Hounsfield scale

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The Hounsfield scale (/ˈhnzfld/ HOWNZ-feeld), named after Sir Godfrey Hounsfield, is a quantitative scale for describing radiodensity. It is frequently used in CT scans, where its value is also termed CT number.

Definition

The Hounsfield unit (HU) scale is a linear transformation of the original

air at STP is defined as −1000 HU. In a voxel
with average linear attenuation coefficient , the corresponding HU value is therefore given by:

where and are respectively the linear attenuation coefficients of water and air.

Thus, a change of one Hounsfield unit (HU) represents a change of 0.1% of the attenuation coefficient of water since the attenuation coefficient of air is nearly zero.[1]: 259 

Calibration tests of HU with reference to water and other materials may be done to ensure standardised response. This is particularly important for CT scans used in

radiotherapy treatment planning, where HU is converted to electron density.[2] Variation in the measured values of reference materials with known composition, and variation between and within slices may be used as part of test procedures.[1]: 283 [3]

Rationale

The above standards were chosen as they are universally available references and suited to the key application for which computed axial tomography was developed: imaging the internal anatomy of living creatures based on organized water structures and mostly living in air, e.g. humans.

Values for different body tissues and material

CT scan of the thorax with window level set to -700 HU (lung)
CT scan of the thorax with window level set to -1,000 HU (air)
CT scan of the thorax with window level set to 0 HU (water)
CT scan of the thorax with window level set to 60 HU (liver)

HU-based differentiation of material applies to

dual-energy CT scans but not to cone beam computed tomography (CBCT) scans, as CBCT scans provide unreliable HU readings.[4]

Values reported here are approximations. Different dynamics are reported from one study to another.

Exact HU dynamics can vary from one CT acquisition to another due to CT acquisition and reconstruction parameters (kV, filters, reconstruction algorithms, etc.). The use of contrast agents modifies HU as well in some body parts (mainly blood).

Substance HU
Air
−1000
Fat −120 to −90[5]
Soft tissue on contrast CT +100 to +300
Bone Cancellous +300 to +400[6]
Cortical +500 to +1900[7][6][8]
Subdural hematoma First hours +75 to +100[9]
After 3 days +65 to +85[9]
After 10–14 days +35 to +40[10]
Other blood Unclotted +13[11] to +50[12]
Clotted +50[13] to +75[11][13]
Pleural effusion Transudate +2 to +15 [14]
Exudate +4 to +33[14]
Other fluids Chyle −30[15]
Water 0
Urine −5 to +15[5]
Bile −5 to +15[5]
CSF +15
Abscess / Pus 0[16] or +20,[17] to +40[17] or +45[16]
Mucus 0[18] - 130[19] ("high attenuating" at over 70 HU)[20][21]
Parenchyma Lung −700 to −600[22]
Kidney +20 to +45[5]
Liver 60 ± 6[23]
Lymph nodes +10 to +20[24]
Muscle +35 to +55[5]
Thymus
  • +20 to +40 in children[25]
  • +20 to +120 in adolescents[25]
White matter +20 to +30
Grey matter +37 to +45
Gallstone Cholesterol stone +30 to +100[26]
Bilirubin stone +90 to +120[26]
Foreign body[27] 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)
Earwax <0

A practical application of this is in evaluation of tumors, where, for example, an

See also

References

  1. ^ .
  2. .
  3. .
  4. .
  5. ^ .
  6. ^ .
  7. .
  8. .
  9. ^ .
  10. ^ Sharma, Rohit; Gaillard, Frank. "Subdural haemorrhage". Radiopaedia. Retrieved 2018-08-14.
  11. ^ .
  12. .
  13. ^ .
  14. ^ .
  15. .
  16. ^ .
  17. ^ .
  18. ^ Saggar, K; Ahluwalia, A; Sandhu, P; Kalia, V (2006). "Mucocoele of the Appendix" (PDF). Ind J Radiol Imag. 16 (2).
  19. S2CID 28755973
    .
  20. ^ Phuyal, Subash S.; Garg, Mandeep Kumar MK; Agarwal, Ritesh R; et al. (2015-09-02). "High-Attenuation Mucus Impaction in Patients With Allergic Bronchopulmonary Aspergillosis: Objective Criteria on High-Resolution Computed Tomography and Correlation With Serologic Parameters". Current Problems in Diagnostic Radiology.
  21. PMID 27055707
    .
  22. .
  23. .
  24. .
  25. ^ .
  26. ^ .
  27. .
  28. ^ Horwich, Perry J. (December 20, 2018). Eugene C Lin (ed.). "Adrenal Adenoma Imaging". Medscape. Retrieved July 28, 2023.
  • Feeman, Timothy G. (2010). The Mathematics of Medical Imaging: A Beginner's Guide. Springer .

External links