**7. Hounsfield unit (CT number)**

Computed Tomography (CT) works on the principle of amount of the X-ray energy absorbed. The amount of X-ray energy absorbed is proportional to the density of the body tissue. The computer generates a grayscale image, where the tissue density is indicated by shades of gray. The Hounsfield Unit (HU) is a relative quantitative measurement of radio density used in the interpretation of computed tomography images. The Hounsfield unit was named after Sir Godfrey Hounsfield, recipient of the Nobel Prize, for the invention of Computed Tomography (CT) [16]. It is proportional to the degree of x-ray attenuation and is defined as:

$$\mathbf{H} \mathbf{U}\_{\text{tissue}} = \left[ \left( \mu\_{\text{tissue}} - \mu\_{\text{water}} \right) / \mu\_{\text{water}} \right] \times \mathbf{1},000 \,\tag{1}$$

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where μ is the linear attenuation coefficient for water and tissue. On the Hounsfield scale, air is represented by a value of −1000 (black on the grayscale) and bone between +700 (cancellous bone) to +3000 (dense bone) (white on the grayscale). The linear attenuation coefficient is a function of both electron density and atomic number of the tissue within a pixel.
