Optical Scanning System for Imaging of Heterogeneity in Biological Tissues

The interaction of electromagnetic radiation with biological tissues depends on their wavelength and medium properties. In therapeutic region (600-1300nm) the penetration of radiation is more due to high scattering and low absorption [1]. During this the backscattered component, originating from a particular depth, contains information on the compositional variation of the tissues. This complex interaction results in emergence of backscattered components from the multiple layers of tissues, at various locations away from the beam entry point, as proved experimentally [2,3] and theoretically [4,5]. The signals detected closer to beam entry point emerge from the tissue layers closer to surface, whereas, components from deeper layers emerge at farther distances. Thus by measurement of the backscattered components at various locations on the surface, for an optical beam incident normal to the tissue surface, the information on composition variation at various depths such as epithelium [6], breast and brain [7,8] and internal organs [9] are determined. The objective of the present work is to develop a noninvasive scanning system with hand-held optical probe for imaging of compositional changes in biological tissues. Methods: The probe is consisting of nine units, functioning simultaneously and each unit is equipped with one LED operating within red region and three photodetectors for measurement of backscattered radiations, placed at distances 7, 12 and 17mm from the photon injection port along the x-axis. For testing purpose three phantoms of paraffin wax are embedded with solid inhomogeneity at depths similar to that of placement of detectors away from the LED. The human in situ biological tissues are consisting of righ hand index finger, four fingers of the same hand and a part of the forearm below elbow joint are used. The data on optical backscattering, by moving the probe within grids on the surface of phantom/tissue are collected and after processing the respective images are obtained. Results: The data analysis with phantoms shows the spatial resolution capability of this procedure. Three images of the index finger show the tissue structural variation in various layers, which are in agreement with anatomical details and radiograph of the finger. The tissue details of four fingers are in agreement with that of individual finger. Similarly the three images of the forearm show the structural variation. Conclusion: These observations suggest that the present technique may be used to detect tissue compositional changes below the skin. This system with spatial resolution capability provides information on structural changes in tissues.        The schematic diagram of the optical scanning system is shown in Fig.1(a). This consisted of a hand-held optical scanning probe, signal conditioning unit, data acquisition unit and data processing unit. Optical Scanning Probe is equipped with 9 red light emitting diodes and 27 photo-diodes as detectors. The size of the scanning probe was 7.5x2cm. Each unit consisted of a source and three detectors. The source and detectors were arranged in a straight line along x-axis. The photo-diodes were placed at distances 7mm, 12mm and 17 mm from the center of the source and the distance between the detectors was 5mm. The output of each photodiode was connected to the amplifier-filter circuit and converted into voltage, digitized and fed to the computer. For preliminary studies related to detection of inhomogeneity by the scanning system, phantoms of paraffin wax embedded with objects at various depths, were made.  Prior to data collection the scanning probe was placed on a sheet of black rubber and dark current was measured. Thereafter it was placed on the surface of the prepared phantom and moved in steps of 1.25mm along a graduated scale and at each step the backscattered light was measured by the respective detectors.  Data were acquired from human forearm and hands and they were reconstructed as images. Results show that the embedded objects inside the phantoms as deep as 17 mm depth was identified by the scanning probe. Table 1 show the absolute value of peak intensity and the full width at half maximum of the images obtained from the scanning probe. The high intensity middle PIP joints of the index, middle, third and fourth fingers are clearly seen from the images obtained using the data obtained by detectors ‘C’ whereas the distal inter-phalangeal (DIP) joint of the middle finger is visible in the images from all the detectors.  The proximal phalanges of the middle three fingers are seen in the images constructed by data collected by detectors ‘B’ and ‘C’. The regions between the dark portions in the images are the regions between the fingers. The contour plots show the constant intensity lines within the images and they also show the presence of bones and joints of the scanned portion of the fingers. This scanning system could be improved in resolution for clinical use.     Fig. 1.(a). Schematic of optical scanning system, (b) placement of electronic components in the scanning probe (i), and their photograph (ii). TABLE I. PEAK INTENSITY (|PI|) AND FULL WIDTH AT HALF MAXIMUM (FWHM) OBTAINED FROM THE IMAGES CONSTRUCTED BY THE BACKSCATTERED DATA OF VARIOUS PHANTOMS.   Depth (mm) White plastic Black plastic Heart |PI| FWHM (mm) |PI| FWHM (mm) |PI| FWHM 7 0.085 4.9 0.410 4.9 0.082 4.8 12 0.017 4.8 0.133 4.4 0.017 3.6 17 0.002 5 0.004 4.3 - -     Images of the human fingers displayed as gray scale images (i) contour plots (ii) and colour images (iii), based on data collected by three detectors: A (a), B (b) and C (c)