How to improve pixel resolution in an image?

Hi all

I have a 951*951 matrix of image after reconstruction. I want to resize them into 512*512 in Matlab.

At the same time, the pixel resolution should be changed to about 2 pixels/mm,which at the moment is 1 pixel /mm (i.e., width of image= 512mm snd ht- 512mm).

I tried with 'imresize'-I could change the size only. Is it possible to reduce the width & height of the image to be 250 mm while maintaining the same pixel numbers (512*512). Is there any way to do it? Also I wonder whether 'imresize' will distort the image quality( I mean making the image more softer)

Answer 1

[Image Analyst]

If you have a 951 wide image with 1 mm per pixel, then the field of view is 951 mm. Now if you reduce this to 512 pixels, the field of view is still the same - 951 mm - so the resolution is now 951 mm / 512 pixels = 1.86 mm per pixel.

If you want 250 mm, then you need to crop out a 250x250 mm chunk from the 951x951 image. So use imcrop to extract a rectangle of that size from the desired location. Once you've done that, you can resize from 250 by 250 pixels to 512 x 512 pixels. Your field of view will stay 250 mm since you are still looking at the same stuff, but it will just have a different image matrix size (512x512). The image may look softer since you are more than doubling the size of a fixed 250x250 mm subimage.

[Kavitha Srinivasan]

Thanks for your answer. But my problem is, I cannot crop out a region from the existing image. Because the entire 951*951 image is required (whole information) for analysis and for future appplications. What could be done if that is the case?

[Image Analyst]

Nothing - the math just doesn't work out. Your entire image is 951 mm wide and there is no way to get the whole field of view and still get a field of view of 250 mm. The field of view is NOT 250 - it's 951.

[Kavitha Srinivasan]

Hi when I resize the image to 512*512 (as dicom image),I got the image information as pixel density of 1*1.i.e., 1 pixel per mm. Width: 512 pixels Height: 512 pixels Pixel size: 1x1 pixel My understanding is, although I reduced the pixel values from 951*951 to 512*512, the number of pixels /mm remains the same. Is it possible to increase or decrease the pixel density by any means? with Matlab? Pls let me know.

[Image Analyst]

No. The only way the pixels per mm could remain the same is if you crop the image, not resize it. I don't know how you got "the image information" but if it still says that there is 1 pixel per millimeter both before and after sizing, then the only way that could be true is by cropping, NOT by resampling the same field of view with fewer pixels (512). Or if it did resize but failed to update the "image information" - wherever it's keeping that. When it says that the Pixel size is 1x1 pixel, well that's pretty redundant - of course it's that! But that's not saying anything about the spatial calibration in pixels per mm.

Of course you can "increase or decrease the pixel density" - just use imresize. But you must realize that doing that changes the spatial calibration in pixels per mm. Not sure why you're having a hard time realizing this. Let's take an example. Let's say that you are aiming your camera out the window at a distant cityscape. Let's say that the camera has 1000 pixels across and that the field of view is 1000 meters. So the resolution is 1 pixel per meter. Now let's say you resize that to a 10 pixel by 10 pixel image that still covers the same 1000 meter field of view. Now how much distance does each pixel cover? 100 meters right? 1000 meter with 10 pixels means each pixel covers 100 meters. It's not one meter for each pixel anymore, don't you agree? Now let's crop. let's say you want 10 pixels by 10 pixels with still 1 pixel per meter. How about if you took the upper left corner so that the image doesn't cover as much. It covers only 10 meters of your distant cityscape scene. But it's 1 meter per pixel. So you can't have it both ways. When you resize or crop SOMETHING changes.

[Kavitha Srinivasan]

Thanks for your clear explanation. I got it now.

[Image Analyst]

You're welcome - go ahead and mark it as "Answered". FYI, you might be interested in the demo code I posted here: http://www.mathworks.com/matlabcentral/answers/56087#answer_67994

Answer 2

[Walter Roberson]

The height and the width of the image as displayed depends on the pixel size of the display. To display a 1 pixel / mm image at 2 pixel / mm, use a higher resolution screen (or possibly use the same screen in a higher resolution mode, if it will support that.)

For example, you could take the image over to an Apple Retina display, which is 326 pixels per inch (about 12.8 pixels per mm)