Problem with 2D fftshift

Hello, everyone,

I hope you can help me

i have a problem with 2d fftshift.

I have to get the noise power spectrum from 18 Rois. I use fftshift but in the center of the picture are higher frequencies that don't belong. as far as i know there are zero frequencies in the middle. can someone help me with this problem.

My Code:---------------------------------------------------------------------------------------------------

clear all

close all

S = load('matlab.mat');

b= cell2mat(struct2cell(S));

[l m n]=size(b);

mm =mean(b,3);

figure(1)

subplot(1,3,1)

imshow(mm,'DisplayRange',[]);title('Original Image');

for i=1:n

spe1(:,:,i)= fftshift(fftn(b(:,:,i)-mm));

spe(:,:,i)=(abs(spe1(:,:,i)).^2);

end

w = (mean(spe,3));

subplot(1,3,2)

imshow(w,'DisplayRange',[]);

title('DFT');

%Radial average

[XX, YY] = meshgrid( (1:m)-m/2, (1:m)-m/2);

R = sqrt(XX.^2 + YY.^2);

profile = [];

for i = 1:1:(m+1)

mask = (i-1<R & R<i+1);

values = w(mask);

profile(i)= mean( values(:) );

end

%Frequenz

frequency=(1/((m)*0.68));

sumb=0:frequency:(((m))*frequency);

subplot(1,3,3)

plot(sumb, profile, '-')

xlabel('Spatial Frquency mm^-1')

ylabel('Noise power spectrum HU^2 mm^2')