Two images with different sizes to same sizes

I am very new to the program, so please excuse me if my questions are silly. I always try to solve them on my own. If my questions were already asked then I did not find them. Sorry if you feel like I am wasting your time. That is not my intention. (Yes, I use this introduction for my other questions, as I did not want to ask them all in the same post since it would be difficult to categorize)
I have an image (a vertebra) taken in different ways: one has a high resolution and the other one a low (different machines). The one with the high resolution has the scale given (0.041 mm pixel width and height) but the other one does not.
I would like to have the vertebra in the center of each image also having the same size. Can you help me do this?
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I am a medical engineering student. I do not know much about programing. Until now I have survived by try and error and copying from different forums. Still I would like to understand the answer, but in the worst case, I’ll be happy just with the solution. If you can give me some reading recommendations I will happily read them and work my way through ImageJ.
In any case, THANK YOU

Hello Diego_Hens,
Well you certainly ask a broad question.
Any image is only a matrix of numbers X,Y and if 3D Z, This is basically a mathematics question which you would apply to an image.
To start at the beginning you need to study and understand matrix math.
It can be done but many variables need to be taken into account. There is pixel count per unit, pixel size per unit area and etc. Once you understand the math you will understand how to apply it to images.
I wish you well, Good Luck


Welcome to the forum.

Some ImageJ commands that may help you (if you don’t already know them).

Image> Scale...
resamples the image, changing the physical pixel/voxel spacing and preserves the field of view of the image.

Image > Crop (after making a rectangle selection)
preserves the pixel spacing and changes (shrinks) the field of view.


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If you do not have an object of know size in each image (or the image already calibrated from the acquisition step), you could try measuring manually the size of the vertebra from the same locations in both images with the line tool to then work out the ratio of sizes, and use the image resize command.


Thank you for your answers :smiley:
@smith_robertj I understand matrix math, but I don’t know how to use it here. I mean, I could do a matrix with the variables you mentioned, but I wouldn’t know what to do with it… I’m lacking the connection between university matrix problems and real life matrix problems:sweat_smile:
@bogovicj Now I get why the crop command was so weird!:joy: I thought it was to cut out a part of the image but it would’nt work. I will use the scale command in combination with @gabriel’s idea of the line tool. It’s good enough, I’m very happy.
Thank you so much!

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