Tomografi keselarasan optik: Perbedaan antara revisi

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== Pendahuluan ==

OCT juga telah digunakan untuk berbagai proyek pelestarian barang seni, di mana ia digunakan untuk menganalisis lapisan yang berbeda dalam sebuah lukisan. OCT telah menarik keuntungan lain dari sistem pencitraan medis.

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OCT delivers high resolution because it is based on light, rather than sound or radio frequency. An optical beam is directed at the tissue, and a small portion of this light that reflects from sub-surface features is collected. Note that most light is not reflected but, rather, scatters off at large angles. In conventional imaging, this diffusely scattered light contributes background that obscures an image. However, in OCT, a technique called interferometry is used to record the optical path length of received photons allowing rejection of most photons that scatter multiple times before detection. Thus OCT can build up clear 3D images of thick samples by rejecting background signal while collecting light directly reflected from surfaces of interest.

 

 

The technique is limited to imaging 1 to 2&nbsp;mm below the surface in biological tissue, because at greater depths the proportion of light that escapes without scattering is too small to be detected. No special preparation of a biological specimen is required, and images can be obtained ‘non-contact’ or through a transparent window or membrane. It is also important to note that the laser output from the instruments is low – eye-safe near-infra-red light is used – and no damage to the sample is therefore likely.

The principle of OCT is white light or low coherence interferometry. The optical setup typically consists of an interferometer (Fig. 1, typically [[Michelson interferometer|Michelson]] type) with a low coherence, broad bandwidth light source. Light is split into and recombined from reference and sample arm, respectively.

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Interferensi dua berkas cahaya yang koheren secara parsial dapat diekspresikan dengan persamaan intensitas sumber cahaya, <math>I_S</math>, sebagai

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===Frequency domain===

In frequency domain OCT (FD-OCT) the broadband interference is acquired with spectrally separated detectors (either by encoding the optical frequency in time with a spectrally scanning source or with a dispersive detector, like a grating and a linear detector array). Due to the [[Fourier analysis|Fourier]] relation ([[Wiener-Khintchine theorem]] between the auto correlation and the spectral power density) the depth scan can be immediately calculated by a Fourier-transform from the acquired spectra, without movement of the reference arm.<ref>{{cite journal|doi=10.1109/2944.796348|title=Optical coherence tomography (OCT): a review|year=1999|last1=Schmitt|first1=J.M.|journal=IEEE Journal of Selected Topics in Quantum Electronics|volume=5|pages=1205–1215|issue=4}}</ref><ref name="Fercher2">{{cite journal|doi=10.1016/0030-4018(95)00119-S|title=Measurement of intraocular distances by backscattering spectral interferometry|year=1995|last1=Fercher|first1=A|last2=Hitzenberger|first2=C.K.|last3=Kamp|first3=G.|last4=El-Zaiat|first4=S.Y.|journal=Optics Communications|volume=117|pages=43–48|bibcode= 1995OptCo.117...43F }}</ref> This feature improves imaging speed dramatically, while the reduced losses during a single scan improve the signal to noise ratio proportional to the number of detection elements. The parallel detection at multiple wavelength ranges limits the scanning range, while the full spectral bandwidth sets the axial resolution.

 

====Spatially encoded====

 

The drawbacks of this technology are found in a strong fall-off of the SNR, which is proportional to the distance from the zero delay and a sinc-type reduction of the depth dependent sensitivity because of limited detection linewidth. (One pixel detects a quasi-rectangular portion of an optical frequency range instead of a single frequency, the Fourier-transform leads to the sinc(z) behavior). Additionally the dispersive elements in the spectroscopic detector usually do not distribute the light equally spaced in frequency on the detector, but mostly have an inverse dependence. Therefore, the signal has to be resampled before processing, which can not take care of the difference in local (pixelwise) bandwidth, which results in further reduction of the signal quality. However, the fall-off is not a serious problem with the development of new generation CCD or photodiode array with a larger number of pixels.

 

== Referensi ==

 

[[Kategori:Peralatan medis]]