Endoscopic Diagnostic System Using Autofluorescence

A fluorescence imaging system (Xillix LIFE – Lung Fluorescence Endoscopy system) using fluorescence for the accurate diagnosis and early detection of lesions through an endosocope has been developed. This system has applied an optical diagnostic technology to functionally diagnose lesions which have been difficult to morphologically recognize or are occult with conventional endoscope. The benefit of this system in the diagnosis of lung cancer has already been confirmed in the US and Japan, and feasibility of the system in the gastric intestinal field has also been evaluated.


INTRODUCTION
By adopting spectroscopic technologies such as fluorescence of light, absorption of light, and Raman-scattered light, abnormalities of living tissue can be detected on the cellular or macroscopic level. Recently, a new technology, optical diagnosis, has been calling attention that applies the above spectroscopic technologies to the "living body as it is" to functionally diagnose lesions that have been difficult to be determined by conventional morphological diagnosis [1][2][3][4]. Particularly, research and development of "fluorescence imaging" and "optical biopsy" have been actively implemented: fluorescence imaging has been developed in view ofearly detection and diagnosis of lesions, that have been difficult to detect with conventional endoscopy.
Corresponding author. This has been done by means of displaying in vivo video images endoscopically using autofluorescence emitted from living tissue. Emitted autofluorescence is a result of when a light of specific wavelength (light between UV and blue region) excites tissue. This can be done without using drugs such as photosensitizing substances [5][6][7]. Optical biopsy has been developed to provide pathological diagnosis with an optical fiber inserted through an endoscope channel, in place of biopsy [8,9]. These autofluorescence technologies are expected to be established in the near future as a new diagnosing method in conjunction with an endoscopic fluorescence imaging endoscopes.
As stated above, development of an endoscopic fluorescence imaging system has been conducted to endoscopically display image of autofluorescence 60 S. TAKEHANA et al.
to provide new diagnostic form by exploiting the difference between the spectrums of normal and abnormal tissue [10,11].

Overview of the Device
The LIFE-Lung system was first developed in the world by Xillix Technologies Corp., Canada, in 1990, and the system was found to be useful in detecting early lesion [10][11][12]. This system has received FDA approval in 1996. Olympus Optical Co., Ltd. started selling the product worldwide in 1997. The outlook appearance of the system is shown in Fig. 1    As stated in the above, this system does not require administration of drugs such as photosensitized substances for the early detection and diagnosis of lesion, thus there is no need to anticipate sideeffects. Also, the laser output of this system is kept low and its safety with regard to bioeffects has been confirmed. Therefore, this system can be usedjust as with conventional white-light bronchoscopy.

Principle of Autofluorescence
Fluorescence imaging is designed for the early detection and diagnosis oflesions by endoscopically illuminating light against tissue, processing the emitted fluorescence into images, and displaying such lesions that have been difficult to detect or diagnose under conventional white-light endoscopy as the difference of fluorescence intensity or color tone. Figure 3 is the result of autofluorescence spectrum measured ofnormal and abnormal tissue when blue light was illuminated to those living tissues through an optical fiber inserted into the endoscope channel [5,6].  of 442 nm (He-Cd laser). While in normal tissue, the shape of fluorescence spectrum demonstrated its peak in green with gradual decrease at longer wavelength, the shape of overall spectrum in abnormal tissue was rather flat with decreased spectrum centering to the short wavelength side and with a small rise in red side.
Although the precise mechanism of different autofluorescence spectrums detected in normal and cancerous tissue has not been established, the following has been considered: It has been considered highly possible that collagen, flavoprotein, NADH, and porphyrin, known as autofluorescence substances existing in the living tissue, have something to do with the different spectrums as has the tissue architecture and the blood content.
With this LIFE-Lung, it has been considered that the system observes the following differences based on the wavelength excited/detected from the above substances.
(1) Cancerous tissue has higher metabolism than normal tissue and therefore, its blood volume increases while oxygen concentration in the cells decreases. Because of the increase of blood volume and accumulation character which is specific to cancer, the amount of porphyrin is increased (red fluorescence is increased), and at the same time, flavin is reduced (green fluorescence is decreased) as oxygen conoentration is decreased.
(2) Autofluorescnece is intensely produced from submucosa stroma (ex. collagen), but epithelium, mucosa, and cancerous tissue emit very little fluorescence. Because of thicker epithelium and mucosa in cancerous region than in normal region and of the presence of cancer, fluorescence of green region is intensely absorbed. (Tissue permeability of light is higher in red region than in green region: compared to conventional white-light bronchoscopy alone, detection ratio of metaplasia and cancer was improved from 65% to 90%, and that specificity was improved from 71% to 77.4% by using autofluorescence bronchoscopy jointly with conventional bronchoscopy. Ikeda et al. also report that detection ratio for metaplasia was improved from 28% to 96% by the joint use of autofluorescence bronchoscopy with conventional bronchoscopy than by the latter only. Since a screening test using sputum cytology has been carried out particularly in Japan, the LIFE-Lung is expected to serve in diagnosis of early lung cancer in subjects who have turned out to be positive in the sputum cytology. Application to GI Figure 4 shows autofluorescence spectrum of normal and abnormal (precancerous) tissue when the light of 437 nm (a combination of high-pressure Halogen lamp and a blue band-pass filter) was emitted into the esophagus. Autofluorescence spectrum similar to that in the bronchi is shown. Based on this fact, Olympus Optical Co., Ltd. and Xillix Technologies Corp. has jointly been developing a fluorescence endoscopy system for gastric intestinal tract (LIFE-GI imaging system) based on LIFE-Lung.
In the gastrointestinal tracts such as stomach and colon where body cavity is larger than in bronchi and whose mucosa contains a lot of capillary blood vessels, good-quality fluorescence image cannot be acquired with the system designed for bronchi due to the shortage of excited light intensity. Therefore, by combining mercury lamp and blue-exiting filter to blue light source, twice as much intensity of light as that of He-Cd laser can be obtained at the tip of the endoscope, which enabled autofluorecence observation in the large body cavity of gastrointestinal tracts. By using a device in which this blue light

DISCUSSION
Various studies have long been carried out in an attempt to diagnose cancer by using fluorescence, but observing the autofluorescence was especially difficult due to its weak intensity. However, recent improvement in illumination and in high-sensitivity camera has enabled real-time display and observation of even subtle changes in autofluorescence endoscopically. The technology was developed and put into practice using the technology developed in the LIFE-Lung system.
The past clinical findings suggests the feasibility of autofluorescence endoscopy to become an effective means of diagnosis for the detection and localization of precancerous tissue such as micro cancer and metaplasia in the bronchi and gastrointestinal tracts. We are also expecting that in the future, combination of this technology for early diagnosis and minimal invasive treatment such as endoscopic treatment and/or PDT will increase opportunity for patients to undergo high QOL treatment of minimally invasive.