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Investigations > OCT > Flashcards

OCT Flashcards

1
Q

What is the function of OCT?

A

provides high-resolution images of the neurosensory retina in a non-invasive manner

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2
Q

What is OCT analogous to and how does it differ?

A

ultrasound but measures light waves rather than sound waves

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3
Q

What principle underpins OCT measurements?

A

they are achieved indirectly using interferometry

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4
Q

How does OCT work?

A
  • combination of light reflected from a tissue of interest, and light reflected from a reference path, produces characteristic inferference patterns depending on the mismatch between the reflected waves
  • time delay and amplitude of one of the waves (i.e. reference path) are known, so time delay and intensity of light returning from sample tissue can be determined
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5
Q

What is the name of the resulting plot of light intensity vs time delay and what does it describe?

A

A scan: anatomy of eye tissue at a specific point

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6
Q

How does an A-scan give rise to a B scan?

A

A-scans are repeated at multiple transverse locations and mapped to a grey or false-colour scale, giving rise to 2D cross-sectional (tomographic) images = B scans

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7
Q

What are 4 indications for OCT?

A
  1. monitoring response to treatment and/or disease activity in patients with chorioretinal vascular and inflammatory diseases (e.g. neovascular AMD, diabetic retinopathy, RVO, CMO)
  2. Diagnosis of clinically occult macular pathology e.g. subtle abnormalities of the vitreoretinal interface
  3. Detection of glaucomatous damage to the RNFL and/or optic nerve head
  4. Assessment and longitudinal monitoring of disc volume, and RNFL and ganglion cell layer (GCL) in disc swelling and papilloedema
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8
Q

What is the colour scheme of OCT false-colour image?

A
  • Highly reflective tissue is reddish white
  • Hyporeflective tissue is blue-black in colour
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9
Q

What is the laternative to false-colour OCT B scans?

A

256 shades of grey - corresponding to differnt optical reflectivities

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10
Q

Which retinal layers are hyporeflective on OCT?

A

inner and outer nuclear layers and ganglion cell layer

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11
Q

Which layers are hyperreflective on OCT?

A

inner and outer plexiform layers and nerve fibre layer (NFL)

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12
Q

How are larger retinal vessels seen on OCT?

A

hyperreflective foci located in the inner retina, with underlying ‘shadowing’

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13
Q

What 3 things do the number of hyperreflective bands in the outer retina consist of?

A
  • external limiting membrane
  • photoreceptor inner segment-outer segment (IS-OS) junction (ellipsoid zone)
  • RPE
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14
Q

What can be used so that the choroid and choroidal-scleral junction may also be seen with OCT?

A

specialised scanning protocols: ‘enhanced depth imaging’

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15
Q

What pathology is shown in the OCT image?

A

full-thickness macular hole (stage 3)

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16
Q

What pathology is shown in the OCT image?

A

central serous retinopathy

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17
Q

What pathology is shown in the OCT image?

A

geographic atrophy

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18
Q

What pathology is shown in the OCT image?

A

soft drusen

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19
Q

What pathology is shown in the OCT image?

A

fibrovascular pigment epithelial detachment (PED)

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20
Q

What pathology is shown in the OCT image?

A

serous pigment epithelial detachment (PED)

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21
Q

What pathology is shown in the OCT image?

A

cystoid macular oedema

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22
Q

How many axial scans per second are achieved with time domain OCT using Stratus OCT?

A

400 axial scans /s

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23
Q

What is the axial resolution fo time domain OCT with Stratus OCT?

A

10 microns

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24
Q

What SD OCT?

A

Spectral domain (or Fourier domain) OCT using Spectralis HRA/OCT or Cirrus scan has a rate of at least 20 000-40 000 A scans per second with axial resolution between 3-8 microns

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25
Q

How can image acquisition speed be further increased in newer swept source OCT devices?

A

employ tunable lasers

26
Q

How is depth penetration be improved in newer forms of OCT devices?

A

use longer wavelengths of light (e.g. 1050nm)

27
Q

How do OCT devices measure retinal thickness?

A
  • each device incorporates image analysis software, measures it via automated detection (‘segmentation’) of inner and outer retinal boundaries.
  • possible to measure thickness at multiple locations and to construct retinal thickness maps corresponding to the ETDRS subfields
28
Q

Quantitative assessment of which structures is possible with newer OCT systems?

A

drusen and geographic atrophy in AMD

29
Q

How can OCT be used for detection/monitoring of glaucoma?

A

specialised circular OCT scanning protocols used with a single circular B-scan, centred on the optic disc and with a fixed diameter can detect/monitor it

30
Q

How can OCT be used to assess peripapillary RNFL thickness + what can this allow detection of?

A

segmentation of the inner and outer boundaries of the RNFL - presence of glaucomatous RNFL thinning can be determined by comparison with normative databases

31
Q

Why can’t measurements from different OCT machines be compared directly?

A

different algorithms are used for acquisition and segmentation between each platform

32
Q

What are 8 factors that can cause error in RNFL analysis?

A
  1. incorrect age of patient entered
  2. poor signal strength
  3. decentration of the scan
  4. long or short axial lengths
  5. interindividual differences in the spatial distribution of nerve fibre bundles (developmental)
  6. cyclotorsion
  7. Peripapillary atrophy (PPA)
  8. segmentation errors
33
Q

What are 5 factors that can cause automated segmentation errors with OCT?

A
  1. poor tear film
  2. dry eyes
  3. corneal opacities
  4. cataract
  5. vitreous opacities
  6. papilloedema
34
Q

What is OCTA (OCT angiography)?

A

non-invasive, non-contact imaging modality allowing detailed visualisation of the retinal and choroidal vasculature,** without need to use IV contrast agent**

35
Q

What is the acquisition time of OCT angiography compared with conventional angiography?

A

it has short acquisition time compared to conventional

36
Q

What is the acquisition time of OCT angiography compared with conventional angiography?

A

it has short acquisition time compared to conventional

36
Q

What is the acquisition time of OCT angiography compared with conventional angiography?

A

it has short acquisition time compared to conventional

36
Q

What is the acquisition time of OCT angiography compared with conventional angiography?

A

it has short acquisition time compared to conventional

37
Q

How repeatable is OCT angiography?

A

easily repeatable

38
Q

How are OCTA images achieved?

A

taking multiple B-scans in the same location over short periods of time; flow of blood through these sections is detected through its effect on reflected light (e.g. changes in phase, amplitude, speckle noise, combination)

39
Q

How may ongoing developments allow variable flow rates (slow and fast) to be detected with OCTA?

A

variable inter-scan time acquisition (VISTA) protocols

40
Q

What are 5 commercially availably OCTA systems?

A
  1. AngioVue
  2. AngioPlex
  3. Triton
  4. OCT2
  5. RS-3000
41
Q

How does the technology of OCTA compare with FFA/ICG?

A

OCTA - technology yet to be established but FFA/ICG technology is well-validated

42
Q

How does OCTA compare with FFA/ICG in terms of invasiveness/physical risk to pt?

A

OCTA is non-invasive, no need for contrast and therefore no risk of anaphylaxis

43
Q

What is the acquisition time like of OCTA vs FFA/ICG?

A

rapid with OCTA whereas FFA/ICG is time consuming

44
Q

How does the information on retinal/choroidal vasculature differ in OCTA vs FFA/ICG?

A

OCTA provides depth information on both retinal and choroidal vasculature but there is no information about individual layers in FFA/ICG

45
Q

What is the resolution of OCTA vs conventional FFA and ICG?

A

OCTA - high resolution with detailed view of the capillaries in the retina, but lower resolution in conventional

46
Q

What is the detection of flow and leakage like in OCTA vs FFA/ICG?

A

OCTA has detection of flow but not leakage; leakage imaged in conventional

47
Q

What is the recognition of artefacts like in OCTA vs conventional angiography?

A

recognition of arterfacts required for interpretation occurs in OCTA; there is less artefact in conventional

47
Q

What is the recognition of artefacts like in OCTA vs conventional angiography?

A

recognition of arterfacts required for interpretation occurs in OCTA; there is less artefact in conventional

48
Q

What are 6 indications for OCTA?

A
  1. diagnosis of CNV (e.g. neovascular AMD)
  2. assessment of chorioretinal disease when findings from conventional angio likely to be equivocal
  3. assessment of retinal non-perfusion
  4. visualisation of retinal vascular abnormalities
  5. assessment of longitudinal monitoring of optic nerve head vasculature
  6. visualisation of anterior segment vascular structure
49
Q

What are 2 examples of chorioretinal disease where disease assessment may be equivocal in conventional angiography but OCTA can be useful?

A
  1. chronic CSC
  2. inflammatory CNV
50
Q

What are 2 examples of retinal non-perfusion that can be assessed with OCTA?

A
  1. RVO
  2. Diabetic retinopathy
51
Q

What are 5 examples of retinal vascular abnormalities that can be visualised with OCTA?

A
  1. retinal artery macroaneurysm
  2. telangiectasia
  3. Intraretinal microvascular abnormalities (RMAs)
  4. new vessels elsewhere (NVE)
  5. new vessels on the optic disc (NVD)
52
Q

What are 5 examples of retinal vascular abnormalities that can be visualised with OCTA?

A
  1. retinal artery macroaneurysm
  2. telangiectasia
  3. Intraretinal microvascular abnormalities (RMAs)
  4. new vessels elsewhere (NVE)
  5. new vessels on the optic disc (NVD)
53
Q

What are 3 limitations of OCTA?

A
  1. yet to be validated across ocular disease
  2. small field of view
  3. imaging artefacts
54
Q

Why do artefacts occur with OCTA?

A

eye movements in patients with poor fixation or due to limitations inherent in the technology (e.g. so-called ‘projection’ artefacts)

55
Q

What is the first stage of interpreting an OCTA?

A

determine in which layer (Retinal vs choroidal) the pathology lies and the area of interest to be scanned

56
Q

How can the OCTA image be assessed for abnormal flow and what is done after this?

A

examine the cross-sectional OCTA - choose the preset segmentation pattern that best captures the area of abnormal flow.
then manual manipulation of the segmentation to optimise the en face OCTA image may be required

57
Q

What is shown in the OCTA image?

A

secondary CNV in L eye of pt with multifocal choroiditis

Investigations (29 decks)

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