Biology Reference
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FIGURE 14.3 DEXA console showing a scan of a femur in progress.
Despite some of the problems of DEXA, it is commonly used for bone density calculation
and has become the gold standard for living populations. Beck and colleagues used DEXA to
determine BMD as well as bone geometry and cortical thickness in a study of living individ-
uals ( Beck et al., 2000 ). They found that areal BMD and bone structural properties were
dependent on body size, with body weight as the single best descriptor of skeletal geometry.
CTcan also provide bone density estimation, but it exposes the patient to much higher levels
of radiation than does DEXA. DEXA provides both T- and Z-scores for quickly diagnosing
osteoporosis. The T-score compares the subject to the optimal bone density of a young
healthy individual. The Z-score compares the subject's density to sex, age, height, weight,
and ancestry-matched individuals. If the T-score falls below 2.5 standard deviations, the indi-
vidual is diagnosed with osteoporosis. See Figure 14.3 .
Ultrasound
Ultrasonography was developed soon after the sinking of the Titanic in 1912 as a way to
use echolocation to find icebergs and other potential underwater hazards; this later became
known as SONAR (SOund Navigation And Ranging) ( Szabo, 2005 ). In medical ultrasonog-
raphy, short pulses of sound waves travel through the body and reflect (i.e., echo) off of
the different tissues. When the echo is received, the delay in time for the reflection is math-
ematically translated into different tissue types interpreted as a grayscale image ( Szabo,
2005 ).
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