Where does the formula for HU to density come from?

[eigenvivek]

I came across this function in the Volume code and was wondering where the piecewise linear formula came from? A quick lit review turned up a lot of different formulas to convert HU to density, so I was wondering if this function is specific to particular scanner / radiation dose. Thank you!

deepdrr/deepdrr/vol/volume.py

Lines 219 to 232 in 2c45632

	@staticmethod
	def _convert_hounsfield_to_density(hu_values: np.ndarray, smooth_air: bool = False):
	# Use two linear interpolations from data: (HU,g/cm^3)
	# use for lower HU: density = 0.001029*HU + 1.03
	# use for upper HU: density = 0.0005886*HU + 1.03
	# set air densities
	if smooth_air:
	hu_values[hu_values <= -900] = -1000
	# hu_values[hu_values > 600] = 5000;
	densities = np.maximum(
	np.minimum(0.001029 * hu_values + 1.030, 0.0005886 * hu_values + 1.03), 0
	)
	return densities

[NotF404]

Have you solved this issue, can you give some info about this formula.

I found that bone and soft tissue dont have enough contrast, when I project spine volume. So I wonder if I cant change this factor to make bone more clear in drr image.