The Hounsfield scale , 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

linear attenuation coefficient The linear attenuation coefficient, attenuation coefficient, or narrow-beam attenuation coefficient characterizes how easily a volume of material can be penetrated by a beam of light, sound, particles, or other energy or matter. A coefficient valu ...

measurement into one in which the radiodensity of

distilled water Distilled water is water that has been boiled into vapor and condensed back into liquid in a separate container. Impurities in the original water that do not boil below or near the boiling point of water remain in the original container. Thus, di ...

at standard

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...

and

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

( STP) is defined as 0 Hounsfield units (HU), while the radiodensity of air at STP is defined as −1000 HU. In a

voxel In 3D computer graphics, a voxel represents a value on a regular grid in three-dimensional space. As with pixels in a 2D bitmap, voxels themselves do not typically have their position (i.e. coordinates) explicitly encoded with their values. I ...

with average linear attenuation coefficient

\mu

, the corresponding HU value is therefore given by:

HU = 1000\times\frac

where

\mu_

and

\mu_

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. 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 Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Rad ...

treatment planning In radiotherapy, radiation treatment planning (RTP) is the process in which a team consisting of radiation oncologists, radiation therapist, medical physicists and medical dosimetrists plan the appropriate external beam radiotherapy or internal ...

, where HU is converted to

electron density In quantum chemistry, electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial va ...

. 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.

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.''

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...

Values for different body tissues and material

CT Scan Thorax Lung -700 HU Window Level

CT Scan Thorax Air -1000 HU Window Level

HU-based differentiation of material applies to medical-grade dual-energy CT scans but not to cone beam computed tomography (CBCT) scans, as CBCT scans provide unreliable HU readings. 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). 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.medscape >Adrenal Adenoma Imaging
Author: Perry J Horwich. Chief Editor: Eugene C Lin. Updated: Apr 21, 2011

References

External links

*
Hounsfield Unit
- fpnotebook.com *{{cite web, url=http://www.intl.elsevierhealth.com/e-books/pdf/940.pdf, title=Introduction to CT physics, publisher=elsevierhealth.com, archive-url=https://web.archive.org/web/20070926231241/http://www.intl.elsevierhealth.com/e-books/pdf/940.pdf, archive-date=2007-09-26
Imaging of deep brain stimulation leads using extended Hounsfield unit CT. Stereotact Funct Neurosurg. 2009;87(3):155-60. doi: 10.1159/000209296
Radiology

Definition

Rationale

Values for different body tissues and material

See also

References

External links