Hounsfield scale
Encyclopedia
The Hounsfield scale, named after Sir Godfrey Newbold Hounsfield, is a quantitative scale for describing radiodensity
.
of distilled water
at standard pressure
and temperature
(STP
) is defined as zero Hounsfield units (HU), while the radiodensity of air at STP is defined as -1000 HU. For a material X with linear attenuation coefficient
, the corresponding HU value is therefore given by
where µ(water) are the linear attenuation coefficients of water. 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.
It is the definition for CT scanners that are calibrated with reference to water.
s.
scans.
A practical application of this is in evaluation of tumors, where, for example, an adrenal tumor
with a radiodensity of less than 10 HU is rather fatty in composition and almost certainly a benign adrenal adenoma
.
of living creatures, initially only as tomographic
reconstructions of slice views or sections. Since the early 1990s, with advances in computer technology and scanners using spiral CT technology, internal three-dimensional anatomy is viewable by three-dimensional software reconstructions, from multiple perspectives, on computer monitors. By comparison, conventional X-ray
images are two-dimensional projections of the true three-dimensional anatomy, i.e. radiodensity
shadow
s.
It was established by Sir Godfrey Newbold Hounsfield, one of the principal engineer
s and developers of computed axial tomography (CAT, or CT scans).
Radiodensity
Radiodensity refers to the relative inability of electromagnetic radiation, particularly X-rays, to pass through a particular material. Radiolucency indicates greater transparency or "transradiancy" to X-ray photons...
.
Definition
The Hounsfield unit (HU) scale is a linear transformation of the original linear attenuation coefficient measurement into one in which the radiodensityRadiodensity
Radiodensity refers to the relative inability of electromagnetic radiation, particularly X-rays, to pass through a particular material. Radiolucency indicates greater transparency or "transradiancy" to X-ray photons...
of distilled water
Distilled water
Distilled water is water that has many of its impurities removed through distillation. Distillation involves boiling the water and then condensing the steam into a clean container.-History:...
at standard pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
(STP
Standard conditions for temperature and pressure
Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...
) is defined as zero Hounsfield units (HU), while the radiodensity of air at STP is defined as -1000 HU. For a material X with linear attenuation coefficient
, the corresponding HU value is therefore given by
where µ(water) are the linear attenuation coefficients of water. 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.
It is the definition for CT scanners that are calibrated with reference to water.
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. humanHuman
Humans are the only living species in the Homo genus...
s.
The HU of common substances
The Hounsfield scale applies to medical grade CT scans but not to cone beam computed tomography (CBCT)Cone beam computed tomography
Cone beam computed tomography is a medical imaging technique that has become increasingly important in treatment planning and diagnosis in implant dentistry, among other things...
scans.
| Substance | HU |
|---|---|
| Air | −1000 |
| Lung | −700 |
| Soft Tissue | −300 to -100 |
| Fat | −84 |
| Water | 0 |
| CSF | 15 |
| Blood | +30 to +45 |
| Muscle | +40 |
| Bone | +700(cancellous bone)to +3000 (dense bone) |
A practical application of this is in evaluation of tumors, where, for example, an adrenal tumor
Adrenal tumor
The term adrenal tumor can refer to one of several benign and malignant neoplasms of the adrenal gland, several of which are notable for their tendency to overproduce endocrine hormones. Adrenal cancer specifically refers to malignant adrenal tumors, which include neuroblastoma, adrenocortical...
with a radiodensity of less than 10 HU is rather fatty in composition and almost certainly a benign adrenal adenoma
Adrenal adenoma
A adrenal adenoma is a benign tumor of the glandular type in the adrenal gland. While some adrenal adenomas do not secrete hormones at all , some secrete cortisol , aldosterone , or androgens ....
.
History
CT machines were the first imaging devices for detailed visualization of the internal three-dimensional anatomyAnatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
of living creatures, initially only as tomographic
Tomography
Tomography refers to imaging by sections or sectioning, through the use of any kind of penetrating wave. A device used in tomography is called a tomograph, while the image produced is a tomogram. The method is used in radiology, archaeology, biology, geophysics, oceanography, materials science,...
reconstructions of slice views or sections. Since the early 1990s, with advances in computer technology and scanners using spiral CT technology, internal three-dimensional anatomy is viewable by three-dimensional software reconstructions, from multiple perspectives, on computer monitors. By comparison, conventional X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
images are two-dimensional projections of the true three-dimensional anatomy, i.e. radiodensity
Radiodensity
Radiodensity refers to the relative inability of electromagnetic radiation, particularly X-rays, to pass through a particular material. Radiolucency indicates greater transparency or "transradiancy" to X-ray photons...
shadow
Shadow
A shadow is an area where direct light from a light source cannot reach due to obstruction by an object. It occupies all of the space behind an opaque object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or reverse projection of the object blocking the...
s.
It was established by Sir Godfrey Newbold Hounsfield, one of the principal engineer
Engineer
An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...
s and developers of computed axial tomography (CAT, or CT scans).
External links
- Hounsfield Unit - GE medcyclopedia
- Hounsfield Unit - fpnotebook.com
- Introduction to CT physics - elsevierhealth.com (Link at the original website is not working anymore, it can be downloaded from Wayback Machine http://web.archive.org/web/20070926231241/http://www.intl.elsevierhealth.com/e-books/pdf/940.pdf)