Why iradon returns negative pixel values?

I took 200 projections at a step angle of 1.8 degrees using LabVIEW software. The size of the image is 2748 x 2748 pixels, unint16. Then using Matlab, I load the projection images, do the flat field correction, resize the image by 1/3 and save the images as .mat file. Then I run the algorithm below for the filtered backprojection.

 interp='linear'; %set interpolation: nearest, linear, spline, pchip, v5cubic

 filter='Hann'; %set filter: Ram-Lak, Shepp-Logan, Cosine, Hamming, Hann, None

 for s=1:916

    for i=1:200
    a(i,:)=proj065(:,s,i);
    end
    a=a';
    %figure(3), imagesc(a)

    b=iradon(a,1.8,interp,filter);

    imagesc(b);
    recon(:,:,s)=b;
    s
    clear a

 end

If I used a filter in this code, I will get negative pixel values.

But, if I run the code without the filter, I will get positive pixel values.

Any idea why iradon returns negative pixel values in filtered back projection?

Thank you.

Nurul