Imaging the Posterior Pole Flashcards
What are the different method sof imaging the posterior pole?
•Optical Coherence Tomography (OCT)
•Confocal Scanning Laser Ophthalmoscopy (cSLO)
- Fundus Autoflurescence - to scan fundus
- Fluorescein Angiography (FA)
- Indocyanine Green (ICG)
•Ultrasound (A- or B- Scan)
Why may we want to image the the posterior pole with more than just an ophthalmoscope?
Direct opthalmoscopy is only 2D - non stereoscopic.
Indirect ophthalmoscopy is stereoscopic - so you do get an element of depth perecption which is better, however, you still cannot see what happens in the retinal layers.
What does the OCT allow us to do?
Image retinal layers as a cross section
How does an OCT machine work?
- It is based on Non-invasive imaging technology
- Based on low-coherence interferometry whereby:
- 830nm light beam (infra-red) directed at target area.
- Magnitude and relative location of the reflected light is used to produce images
- Image based on optical properties of the microstructure of the tissue.
- Each imaged point generates an axial A-scan
-The more A-scans, the higher the resolution
•As the scanning beam moves across the tissue a cross sectional image is created = this is called a tomogram = also known as a B-scan.
(Basically A scan shows us resolution and whats there pictorally then the B scan shows us cross sectional images which indicate depth thus if you put all the B scans together you create a 3D image called a C scan).
How do OCT machines measure the light that is reflected back in each type of scan?
•Time domain OCT (TD-OCT à this is an older machine now) uses a moving mirror to measure time taken for light to be reflected (as time taken to get back would indicate depth)
-[400 A-scans per second - point of this info is to show you its slower]
- Spectral domain OCT (SD-OCT) measures multiple wavelengths of reflected light across a spectrum (produces better image quality than old machine).
- [40,000 A-scans per second - a.k.a faster than old machine]
How does the OCT machine allow us to view the different layers of the retina and any pathology if present?
What machine has been used to produce this image and what is it an image of?
OCT machine depicting layers of the retina.
[Just so you know choroid doesn’t have that highly structured layer thing going on that the retina does - it instead is highly fenestrated and thats why you get the bubbly appearance]
Can you tell me whether this image has been taken from a right or left eye?
Nerve fibre layer is thickest towards the optic disc.
In the image the nerve fibre layer is thickest on the left.
Thus this means the optic disc is on the left hand side.
We therefore know this is a left eye.
In a right eye what side is the optic disc on?
Right side ( nasally).
[Macula is always on the temporal side]
How are blood vessels viewed by in OCT?
As white specs/dots. (They are hyper reflective )
As the 830nm light passes through the structure of the blood vessel they cast a shadow on anything beneath them so you end up with a vertical shadow on the OCT image.
Can you view a foveal reflex on an OCT scan?
Yes - its the single white dot in the foveola region- have a look on the image. The green line in the middle of the box on the picture displaying the view via opthalmoscopy is where a single line section of the retina was taken via an OCT.
Why is there a number on the OCT scan?
It is where the OCT scan has been used to measure the size of a structure.
What are the three types of scans we can produce using an OCT machine?
A single line scan - so here we are looking at one cross section.
A raster scan - so this is basically a collection of single scans taken at different points (e.g. where the rows of green lines are on the picture) in order to show change across a structure - this scan would be viewed as a mini video.
OCT 3D scan- produces 3D image that moves.
[Presence of blood vessels can be seen as vertical shadows in the image in the OCT image]
What colour are OCT scans?
Black and white (although false colour can be added).
What are the advantages and disadvantages of adding false colour to an OCT scan?
Advanatges - Adding colour allows clear distinction between retinal layers
Disadvanatges - Loose resolution between definition of layers. Can be confusing due to what colours they use.
What does an OCT 3D/ C scan look like?
Imagine it moves as you scan along
Can an OCT machine allow for anterior chamber imaging?
Yes (using a seperate attachment).
Is this a normal OCT scan?
NO.
We have lost the regular structure of the retina (the linear appearnce).
Can see lots of hyperreflections (bright white specs) in the middle of the retina.
Lots of spaces which indicate seperation of the retina.
The circular spaces look like cysts.
Even at RPE level at Bruch’s complex we have lost that defining line (in the middle of the scan).
In this image which layer of the retina is affected?
What is Fundus AutoFluorescence (FAF) used for and how ?
It is used to RPE health.
- Lipofuscin is a byproduct of cell function
- It Accumulates in RPE with age or disease
- It Eventually leads to cell death of RPE cells
- Blue light at 488nm causes lipofuscin to autofluoresce
- A B&W image created.
[In the FAF image the dark spots represent the ONH (optic nerve head) . BVs and macula as blue light is absorbed by macula pigment.]
How does Fluorescin Angiography work?
•Fluorescein sodium dye injected as a bolus (a small mass) into a peripheral vein
(Dye metabolized by kidneys and eliminated via urine within 24 to 36 hrs)
- Fluorescein excites at 490 nm and emits at 530 nm
- Specialized fundus camera or cSLO capture rapid-sequence photographs of retina
- Photographs or video images are taken as dye passes through blood vessels in eye,
What are the phases of Fluorescin angiography as in when can the images be seen?
