Perfect Pachymetry with Dual Scheimpflug Technology 1 Dual vs. Single Scheimpflug The principal advantage of Dual Scheimpflug over Single Scheimpflug is the compensation of misalignment or decentration which can occur when performing pachymetry or measuring posterior heights. The GALILEI device employs 2 opposing Scheimpflug cameras, each rotating 180 degrees, which compensate for the effect of x–y decentration in the pachymetry measurement. With Dual Scheimpflug imaging, corresponding corneal thickness data from each view are simply averaged to compensate for unintentional misalignment which results in a corrected measurement value at the corresponding location (figure 1). The software interface allows the user to view the acquired Placido or Top View image and simultaneously acquired Scheimpflug images, either left or right. Averaging the corresponding thickness values reduces the deviation by a factor of ten without needing to correct for decentration (figure 2). The Dual Scheimpflug imaging principle is independent of inclined surfaces giving it the possibility to produce repeatable measurements without knowledge of the actual decentration of the slit from the apex. This is an important feature, particularly as eyes are always in motion. References: 1 Cynthia J. Roberts, Ph.D. and Benno J. Züger, Ph.D. The Advan- tage and Principle of Dual Scheimpflug Imaging for Analyzing the Anterior Segment of the Human Eye. SIS Surgical Instrument Systems AG Port, Switzerland. April, 2006. Technical bulletin -2.6 -0.64 -0.16 -0.16 -0.64 -2.6 -33.1 27.9 -40 -20 0 20 40 -1 -0.5 0 0.5 1 [μm] Averaged Deviation left Deviation right Decentration (mm) Fig 2. This graphic simulates the apparent thickness deviation of a spherical cornea with a thickness of 500 μm and a decentration of up to ±1 mm. The thickness deviations from the true value are represented in red and blue, as seen from the left view and from the right view, respectively. GALILEI Dual Scheimpflug technology Ziemer Ophthalmic Systems AG, CH-2562 Port, Switzerland | www.ziemergroup.com | [email protected] Summary • Dual Scheimpflug compensates for decentered eyes and delivers precise pachymetry data – even with the eye decentered. • Simple averaging of the thicknesses in the two corresponding Scheimpflug views reduces error by a factor of 10 without the need for correcting the misalignment. • Using Placido topography with Scheimpflug imaging improves the accuracy of the central anterior corneal curvature calculation. Fig 1. A: In the centered instrument condition, the apparent slit images in both Scheimpflug views are identical. The slit light is perpendicular to the spherical surface, therefore the viewing angles for both cameras are equal. B: In the decentered condition, either left or right, the apparent slit images are no longer identical. The slit light is not perpendicular to the surface. Therefore, the apparent slit image is thicker in the left view and thinner in the right view, or vice versa, depending on the direction of decentration. A B 45˚ Slit lamp Apparent slit lamp images Cornea L R X Apex

GALILEI Clinical Study Dual Scheimpflug A4...imaging improves the accuracy of the central anterior corneal curvature calculation. Fig 1. A: In the centered instrument condition, the

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Page 1: GALILEI Clinical Study Dual Scheimpflug A4...imaging improves the accuracy of the central anterior corneal curvature calculation. Fig 1. A: In the centered instrument condition, the

Perfect Pachymetry with Dual Scheimpflug Technology1

Dual vs. Single ScheimpflugThe principal advantage of Dual Scheimpflug over Single Scheimpflug is the compensation of misalignment or decentration which can occur when performing pachymetry or measuring posterior heights. The GALILEI device employs 2 opposing Scheimpflug cameras, each rotating 180 degrees, which compensate for the effect of x–y decentration in the pachymetry measurement. With Dual Scheimpflug imaging, corresponding corneal thickness data from each view are simply averaged to compensate for unintentional misalignment which results in a corrected measurement value at the corresponding location (figure 1). The software interface allows the user to view the acquired Placido or Top View image and simultaneously acquired Scheimpflug images, either left or right. Averaging the corresponding thickness values reduces the deviation by a factor of ten without needing to correct for decentration (figure 2). The Dual Scheimpflug imaging principle is independent of inclined surfaces giving it the possibility to produce repeatable measurements without knowledge of the actual decentration of the slit from the apex. This is an important feature, particularly as eyes are always in motion.

References:

1 Cynthia J. Roberts, Ph.D. and Benno J. Züger, Ph.D. The Advantage and Principle of Dual Scheimpflug Imaging for Analyzing the Anterior Segment of the Human Eye. SIS Surgical Instrument Systems AG Port, Switzerland. April, 2006.

Technical bulletin

-2.6-0.64-0.16-0.16-0.64-2.6

-33.1

27.9

-40

-20

-1 -0.5 0 0.5 1Dezentration x [mm]

[µm

]

Averaged Deviation left Deviation right

Decentration (mm)

Fig 2. This graphic simulates the apparent thickness deviation of a spherical cornea with a thickness of 500 μm and a decentration of up to ±1 mm. The thickness deviations from the true value are represented in red and blue, as seen from the left view and from the right view, respectively.

GALILEIDual Scheimpflug technology

Ziemer Ophthalmic Systems AG, CH-2562 Port, Switzerland | www.ziemergroup.com | [email protected]

Summary

• Dual Scheimpflug compensates for decentered eyes and delivers precise pachymetry data – even with the eye decentered.

• Simple averaging of the thicknesses in the two corresponding Scheimpflug views reduces error by a factor of 10 without the need for correcting the misalignment.

• Using Placido topography with Scheimpflug imaging improves the accuracy of the central anterior corneal curvature calculation.

Fig 1. A: In the centered instrument condition, the apparent slit images in both Scheimpflug views are identical. The slit light is perpendicular to the spherical surface, therefore the viewing angles for both cameras are equal. B: In the decentered condition, either left or right, the apparent slit images are no longer identical. The slit light is not perpendicular to the surface. Therefore, the apparent slit image is thicker in the left view and thinner in the right view, or vice versa, depending on the direction of decentration.

A B

45˚

Slit lamp

Apparent slit lamp images

Cornea

L R

Apex