CT Number= K × (µ tissue – µ water)/ µ water

Where

µ tissue= Tissue linear attenuation coefficient.

µ water= water linear attenuation coefficient. The attenuation coefficient of water is 0.195.

K= Magnification constant that determines the scale Factor for the range of CT number.

( K = 1000).

CT Number=1000× (µ tissue – µ water)/ µ water

The pixel is displayed on the monitor as a level of brightness. Tje CT number is also called Hounsfield Units. The range of CT Number from -1000 To +3000. The CT Number of -1000 corresponding to air and CT number of +3000 corresponding to dense bone. The CT from +300 to +3000 corresponding to bone The CT number of 0 corresponding to water.

CT numbers are quantitative, and this property leads to more accurate diagnosis in some clinical settings. For example, pulmonary nodules that are calcified are typically benign, and the amount of calcification can be determined from the CT image based on the mean CT number of the nodule. Measuring the CT number of

a single pulmonary nodule is therefore common practice, and it is an important part of the diagnostic work-up. CT scanners measure bone density with good accuracy, and when phantoms are placed in the scan field along with the patient, quantitative CT techniques can be used to estimate bone density, which is useful in assessing fracture risk. CT is also quantitative in terms of linear dimensions, and therefore it can be used to accurately assess tumor volume or lesion diameter.

Substance		HU
Air		−1000
Fat		−120 to −90
Soft tissue on contrast CT		+100 to +300
Bone	Cancellous	+300 to +400
	Cortical	+500 to +1900
Subdural hematoma	First hours	+75 to +100
	After 3 days	+65 to +85
	After 10–14 days	+35 to +40
Other blood	Unclotted	+13 to +50
	Clotted	+50 to +75
Pleural effusion	Transudate	+2 to +15
	Exudate	+4 to +33
Other fluids	Chyle	−30
	Water	0
	Urine	−5 to +15
	Bile	−5 to +15
	CSF	+15
	Abscess / Pus	0[ or +20, to +40 or +45
	Mucus	0[ – 130[ (“high attenuating” at over 70 HU)
Parenchyma	Lung	−700 to −600
	Kidney	+20 to +45
	Liver	60 ± 6
	Lymph nodes	+10 to +20
	Muscle	+35 to +55
	Thymus	+20 to +40 in children +20 to +120 in adolescents
	White matter	+20 to +30
	Grey matter	+37 to +45
Gallstone	Cholesterol stone	+30 to +100
	Bilirubin stone	+90 to +120
Foreign body	Windowpane glass	+500
	Aluminum, tarmac, car window glass, bottle glass, and other rocks	+2,100 to +2,300
	Limestone	+2,800
	Copper	+14,000
	Silver	+17,000
	Steel	+20,000
	Gold, steel, and brass	+30,000 (upper measurable limit)