Ординатура / Офтальмология / Английские материалы / Automated Image Detection of Retinal Pathology_Jelinek, Cree_2009
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11
Tele-Diabetic Retinopathy Screening and
Image-Based Clinical Decision Support
Kanagasingam Yogesan, Fred Reinholz, and Ian J. Constable
CONTENTS |
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11.1 |
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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11.2 |
Telemedicine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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11.3 |
Telemedicine Screening for Diabetic Retinopathy . . . . . . . . . . . . . . . . . . . . . . . . . |
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11.4 |
Image-Based Clinical Decision Support Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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11.5 |
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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11.1Introduction
Telemedicine services such as specialist referral services, patient consultation, remote patient monitoring, medical education, and consumer medical and health information use provide major benefits to health systems. Telemedicine services reduce health care costs and enables early detection of blinding eye conditions, e.g., diabetic retinopathy. Several feasibility studies of telemedicine screening for diabetic retinopathy have been reported. These studies demonstrate the enormous usefulness of the technology for the communities living in rural and remote areas. In combination with automated image analysis tools and clinical decision support systems, telemedicine could provide widespread screening with the help of less expensive staff and empower local clinicians in the decision making process for eye conditions.
11.2Telemedicine
With an increasing population it is a challenge to provide specialist health care to all. The advent of telemedicine has opened new vistas in patient care and disease
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management. Telemedicine means exchange of clinical data via communication networks to improve patient care. Application of telemedicine in ophthalmology and teleophthalmology has already become a common tool in mainstream eye care delivery in many countries [1]. Telemedicine service delivery offers a number of significant advantages over conventional face-to-face consultations. Major benefits of telemedicine are: (1) service access can be improved considerably in remote areas,
(2)telemedicine is cost effective due to reduction in patient or staff transfers [2],
(3)patients have the benefit of receiving treatment where they live without travelling, and (4) training of local medical officers and nurses to gain knowledge about eye diseases and diagnosis.
There are two modes of delivery in telemedicine. One is real-time video conferencing, which is widely used for emergency and psychiatry consultations. Real-time consultations require high-bandwidth and scheduling. They need both clinician and patient to be present at the same time. This may be not possible in the case of specialists like ophthalmologists. In ophthalmology, high-resolution color images are required to capture the fine detail in the eye. Image files can be large and transmission of these high-resolution images can be time intense. Therefore, teleophthalmology is normally not suitable for video conferencing.
The second mode is called store-and-forward, where images are captured, compressed, and stored in the computer for transmission at a later stage. This is a costeffective mode that utilizes low bandwidth (Internet) and can send high resolution images. Ophthalmology is an image-based diagnostic field, and most of the diseases can be identified from retinal and anterior segment images. Therefore, specialist ophthalmic care can be easily delivered using the store-and-forward method.
11.2.1Image capture
One of the procedures of the store-and-forward method is image capture. It is the most important part of the system. There are three different imaging devices, namely the slitlamp, fundus camera, and ophthalmoscope, to image the anterior segment and retina. Corneal opacification from trachomatous scarring, vitamin A deficiency, injuries, or bacterial and viral keratitis can be readily imaged using a video slitlamp biomicroscope or external macro-photography. Cataract can also be documented by each of these methods, or by the use of more complicated photographic systems. Glaucomatous cupping of the optic disc can be detected by standard ophthalmoscopy, fundus photography, and stereo fundus photographic systems or by scanning laser ophthalmoscopy [3; 4]. Diabetic retinopathy is detected by slitlamp biomicroscopy with a fundus lens (+78 to +90D), fundus photography (large or small pupil instruments), or by conventional ophthalmoscopy. The relative efficacy of these methods has been examined in several field trials [5; 6]. Each examination method has advantages and disadvantages related to portability, cost of equipment, ability to obtain hard copy or digitized records, resolution of images, and ease of use. There are other, more advanced but also very expensive imaging devices such as OCT (Optical Coherent Tomography) and SLO (Scanning Laser Ophthalmoscope) to study the retina. These devices are limited to major eye clinics.
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Table 11.1: Camera Pixel Number Required to Resolve Detail of |
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Various Resolutions at the Retina |
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Resolution (mm) |
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Pixel number (megapixels) |
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30° FOV |
50° FOV |
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3.5 |
(Sparrow limit) |
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12 |
32 |
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4.5 |
(Rayleigh criterion) |
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7 |
20 |
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10 |
(Typical photographic |
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fundus camera resolution) |
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1.5 |
4.5 |
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20 |
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0.4 |
1 |
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11.2.2Image resolution
Image resolution, which impacts on image quality, is very important for the diagnosis of eye conditions. Poor quality images may lead to poor diagnosis. The resolution of any optical instrument is defined as the shortest distance between two points on a specimen that can still be distinguished as separate entities, first by the optical camera system and second by the observer. The pixelated camera sensor samples the continuous optical images of the retina that the human ocular system and optical components form on the surface of the sensor. For both, the optical image formation and the periodic sampling at discrete points in time and space, there are well established theories that describe the dependence of the achievable resolution on myriads of imaging conditions and parameters (such as wavelength, illumination or imaging cone, spatial or temporal cut-off frequencies, signal-to-noise ratio, and signal strengths).
It is an important design consideration for ophthalmoscopic instruments to match the optical and the digital resolutions. If the digital resolution is too small, then information may be lost. On the other hand, an increase in digital resolution above the optimum value will not improve the definition of the image. However, the disadvantages are: (1) a sensor with a larger pixel number is more expensive; (2) the light sensed per pixel is less, which gives rise to more noise in the image; and (3) the image file size increases, making it more time consuming to process and transmit the file.
Due to considerations for eye safety (maximum permissible exposure) and patient comfort the power of the illuminating light is restricted to certain upper limits, depending on wavelengths and exposure time. As a consequence, the information being reflected off the retina containing light is very weak. Or in other words, the number of signal photons arriving at the sensor for detection is small. This, together with the inherent shot noise (Poisson statistics) of light bursts, limits the number of distinguishable grey levels in digital ophthalmic instruments. A dynamic range of 8-bit in each color band is therefore more than sufficient for the presentation of such images to ophthalmologists or for quantitative data analyses.
In Table 11.1, the required digital image resolution (pixel number) of the camera is given in terms of the desired retinal feature size to be resolved (the smallest value of 3.5 mm results from the maximum obtainable optical resolution with normal human
