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.