OCT scans Flashcards
what are the SSI values for the following: retina map, ONH and GCC
retina map: >39
ONH: >28
GCC: >32
what is the average RNFL thickness
what is the inter eye symmetry for rnfl
> 75 µm
Inter-eye asymmetry should be <9 µm
4 factors affecting RNFL thickness
- higher Age, thinner RNFL
- bigger Optic Disc size, thicker RNFL
- Females = thicker RNFL
- High Myopes = thinner RNFL
what is the TSNIT RNFL thickness graph used for
TSNIT plot compares the RNFL thickness to the normative data base
to detect locallisied RNFL thickness
what are the 3limitations of using OCT RNFL in glaucoma
large overlap exists between normal and early glaucomatous eyes
averaging of measurements within a fixed area results in an underestimation of results
The effects of MYOPIA are NOT considered in current normative databases in OCT
what is the abnormality seen in TSNIT graph for high myopes and tilted disk?
BLACK LINE: High myope; peak has been shifted towards the temporal region (see BLUE arrows), resulting an abnormal dipping picked up by the OCT (RED arrows)
2 things to take note of in GCC
- Inter-eye asymmetry is GREATER than 5µm
- Asymmetry pattern is across the temporal horizontal raphe
what is the man structure of reference in anterior angle
scleral spur
4 limitations of RTVue
The shorter wavelength of 830 nm used in RTVue encounters signal loss due to scattering at the limbus and angle.
This causes poorer visualization of the scleral spur and iris root, compared to longer wavelength (1310 nm) OCT systems
The Schwalbe’s line, which represents the termination of Descemet’s membrane, is an anatomic feature that is consistently visible on all high quality images
Measurement of the distance between Schwalbe’s line and the anterior surface of the iris defines the AOD at Schwalbe’s line (AOD-SL), a method of quantifying the angle width on RTVue images
explain pre-retinal, intra-retinal and sub-retinal
pre-retinal: between vit and retina or within vit
intra-retinal: within retina (ILM to ELM)
sub-retinal: slightly above RPE
in retina thickness map, what regions of the retina are present in full, inner and outer
Full: ILM to RPE
Inner: ILM to IPL
Outer: IPL to RPE
what are the 5 artifacts seen in OCT?
Mirror artifact
Vignetting
Misalignment
Blink
Motion
what causes mirror artifact? identification? limitation?
due to the eye is positioned INCORRECTLY or when the retinal features of interest span a large depth range
Inverted, partly inverted, possibly poor resolution image
can occur in highly myopic eye with a long axial length (nothing we can do to mitigate)
what causes vignetting artifact? identification? remedy?
occurs when part of the OCT beam is blocked by the iris (px could have small pupils)
characterized by a loss of signal over one side of the image
reposition the OCT machine so that it is the correct distance from the eye, dim room lights and close room doors
what causes misalignment artifact? identification? remedy?
occurs when the ETDRS grid in a quantitative volumetric scan is NOT centered on the fovea
(poor fixation, poor attention)
normal foveal depression that usually appears blue on the ETDRS map is not aligned with the centre of the ETDRS macular grid
manually correct misalignment on most SD-OCT machines. Alternatively, the patient can be rescanned using external fixation
