256x256 medical image gives me 256x256x3

I need a 256x256 pixels image to use it in my project. I downloaded images from the internet, but when I read the image, it stored in a 256x256x3 variable (color image format PET images). the NSCT fusion algorthms gives me errors (dimension of matrices)

What is the reason for this and what can I do? here is the code to read the image

img1=imread('pet.jpg');

img2=imread('ct.jpg');

Answer 1

[KSSV]

You can convert it into gray image using rgb2gray.

I1 = rgb2gray(I) ;   % where I is RGB image 

[Majid Nullo]

Thank you, but I need the color image output not grayscale

[KSSV]

You can save the colormap using hsv. Read about it.

Also, you can send each channel seperately, process it and gather into 3D again. If this is the case.