. Some tips?

My current steps for the first example:

1) Clahe: with clip_limit = 10 and grid_size = (8,8)

2) Box Filter: with size = (5,5)

3) Inverted Image: 255 - image

4) Threshold: when inverted_image < 64

UPDATE

I have performed some preprocessing steps to improve the quality of tested images. I adjusted my ROI mask to crop top and down (because they are low intensities) and added a illumination correction to see better the line. Follow below the current images:

Answer 1

Even though the images are noisy, you are only looking for straight lines towards the north of image. So, why don't use some kind of matched filter with morphological operations?

EDIT: I have modified it.

1) Use median filter along the x and y axis, and normalize the images. 2) Matched filter with all possible orientations of lines.

% im=imread('JwXON.png');
% im=imread('Fiy72.png');
% im=imread('Ya9AN.png');
im=imread('OcgaIt8.png');

imOrig=im;

matchesx = fl(im, 1);
matchesy = fl(im, 0);

matches = matchesx + matchesy;

[x, y] = find(matches);

figure(1);
imagesc(imOrig), axis image
hold on, plot(y, x, 'r.', 'MarkerSize',5)
colormap gray


%----------

function matches = fl(im, direc)

if size(im,3)~=1
    im = double(rgb2gray(im));
else
    im=double(im);
end
[n, m] = size(im);

mask = bwmorph(imfill(im>0,'holes'),'thin',10);
indNaN=find(im==0); im=255-im; im(indNaN)=0;

N = n - numel(find(im(:,ceil(m/2))==0));
N = ceil(N*0.8); % possible line length

% Normalize the image with median filter
if direc
    background= medfilt2(im,[1,30],'symmetric');
    thetas = 31:149;
else
    background= medfilt2(im,[30,1],'symmetric');
    thetas = [1:30 150:179];
end

normIm = im - background;
normIm(normIm<0)=0;

% initialize matched filter result
matches=im*0;

% search for different angles of lines
for theta=thetas
    normIm2 = imclose(normIm>0,strel('line',5,theta));
    normIm3 = imopen(normIm2>0,strel('line',N,theta));
    matches = matches + normIm3;
end

% eliminate false alarms
matches = imclose(matches,strel('disk',2));
matches = matches>3 & mask;
matches = bwareaopen(matches,100);