Biometry/ Ocular Imaging Flashcards
Measurement of basic data relating to the anatomical dimensions of the eye
Biometry
Is biometry quantitative or qualitative
Quantitative
Is ocular imaging qualitative or quantitative
Qualitative
Techniques that provide imaging of any part of the eye. Help document and facilitate diagnosis and treatment
Ocular imaging
What is the oldest form of ocular imaging
Slit lamp
What is now an integral part of the eye exam today
Ocular imaging
One dimensional depth plot
The A scan
- A=axial
- time of flight converted to distance
Axial resolution
-smallest distinction we can make along axis of propagation
Things we can measure with A scan
- axial length
- AC depth
- corneal thickness
What is axial resolution dependent on
Wavelength
Two-D plot for anatomical resemblance
B-scan
B for brightness
Combination of multiple A scans
B scan
What is all different between optical and acoustical
Wavelength, physics, results
What are some constraints on ocular imaging
- absoprtion by tissue
- wavelength
- focal length
- F#
- resolution
- depth range
What is the F#
Related to the resolution of the eye
F# of the eye=8.3
What is confocal good for
Powerful to show you cell morphology of the cornea
- limited to thin layer
- not as much depth
Acoustical modalities of imaging
- ultrasound waves (8-80MHz)
- axial res dependent on wavelength
- good penetration
- high speed in dense material
- signal dependent on change in impedance
- res better for small wavelengths (higher freq)
- time of flight measurement converted to depth
- requires immersion coupling and is often contact technique
What the main difference between acoustical vs optical modalities
Acoustical is contact, optical is noncontact
Optical modalities
- visible or infrared
- axial res dependent on wavelength
- poor penetration
- signal depended on change in refractive index
- slower speed in dense media
- res is better for smaller wavelengths, typically better than acoustical
- interferometric measurements converted to depth
- no contact
what is a qualitative mode of imaging that detects tumors
MRI
Reflection of elastic wave due to different elastic modulus
Ultrasound/ultrasonography
Is sound slower or faster in water
Faster
Is light slower or faster in water
Slower
An ultrasound system operating at 10MHz has axial resolution of 120 microns. A high frequency US operates at 30 HMz. It’s axial res is
40 microns
What kind of sound does ultrasound use
Very high frequency, 1000x higher than what humans can hear
What happens to wavelength when frequency increases
Decreases
What happens to resolution when wavelength is lower
Increased
A scan or B scan: quantitative, numbers
A scan
A scan or B scan: cross section, morphology, qualitative, shapes
B scan
Traditional ultrasound
- 8-20MHz sound waves
- contact
- no sublayer pach
- low res, no 2D
- not reliable peripherally
- most common method used clinically
- fast, simple technique
- not accurate for edematous corneas
- difficult to reposition
Pros of ultrasound
- penetration through opaque ocular media
- portability
- fast, simple
- widely established, common clinical method
Cons of ultrasound
- repeatability, dissimulation to reposition, stadaradized
- not reliable for periphery
- possible risk of corneal abrasion
- requires contact (need for anesthetic)
Traditionally employed for axial length measurements and corneal pachymetry
Ultrasound imaging
Uses a probe stylus and requires immersion coupling and applanation
Ultrasound
Helps assess various ocular and orbital diseases
Ultrasound
What can penetrate behind the iris
Ultrasound
A UBM system operates on a 30 micron wavelength, its axial resolution is
30
Should be about the same as wavelght
In UBM, what is the reflection amplitude proportional to
Impedance gradient
Resolution of UBM
Dependent on wavelength; the higher the frequency, the better the resolution
What is the limitation of optica imaging
Can’t penetrate tissue
Visualization of the iridocorneal angle, anterior lens, ciliary sulcus
UBM
LASIK flap and epithelial visualization
Epithelium is the small layer on top
Flap is about 2x thick as epithelium
CCT is the whole thing
When you add the epithelial thickness and the stromal thickness in the center, what do you get on the pentacam
CCT
Pros of arcscan UBM
- high res
- epithelial and stromal thickness maps
- corneal sublayer pachymetry, lamellar graft, and flap imaging
Cons of arcscan UBM
Patient uncomfortable data acquisition process
- difficult to maintain centration, long acquisition time
- significant capital acquisition cost
What’s the difference between A scan and optical lower coherence reflectomerty
Optical low coherence reflectometyr is higher res
How is the time of flight measure in optical low coherence reflectometery
Interferometry
Examples of optical low coherence reflectometry
- IOL master
- LenSta Ls900
Infrared interferometry (similar to OCT), non contact
Optical low coherence reflectometry
Sublayer pachymetry and optical low coherence reflectomerty
Limited potential
What does optical low coherence reflectometry provide
A number of biometry data including axial length, anterior chamber depth, and pachymetry
Tight focusing and increased contrast through equal-optical length illumination and detection optical paths
Confocal microscopy
What is the confocal microscopy important for
Cellular morphology, detects microbial presence
Can confocal microscopy do pachymetry?
Yes, but VERY slowly
Things about confocal microscopy
-tight focus though corneal structures
-contact
-high res with sublayer pachymetry and cellular debris
-2D
=periphery requires repositioning
-cellular morphology; detects microbial presence
-high res, can quantify haze/scatter
-slow, poor penetration of opacity
Which modality to use for intraorbital features
MRI
Which modality or use to image cell morphology
Confocal microscopy
Which modality to sue to image the epithelial layer
Ultrasound biomicroscopy
Which modality to use for quick pachymetry
Ultrasound
Which modality to use to image the vitreous when corneal opacity is present
If you want numbers: ultrasounds
If you want pictures: UBM
Which modality to sue to image the ciliary sulcus structures
UBM
Which modality to use to measure axial length with the best precision
Optica low coherence interferometry
