docs: def ct2mu(im):
      '''HU units to 511keV PET mu-values
          https://link.springer.com/content/pdf/10.1007%2Fs00259-002-0796-3.pdf
          C. Burger, et al., PET attenuation coefficients from CT images, 
      '''
  
      # convert nans to -1024 for the HU values only
      im[np.isnan(im)] = -1024
      # constants
      muwater  = 0.096
      mubone   = 0.172
      rhowater = 0.184
      rhobone  = 0.428
      uim = np.zeros(im.shape, dtype=np.float32)
      uim[im<=0] = muwater * ( 1+im[im<=0]*1e-3 )
      uim[im> 0] = muwater+im[im>0]*(rhowater*(mubone-muwater)/(1e3*(rhobone-rhowater)))
      # remove negative values
      uim[uim<0] = 0
      return uim