Use of the Free Electron Laser for the
Noninvasive Determination of Retinal
Oxyhemoglobin Saturation by Near
Infrared Reflectance Spectrophotometry
Ref: Eye, M.C. Escher, 1946
Ref: Eye, M.C. Escher, 1946
Ref: Eye, M.C. Escher, 1946
Ref: Eye, M.C. Escher, 1946
Why Is It Important to Determine Retinal Oxygenation ?
Several Types of Ischemic Retinopathies:
Arterial Occlusion
Venous Occlusion
Ocular ischemic syndrome
Retinopathy of prematurity
Diabetic retinopathy
2000 National Estimates of the Burden of Diabetes
Diabetes
Prevalence:
Diagnosed
Undiagnosed
Total
Diabetic
Retinopathy
Prevalence:
11.1 million
5.9 million
17.0 million (6.2% of US population)
65,000 diabetics per year
develop proliferative DR
Leading cause of blindness
among 20 to 74 yr old
Ref: National Diabetes Information Clearinghouse
Research to Prevent Blindness
Ref: G.W. Robison, Jr. et al., 1995
Hyperglycemia
Coagulation Elevated
factors
PAI-1
Arteriolar
hyalinosis
Microthrombi
formation
Polyol Pathway
Protein
Glycation
Areas of
Non-perfusion
Ischemia
<ANG-1
>ANG-2
Increased
DAG
PKCß2
Ref: Jacot et al., 1999
BM
Thickening
Loss of
contractility
Changes in
Vascular
Resistance/BF
Reduced
angiostatic
Functional changes
factors
TIMPs
In EC
VEGF
Gradual Sudden
Cotton-wool
spots
Capillary
closure
Loss of
Peri-endothelial
Cell contact
AGEs
Capillary
occlusion
Occlusion
of
terminal
arterioles
Pericyte
Degeneration
(pericyte ghost)
Loss of structural
support
Hypoxic
changes
Activated
EC
Proliferation MMPs
ECM
proteolysis
Increased vascular
Permeability
(leakage)
Edema
Hemorrhage
Exudates
Angiogenesis
Preretinal IRMA Microaneurysm
angiogenesis
Increased growth factors
In vitreous
Breakdown
Of BRB
Extravasation of
Pr and growth factors
Incident light
Backscatter
Favorable Attributes of the FEL
• Short pulsation of the emitted light
» Minimizes total retinal irradiance
» Compensates for eye movements
Vessel Oximetry
Incident light
Backscatter
Scattering Components of
the Fundus
Sclera
Choroid
RPE
Internal limiting membrane
Vitreous humor
n = 1.336
Focal length
Approximately 23 mm
n= 1.427
Diameter of Retinal Vessels:
Veins: 212 um
Arteries: 150 um
Rate of flow: 34 ul/min
Cornea
n=1.377
A-V oxygen difference 2%
Vitreous
50% v/v Standard type A immersion oil n =1.515
50% Mineral spirits n=1.438
n=1.476
Retinal vessels
Capillary pipette Internal diameter 110-268 um
Borosilicate glass n=1.472
Phantom retina
Spectralon
Diffuse reflection
Cornea
f = 17.2 mm
Plano-convex lens
22.6 mm
•Optical geometry
•Reflectance properties
Rate of flow
34ul/min
Aluminum
Backing
Optical Absorption of Hemoglobin
Molar Extinction Coefficient (cm-1/M)
1000000
Oxyhemoglobin
Deoxyhemoglobin
100000
10000
1000
100
250
350
450
550
650
Wavelength (nm)
750
850
950
Optical Absorption of Hemoglobin
Molar Extinction Coefficient (cm-1/M)
1000000
Oxyhemoglobin
Deoxyhemoglobin
100000
10000
1000
™High absorption
™Isobestic points
100
250
350
450
550
650
Wavelength (nm)
750
850
950
Optical Absorption of Hemoglobin
Molar Extinction Coefficient (cm-1/M)
1000000
Oxyhemoglobin
Deoxyhemoglobin
100000
10000
™Weaker absorption
™Single Isobestic point
1000
™Reduced scattering
™Outside of visible range
100
250
350
450
550
650
Wavelength (nm)
750
850
950
Favorable Attributes of the FEL
•
Broad tunability of the emitted light.
» To rapidly scan a range of 70 to 100 nm
» Alternate between visible and near-infrared
•
Strong energy output in the near-infrared
region of the spectrum (700 to 900 nm).
» Where hemoglobin chromophore signal is
attenuated
Derivation of Algorithm
Consulted References
•R. Pittman and B. Duling (1975) described the relationship
between spectral absorption and oxygen saturation of
hemoglobin in microvasculature within visible range.
•A.P. Sheperd (1975) developed an empirically derived
analytical expression relating OD and Hb concentration.
•Delori (1988) Determined contribution of fundus scattering
to OD of hemoglobin at three wavelengths (555nm).
•Ferrari et al (1989) utilized DNIRS to noninvasively
determine cerebral venous Hb saturation (sagittal sinus).
r2 = 0.993
Dark Adapted–High Oxygen Extraction
Profile of
Oxygen Tension
Choroidal
Vessels
Outer
Segment
Choriocapillaris
Inner
Segment
Retinal
Photoreceptor
Layer
ROD
Dark-Current
Retinal
Arteriole
Retinal
Capillaries
Light Adapted–Low Oxygen Extraction
Profile of
Oxygen Tension
Choroidal
Vessels
Outer
Segment
Choriocapillaris
Inner
Segment
Retinal
Photoreceptor
Layer
ROD
Light-Current
Retinal
Arteriole
Retinal
Capillaries
RPE
OS
IS
ONL
OPL
INL
IPL
GC
Light-adapted
Dark-adapted
Ref: Padnick-Silver, Linsenmeier RA. IOVS 44(2):745-750, 2003.
Linsenmeier RA. J. Gen. Physiol. 88:521-542, 1986
In Situ Schematics of Optical System
Detector
Electro-optic
or Acousto-optic
Deflector Device
Polarizing
Prism
Output
Fiber optic laser light
FEL
LASER
Source
Deflected
Beam
Power Supply
Fundus camera
Y
Regional Oxygen Map
High O2
Low O2
X
Summary
• Presents the opportunity to study retinal metabolism
in a yet unprecedented fashion.
•Technique may reveal subtle alterations in retinal metabolism
that are not discernable with present routine function tests.
•Could promote a more rational physiologic basis for
clinical intervention and delve into the pathogenic mechanisms
and possible early diagnosis of diabetic retinopathy, and other
retinal ischemic/vascular diseases.
•Collectively these retinopathies comprise the leading causes
of blindness in the industrialized world.
THE
END
Ref: Drawing Hands, M.C. Escher, 1948