What Is CT Number or Hounsfield Units ?
The digital value describe to each pixel is called the CT number or Hounsfield Unit ( HU). The CT number calculate from the scanner data from the linear attenuation coefficient (µ) of each pixel. After reconstruction, the attenuation values of each pixel is normalized to that of water to gate integer values as follows:-
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 |
|
|
| 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) | |
Very interested my friend,keep up the good work.
Thank you for appreciation bro .