Optical Imaging

The evolution of technology has given impetus to optical imaging, which is a valuable tool for the diagnosis and treatment of cancer. Generating and measuring an optical response in tissue is desired in order to obtain specific information pertaining to the diseased state of that particular tissue. The visual representation of an optical response enables the tissue to be interrogated with respect to cellular organisation and structure.

In vivooptical imaging has the potential to negate invasive measures such as surgery, which is commonly advocated in many PDT centres. Imaging systems can illuminate large tissue surface areas and therefore can provide pertinent information at numerous locations over the total area for a single image acquisition. Optical coherence tomography (OCT) and fluorescence imaging (FI) are optical imaging techniques, which provide diagnostic information on the spatial distribution and localisation of tumours [6].

2.2.1.1 Optical Coherence Tomography (OCT)

Optical coherence tomography (OCT) is a promising non-invasive optical imaging technique operating on the principle of low coherence interferometry (LCI), which produces (2D) two-dimensional cross-sectional images [7]. It offers morphological information pertaining to the tissue under investigation. Typically a light source – superluminescent diode (SLD) – with a short coherence length and a broad spectral bandwidth is used. The power from this light source is split into a sample wave and a reference wave. Light reflected by the sample, i.e., tissue, is combined to the light reflected by the reference mirror and differences in their respective pathlengths allow reconstruction of tissue reflectivity to represent depth [8]. The position of the mirror can be moved, which offers a measure of depth within the tissue from where the reflected signal came from [9]. For OCT, near-infrared electromagnetic radiation is employed while multiple LCI scans are acquired, which generate a 2D optical image. OCT has been widely used in studies of the human eye [10]. However, it is an

emerging imaging technique that may be used for skin studies. Cross sectional anatomy imaging of skin tissue can be performed on a micrometer scale, with high spatial resolution providing instant diagnostic information pertaining to the region of interest [11]. There are several reports on OCT and its application to PDT for skin cancer; Hamdoon et al. [12], have investigated OCT in lesion mapping, assisting in the correct delivery of PDT and monitoring the outcome. Results from this study indicate that OCT-guided PDT is a promising procedure, which attempts to efficiently discriminate between tumour and non-tumour margins, while monitoring and assessing the healing process. Gambichler et al. [13] reported that OCT images were capable of visualising altered skin architecture pertaining to BCCs. Furthermore, features that were observed only in images of lesional skin correlated well with corresponding sections of biopsy samples. While OCT has exceptionally high resolution, it has a limited penetration/detection depth of 1 – 1.5 mm; which is typical for OCT of skin tumours [10]. Further limitations of OCT include restricted sampled field sizes and multiple scans are necessary in order to acquire images. The practical application of OCT to the clinical environment is limited when compared to other optical diagnostic techniques particularly those involved in FD, such as fluorescence spectroscopy and fluorescence imaging. Therefore OCT is rarely used in PDT. Moreover, unlike OCT, FD may also be applied to monitor PDT treatment efficacy and investigate the rate change of fluorescence during and after treatment.

2.2.1.2 Fluorescence Imaging (FI)

Fluorescence imaging (FI) is based on the principle of detecting fluorescence from endogenous species, such as collagen, tryptophan, NADH and PpIX. The detection of fluorescence finds its origins as far back as 1924, when fluorescence emission from tumours under Woods light exposure was reported [14].In vivofluorescence imaging measurements involve illumination of a significant area of tissue surface and collection of the emitted light with a filtered camera for spectral discrimination [3]. When the tissue surface is imaged there is potential to extract relevant diagnostic information using fluorescence, which can identify precancerous and cancerous lesions, define lesion margins and guide localised treatment [15]. Fluorescence differences observed between adjacent tissues provide a description about the biochemical state of the tissue and the changes associated with disease development.

The advent of fluorescence biochemical markers has greatly assisted in optical non- invasive tumour detection and diagnosis [16]. As mentioned earlier in Chapter 1, Section 1.2.3 the photosensitiser pro-drug – ALA – is responsible for causing the build up of the potent photosensitiser, PpIX in tissue. Consequently, the fluorescence signal will be increased dramatically particularly when compared with intrinsic autofluorescence signals. The spatial distribution of fluorescence intensity levels can be recorded and subsequently report on specific locations of the lesion. Fluorescence images provide a visual inspection of the tissue, which allows for the demarcation between non-lesional and lesional tissue sites with high sensitivity. By combining an excitation light source together with a multispectral imaging system consisting of an intensified camera, image splitter and an image intensifier – to amplify the faint fluorescence signal – it is possible to not only obtain a visual inspection but also acquire quantitative fluorescence measurements [17]. Ultimately, fluorescence imaging may be used for dosimetry in PDT, guide tissue biopsy and surgical resection as it offers diagnostic feedback regarding a large area of suspicious tissue [1].