black pixels when save MRI dicom after applying lut

Question

I'm trying to apply LUT to MR images using following codes. the original dicom pixel intensities ranges from 0 to 4334 and the pixel intensities after applying LUT function ranges from 0 to 254. I need to preserve the metadata info for the resulted image. When I plot the resulted image, it displays correct but when I save the image, it is all black pixels. I know I need to change some meta data tags but I'm new to dicom image processing and couldn't figure out what is causing the problem.

def make_lut(dcm_data, width, center, p_i):
"""
LUT: look-up tables
VOI: volume of interest
"""
slope= 1.0
intercept= 0.0
min_px= int(np.amin(dcm_data))
max_px= int(np.amax(dcm_data))
lut= [0] * (max_px + 1)
invert= False
if p_i == "MONOCHROME1":
    invert= True
else:
    center = (max_px - min_px) - center
for px_value in range(min_px, max_px):
    lut_value = px_value * slope + intercept
    voi_value= (((lut_value - center) / width + 0.5) * 255.0)
    clamped_value= min(max(voi_value, 0), 255)
    if invert: 
        lut[px_value] = round(255 - clamped_value)
    else:
        lut[px_value] = round(clamped_value)
return lut

def apply_lut(pixels_in, lut):
pixels= pixels_in.flatten()
pixels_out= [0] * len(pixels)
for i in range (0, len(pixels)):
    pixel= pixels[i]
    if pixel > 0:
        pixels_out[i] = int(lut[pixel])
return np.reshape(pixels_out, (pixels_in.shape[0], pixels_in.shape[1]))

# apply the function
idx= 30
ds= pydicom.dcmread(dcm_files[idx])
raw_pixels= dcm_data.pixel_array
if dcm_data.WindowWidth != '' and dcm_data.WindowCenter != '':
    window_width = dcm_data.WindowWidth
    window_center = dcm_data.WindowCenter

lut = make_lut(raw_pixels, window_width, window_center, dcm_data.PhotometricInterpretation)
dcm_default_windowing = apply_lut(raw_pixels, lut)

# save the result
ds.PixelData = dcm_default_windowing.tobytes()
ds.save_as("test_luted.dcm")