Maximum Detection Depth Research Articles

Optical coherence tomography in dermatology: a review.

Optical coherence tomography (OCT) is a non-invasive technique for morphological investigation of tissue. Since its development in the late 1980s it is mainly used as a diagnostic tool in ophthalmology. For examination of a highly scattering tissue like the skin, it was necessary to modify the method. Early studies on the value of OCT for skin diagnosis gave promising results. The OCT technique is based on the principle of Michelson interferometry. The light sources used for OCT are low coherent superluminescent diodes operating at a wavelength of about 1300 nm. OCT provides two-dimensional images with a scan length of a few millimeters (mm), a resolution of about 15 microns and a maximum detection depth of 1.5 mm. The image acquisition can be performed nearly in real time. The measurement is non-invasive and with no side effects. The in vivo OCT images of human skin show a strong scattering from tissue with a few layers and some optical inhomogeneities. The resolution enables the visualization of architectural changes, but not of single cells. In palmoplantar skin, the thick stratum comeum is visible as a low-scattering superficial well defined layer with spiral sweat gland ducts inside. The epidermis can be distinguished from the dermis. Adnexal structures and blood vessels are low-scattering regions in the upper dermis. Skin tumors show a homogenous signal distribution. In some cases, tumor borders to healthy skin are detectable. Inflammatory skin diseases lead to changes of the OCT image, such as thickening of the epidermis and reduction of the light attenuation in the dermis. A quantification of treatment effects, such as swelling of the horny layer due to application of a moisturizer, is possible. Repeated measurements allow a monitoring of the changes over time. OCT is a promising new bioengineering method for investigation of skin morphology. In some cases it may be useful for diagnosis of skin diseases. Because of its non-invasive character, the technique allows monitoring of inflammatory diseases over time. An objective quantification of the efficacy and tolerance of topical treatment is also possible. Due to the high resolution and simple application, OCT is an interesting addition to other morphological techniques in dermatology.

Detection of inhomogeneities in biological tissues by the methods of confocal microscopy

The model of detection of local inhomogeneities of scattering and absorbing types in biological tissues by the methods of reflection and transmission confocal microscopy has been developed on the basis of the theory of vision in scattering media. General equations for calculation of the image contrast of an inhomogeneity against the background of a scattering medium are derived. The influence of the object characteristics and observation-system parameters on the maximum detection depth of inhomogeneities is analyzed.

Near-Surface Ice on Mercury and the Moon: A Topographic Thermal Model

A thermal model that can be easily adapted to craters of arbitrary shape is developed and applied to high-latitude impact craters on Mercury and the Moon, Chao Meng Fu crater at -87.5°L on Mercury, an unnamed bowl-shaped crater at 86.7°L on Mercury, and Peary crater at 88.6°L on the Moon. For an assumed input topography and grid of surface elements, the model computes for each element the irradiation from direct insolation and reflected and emitted radiation from other elements, taking into account shadowing by the walls of the crater, partial obscuration of the solar disk near the poles and the diurnal, orbital, and seasonal cycles. Temperatures are computed over the surface grid as functions of depth and time from the surface to a specified depth and over the pertinent astronomical cycles, including the effects of direct and indirect surface irradiation, infrared radiation, heat conduction, and interior heating. Vapor fluxes and ice recession times are computed as functions of ice depth over the surface grid. Temperatures profiles, vapor fluxes, and ice recession times were computed for flat surfaces not associated with craters near the poles of Mercury and the Moon. It was found that water ice could have existed throughout geologic time within the maximum radar detection depth of recent observation of Mercury (J. K. Harmon and M. A. Slade, 1992, Science 258, 640-643) poleward of ∼87-88°L on Mercury and poleward of ∼73°L on the moon. For Chao Meng Fu crater it was found that ∼40% of the crater floor is permanently shadowed from direct solar insolation, while the remainder of the crater floor is periodically illuminated by a partially obscured Sun. Temperatures at the upper levels of the south wall can slightly exceed 550 K. Surface temperatures in the permanently shadowed region of the crater floor are under ∼130 K, which could have allowed water ice to exist throughout geologic time within the radar detection depth of recent observation of Mercury. For the small bowl-shaped crater on Mercury, it was found that most of the crater is permanently shadowed from direct solar radiation, except for a narrow semicircular band bordering the north rim. However, temperatures in the permanently shadowed region periodically reach a maximum near ∼315 K due to efficient heating of the small crater by thermal emission and reflection from the small sunlit region, which periodically reaches temperatures exceeding 630 K. Water ice could not have existed throughout geologic time anywhere in this crater within the radar detection depth. For Peary crater on the Moon, the entire crater floor is permanently shadowed from direct solar insolation with maximum temperatures under 120 K. The upper level of the north wall periodically reaches a maximum temperature near 310 K. The low temperatures on the crater floor would have allowed water ice to exist near the surface throughout geologic time, provided that the Moon's obliquity was always as low as it is at present.

An evaluation of Bristow’s method for the detection of subsurface cavities

The Bristow method, an electrical resistivity technique employing a pole‐dipole measurement array in conjunction with a simple graphical method of interpretation, has proven an effective means of locating subsurface cavities. There have been questions, however, regarding the limits of the method and whether the Bristow method is indeed the most suitable of the various electrical resistivity techniques for cavity detection. In hopes of resolving some of the controversy surrounding Bristow’s method, resistivity traverses are numerically modeled over spherical and cylindrical cavities given a variety of circumstances. Using a slight variation of Bristow’s original interpretive technique on modeled data, the size and location of subsurface cavities can be determined with surprising accuracy. However, when the simulation is altered to incorporate geologic noise, the maximum depth at which a cavity can be detected is found to be far less than has been reported in field investigations. In this instance the presence of a cylindrical cavity cannot be discerned beyond a depth to the top approximately equal to the diameter of the cavity, and spherical cavities are indistinguishable at depths much greater than the radius. One should note that the noise field generated for this model may not be representative of what would normally be found in the real earth. In the field, the maximum achievable depth of detection will vary depending on the actual geologic conditions and whether some technique is employed to reduce the effects of noise. In any case, a comparison of traverses using various electrode arrays confirms that the Bristow method is the most satisfactory of the applicable electrical resistivity techniques.

Measurement of Fatigue Crack Propagation by Eddy Current Method

The eddy current method was applied to the detection of fatigue crack propagation of notched specimens under rotating bending. The practical device to apply the eddy current method and the test results are described. The depth of fatigue cracks can be measured continuously by the eddy current method, and the accurate value of crack depth is obtained without breaking the test pieces. The method can be applied to ferromagnetic substance as well as to paramagnetic substance, though the detecting probe must be varied so as to suit the materials. A linear relation is obtained between the crack depth and the response of the eddy current circuit irrespective of the environment in which fatigue tests are carried out. The circumferential crack propagation can also be measured by rotating the test piece once or by reciprocating the detecting probe over the notch at several circumferential positions, and the latter method shows better agreement with the crack depth measured by temper coloring. The sensibility of the crack detection is 0.1∼0.05mm. The maximum detectable crack depth is more than 2.5mm in the case of carbon steel (ferromagnetic substance) and is more than 4.0mm in the case of Al alloy (paramagnetic substance).