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Optical Computed Tomography Essay

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Development of optical computed tomography is traceable back to slightly more than a decade ago. ( Doran & Krstajic, 2006:45) The study of tissues using conventional methods and more specifically optical microscopy often suffered major problems given its inherent representation of 3-D materials in 2-D. it is on this basis that there arose the need for more advanced representation techniques. (Doran & Krstajic, 2006:47) Optimal tomography has been able to fill this scientific gap through its ability to combine a series of 2-D images to give a 3-D image.

This process involves construction of a spatial 3-D distribution regarding the linear attenuation of an object with the use of samples recorded from various angles of the illumination beam transmitted. (Zhang G. Et al, 2008:2738) Significantly, optical CT has independently emerged within three differing fields. Documented evidence reveal that the earliest emergence of CT is traceable to late 1990’s discovery by Maryanski and Gore similar to the CCD-based optical scanner applicable in investigation of chemical structures that often engage in self organization was presented by Winfree.

(Chamgoulov, 2006:1) Later in 2002 though Sharpe published micro-CT images of which he refers to as optical projection tomography. However, recent technologies have seen introduction of optical technology applying the use of trans-illumination tomography in the study of tissues. Optical CT thus falls within the class of optical imaging devices. Often it’s referred t as an interferometric mechanism which brings together scattered light emerging from the sample with regard to a reference beam. (Zhang G. Et al, 2008:2740) OCT’s use in tissue study experiences limitation regarding the maximum penetrable depth.

The operations of optical CT originates from the beers law which describes the attenuation of light and X-rays as they move through a given medium. The law states that for uniformly distributed substance bearing a linear attenuation coefficient ? with a measuring detector at a depth d is described by the equation below l(d)= l_0 exp?? (-? d),l? Where l0 represents the measured intensity at depth equals to zero. Where N mediums are to be bypassed by the rays, then the equation translates as described below; l(N? y) ? = l? _0 exp{-? _(i=1)^N-?? _i ? y? }

The most recent development of Optic CT application has been the optical CT microscopes which uses of DMD (Digital micro-mirror device). This involves an array consisting of thousands of very small micro-mirrors designed in a way that allows each to be individually controlled. This allows illumination of the specimen at different angles or rather they act as spatial modulators of light. (Chamgoulov, 2006:2) Another form of optical CT microscope employs the use of an optical scanner primarily controlled by a computer in moving of light across the given sample.

(see figure 1) Such a system is made up of a pair of objective lenses with high numerical apertures, a source of light, and a detector. The optical scanner is made up of a two axis mirror consisting of motorized linear actuators. (Doran & Krstajic, 2006:50) The objective lenses numerical aperture limits the angular range. The microscope applies greater use of the radon’s transformations which is described by the equation below; l=l_(0 ) exp {–? -? (y)dy} across a given sample Figure 1: Optical CT microscope schematic representation

The optical CT microscope gives 3-D images of sample being studied with the samples dimensions being visibly seen. This has thus extended its application as a way of studying tissues and body cells more effectively as previously only two dimensional images could be visualized. Data from optical CT imaging is often presented in Radon space, a 2-D form of image referred to as a sinogram. (Doran & Krstajic, 2006:51) The samples various points are said to each produce their own sinogram tracks overlaid to each other.

The source and the detector are each placed on either side of the object being imaged. When the source detector is at position x, then the attenuated laser beam intensity that the detector receives is given by the equation l(x)=l_0 exp {–? -? (x,y) dy} Often the source detector track is rotated around the sample being imaged. However some designed allow the sample to rotate instead. This change does not result into a change in design except for mathematical calculations that are altered to take into consideration the resulting sample rotational angles.

The equation of radon transforms then takes the form describes below; ?_? (x)= ? -? (x_? y_? )dy= -(In (l_? x)/l_0 ) Advantages and disadvantages of using optical computed tomography This technology has attracted widespread use by industrialist and other end users due to its high speed and precision. In addition it provides more detailed subsurface images of both 2-D and 3-D structures. (Sakhalkar & Oldham, 2007:104) Notably also is its capability of producing high resolution and in-depth images of tissues.

This has led to its gaining popularity with ophthalmologists. Non-biological application too employs the use of optical CT as it is applicable in non-destructive testing and material evaluation. In 2007 for instance, David Stifter in his paper cited the applicability of the technology in detection of ceramic defects (Stifter et al, 2005) and other non-contact materials including glass, polymers among others. However, the technology is still considered expensive and unaffordable to most of its desirable users.

Additionally, the highly scattering nature of biological tissues has limited the technology to only shallow depths in imaging penetration. References Doran, S. J. & Krstajic, N. (2006) The history and principles of optical computed tomography for scanning 3-D radiation dosimeters. Journal of physics, 56:45–57 Zhang G. Et al (2008) Use of three-dimensional (3D) optical flow method in mapping 3D anatomic structure and tumor contours across four-dimensional computed tomography data. Journal of Applied Clinical Medicine Physics, 9(1):2738 Chamgoulov, R. , Pierre L. & Calum, M.(2006)

Computed tomography generates three-dimensional microscopic images of cells, Journal of Optical Engineering, 2(2):1-3. J. Sharpe, (2008) Optical Projection Tomography. Annual Review of Biomedical Engineering, 8: 209-228, Sakhalkar H. S. & Oldham, M (2007) Fast, high-resolution 3D dosimetry utilizing a novel optical-CT scanner incorporating tertiary telecentric collimation. Med. Phys. 35(1):101-111 Stifter, D. et al (2005) En-face scanning optical coherence tomography with ultra-high resolution for material investigation, Journal of Optics, 13( 3):1015-1024

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