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a
b
c
d
Fig. 7 Demonstration of standardized Houns eld density units for body tissues on a CT cross
section. In the CT axial cross section, measured HU in a sample ROI were a 999 within the
trachea; b 105 in the fatty tissue of the axilla; c 45 in the muscle tissue of the shoulder; and
d 1019 at the cortex of the scapula
As noted, both CT and radiographic images are produced with X-rays, and so
represent surrogate maps of body organ density obtained via X-ray attenuation.
Following the convention of previous generation radiographs taken on photographic
film, images are mapped by grayscale, with dense structures such as bone scaled
toward the white or bright end of the scale, and lower density materials scaled toward
the dark end of the scale. With the invention of computed tomography, an intrinsic
grayscale was created, with the scale subdivided into Houns
eld units (HU) [ 4 , 5 ]. In
the Houns
eld system of units, very low density structures are scaled in negative
units, as in the case of air (approximately
100
HU). Water is set at the standard of 0 HU, with muscle tissue scaled at approximately
40 HU and bone scaled at approximately 1,000 HU (Fig. 7 ).
On the computer monitor, Houns
1,000 HU) and fat (approximately
eld units are scaled to pixel intensity. Medical
diagnostic image display systems typically allow 10
-
12 bit depth, allowing the
display of 1,024
4,096 shades in grayscale. Now, as the human eye can only dif-
ferentiate approximately 30
-
40 grayscale shades, sets of restricted range brightness
setting windows are created, centered about the tissue density of interest (Fig. 8 ).
In materials such as bone, these windows isolate and amplify details of the anatomy
of interest. The uses of CT imaging in clinical medical practice are multifold, being
particularly useful for complex anatomic structures such as the spine, and for
assessing certain classes of pathology such as acute fractures in non-osteopenic
patients. In cases such as these, pathology on radiographic studies may be obscured
or vague conferring a diagnostic advantage on CT (Fig. 9 ), or more complex
-
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