SCHEIMPFLUG IMAGING
The
Scheimpflug imaging principle is named after an Austrian army Captain Theodore
Scheimpflug, who used it to devise a systematic method and apparatus for
correcting perspective distortion in aerial photographs.
 |
| Theodore Scheimpflug (1865-1911) |
It
is a geometric rule that describes the orientation of the plane of focus of an
optical system (such as a camera) when the lens plane is not parallel to the
image plane.
The Scheimpflug principle refers to a concept in geometric optics whereby a photograph of an object plane that is not parallel to the image plane can be rendered maximally focused given certain angular relations among the object plane, the lens, and the image plane.
Normally,
the lens and image (film or sensor) planes of a camera are parallel to each
other, and the plane of focus (PoF) is parallel to the lens and image planes.
If
the subject plane is not parallel to the image plane, it will be in focus only
along a line where it intersects the PoF.
When
a lens is tilted with respect to the image plane, an oblique tangent extended
from the image plane and another extended from the lens plane meet at a line
through which the PoF also passes. With this condition, a planar subject that
is not parallel to the image plane can be completely in focus.
It
is commonly applied to the use of camera movements on a view camera. It is also
the principle used in corneal pachymetry, corneal topography, and for early
detection of keratoconus.
Scheimpflug-based
devices can provide three dimensional image representations of the anterior
segment, which may be useful for screening narrow angles.
It
allows for photographic documentation of the anterior segment with a depth of
focus ranging from the anterior cornea to the posterior lens surface.
A
number of devices based on the Scheimpflug principle are available. These
include the following:
Device
|
Manufacturer
|
Image
acquisition
|
Orbscan II
|
Bausch
& Lomb, USA
|
Horizontal cross section
|
Pentacam
|
Oculus,
Germany
|
Single rotating camera
|
Galilei
|
Ziemer, Switzerland
|
Dual rotational camera
|
Sirius
|
CSO,
Italy
|
Single rotating camera
|
TMS-5
|
Tomey,
Japan
|
Single rotating camera
|
Precisio
|
Ivis, Italy
|
Single rotating camera
|
The
Orbscan is based on a concept referred to as slit-scan triangulation to obtain
topographic data.
It
projects 40 slit beams (20 nasal and 20 temporal) at the anterior segment at an
angle of 450 from the axis of the camera. In order
for the cornea, the iris, and the lens to be captured in focus, the image plane
of the camera is tilted to satisfy the Scheimpflug condition. The measurements
obtained by triangulation can then be integrated to provide three-dimensional
information regarding the anterior segment.
The
Orbscan can estimate the iridocorneal angle and the anterior chamber depth
(ACD). In normal subjects, these measurements have been shown to be highly
reproducible. However, studies validating the utility of the Orbscan in
assessing angle closure are still needed.
Rotational
Scheimpflug cameras have been found to have good reproducibility in estimating
angle closure when compared with Anterior Segment OCT (AS-OCT) and Ultrasound
Biomicroscopy (UBM).
The
Pentacam (Oculus, Wetzlar, Germany) is equipped with two cameras: a rotational
camera that captures the Scheimpflug image and a front camera that is used to
evaluate the pupillary opening. Information obtained by the front camera aids
with measurement corrections as well as the three-dimensional reconstruction.
A
drawback for Scheimpflug devices is that due to total internal reflection,
photographs of the innermost aspects of the iridocorneal angle cannot be
obtained and therefore direct visualization of the angle is not possible.
Scheimpflug devices rely on extrapolated measurements from surrounding
structures. In this instance AS-OCT and UBM are better suited to obtain images.
Some
of the biometric parameters obtained by Scheimpflug imaging correlate well with
gonioscopy. It is capable of estimating the anterior chamber depth (ACD),
anterior chamber volume (ACV), and anterior chamber angle (ACA).
However, the Pentacam’s ACA measurement
was not reliable for evaluating eyes with a Shaffer grade of 2 or less. The
correlation between ACA measurement and gonioscopic grade was also weaker by
Schiempflug photography when compared to UBM. The unreliability of the
Pentacam’s ACA measurement is likely due to limited angle visualization.
Scheimpflug imaging is also capable of
measuring central corneal thickness. Galilei, with its two rotating cameras is
especially suitable for CCT measurements.
Hysteresis and other biomechanical
properties of the cornea are being increasingly studied regarding measurement
of IOP as well as risk factors for the development of glaucoma. The two devices
capable of quantifying biomechanical features of the cornea include the Ocular
Response Analyzer or ORA and the Scheimpflug-based noncontact tonometer Corvis
ST.
The ORA can measure corneal hysteresis,
which may be an indicator of the cornea’s viscoelasticity. Patients with POAG
and normal tension glaucoma have been shown to have lower-than-average corneal
hysteresis values. Furthermore, low corneal hysteresis has also been implicated
in glaucomatous field progression. The Corvis ST provides a noncontact method
for evaluating IOP, CCT, and the cornea’s biomechanical response to a
collimated puff of air. It is equipped with an ultra-high-speed Scheimpflug
camera that is capable of recording 4330 frames/second.
The ORA functions in a very different way
from the Corvis ST; while the air puff of the Corvis ST is applied with a fixed
force, the ORA air puff is delivered with a variable force. Although both
instruments assess corneal biomechanics, it is difficult to compare the metrics
obtained by these two devices.
Scheimpflug systems such as the Pentacam
can measure lens densitometry with specific metrics that include average
density and maximum density. Based on these measurements, the Pentacam can be
equipped with software that then assigns a grade of nuclear sclerosis on a
scale of 1–5 (Pentacam Nuclear Staging or PNS).
The Pentacam’s densitometric parameters
correlate well with higher-order aberrations (HOAs) obtained from wavefront
analyses. This is useful to assess patients who have suboptimal quality of
vision.
The LENSAR (LENSAR Inc., Winter Park,
USA) is a femtosecond laser equipped with Scheimpflug imaging capabilities.
Similar to the Pentacam, the device can automatically grade lens density on a
scale of 1–5. It also has an imaging system that enables the detection of any
crystalline lens tilt; this feature
maximizes the likelihood of producing a precise, freefloating capsulotomy.
