Page 36 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

2.4 Nonionizing electromagnetic imaging 33

2.4.1.1 Thermo-acoustic imaging
Thermo-acoustic imaging systems use nonionizing radiation, such as RF, light
absorption, and microwave to induce ultrasound waves in the targeted tissues.
The basic steps of image formation in this technic are in five steps. First, the
electromagnetic energy is emitted as uniformly as possible. Second, the depos-
ited energy is absorbed by the tissue leading to increase the temperature (less
than 0.001°C). Third, increase in temperature causes thermal expansion, how-
ever slightly. Fourth, this mechanical expansion leads to produce acoustic wave
propagating in all directions and detect by transducer surrounding the target.
Fifth, the transducers, which is mainly piezoelectric crystals, scan the target and
collect the tomographic data. (The summary of image formation is shown in
Fig. 2.5.)
Photo-acoustic is a type of thermoacoustic which the generating radiation is
optical. Typically, the pulse duration in the photo-acoustic imaging is around
5–10 ns. Due to the increase in the amount of hemoglobin and water around
a tumorogenic tissue, more absorption in electromagnetic energy and conse-
quently more thermal expansion has occurred than surrounded healthy tissues
[29]. Reports have been shown that photo-acoustic imaging with very short wave-
lengths has created a high contrast between tumorogenic and nontumorogenic
tissues [30].
The features of emitted sources are different. For example, the penetration
depth of RF and microwave are more than optical pulses, while microwave pulses
are less uniform as compared with the others and are mainly used for preclinical
studies [31].

FIGURE 2.5 Principle of thermoacoustic imaging.