- Summary
- Types of Retinal Imaging Tests
- Optical coherence tomography (OCT)
- Fundus photography
- Wide-field retinal imaging systems
- Fluorescein angiography
- Ultrasonography
- References
- Author(s)
Summary
Retinal imaging tests are a valuable tool for ophthalmologists in the diagnosis and management of retinal conditions. These tests allow ophthalmologists to evaluate the health of the retina and detect any abnormalities or issues that may affect vision. Regular retinal imaging tests can help to detect issues early on and allow for timely treatment, which can help to preserve vision and prevent further vision loss.
Types of Retinal Imaging Tests
Optical coherence tomography (OCT)
A non-invasive diagnostic technique that renders an in vivo cross-sectional view of the retina. OCT utilizes a concept known as interferometry to create a cross-sectional map of the retina. OCT is excellent at measuring retinal thickness, so it’s helpful for diseases that cause fluid buildup, such as retinal vein occlusion (RVO) and diabetic macular oedema.
Fundus photography
Traditional fundus photography captures 30- to 50-degree views of the retina and optic nerve. Imaging of the retina poses difficulty as the retina must be illuminated and imaged simultaneously through a narrow pathway (the pupil). The camera uses the principle of reflex-free indirect ophthalmoscopy - the rim of the pupillary aperture is used for illumination and the centre of the pupil for imaging, hence the overlapping reflections are minimised.
Wide-field retinal imaging systems
WFI refers to imaging beyond 50 degrees field area. UWFI systems can image upto 200 degrees. These allow simultaneous imaging of central and peripheral retina.
Confocal scanning laser ophthalmoscopy imaging (CSLO) systems
The integration of the confocal scanning laser ophthalmoscope (CSLO) and OCT has produced a dynamic new instrument, the OCT ophthalmoscope. SLO systems use laser light to illuminate the retina, instead of bright flashes of light. This reduces scatter of light in images acquired. Examples include Optos® and Spectralis®.
Fluorescein angiography
Fundus fluorescein angiography (FFA) is an invasive diagnostic procedure. This test involves injecting a dye into the patient's bloodstream. The fundus camera is equipped with two filters. An excitation filter (465 to 490 nm) allows only blue light to enter the eye to excite the fluorescein molecules of the dye. A barrier filter (520 to 530 nm) allows only the green light emitted by the dye to pass through.[4] It blocks the light of any other range that is reflected from the eye. Injection of the dye is timely coordinated with the process of taking photographs.
Fluorescein angiography is relatively contraindicated in pregnancy, and a history of severe allergic reactions is an absolute contraindication.
Ultrasonography
There are two main types of ultrasound used in ophthalmologic practice currently, A-Scan (8 MHz) and B-scan (10 MHz).The data collected by the transducer produces a corresponding image. In the A-scan, spikes form that correspond with tissue interface zones which can help characterize tumor structure and also measure eye length. Measurements derived from B-scan include visualization of the lesion, including anatomic location, shape, borders, and size.
References
- John F. Salmon MD FRCS. Kanski’s Clinical Ophthalmology: A Systematic Approach. 10th edition, Elsevier, 2024.
- ‘Retinal Imaging: Choosing the Right Method’. American Academy of Ophthalmology, 1 July 2014, https://www.aao.org/eyenet/article/retinal-imaging-choosing-right-method.
- Ruia, Surabhi, and Koushik Tripathy. ‘Fluorescein Angiography’. StatPearls, StatPearls Publishing, 2023. PubMed, http://www.ncbi.nlm.nih.gov/books/NBK576378/.
- Mishra, Chitaranjan, and Koushik Tripathy. ‘Fundus Camera’. StatPearls, StatPearls Publishing, 2023. PubMed, http://www.ncbi.nlm.nih.gov/books/NBK585111/
- Nagaraju, Rashmi M., et al. ‘Efficacy of High Frequency Ultrasound in Localization and Characterization of Orbital Lesions’. Journal of Clinical and Diagnostic Research : JCDR, vol. 9, no. 9, Sept. 2015, pp. TC01–6. PubMed Central, https://doi.org/10.7860/JCDR/2015/13021.6428.
Author(s)
Dr Sara Memon
Sara is the Co-Founder of Ophtnotes. She is a doctor who graduated from UCL Medical School in London. She won the Allen Goldsmith Prize in Ophthalmology. Sara is also the co-founder of PAMSA: an organisation linking doctors and medical students of Pakistani origin. She’s especially passionate about teaching and education, having presented a workshop she designed herself at the 2019 Annual GMC Conference.