How do I extract the intensity values of a 3D ROI of an image and plot them in a histogram?

Hi all,

I want to extract the index numbers (intensity value) of an image in a 3D ROI. I have the ROI coordinates (X,Y,Z) and the 3D PET image.

After I have read in the image:

%Define the X and Y coordinate of the 3D ROI: xi=3Droi(:,1); yi=3Droi(:,2); zi=3Droi(:,3);

% Look at the values of uptake in the 3Droi voxels pixelsWithinRoi = petImage(xi,yi,zi); *When I get to this point I am told that I run out of memory*** pixelsWithinRoi = petImage(xi,yi) works but don't think it gives the right voxels.

% Plot a histogram of these values hist(pixelsWithinRoi/threshold); % do I use imhist?

Thanks in advance for your time and help,

ojk