Ординатура / Офтальмология / Английские материалы / Handbook of Optical Coherence Tomography_Bouma, Tearney_2002

Figure 2 Fiber-optic implementation of spectral radar.

regions of the object [11]. The waves scattered back from different depths of the object are coaxially observed.

Imaging with a finite aperture causes subjective speckle at the entrance of the fiber. The speckle contrast is the actual carrier of the information. With an accepting aperture NA ¼ 0:1 of the fiber core, there is only one speckle on the core. The confocal imaging of the backscattered photons onto the fiber core also has the advantage of spatially separating the photons. For photons that have been scattered several times, there exists hardly any correlation between run time and depth information [22]. Now just these photons will no longer hit the fiber.

The backscattered wave is superimposed with the reference wave. At the interferometer exit we use a grating spectrometer to locally separate the different wavelengths. The resolution of the spectrometer is 0.05 nm. The spectrum is imaged onto an array of 1024 photodiodes, each 2:5 mm 25 m. Each photodiode can collect 6 107 photons to give low photon shot noise and a large dynamic range. The signal is transferred to the host computer by a 14 bit A/D converter. Then the Fourier transformation is performed (which actually limits the dynamic range).

12.5.2 Signal Formation

The measuring principle is based on spectral interferometry. The signal from the object consists of many elementary waves emanating from different depths z. We neglect the dispersion in the object. The scattering amplitude of the elementary waves versus depth is aðzÞ. The object signal is superimposed with the plane reference wave aR. At the exit of the interferometer a spectrometer locally separates the different wavenumbers kð¼ 2=Þ. The interference signal IðkÞ is

12.5.3 Measuring Range and Measuring Uncertainty

The measuring range z of spectral radar is basically limited by the resolution of the spectrometer [17]. A large difference between the object and reference optical paths will cause a high frequency in the spectrum. According to the sampling theorem the sample frequency of the photodiode array has to be twice as large as the highest occurring frequency in the spectrum. For z ¼ zMAX, the period of the cosine fringes is k ¼ =nzMAX. Therefore the spectrometer has to resolve at least k=2. With j kj ¼ 2=2, we get

With the parameters in our setup the measuring range is z ¼ 2:4 mm (n ¼ 1:5). However, if we measure a mirror at variable distance z, we observe a decay of

the signal peak with increasing distance. This decay is caused by the finite width of the photodiodes (1=) of the spectrometer, although the reflected amplitude stays the same. To get the real scattering amplitude aðzÞ we must first compensate for this decay by normalization. Each measurement is normalized by dividing the measured signal by the measured decay.

The achievable spatial resolution depends on the coherence length of the light source and the scattering characteristics of the object. According to our experiments on the behavior of the spatial impulse response during signal propagation in volume scatterers with OCT methods [22], the minimum resolvable distance decreases with decreasing coherence length. However, it is finally limited by the scattering characteristics of the object. To determine the spatial resolution of spectral radar we measured a well-defined multilayer object with scattering coefficients adapted to human skin [26]. It turns out that the achievable spatial resolution within a range of 1000 m is about 10 m or less.

12.6EXPERIMENTAL RESULTS

12.6.1 Measurements on Biological Objects

Spectral radar was implemented as a fiber-optic system, and in vivo measurements were performed [4,18,19]. Figure 5 shows the head of the spectral radar including the collimation optics, the devices to adjust the reference plane and the depth of focus, and the lateral scanner.

Figure 6 displays two optograms comparing human skin at the forearm and the hand. We can distinguish the typical layer structure of human skin in both measurements. The scattering amplitudes along two A-scans (lines A and B) are shown at the bottom of the figure. Along each A-scan we see first a high peak obtained from direct reflection at the surface (position 100 m) and the strongly scattering stratum corneum (s.c.). The next layer of the skin is the boundary of the weakly scattering stratum germinativum (s.g.) followed by the strongly scattering stratum papillare (s.p.). In Fig. 6 the difference between different types of skin is evident. The structure in the forearm (e.g., thickness of the s.g.: 70–80 m) is thinner than the structure at the hand (thickness of the s.g.: 300–350 m).

Figure 5 Spectral radar is used to check a patient’s arm.

The main aim of OCT is to investigate alterations of the skin. We investigated the influence of increased moisture content on the morphological structure of the skin by OCT. In the result the thickness of the epidermis with increased moisture content leads to an average extension of the epidermis of about 10% [4]. We can demonstrate the difference between healthy skin and a superficial spreading melanoma [4]. Further, we measured the morphological structure of a fingernail and that of the growing nail beneath the skin [19].

In Fig. 7, a malignant melanoma and Bowen’s disease, a preliminary stage of skin cancer, are compared. The melanoma causes an accumulation of melanin in the cells of the epidermis and therefore high backscattering. In addition, the layer structure of the healthy skin is destroyed. The epidermis is extended, and the backscattering caused by the melanin is very strong and homogeneous. The layer structure of the skin has vanished in the malignant melanoma as in the Bowen’s disease (Fig. 7). Differently from the melanoma, the scattering amplitude in the Bowen’s disease decays exponentially. This picture shows the possible first step of spectral radar to differentiate alterations in the skin. Many further measurements have to be done to confirm the result.

The optogram of Fig. 8 displays a tunnel in the epidermis built by a larva migrane transferred by a midge bite. The tunnel in the epidermis is characterized by an almost vanishing scattering amplitude due to tissue missing from the longitudinal position beneath 150 m. The tunnel was too deep for its end to be detected.

Our most recent measurements have been performed with a more powerful SLD (output power in the fiber P ¼ 10 mW; central wavelength ¼ 840 nm; FWHM ¼ 20 nm; coherence length lC ¼ 35 m). According to our considerations

Figure 6 Optograms from two different parts of the human skin. Left: Skin at the forearm. Right: Skin at the hand.

about the dynamic range, the measuring range increases with increasing output power. In measurement with the former SLD (output power P ¼ 1:7 mW) we could achieve a depth of only about 600 m in the skin of the fingertip (Fig. 6). Now we are able to measure scattering amplitudes up to a depth of more than 1 mm with the new SLD. Figure 9 displays an optogram of human skin at the fingertip obtained with the new SLD. The epidermis (first two layers) has a thickness of about 250 m. The third layer with high signal due to backscattering and dark zones below is the stratum papillare. The surface of the skin follows the waves of papillary ridges in the depth. The lateral B-scan is performed over the range of 2 mm. The measuring time for the whole optogram is 21 s. (About half of the time is for scan and control of the spectrometer. The exposure time for one A-scan is 20 ms.)

The lateral boundary between normal skin and a malignant melanoma can be seen in Fig. 10. The melanoma causes an accumulation of melanin in the cells of the epidermis and therefore high backscattering (white areas at the bottom of the image). The epidermis is extended, and the typical layer structure is destroyed. At the top of the image you see healthy human skin with a three-layer structure. The result delivered in Fig. 10 corresponds well to the histology. The melanoma has an average

Figure 7 Optograms of malignant melanoma (left) and Bowen’s disease (epithelioma) (right). Both skin alterations destroy the layer structure of the healthy skin. The epidermis is extended. The scattering amplitude in the malignant melanoma is strong and homogeneous. The scattering amplitude of the Bowen’s disease decays exponentially.

thickness of about 550 m. This depth was confirmed by further measurements. The measurement was performed with the 10 mW SLD.

12.6.2 Technical Applications

Although spectral radar was developed for the acquisition of 3-D data of biological tissue, it can be used for the examination of technological objects as well [18]. In Fig. 11 we show the structure of a multilayer printed circuit board. The board contains four layers of conducting lines, at depths of about 200, 400, 600, and 700 m. The first layer is on the surface. The last one is the copper baseplate. The conduction lines are also made of copper. Because the light cannot penetrate copper, no signal can be acquired from underneath the metal. Another application is the measurement of the thickness of paint. Three layers of paint on a car body can be detected and measured.