4D dicom matrix? Why not 3D?

I converted a mha file to dicom format (and got this file: https://www.dropbox.com/s/i0cr2910ie4k5zy/TumorSimOutput2_T1.dcm ) via a Linux utility program (this one: http://manpages.ubuntu.com/manpages/lucid/man1/gdcm2vtk.1.html ) and visualize it via imshow using threedimensional indexing:

    imshow(image_data(:,:,index),'DisplayRange',[]);

when I examine the dimensions of image_data I get:

256 256 1 181

I don't understand the purpose of this dimension of 1 element and why isn't required to specify it via imshow. Is this the gray level intensity? How can I check that?

Answer 1

[Walter Roberson]

Some MATLAB routines recognizes four-dimensional arrays as being arrays of images when the third dimension is 1 (grayscale) or 3 (RGB)

However, when you use

image_data(:,:,index)

on a 4 dimensional array, the result is defined to be the same as if you had specified

image_data(:,:,index,1)

As the final two dimensions are both 1, the result of this would be automatically collapsed in representation to two dimensions.

[Antonio]

Thanks, this clarifies my doubt. I realized that converting this grayscale dimension to RGB may be useful for my image processing task.

How can I do this conversion on this 4D Matrix?

Answer 2

[ChristianW]

"Is this the gray level intensity?" - Yes, color reserved dimensions.

It's mostly not about the gray scale. These values just reference to a color in the colormap.

[X, map] = dicomread('US-PAL-8-10x-echo.dcm'); 
montage(X, map, 'Size', [2 5]);