32
D. Cabrera DeBuc et al.
Confocal scanning laser ophthalmoscopy (SLO) is integrated with OCT in most
of commercial ophthalmology OCT systems. SLO utilizes two-dimensional scanning mirrors to scan a specific fundus area. This is same as the B-mode imaging
in standard OCT. SLO and OCT share the same optical design in the sample arm
and use a switchable reflector to guide backscattered light to a different detection
arm. SLO, especially adaptive optics SLO (AOSLO) can provide better transverse
resolution than OCT and needs no pupil dilation measurements [25]. SLO technology also enables accurate eye movement tracking which can guide and correct the
scanning procedure of OCT. Fundus autofluorescence (FAF), a further SLO modality whose excitation light and detection light are in different wavelength regime are
also introduced to integrate with OCT [26]. FAF images reflect density distribution
of lipofuscin which is a biomarker of the retinal pigment epithelium. Also, adaptive
optics OCT (AO-OCT) was introduced to compensate for aberrations to the imaging beam caused by the optics of the eye, which limits the transverse resolution in
OCT [27]. In the sample arm of AO-OCT, a wavefront sensor measures the aberrations of light and then information is fed back to a wavefront corrector such as a
deformable mirror which compensates the wavefront aberrations. It can improve the
lateral resolution of retinal imagery near the diffraction limit. A combination of AO
with PS-OCT has also been introduced by Cense et al. [28].
Full-field OCT (also called en-face OCT) is another way to achieve high lateral
resolution retinal images [29, 30]. It employs full field illuminating and parallel
detector to produce en-face imaging of tissues like a conventional microscope. Without the transverse scanning of sample light beam, the lateral resolution of en-face
OCT is limited by the objective numerical aperture instead of scanning beam spot
size. However, the lateral resolution can exceed 1 µ m with the use of objectives with
high numerical aperture such as water-immersion objectives. The B-mode images of
en-face OCT are continuous which benefits the visualization of small lesions due to
the scanning direction parallels the anatomic fabric of retina.
OCT systems are still pursuing powerful imaging abilities to meet the requirements of medical diagnosis, especially the diagnosis of disease at the early stage
when the structural images do not show obvious changes but the function of tissue becomes abnormal. The total oxygen metabolic rate is supposed to be a potential
biomarker for early diagnosis of glaucoma, diabetic retinopathy and age-related macular degeneration [31]. A multi-modality OCT system combining the advantage of
photoacoustic (PA) imaging and OCT is introduced to measure the oxygen metabolic
rate where PA imaging could measure the oxygen saturation (SO 2 ) in the retinal and
choroidal vasculature and Doppler OCT could obtain the blood flow velocity [32].
However, PA-OCT is currently limited to animal eye experiments due to the higher
excitation laser energy which may cause unnecessary damage. Alternatively, visible
light OCT (vis-OCT) is developed to obtain structural images, blood flow, oxygen
saturation and oxygen metabolic rate of the human retina simultaneously with a single technique [33, 34]. The principle of vis-OCT is based on the absorption spectrum
method which utilizes different absorption spectra of deoxygenated and oxygenated
hemoglobin in the visible light regime.
D. Cabrera DeBuc et al.
Confocal scanning laser ophthalmoscopy (SLO) is integrated with OCT in most
of commercial ophthalmology OCT systems. SLO utilizes two-dimensional scanning mirrors to scan a specific fundus area. This is same as the B-mode imaging
in standard OCT. SLO and OCT share the same optical design in the sample arm
and use a switchable reflector to guide backscattered light to a different detection
arm. SLO, especially adaptive optics SLO (AOSLO) can provide better transverse
resolution than OCT and needs no pupil dilation measurements [25]. SLO technology also enables accurate eye movement tracking which can guide and correct the
scanning procedure of OCT. Fundus autofluorescence (FAF), a further SLO modality whose excitation light and detection light are in different wavelength regime are
also introduced to integrate with OCT [26]. FAF images reflect density distribution
of lipofuscin which is a biomarker of the retinal pigment epithelium. Also, adaptive
optics OCT (AO-OCT) was introduced to compensate for aberrations to the imaging beam caused by the optics of the eye, which limits the transverse resolution in
OCT [27]. In the sample arm of AO-OCT, a wavefront sensor measures the aberrations of light and then information is fed back to a wavefront corrector such as a
deformable mirror which compensates the wavefront aberrations. It can improve the
lateral resolution of retinal imagery near the diffraction limit. A combination of AO
with PS-OCT has also been introduced by Cense et al. [28].
Full-field OCT (also called en-face OCT) is another way to achieve high lateral
resolution retinal images [29, 30]. It employs full field illuminating and parallel
detector to produce en-face imaging of tissues like a conventional microscope. Without the transverse scanning of sample light beam, the lateral resolution of en-face
OCT is limited by the objective numerical aperture instead of scanning beam spot
size. However, the lateral resolution can exceed 1 µ m with the use of objectives with
high numerical aperture such as water-immersion objectives. The B-mode images of
en-face OCT are continuous which benefits the visualization of small lesions due to
the scanning direction parallels the anatomic fabric of retina.
OCT systems are still pursuing powerful imaging abilities to meet the requirements of medical diagnosis, especially the diagnosis of disease at the early stage
when the structural images do not show obvious changes but the function of tissue becomes abnormal. The total oxygen metabolic rate is supposed to be a potential
biomarker for early diagnosis of glaucoma, diabetic retinopathy and age-related macular degeneration [31]. A multi-modality OCT system combining the advantage of
photoacoustic (PA) imaging and OCT is introduced to measure the oxygen metabolic
rate where PA imaging could measure the oxygen saturation (SO 2 ) in the retinal and
choroidal vasculature and Doppler OCT could obtain the blood flow velocity [32].
However, PA-OCT is currently limited to animal eye experiments due to the higher
excitation laser energy which may cause unnecessary damage. Alternatively, visible
light OCT (vis-OCT) is developed to obtain structural images, blood flow, oxygen
saturation and oxygen metabolic rate of the human retina simultaneously with a single technique [33, 34]. The principle of vis-OCT is based on the absorption spectrum
method which utilizes different absorption spectra of deoxygenated and oxygenated
hemoglobin in the visible light regime.