Why does imshow display different image when I try to show a CT image?

Question

S C.Carl 2023-5-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1962554-why-does-imshow-display-different-image-when-i-try-to-show-a-ct-image

编辑： DGM 2023-5-14

Hi,

I want to show an original abdominal CT (Computed Tomography) image (which is uint16 bit ) with these codes;

im = imread('Slice_87.png');
imshow(im);

The original image and the figure shown by the above command are different.

For instance, the figure shown by the above command has a grayscale background. However, the background in the original image seems as black when I open it with ImageJ or paint.

Why are they different?.

How can I show the CT image as it is seen by ImageJ or paint?

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Answer 1

KALYAN ACHARJYA 2023-5-13

1
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1962554-why-does-imshow-display-different-image-when-i-try-to-show-a-ct-image#answer_1234164

在 MATLAB Online 中打开

Can you try this way-

[img,map] = imread('Slice_87.png');
imshow(img,map);

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DGM 2023-5-13

在 MATLAB Online 中打开

cmantifmk.png

If you're trying to apply the mask to the image, then the mask must be unit-scale. Unless the mask is binarized, it must also be floating-point. If it is required that the mask is floating point, then the image must also be floating point.

All throughout these conversions between classes, you either rescale your data according to the class, or you need to keep track of your data scale and make sure that it doesn't cause problems (e.g. truncation). If you're using uint8(), you aren't rescaling anything, and it's not possible for me to infer what the actual data scaling is, so chances are your data scaling is inappropriate and it's resulting in severe truncation.

inpict = imread('cameraman.tif'); % uint8-scale uint8

mask = imread('cmantifmk.png'); % uint8-scale uint8

% multiply two uint8-scale images

% nothing left but the mask due to severe truncation

outpict1 = mask.*inpict;

imshow(outpict1)

% multiply a unit-scale uint8 mask with a uint8-scale image

% works, but will break if mask or inpict aren't uint8

% also won't work if mask is not binarized

outpict1 = mask/255.*inpict;

imshow(outpict1)

% multiply two unit-scale float images

% that works regardless of input class

% works for binarized masks and graduated masks

outpict3 = im2double(mask).*im2double(inpict);

outpict3 = im2uint8(outpict3); % cast _and rescale_

imshow(outpict3)

Simon Chan 2023-5-14

在 MATLAB Online 中打开

One possible way is use function getframe as follows:

I=imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1382669/wholeCTgray_87.png');
mask = imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1382664/MASK_87.png');
outpict3 = im2double(mask).*im2double(I);
outpict3 = im2uint16(outpict3);                         % Your original image is uint16
[minValue,maxValue] = bounds(outpict3(outpict3~=0));
imshow(outpict3,[minValue,maxValue]);
F = getframe(gca);
imwrite(F.cdata,'ScreenCapture.png');

DGM 2023-5-14

编辑：DGM 2023-5-14

在 MATLAB Online 中打开

Use mat2gray() to do the normalization instead of relying on figure capture and all the problems that causes.

I=imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1382669/wholeCTgray_87.png');

mask = imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1382664/MASK_87.png');

outpict3 = im2double(mask).*im2double(I); % multiply

[minValue,maxValue] = bounds(outpict3(outpict3~=0)); % get data range

outpict3 = mat2gray(outpict3,[minValue,maxValue]); % normalize to extrema

outpict3 = im2uint16(outpict3); % Your original image is uint16

imshow(outpict3); % now it's scaled as expected

imwrite(outpict3,'ScreenCapture.png');

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Why does imshow display different image when I try to show a CT image?

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Why does imshow display different image when I try to show a CT image?

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