Ansgar Kaupp

Objective Measurement of Small Angles of Strabismus in Infants and Children with Photographic Reflection Pattern Evaluation

A new photographic method for the precise measurement of the angle of squint in children and infants is presented. The apparatus consists of a reflex camera with three horizontally aligned flashes and a small fixation light. The subjects are photographed while fixating binocularly or monocularly in the primary or secondary position. Six reflections can be seen on the photograph of each pupil. These are the first and fourth Purkinje images of each light source. From the reflection patterns, the squint angle or the angle kappa or alpha can be computed using a simple formula. A vertical angle of strabismus may be calculated from the same photograph using the same principle. Results from orthotropic and strabismic adults and from children are evaluated to establish reference values. Cooperation from the children is generally very good. The accuracy of the new method is between between 2 and 4.5 prism diopters (between 1 degree and 2.5 degrees), depending on the measuring strategy.

Rotation-extended cepstrum technique optimized by systematic analysis of various sets of x-ray images

Spatial registration is a major problem arising whenever several images of similar contents are to be compared. Considering translations only, two-dimensional cepstral techniques have been proven to be exact and robust against noise or intensity variations. Furthermore, the cepstral filtering is numerically more efficient than most common approaches to image registration based on cross-correlation or template matching. In a previous paper, we proposed a two- dimensional cepstrum based matching technique accessing rotations and translations. The logarithmic polar mapping of the power spectra of both images to be registered is used for the decoupling of rotations and translations (similar to the Fourier-Mellin transform). Rotations are detected first matching the mapped spectra by two-dimensional cepstrum analysis. After rotating back one image, the relative shift is determined using the same cepstrum technique. In clinical practice, the rotation detection step was discovered as the weakness of this registration technique. Based on 855 pairs of dental radiographs acquired in known positions, three different approaches of matching the mapped spectra are compared: the cepstrum technique, the cross-correlation, and the entropy of the one-dimensional histogram distribution function of the substraction image of the mapped spectra. The combination of the log-polar mapped power spectra of both x rays with the entropy-measure allows the best detection of rotations. The union with common cepstrum methods correcting translations results in a robust rotation- extended cepstrum technique.

Measuring morphologic properties of the human retinal vessel system using a two-stage image processing approach

The scanning laser technique in combination with digital image analysis can be used to assess the morphology of the retinal vascular tree. Quantitative description of the retinal vascular network may provide further knowledge in pathophysiology of retinal and systemic vascular disease. Especially, for monitoring of vascular alteration in follow-up studies quantitative reproducible methods to assess the vascular morphology are essential. Therefore we developed an automatic scheme allowing the measurement of morphological properties for the use in diagnosis and therapy control. To extract the morphological properties of the retina a two-stage image analysis procedure is employed. First the image is segmented in objects using a model-based top-down, image segmentation scheme. Then the obtained objects are classified with a neural net, the result being the tree of the arteries and veins. The third step is a measurement process which yields the desired information of arterial diameter, tortuosity and other morphological properties. As an example we show a functional image of the diameters and present a pilot-study in patients with arterial hypertension to demonstrate the ability of the new method for computerized analysis of the retinal vascular tree to detect arteriolar vascular alterations.<<ETX>>

Automatic strabometry by Hough-transformation and covariance-filtering

In order to measure the angle of squint, the eyes of a patient are photographed using two CCD-cameras. Three hashes are activated at the moment of exposure. By measuring the positions of the reflexes in the digital image the squint angle can be determined. This paper presents a segmentation method for the automatic detection of the reflexes without the use of fixed threshold values. With an algorithm based on Hough-transformation the centre of the iris is localized. In the region of interest the reflex positions are located using cross-covariance-filtering. A classification and plausibility control follow to make sure there are no faulty measurements. With 191 clinical images arbitrarily chosen the algorithm was tested showing perfect results. It is already used in clinical practice.<<ETX>>

Self-assessment of angles of strabismus with photographic Purkinje I and IV reflection pattern evaluation

Abstract• Background: Accurate assessment of the angle of strabismus, e.g. of variable angles of strabismus, is crucial in preoperative patient management and is usually performed in a clinical environment. Objective assessment by patients themselves, under everyday conditions, could contribute to a better preoperative work-up. A new objective evaluation procedure for the measurement of manifest angles of strabismus for near and distance fixation by the patient himself is presented. • Methods: To account for the modified experimental setup used for the self-assessment, an amended computation procedure of Purkinje reflection pattern evaluation was developed. For measurement, patients and controls placed their head on a head/chin rest and fixated at 33 cm or 4 m distance in primary position. A reflex camera and three photo flash units were positioned on a special frame underneath the visual axis and in front of the subject so that both eyes could be photographed simultaneously. The cameras remote shutter control was released together with the photo flash units by the properly fixating subject. The angles of strabismus were obtained from the series of pictures through later evaluation of the Purkinje I and IV reflection patterns recorded in the photographs of the eyes.• Results: Measurements of the ocular alignment in two control groups and in a group of strabismic subjects showed satisfactory accuracy of the “self-assessment” method compared to “standard” Purkinje reflection pattern evaluation and orthoptic measurements of the angle of strabismus.• Conclusion: The modified “self-assessment” method can be used for the objective recording of angles of strabismus as needed in the preoperative work-up of patients with variable angles of strabismus, over prolonged periods of time, and outside a clinical setting.

Automatic measurement of the angle of squint by Hough-transformation and covariance-filtering

Describes the image processing method for the automatisation of the measurement of squint using the first and fourth Purkinje reflex. This method is already in use in clinical practice in comparison to the standard method of the cover test. The new method, based on image processing, allows the exact and objective determination of the angle of squint. Children from the age of six months as well as adults can be examined. The automatisation of the previously interactive method is a prerequisite for the screening of large collectives of children for microstrabism. The method uses a two-stage abstraction to identify six reflexes in the iris of each eye. The method based on the Hough-transform and covariance-filtering is robust against image distortions and additional reflexes, as produced by wearing glasses. The reliability of the method has been proven in 191 cases and is tested in routine use now.

PACS: digitale Bildarbeitsplätze in der medizinischen Diagnostik

PACS — picture archiving and communication systems — steht fur den Ersatz des photographischen Bildes durch Technologien, mit denen diagnostisches Bildmaterial durchgangig digital transportiert und gespeichert werden kann. Digitale Massenspeicher werden die traditionellen Filmarchive ablosen. Bildspeicher und Bildquellensystem werden uber digitale Datenubertragungssysteme untereinander vernetzt sein. Die fur den Radiologen sichtbarste Konsequenz jedoch wird ein neuartiges Arbeitsgerat sein: die diagnostic image workstation. Bilder werden nicht mehr am Lichtkasten prasentiert sondern auf Bildschirmen wiedergegeben werden. Zusatzlich wird eine Vielzahl neuer Arbeitshilfen fur die Bildhandhabung angeboten werden. Der Umgang mit Bildmaterial wird sich drastisch andern, was nicht ohne Einflus auf Arbeitsweise und Arbeitsbedingungen des Radiologen bleiben wird.