12bit to 8 bit and Windowing

Hallo,

I try to dicomread a 12bit-pic, change it to 8bit and windows it.

With

im = dicomread("CTA_Head.dcm");
im8 = uint8(im-1);
imshow(im8, [LOW,HIGH]);

Everything works fine. Matlab cuts all the values that aren't relevant and turns them into white or black.

But is there a function that does the same without directly displaying the picture?

When I use the following pictures do not look the same(it's darker):

    im = dicomread("CTA_Head.dcm");
    [nrows ncols] = size(im);
    for c = 1:ncols
        for r = 1:nrows
            if (im(r,c) < LOW)
               im2(r,c) = 0;
            elseif (im(r,c) > HIGH)
                im2(r,c) = 255;
            else
                im2(r,c) = im(r,c);
            end
        end    
    end
    im8er = uint8(im2);
    imshow(im8er);
    

Shall I cast to uint8 before or after cutting the values?

Thank You!

[Guillaume]

If im is 12-bit (integer I presume), this is a very odd thing to do:

im8 = uint8(im-1);

If the 12 bit is signed (range [-2048, 2047]), this rescale im to the range [-2049, 2046] (assuming the 12-bits are stored in a 16-bit integer since matlab doesn't have 12 bit type) which is then clipped to [0 255] by the conversion to uint8. So it maps all values between -2048 and 1 included to 0, maps 1-256 to 0-255 and maps 256+ to 255.

If the 12 bit is unsigned (range [0 4095]), this rescale im to [0 4094] which is then clipped to [0 255]. So it maps 0 and 1 to 0, 2-256 to 1-255 and 256+ to 255.

Answer 1

[Walter Roberson]

In older MATLAB see mat2gray, which has a deceptive function name and is not restricted to grayscale like the name suggests.

In newer MATLAB rescale()

[David Großmann]

Thank you, but sadly I'm not allowed to use these. So have you got a different idea?

[Walter Roberson]

scaled = (double(im) - LOW) ./ (HIGH - LOW);
im8er = im2uint8(scaled);

[David Großmann]

Wow thanks, this looks perfectly fine!