Biomedical Engineering Reference
In-Depth Information
difference in energy at 5 cm? Assume the density of bone to be 2 g/cm
3
and density of air
to be 1.1
×
10
−
3
g/cm
3
.
Solution: Since the number of photons are the same and assuming the same cross-
sectional area, (8.13) can be written in terms of energy.
2
3
3
5
1
0.095[cm /g] * 1.1
10
−
[gm/cm ]
9.9
10
−
cm
−
μ
μ
=
×
=
×
air
2
3
1
0.665[cm /g] * 2[gm/cm ]
1.33 cm
−
=
=
bone
5
9.9 10
−
*5
E
40[keV] *
e
−×
39.980 keV
=
=
air
−
1.33*5
E
=
40[keV] *
e
=
0.052 keV
air
Clearly, there is a significant difference in the transmitted energy.
X-rays, CT scans, and fluoroscopy images are produced by transmission. For
example, the X-ray tube is placed one or more meters from the patient. As the
emitted X-rays pass through the patient, some X-rays pass straight through the
body (primary radiation), some are scattered by the tissue (secondary radiation)
and some are completely absorbed by the tissue (absorbed radiation). Transmit-
ted X-rays are recorded by a detector on the opposite side. Hence, the intensity
of photons reaching a detector decreases as
μ
x
increases for the same absorber
thickness and photon energy. Muscle and connective tissues (ligaments, tendons,
and cartilage) appear gray. Bones appear white. Scattered light may interfere con-
structively with the incident light in certain directions, forming beams that have
been reflected and/or transmitted. The constructive interference of the transmitted
beam occurs at the angle that satisfies Snell's law, while the after reflection occurs
for
θ
incident
.
Fluorescence-based techniques are widely used for studying cellular structure
and function, and interactions of biomolecules in the detection and quantitation
of nucleic acids and proteins, microarray technology, and fluorescence spectros-
copy.
Emission
also occurs when tiny, nuclear particles or magnetic energy are
detected by a scanner and analyzed by a computer to produce an image of the
body structure or organ being examined. In nuclear medicine, radioactive agents
are ingested, inhaled, or injected into the body, and the resulting image reflects
θ
reflected
=
Table 8.1
Attenuation Coefficients for Typical Tissues [1]
μ
x
/
ρ
[cm
2
/g]
Density [g/cm
3
] 15 keV 30 keV
50 keV
100 keV
150 keV
Air
0.0013
1.614
0.354
0.208
0.154
0.136
Adipose tissue
0.95
1.083
0.306
0.212
0.169
0.150
Whole blood
1.06
1.744
0.382
0.228
0.169
0.149
Cortical bone
1.92
9.032
1.331
0.424
0.186
0.148
Lead
11.35
111.6
30.32
8.041
5.549
2.014
Lung
1.05
1.721
0.382
0.227
0.169
0.149
Skeletal muscle 1.05
1.693
0.378
0.226
0.169
0.149
Water
1.00
1.673
0.376
0.227
0.171
0.137
