Clinical Background

Glaucoma is the leading reason of irreversible blindness in the world. It is a disease involving both anterior and posterior segments of the eye. Abnormal anatomical structures of the chamber angle may cause elevated IOP and gradually leads to glaucomatous optic neuropathy. Such kind of condition is named as “primary angle-closure diseases” (PACD), which is the major type of glaucoma in Asia.

PACD patients have a characteristic structural difference from open-angle subjects in chamber angle and ocular biometric parameters, including narrow chamber angles, short axial length, thick lens, etc.

In clinical practice, we have various ways to image the angle structures: gonioscopy, ultrasonography, eyecam, optical coherence tomography (OCT), etc. Among them, gonioscopy is the gold standard in the classification of angle structure. Ophthalmologists grade angle width into different levels according to the angle structures seen under gonioscopy. However, gonioscopy is a contact examination and relies on ophthalmologists’ personal experience.

OCT provides us with non-contact imaging of the whole anterior chamber. In the old days, due to the limitation of scanning speed, only line scans of the anterior chamber could be obtained. With the invention of the swept-source light source, fast imaging of 3D anterior chamber structures became possible. OCT is able to clearly show the anatomical details of the chamber angle.

OCT scanning protocol and manufacturers

The images available in this Challenge are acquired with CASIA I (SS-1000, Nagoya, Japan). SS-OCT examination was performed in a standardized darkroom with a light intensity of 0.16lux. The anterior chamber was divided into 128 meridians. 16 slices were extracted from each volume scan. Angle structure was classified into open, narrow, closure and unreadable. Gonioscopy was used as the reference standard. The scleral spur was localized on each side of the anterior chamber by 4 ophthalmologists independently.

Clinical Significance

OCT examination is a fast and non-contact method to capture the morphology of the anterior chamber. We can easily identify open and narrow/closed angles based on OCT images. Besides, we could perform measurement of angle parameters, including angle open distance (AOD), trabecular iris space area (TISA), etc. Quantification of these parameters relies on the localization of specific mark — scleral spur (SS), which appears as a wedge projecting from the inner aspect of the anterior sclera in cross-sectional images. SS is attached anteriorly to the trabecular meshwork and posteriorly to the sclera and the longitudinal portion of the ciliary muscle.    

Diagnostic Criteria and Ground-truth Annotation

Classification of angle width (open/closed) was based on gonioscopy by glaucoma experts. SS localization was determined by the mean of 4 independent annotations from a group of ophthalmologists, followed by a manual adjustment from a senior glaucoma expert.

Acknowledgement

Thanks for iChallenge-PACG study group.

iChallenge-PACG study group includes:

Feiyan Deng;

Nuhui Li;

Xingyi Li;

Huang Luo;

Yi Sun;

Chunman Yang;

Yichi Zhang;

Rouxi Zhou.

Attention: Group members are ranked by alphabetical order of last name; if the last names are the same, then they will be sorted by alphabetical order of first name. If you have any questions, please contact us.