ATLAS Optical Coherence Tomography of Macular Diseases and Glaucoma Amod Gupta, Vishali Gupta, Mangat Ram Dogra
INDEX
A
Acquired ocular toxoplasmosis 465
Acquired toxoplasmic retinochoroiditis 462
Acute posterior multifocal placoid pigment epitheliopathy 502
Adult–onset foveomacular vitelliform dystrophy 394, 404
Advanced interactive analysis 39
Advantages of spectral domain over time domain OCT 671
Age-related macular degeneration 296
choroidal neovascular membrane contiguous to geographic atrophy 300
classic CNVM 346
cystoid macular edema 311
disciform scar in ARMD 303
distinct RPE excrescences 324
drusen 296
fibrovascular PED 302
geographic atrophy 296
intermediate-risk non-neovascular ARMD 298
neovascular ARMD 296
nodular pattern 326
non-neovascular ARMD 296
occult CNV with minimally classic component 307
occult CNVM 345
patterns of drusen 324
persistent CNVM following thermal laser for extrafoveal classic CNVM 313
photodynamic therapy for juxtafoveal CNVM 315
pigment epithelium detachment without neovasculari-zation 334
point-to-point registration in dry ARMD 330
preapoptotic stage of geographic atrophy 329
progression of AMD from dry to wet 340
response to anti-VEGF treatment 347
retinal pigment epithelial changes 327
RPE changes with RPE cell migration 328
RPE changes without RPE cell migration 327
RPE rip following intravitreal anti-VEGF injection 350
saw tooth pattern 325
spectral domain OCT 318
transpupillary thermotherapy 307,311
Alignment and focus 22
Analysis of scan : spectral domain OCT 38
Analysis protocols for glaucoma 604
Assessment of progression 596
B
Basics of time domain (TD) optical coherence tomography 3
Behçet's disease 475
Best disease with choroidal neovascular membrane 395
Best's disease 398, 401
Better reproducibility 673
Bietti's crystalline tapetoretinal dystrophy 390
Branch retinal artery occlusion 241
Bull's eye maculopathy due to hydroquinine toxicity 408
C
Calculation of hole form factor 204
Case illustrations 622
early glaucoma in one eye 631
fallacious progression on visual fields 640
glaucoma suspects 622
glaucoma with macular pathology 646
ocular hypertension 627
pre-perimetric glaucoma in fellow eye 631
retinal pathology 549
with high IOP 652
with suspicious optic 649
suspicious optic 622
follow-up 627
Heidelberg retinal tomogram 627
visual fields 623
unable to understand 636
unreliable visual fields 636
Central areolar choroidal dystrophy 393
Central retinal vein occlusion with branch retinal artery occlusion 244
Choloroquinine-induced foveal toxicity 432
Choosing appropriate analysis protocol 610
Chorioretinal inflammations 458
Choroidal neovascular membrane 531
Choroidal neovascular membrane in toxoplasmosis 467
Choroidal neovascular membrane contiguous to geographic atrophy 300
Choroidal ruptures 558
Chronic ICSC with cystoid macular edema 132
Cirrus HD OCT TM 20
Cirrus OCT machine 14
Classic CNVM 346
Closure of macular hole following pars plana vitrectomy 195
CNVM in angiod streaks 356
CNVM with choroiditis 358
CNVM with toxoplasmosis scar 531
Commotio retinae 550
Commotio retinae with reversible retinal changes 551
Commotio retinae/berlins edema 550
Comparison of RNFL measurements with the Cirrus and Stratus OCT 673
Complete Behçet's disease with CME 475
Cone dystrophy 387, 426
Congenital toxoplasmic scar 459
Congenital toxoplasmosis scar with central serous chorio-retinopathy 469
Correlation with other technology 619
Crinkled cellophane maculopathy 267,271
ERM with PVD and true macular hole 273
ERM with vitreomacular traction 289
focally adherent ERM with traction on ILM surface 286
globally adherent ERM 274
globally adherent ERM with smooth ILM surface 281
globally adherent ERM with traction on ILM 284
pars plana vitrectomy for ERM 292
secondary ERM following scleral buckle surgery 276
spectral domain OCT in ERMS 280
CRVO with Taut posterior hyaloid membrane 239
CSME with cystoid macular edema 89
CSME with foveal tractional retinal detachment 96
CSME with serous retinal detachment 91
CSME with sponge-like thickening 87
CSME with Taut posterior hyaloid membrane 100
Cystoid macular edema 526
Cystoid macular edema following PPV for endophthalmitis 589
Cystoid macular edema in branch retinal vein occlusion 229, 258
Cystoid macular edema in CRVO 248
Cystoid macular edema in retinitis pigmentosa 382
D
Degenerative myopia 576
foveal retinoschisis in myopia 578
Fuchs’ spot 581
intravitreal bevacizumab injection for CNVM in myopia 582
myopia with choroidal neovascular membrane 576
Diabetic macular edema 49
laser photocoagulation for diffuse macular edema 77
macular hole following pars plana vitrectomy for diabetic macular edema 81
posterior sub-Tenon triamcinolone acetonide and grid 77
Diabetic macular edema with central serous chorioretino-pathy 84
CSME with cystoid macular edema 89
CSME with foveal tractional retinal detachment 96
CSME with serous retinal detachment 91
CSME with sponge-like thickening 87
CSME with Taut posterior hyaloid membrane 100
focal traction with vitreoschisis 94
management of bilateral CSME with serous retinal detachment 108
response of CSME to intervention 102
spectral domain OCT 86
Diabetic macular edema with central serous chorioretinopathy 84
Disciform scar in ARMD 303
Distinct RPE excrescences 324
Drusen 296,319
E
Epiretinal membrane following PPV for endophthalmitis 590
Epiretinal membrane with underlying retinal edema 269
Epiretinal membranes 266
crinkled cellophane maculopathy 267
epiretinal membrane with underlying retinal edema 269
ERM with PVD and true macular hole 273
ERM with vitreomacular traction 289
ERM: macular pucker 268
ERMS 280
F
Fast macular thickness map 18, 602
Fast optical disc 602
Fast RNFL thickness 601
Fibrovascular PED 302
Focal traction with vitreoschisis 94
Focally adherent ERM with traction on ILM surface 286
Foveal atrophy in ICSC 147
Foveal hemorrhage 436
subhyaloid foveal hemorrhage 437
sub-internal limiting membrane hemorrhage 436
subretinal bleed 438
Foveal retinoschisis in myopia 578
Fuchs’ spot 581
Full thickness macular hole with PVD 190
Full thickness macular hole without complete PVD 215
Full thicknesses macular hole without PVD 188
G
Geographic atrophy 296
Giant retinal tear 571
Glaucoma 613, 666
Glaucoma analysis protocol 669
Glaucoma scanning protocols 600
Glaucoma suspects 622
Globally adherent ERM 274
Globally adherent ERM with smooth ILM surface 281
Globally adherent ERM with traction on ILM 284
H
Heidelberg retinal tomogram 627
Hemorrhagic pigment epithelial detachment 355
Heredodystrophic disorders 381
adult–onset foveomacular vitelli-form dystrophy 394,404
Best disease with choroidal neovascular membrane 395
Best's disease 398, 401
Bietti's crystalline tapetoretinal dystrophy 390
Bull's eye maculopathy due to hy-droquinine toxicity 408
central areolar choroidal dystrophy 393
choloroquinine-induced foveal toxicity 432
cone dystrophy 426
cystoid macular edema in retinitis pigmentosa 382
OCT in heredodystrophic disorders 381
OCT in retinitis pigmentosa 381
pattern dystrophy 406
pattern dystrophy with choroidal neovascular membrane 428
retinitis pigmentosa with elongation of photoreceptors 415
retinitis pigmentosa with photo-receptor atrophy 412
retinitis pigmentosa with RPE atrophy 385
spectral domain OCT 411
Stargardt's disease 389,418
High definition image analyses for 5 line raster scan 38
Higher resolution 672
Histopathologic correlation of Stratus OCT™ image 28
Hydration theory of pathogenesis of macular hole 193
Hypertrophic scar of neovascular membrane 367
I
ICSC in an elderly patient 148
ICSC in diabetes 150
ICSC with multiple peds 123
ICSC with polypoidal choroidal vasculopathy 135
ICSC with RPE rip 141
Idiopathic central serous chorioreti-nopathy 119
chronic ICSC with cystoid macular edema 132
foveal atrophy in ICSC 147
ICSC in an elderly patient 148
ICSC in diabetes 150
multifocal ICSC 152
ICSC with multiple PEDS 123
ICSC with polypoidal choroidal vasculopathy 135
ICSC with RPE rip 141
interpretation of transverse C-scans in ICSC 158
morphological alterations in opposite asymptomatic eye 178
OCT characteristics in ICSC 119
OCT in diagnosing complications or associations of ICSC 119
OCT charecteristics in typical ICSC 121
OCT in diagnosing complications/associations of ICSC 125
ICSC showing serous RD with pigment epithelium detachment 125
OCT in ICSC 119
photoreceptor elongation in acute ICSC 171
pregnancy induced ICSC 155
resolution of ICSC with fibrin 175
RPE bumps and microrip in ICSC 161
spectral-domain OCT in ICSC 157
spontaneous resolution of RPE microrip 166
Idiopathic macular hole with retinal detachment 191
Image acquisition 602
Image analysis 30
Image interpretation 29
Image processing protocols 30
Impending macular hole 184, 208
Incomplete Vogt-Koyanagi-Harada disease 490
Inflammatory diseases of retina-choroid 458
acquired ocular toxoplasmosis 465
acquired toxoplasmic retino-choroiditis 462
acute posterior multifocal placoid pigment epitheliopathy 502
APMPPE 502
Behçet's disease 475
chorioretinal inflammations 458
choroidal neovascular membrane 531
CNVM with toxoplasmosis scar 531
complete Behçet's disease with CME 475
congenital toxoplasmic scar 459
congenital toxoplasmosis scar with central serous choriore-tinopathy 469
cystoid macular edema 526
incomplete Vogt-Koyanagi-Harada disease 490
intermediate uveitis 472
intraocular cysticercosis 518
MEWDS 508
multifocal choroiditis with choroidal neovascular membrane 505
multiple evanescent white dot syndrome 508
pars planitis with cystoid macular edema 472
probable Vogt-Koyanagi-Harada disease 484
pseudophakic cystoid macular edema 522
recurrent multifocal choroiditis/ multifocal choroiditis and panuveitis 505
serous retinal detachment in Vogt-Koyanagi-Harada syndrome 527
serpiginous choroidopathy with choroidal neovascular membrane 513
serpiginous choroidopathy with cystoid macular edema 516
spectral domain OCT 525
striations in acute VKH disease 537,539
sympathetic ophthalmia 495,529
toxoplasmic retinochoroiditis 458
toxoplasmic retinochoroiditis with serous fluid 535
tuberculoma 520
type II choroidal neovascular membrane in toxoplasmosis 467
vitreomacular traction in uveitis 534
Vogt-Koyanagi-Harada syndrome (VKH syndrome) 480
Initial preparation 602
Intermediate uveitis 472
Intermediate-risk non-neovascular ARMD 298
Interpolated data 667
Interpretation of OCT scan 27
advanced interactive analysis 39
analysis of scan: spectral domain OCT 38
high definition image analyses for 5 line raster scan 38
histopathologic correlation of stratus OCT™ image 28
image interpretation 29
image analysis 30
image processing protocols 30
objective 29
quantitative analysis 34
macular thickness change analysis 45
normal macula scan 27
time-domain OCT: analysis of scan 27
Interpretation of transverse C-scans in ICSC 158
Intraocular cysticercosis 518
Intravitreal bevacizumab 250
Intravitreal bevacizumab injection for CNVM in myopia 582
Intravitreal triamcinolone acetonide 369
J
Juxtafoveal telangiectasia 360
intravitreal triamcinolone acetonide 369
juxtafoveal telangiectasia 364
parafoveal telangiectasia with choroidal neovascular membrane 366
spectral domain OCT 372
time domain OCT in JFT 361
with hypertrophic scar of neovas-cular membrane 367
L
Lack of registration 667
Lamellar macular hole 186
Lamellar macular hole with intact photoreceptor 211
Laser photocoagulation for diffuse macular edema 77
Laser-induced foveal burn 445
Leber's hereditary optic neuropathy 659
Lightening maculopathy 443
Limitations of OCT 657
all ‘normal’ and ‘green’ may not be normal 661
all cupping and red color may not be glaucoma 657
disc hemorrhage with early thickening of RNFL layer 663
Leber's hereditary optic neuropathy 659
RNFL defect 661
traumatic optic neuropathy 657
Line scan 16
M
Macula post scleral buckling surgery 568
Macular edema in hemispheric CRVO 235
Macular edema in macular BRVO 231
Macular evaluation following retinal detachment surgery 567
macula post scleral buckling surgery 568
retinal tear 571
Macular hole 182
calculation of hole form factor 204
closure of macular hole following pars plana vitrectomy 195,221
with restoration of photoreceptor 221
without restoration of photoreceptor layer 225
closure of macular hole without restoration of photoreceptor layer 198
closure with restoration of photoreceptor layer 196
full thickness macular hole with PVD 190
full thickness macular hole without complete PVD 215
full thicknesses macular hole without PVD 188
hydration theory of pathogenesis of macular hole 193
idiopathic macular hole with retinal detachment 191
impending macular hole 184,208
lamellar macular hole 186
lamellar macular hole with intact photoreceptor 211
macular hole in tractional retinal detachment 192
macular hole with complete PVD 217
macular hole with retinal detachment 219
macular hole: foveal pseudocyst 183,206
operculated lamellar macular hole with photoreceptor layer defect 213
patterns of closure of macular holes 196
persistent subfoveal fluid following successful closure of macular hole 203
prognosticating success of macular hole surgery 204
spectral-domain OCT 205
time domain OCT 182
U-shaped hole closure 199
vitreomacular traction syndrome 185
V-shaped hole closure 201
Macular hole following pars plana vitrectomy 81
Macular hole in tractional retinal detachment 192
Macular hole with complete PVD 217
Macular hole with retinal detachment 219
Macular hole: foveal pseudocyst 183,206
Macular thickness 600
Macular thickness change analysis 45
Macular thickness map 16,602,609
Macular thickness protocols 602
Management of bilateral CSME with serous retinal detachment 108
Media clarity 21
MEWDS 508
More speed- lesser motion artifacts 671
Motion artifacts 666
Multifocal choroiditis with choroidal neovascular membrane 505
Multiple evanescent white dot syndrome 508
Myopia with choroidal neovascular membrane 576
N
Neovascular ARMD 296
Neurosensory atrophy 573
endophthalmitis 586
scleral buckling surgery 573
Nodular pattern 326
Non-ischemic CRVO with cystoid macular edema 237
Non-neovascular ARMD 296
Normal macula scan 27
Normative database 670
O
Occult CNV with minimally classic component 307
Occult CNVM 345
OCT characteristics in ICSC 119
OCT charecteristics in typical ICSC 121
OCT in choroidal neovascular membranes 354
CNVM in angiod streaks 356
CNVM with choroiditis 358
hemorrhagic pigment epithelial detachment 355
OCT in diagnosing complications or associations of ICSC 119,125
OCT in glaucoma 595
assessment of progression 596
early diagnosis 596
histology 596
pathologic basis of glaucoma 595
OCT in heredodystrophic disorders 381
OCT in ICSC 119
OCT in retinitis pigmentosa 381
OCT scan for glaucoma 599
analysis protocols for glaucoma 604
appropriate analysis protocol 610
glaucoma scanning protocols 600
image acquisition 602
initial preparation 602
macular thickness map 609
macular thickness protocols 602
OCT scans for glaucoma 599
optic nerve head analysis 600
optic nerve head protocols 602
optic nerve head (single eye) 607
RNFL measurements 599
RNFL protocols 600
fast RNFL thickness 601
RNFL map 601
RNFL thickness 600
RNFL thickness (single eye) 604
RNFL thickness average 604
RNFL thickness map 609
RNFL thickness serial analysis 607
scan acquisition 602
scan placement 603
OCT scans for glaucoma 599
Ocular hypertension 627
Operculated lamellar macular hole with photoreceptor layer defect 213
Optic disc pit with no clinical macular detachment 448
Optic disc pit with serous macular detachment 450
Optic disc pits 448
pars plana vitrectomy for optic disc pit and serous macular detachment 451
spectral domain OCT for optic disc pit 454
with no clinical macular detachment 448
with serous macular detachment 450
Optic nerve analysis 671
Optic nerve head analysis 600
Optic nerve head protocols 602
Optic nerve head (single eye) 607
Optic nerve head on the OCT 615
Optical coherence tomography 3
basics of time domain optical coherence tomography 3
limitations of time domain technology 4
spectral/Fourier domain technology 4
OCT equipment 7
spectral/fourier domain OCT 7
principle 3
spectral OCT 5
switching from time to spectral domain 5
time domain OCT 3
Optical disc 602
P
Parafoveal telangiectasia with choroidal neovascular membrane 366
Pars plana vitrectomy for ERM 292
Pars plana vitrectomy for optic disc pit and serous macular detachment 451
Pars planitis with cystoid macular edema 472
Pathologic basis of glaucoma 595
Patient preparation 14
Pattern dystrophy 406
Pattern dystrophy with choroidal neovascular membrane 428
Patterns of closure of macular holes 196
Patterns of Drusen 324
Persistent CNVM following thermal laser for extrafoveal classic CNVM 313
Persistent subfoveal fluid following successful closure of macular hole 203
Photic maculopathy 440
laser-induced foveal burn 445
lightening maculopathy 443
solar retinopathy following eclipse viewing 440
welding arc maculopathy 442
Photodynamic therapy for juxtafoveal CNVM 315
Photoreceptor elongation in acute ICSC 171
Pigment epithelium detachment without neovasculari-zation 334
Point-to-point registration in dry ARMD 330
Posterior sub-Tenon triamcinolone acetonide and grid 77
Postoperative endophthalmitis 585
cystoid macular edema following PPV for endophthalmitis 589
epiretinal membrane following PPV for endophthalmitis 590
neurosensory atrophy following endophthalmitis 586
serous detachment following PPV for endophthalmitis 587
Preapoptotic stage of geographic atrophy 329
Pregnancy induced ICSC 155
Probable Vogt-Koyanagi-Harada disease 484
Prognosticating success of macular hole surgery 204
Progression of AMD from dry to wet 340
Pseudophakic cystoid macular edema 522
Q
Quantitative analysis 34
R
Radial lines 16
RAP in idiopathic perifoveal telangiectasia 542
Raster lines 18
Recurrent multifocal choroiditis/ multifocal choroiditis and panuveitis 505
Resolution of ICSC with fibrin 175
Response of CSME to intervention 102
Response to anti-VEGF treatment 347
Retinal angiomatosis proliferation 541
RAP in idiopathic perifoveal telangiectasia 542, 544
RAPs in the same eye 546
spectral domain OCT 544
Retinal pigment epithelial changes 327
Retinal pigment epithelial irregularity following scleral buckling surgery 572
neurosensory atrophy following scleral buckling surgery 573
Retinal pigment epithelial irregularity following scleral buckling surgery 572
Retinal pigment epithelium hyperplasia following pars plana vitrectomy for retinal detachment 574
Retinal trauma 549
choroidal ruptures 558
commotio retinae with reversible retinal changes 551
commotio retinae/berlins edema 550
reversible commotio retinae 553
subretinal neovascularization 563
traumatic choroidal rupture 558
traumatic choroidopathy 561
traumatic macular cyst 555
traumatic macular holes 556
traumatic retinal detachment 559
traumatic subretinal hemorrhage 563
Retinal vascular occlusions 228
branch retinal artery occlusion 241
central retinal vein occlusion with branch retinal artery occlusion 44
CRVO with Taut posterior hyaloid membrane 239
cystoid macular edema in branch retinal vein occlusion 229,258
cystoid macular edema in CRVO 248
macular edema in hemispheric CRVO 235
macular edema in macular BRVO 231
non-ischemic CRVO with cystoid macular edema 237
spectral domain OCT in vascular occlusions 247
Retinal vasculitis 262
Retinitis pigmentosa with elongation of photoreceptors 415
Retinitis pigmentosa with photore-ceptor atrophy 412
Retinitis pigmentosa with RPE atrophy 385
Reversible commotio retinae 553
RNFL map 601
RNFL measurements 599
RNFL protocols 600
RNFL thickness 600,669
RNFL thickness (single eye) 604
RNFL thickness average 604
RNFL thickness map 609
RNFL thickness serial analysis 607
RPE bumps and microrip in ICSC 161
RPE changes with RPE cell migration 328
RPE changes without RPE cell migration 327
RPE rip following intravitreal anti-VEGF injection 350
S
Saw tooth pattern 325
Scan acquisition 14,602
Scan placement 603
Scan protocols suitable for macula 16
Secondary ERM following scleral buckle surgery 276
Secondary ERM with lamellar macular hole following endogenous endophthalmitis 278
Selection of scan protocols 15
scan protocols suitable for macula 16
fast macular thickness map 18
line scan 16
macular thickness map 16
radial lines 16
raster lines 18
repeat 18
spectral domain OCT 20
variables affecting the scan quality 20
alignment and focus 22
media clarity 21
video window 22
Serous detachment following PPV for endophthalmitis 587
Serous retinal detachment 121
Serous retinal detachment in Vogt-Koyanagi-Harada syndrome 527
Serpiginous choroidopathy 510
Serpiginous choroidopathy with choroidal neovascular membrane 513
Serpiginous choroidopathy with cystoid macular edema 516
Solar retinopathy following eclipse viewing 440
Spectral domain OCT for glaucoma 666
advantages of spectral domain over time domain OCT 671
better reproducibility 673
comparison of RNFL measurements with the Cirrus and Stratus OCT 673
glaucoma analysis protocol 669
higher resolution 672
interpolated data 667
lack of registration 667
low signal-to-noise ratio scans 667
more speed-lesser motion artifacts 671
motion artifacts 666
normative database 670
optic nerve analysis 671
RNFL thickness 669
shortcomings of Stratus OCT 666
three-dimensional data sets 672
Spectral/Fourier domain OCT 7
Spectral/Fourier domain technology 4
Spontaneous resolution of RPE microrip 166
Stargardt's disease 418
atrophic maculopathy with flecks 389
atrophic maculopathy without flecks 388
Stratus OCT 666
Stratus OCT in glaucoma 612
correlation with other technology 619
current status of the Stratus OCT in glaucoma 621
disc analysis algorithm 616
discriminators for glaucoma 613
glaucoma suspects 613
manual correction 615
normative database 618
ocular hypertensives 613
optic nerve head measurements 619
optic nerve head on OCT 615
optimum conditions 617
peripapillary atrophy 615
RNFL measurement 617
RNFL thickness 619
scan protocol 617
serial follow-up of same patient 619
signal strength 618
structure-function correlation 614
suspicious discs 613
Striations in acute VKH disease 537
Subhyaloid foveal hemorrhage 437
Sub-internal limiting membrane hemorrhage 436
Subretinal bleed 438
Subretinal neovascularization 563
Suspicious discs and ocular hypertensives 613
Sympathetic ophthalmia 495
T
Technique of acquiring OCT 9
cirrus OCT machine 14
patient preparation 14
scan acquisition 14
with time-domain stratus OCT 9
spectral domain the cirrus HD-OCT 14
Technique of acquiring OCT with spectral domain 14
Technique of scans acquisition 14
Three-dimensional data sets 672
Time domain technology 4
Time-domain OCT: analysis of scan 27
Toxoplasmic retinochoroiditis 458
Toxoplasmic retinochoroiditis with serous fluid 535
Transpupillary thermotherapy for juxtafoveal classic CNVM 311
Transpupillary thermotherapy for sub-foveal 307
Traumatic choroidal rupture 558
Traumatic choroidopathy 561
Traumatic macular cyst 555
Traumatic macular hole 556
Traumatic retinal detachment 559
Traumatic subretinal hemorrhage with choroidal neovas-cular membrane 563
Tuberculoma 520
U
Unreliable visual fields 636
U-shaped hole closure 199
V
Variables affecting scan quality 20
Vascular occlusions 247
Video window 22
Visual fields 640
Vitreomacular traction in uveitis 534
Vitreomacular traction syndrome 185
Vogt-Koyanagi-Harada syndrome 480
V-shaped hole closure 201
W
Welding arc maculopathy 442
×
Chapter Notes

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1Introduction to OCT2

Introduction to Optical Coherence Tomography1

 
Time Domain OCT
 
BASICS OF TIME DOMAIN (TD) OPTICAL COHERENCE TOMOGRAPHY
 
What is OCT?
Optical Coherence Tomography (OCT) is a new diagnostic tool that can perform tomography/cross-sectional imaging of biologic tissues with ≤ 10 microns axial resolution using light waves. Since retina is easily accessible to the external light, it is especially suited for retinal disorders. This imaging technique provides information regarding the retinal tomography and is akin to in vivo histopathology of the retina. The conventional imaging techniques including fundus photography and fluorescein angiography yield diagnostic information about retinal topography. OCT yields information about retinal tomography that is complementary to the conventional topographic techniques.
 
PRINCIPLE
We are all familiar with principle on which ultrasound works where the high frequency sound wave is launched into the eye with the help of a probe. The sound wave is reflected from different boundaries between microstructures. The working principle of OCT is similar to the ultrasound with two major differences:
  1. It uses light instead of sound. The speed of light being almost a million times faster than sound allows the measurement of structures with a resolution of ≤ 10 microns as compared to 100-micron scale for ultrasound.
  2. Ultrasound needs contact with the tissue under study whereas OCT does not require any contact.
 
HOW DOES TIME DOMAIN OCT WORK?
It is a non-contact, non-invasive device where a broad bandwidth near-infrared light beam (820 nm) is projected onto the retina. The light gets reflected from the boundaries between the microstructures and also gets scattered differently from tissues with different optical properties. It then compares the time delay of the light reflecting from the various layers of the retina with the time delay of the light reflected from a reference mirror at a known distance. The interferometer then combines the reflected pulses from the retina as well as reflecting mirrors, resulting in a phenomenon known as interference. This interference is then measured by a photodetector, which determines the distance traveled by various light pulses by varying the distance to the reference mirror. This finally produces a range of time delays for comparison.
The interferometer integrates several data points over 2 mm of depth to construct a tomogram of retinal structures. It is a real time tomogram using false color scale. Different colors represent the degree of light back scattering from different depths of retina. The image, thus, produced has axial resolution of ≤ 10 microns and transverse resolution of 20 microns.4
The only commercially available equipment based on time domain technology is Stratus OCT (Carl Zeiss Meditech, Dublin CA).
 
LIMITATIONS OF TIME DOMAIN TECHNOLOGY
The time domain based technology had significant limits in signal acquisition time and thus comprehensive three-dimensional imaging of the retina was not possible. The signal acquisition time was prolonged because of the fact that this technology relied on the mechanical movements of the internal components for thickness measurements, thus limiting the speed of acquisition. In addition, there was no proper recording of the eye movement during acquisition and movement of the device or patient during scanning would reduce the quality and also markedly decrease the spatial registration (the ability to determine the precise location of the B-scan OCT image). Also, there was no comprehensive coverage of retina that could result in missing a pathology if the scan line would not pass through the area of pathology. Recently, these shortcomings of the time-domain technology have been overcome by the introduction of spectral-domain OCT that allows cross-sectional and three-dimensional tomograms of the posterior segment of the eye.
 
Spectral/Fourier Domain Technology
Spectral/Fourier domain detection techniques measure the echo time delay of light by measuring the spectrum of the interference between light from the tissue and light from a stationary unscanned reference arm. Light returning from the sample and reference paths is combined at the detector, which is a spectrometer in Spectral domain (SD)-OCT. The spectrometer resolves the interference signals throughout the depth of each A-scan immediately by means of a Fourier transformation. This is possible because the spectrometer resolves the relative amplitudes and phases of the spectral components scattered back from all depths of each A-scan tissue sample, without varying the length of the reference path. The depth is analyzed by inverse fourier transformation of the back-scattered light spectrum. Eliminating the necessity of moving a mechanical reference arm makes it possible to acquire OCT image data about 70 times faster than conventional (time domain) OCT (Figures 1.1A and B).
zoom view
Figure 1.1A: Line diagram showing the difference in the techniques measuring the echo time delay of light(Courtesy: Rick Torney, Carl Zeiss Meditec).
The vast increase in scan speed makes it possible for these new generation OCTs to acquire three-dimensional data sets, or entire cubes of data in about the same time (depending on the selected scan type) than conventional OCT. Data over a 6 × 6 mm cube (upto 128 lines and 512k resolution) can be acquired in about 2 seconds with an axial resolution of 5-7 µm and transverse resolution of about 10 µm. The comprehensive coverage of the retina by a 6 × 6 mm cube allows better delineation of pathology with less sampling errors.
5
zoom view
Figure 1.1B: Principle of Spectral domain OCT(Courtesy: Rick Torney, Carl Zeiss Meditec).
 
WHAT ADDITIONAL INFORMATION DOES SPECTRAL OCT GIVE?
SD OCT gives cross-sectional images that can be useful in studying several pathologies including choroidal neovascularization, vitreomacular traction, macular hole, epiretinal membranes, retinal detachment, etc. In addition, the 3D surface maps can be obtained that help in mapping the surface topography. The quantification of retinal thickness data can be done as in time domain OCT; however, the image registration is superior in SD OCT due to the incorporation of eye tracker. The retinal thickness measured by SD OCT tends to be higher for any given situation compared to time-domain because time domain OCT measures from inner retina to inner segment/outer segment (IS/OS) junction while SD OCT measures from inner retina to inner or outer retinal pigment epithelium (RPE) band. Table 1.1 shows the comparison between the two OCT systems.
 
Switching from Time to Spectral Domain
It is important to note that one cannot switch from TD OCT to SD OCT in a given patient during follow-up. The foveal thickness measured by spectral domain OCT has been found to be higher than that measured with the time domain OCT both in normal eyes as well as eyes with retinal pathologies. There could be several reasons for this variability. Firstly, the spectral domain OCT segmentation measures the retinal thickness from RPE to the ILM, whereas time domain Stratus OCT measures the retinal thickness from IS/OS to the ILM. It has been demonstrated that the outer boundary of the neural retina was inaccurately delineated in time domain OCT using the automated measurement tool that considers the inner hyper-reflective line (HRL), whenever present, as the outer boundary of the neural retina, thus excluding a significant part of the structure for ultimate retinal thickness measurements at the specific regions of the normal as well as abnormal maculas. The distance between IS/OS and RPE is approximately 45 microns, thus spectral domain OCT would be inherently thicker compared to time domain OCT. Secondly, the retinal thickness measurement in Stratus OCT may be biased by the retinal thickness in the central subfield alone as the scan is obtained by ‘Fast Retinal Thickness scan’ wherein only six linear line-scans pass through the foveal center. The spectral domain OCT, on the other hand, measures the retinal thickness by an evenly distributed square centered on the fovea and is likely to indicate true retinal thickness.
So far there is no conversion factor from time to spectral domain OCT and thus it is suggested that any particular patient maybe followed on one machine only.
6
Table 1.1   Comparison between time and spectral domain OCT (Courtesy: Rick Torney, Carl Zeiss Meditec)
Spectral Domain OCT
Time Domain Stratus OCT
Benefit of Spectral Domain
Light source
840 nm Broader bandwidth
820 nm
Provides higher resolution
Detector
Spectrometer
Single detector
No moving parts—faster scan acquisition
Axial resolution
6-7 μm
10 μm
Better visualization of retinal
Transverse resolution
10 μm
20 μm
layers and pathology
Maximum A-scans per B-scan
8,000
512
Better visualization of tissue/pathology
Scan depth
2 mm
2 mm
Slightly better penetration of light
Scanning speed
About 28,000 A-scans per second
400 A-scans/sec
Better registration, 3D scanning and analysis
Suggested Reading
  1. Costa RA, Calucci D, Skaf M, et al. Optical coherence tomography 3: Automatic delineation of the outer neural retinal boundary and its influence on retinal thickness measurements. Invest Ophthalmol Vis Sci 2004; 45: 2399–2406.
  1. Forooghian F, Cukras C, Meyerle CB, Chew EY, Wong WT. Evaluation of time domain and spectral domain optical coherence tomography in the measurement of diabetic macular edema. Invest Ophthalmol Vis Sci. 2008 May 30. [Epub ahead of print].
  1. Gupta V, Gupta P, Singh R, Dogra MR, Gupta A. Spectral-domain cirrus high-definition optical coherence tomography is better than time-domain stratus optical coherence tomography for evaluation of macular pathologic features in uveitis. Am J Ophthalmol 2008;145: 1018–22.
  1. Hee MR, Izatt JA, Swanson EA, et al. Optical coherence tomography of the human retina. Arch Ophthalmol 1995; 113: 325–32.
  1. Huang D, Swanson EA, Lin CP, et al. Optical coherence tomography. Science 1999;254:1178–81.
  1. Leung CK, Cheung CY, Weinreb RN, et al. Comparison of macular thickness measurements between time domain and spectral domain optical coherence tomography. Invest Ophthalmol Vis Sci 2008 Apr 30 [Epub ahead of print].
  1. Schuman JS, Puliafito CA, Fujimoto JG. Optical Coherence Tomography of Ocular Diseases (2nd edn). Slack Incorporated 2004.