389
Terahertz Radiation
cancer, THz imaging approaches are also being developed to better identify skin-surface
abnormalities, such as burns, scars, and wounds (Bogue 2009; Oh et al. 2009; PickwellMacPherson and Wallace 2009; Suen et al. 2009). In 2008, a study demonstrated that a
reflective THz-pulsed imaging system could be used to obtain high-resolution images
of skin burns through 10 layers of dry medical gauze. Figure 7.13f contains the photographs of the experimental setup, visible burn images, and THz images for burned skin
through gauze. In summary, THz spectroscopy and imaging approaches are being used
in many biomedical, medical, and security applications.
7.4 Terahertz Skin Interactions
In this section, we provide details regarding the composition of skin, the primary biologic target for THz radiation. Then, we outline the fundamental principles of tissue
optics that govern THz skin interactions, including a discussion of the optical properties
of skin at THz frequencies. We then discuss photothermal mechanisms and the thermal
effects associated with each temperature damage zone. Finally, we discuss computational and empirical approaches to measure THz-induced tissue damage. The importance of tissue damage measurements is then discussed in the context of determining
safe exposure standards at THz frequencies.
7.4.1 Skin Anatomy
Skin covers the entire human body and is therefore our largest organ. At the most basic
level, skin provides several vital functions: (1) provide protective barrier from the outside
world, (2) receive sensory information, (3) control evaporation and prevent fluid loss, (4)
regulate heat loss, and (5) provide a water-resistant barrier to minimize loss of essential
nutrients. The skin barrier can be breached by a host of common chemical, mechanical,
and thermal stressors. In addition, unique stressors, such as ultraviolet (UV), IR, and
THz radiation can also cause damage to the skin barrier. When skin is exposed to external stressors it responds by eliciting a dynamic repair response to combat the insult. This
response takes place at a systemic, tissue, cellular, and molecular level.
Skin tissue consists of an outer epidermis, an underlying dermis, and a hypodermis
(Figure 7.14). The epidermis consists primarily of water, keratin proteins, keratinocytes,
Langerhans, and melanocytes. In order to provide various levels of protection, the thickness of the epidermis varies across the human body between 50 and 1000 μm. Epidermal
thickness is controlled by the keratinocytes, which are genetically programmed to
undergo kertinization or stratification processes. Figure 7.14a contains a histological
cross section of the five epithelial layers. From the outside moving inward, the layers
include the following: stratum corneum (sc), stratum lucidum (sl), stratum granulosum
(sg), stratum spinosum (ss), and stratum basale (sb).
The inner sb layer consists of a single layer of cells, which are highly concentrated
with melanin pigment granules. Melanin granules are produced by melanocytes and
are transferred to keratinocytes by cytocrine secretion. The color of human skin (i.e.,
Fitzpatrick type) varies with the concentration and type of melanin granules. Although
Terahertz Radiation
cancer, THz imaging approaches are also being developed to better identify skin-surface
abnormalities, such as burns, scars, and wounds (Bogue 2009; Oh et al. 2009; PickwellMacPherson and Wallace 2009; Suen et al. 2009). In 2008, a study demonstrated that a
reflective THz-pulsed imaging system could be used to obtain high-resolution images
of skin burns through 10 layers of dry medical gauze. Figure 7.13f contains the photographs of the experimental setup, visible burn images, and THz images for burned skin
through gauze. In summary, THz spectroscopy and imaging approaches are being used
in many biomedical, medical, and security applications.
7.4 Terahertz Skin Interactions
In this section, we provide details regarding the composition of skin, the primary biologic target for THz radiation. Then, we outline the fundamental principles of tissue
optics that govern THz skin interactions, including a discussion of the optical properties
of skin at THz frequencies. We then discuss photothermal mechanisms and the thermal
effects associated with each temperature damage zone. Finally, we discuss computational and empirical approaches to measure THz-induced tissue damage. The importance of tissue damage measurements is then discussed in the context of determining
safe exposure standards at THz frequencies.
7.4.1 Skin Anatomy
Skin covers the entire human body and is therefore our largest organ. At the most basic
level, skin provides several vital functions: (1) provide protective barrier from the outside
world, (2) receive sensory information, (3) control evaporation and prevent fluid loss, (4)
regulate heat loss, and (5) provide a water-resistant barrier to minimize loss of essential
nutrients. The skin barrier can be breached by a host of common chemical, mechanical,
and thermal stressors. In addition, unique stressors, such as ultraviolet (UV), IR, and
THz radiation can also cause damage to the skin barrier. When skin is exposed to external stressors it responds by eliciting a dynamic repair response to combat the insult. This
response takes place at a systemic, tissue, cellular, and molecular level.
Skin tissue consists of an outer epidermis, an underlying dermis, and a hypodermis
(Figure 7.14). The epidermis consists primarily of water, keratin proteins, keratinocytes,
Langerhans, and melanocytes. In order to provide various levels of protection, the thickness of the epidermis varies across the human body between 50 and 1000 μm. Epidermal
thickness is controlled by the keratinocytes, which are genetically programmed to
undergo kertinization or stratification processes. Figure 7.14a contains a histological
cross section of the five epithelial layers. From the outside moving inward, the layers
include the following: stratum corneum (sc), stratum lucidum (sl), stratum granulosum
(sg), stratum spinosum (ss), and stratum basale (sb).
The inner sb layer consists of a single layer of cells, which are highly concentrated
with melanin pigment granules. Melanin granules are produced by melanocytes and
are transferred to keratinocytes by cytocrine secretion. The color of human skin (i.e.,
Fitzpatrick type) varies with the concentration and type of melanin granules. Although