What is OCT?

1
Volumetric multiple wavelength ultrahigh
resolution imaging in glaucomatous retina Marilyn
Puah
Biomedical Imaging Group, School of Optometry and
Vision Sciences

• Eye
• Retina of eye consists of 10 layers
• Nerve fibres in the eye exit via the optic nerve
  head to the brain
• A flat disc or disc or shallow depth is
  indication of a healthy optic nerve head

• Principle of OCT
• Measures time delay and magnitude of optical
  echoes at different transverse positions
• Dimensions of structures at different depths are
  based on echo time for light to be
  backscattered
• By scanning optical beam in the transverse
  direction and performing successive axial scan
  measurements, cross sectional image is produced
• A 2D grey scale or false colour image is produced

• What is OCT?
• Ultrahigh resolution optical coherence tomography
  (UHR OCT) is an upcoming new technology that
  allows non-invasive, optical medical diagnostic
  imaging.
• Analogue to ultrasound but it allows real-time in
  situ imaging with higher resolution of 1 to 15
  microns.
• It enables a three-dimensional in vivo direct
  visualization of the microstructure of the
  retina, allowing both qualitative and
  quantitative changes to be seen which are
  beneficial in diagnosing optical diseases
  especially in early stages.
• OCT measures by echo time delay and intensity of
  back reflected light

Figure 5. Cross section of the eye, optic nerve
head and retina

• What is glaucoma?
• Glaucoma is one common disease of the eye in
  human
• Characterized by increase in intraocular pressure
  and atrophy in the optic nerve head with
  corresponding visual field loss which is usually
  not noticeable by patient until a late stage
• Optic atrophy results in a larger cup disc size
  and depth, which is seen as a dip from the cross
  section of the optic nerve head.
• Several studies done have proposed that
  structural changes of the optic nerve head and
  nerve fibre layer will precede before visual
  field loss appear.
• Therefore it is crucial to detect it early so
  that appropriate treatment can be done to prevent
  further damage to the optic nerves.

Figure 2. How OCT generates images

• Methods
• In vitro measurements of glaucomatous animal
  retinas were taken with microscope OCT
• Allows three-dimensional imaging and
  visualisation of the morphological changes of the
  optic nerve head
• Images intraretinal contrast were then compared
  to histology
• Allows the improvement of OCT system in regards
  to resolution, contrast and penetration depth for
  future studies
• 3 pairs of tree shrews eyes and 5 pairs of
  rats eyes were imaged
• Glaucoma introduced in the left eye and right eye
  as a control
• All retinas were processed using ImageJ
• Best 3 pairs of eyes were chosen for this
  presentation

• Comparison with other imaging techniques
• Ultrasound allows increased penetration depth for
  imaging with higher frequency sound waves
• But resolution is reduced due to increased
  ultrasonic attenuation
• Confocal microscopy allows submicron resolution
• But image penetration is limited
• OCT strikes a balance between confocal microscopy
  and ultrasound imaging
• OCT allows image penetration up to 2-3mm and
  resolution ranging from 15 to 1µm which is
  dependent on bandwidth of light source

Results
Tree shrew CTS1 normal RE
Rat CR312 normal RE
Rat CR337 normal RE
Rat CR312 glaucomatous LE
Tree shrew glaucomatous LE
Rat CR337 glaucomatous LE

• Discussion
• Glaucoma introduced in left eye
• Optic atrophy and morphological changes seen in
  left eye
• Tree shrews and rats retinas differ
• Hyaloid artery interfere with ONH imaging in tree
  shrews
• Orbital fats caused a layer oil film, disrupting
  images
• Careful handling of retinas required
• Hyaloid artery interferes with imaging
• Detection of glaucoma in animals retina
  correspond to that in human

• Future outlook
• Visualise structure without sacrificing animal
• Extend knowledge to human
• Beneficial in early detection of glaucoma
• Pick up other ocular diseases due to early
  morphological changes
• Applies to other areas of biomedical research

• Acknowledgements
• I would like to thank Prof Wolfgang, Dr Boris
  Povazay and the OCT team for their help and
  guidance throughout the 6 weeks of the placement
  which made the experience amazing
• I would also like to extend my upmost gratitude
  to CUROP and Cardiff University for funding me
  during the 6 weeks of placement

• Reflections
• Provide an insight to research work
• Exposed to new technology
• Rewarding, fruitful experience
• Allowed to be part of a promising research team

References Figure 5 taken from http//www.healthye
yes.org.uk/uploads/pics/Eye-1.jpg and
http//www.e-sunbear.com/images/glopticnerve.jpg
and http//www.answersingenesis.org/Home/Area/Maga
zines/tj/images/v13n1retina3.gif Figure 6 taken
from http//www.meei.harvard.edu/patient/images/op
tic-nerve-head.jpg Figure 7 taken from
http//www.intecheye.com/Upload/glaucoma1.jpg

• Limitations of OCT
• Optical imaging restricted to surface tissues but
  not affected in the eye
• Less penetration than ultrasound
• Optical artefacts interfere with quality of images