Statistical signal and image processing techniques in corneal modeling [Elektronische Ressource] / von Weaam F. H. Alkhaldi
152 pages
English
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Statistical signal and image processing techniques in corneal modeling [Elektronische Ressource] / von Weaam F. H. Alkhaldi

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152 pages
English

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Publié le 01 janvier 2010
Nombre de lectures 29
Langue English
Poids de l'ouvrage 5 Mo

Exrait

Statistical Signal and Image Processing Techniques
in Corneal Modeling
Vom Fachbereich 18
Elektrotechnik und Informationstechnik
der Technischen Universit¨at Darmstadt
zur Erlangung der Wu¨rde eines
Doktor-Ingenieurs (Dr.-Ing.)
genehmigte Dissertation
von
Weaam F. H. Alkhaldi, M. Sc.
¨geboren am 10.06.1974 in Alexandria, Agypten
Referent: Prof. Dr.-Ing. Abdelhak M. Zoubir
Korreferent: Dr. D. Robert Iskander
Korreferent: Prof. Dr. Henryk Kasprzak
Tag der Einreichung: 22.04.2010
Tag der mu¨ndlichen Pru¨fung: 18.06.2010
D 17
Darmsta¨dter Dissertation
Darmstadt, 2010I
Acknowledgments
This thesis was a result of years of research that included turbulent times, but also
times of happiness and success. I am greatly indebted to many people, who have
provided endless support and help.
First, I wish to express my sincerest gratitude to the German Academic Exchange
Service (DAAD). They have offered me the great opportunity to come to Germany
and start a new step in my life. Throughout the years, they were always providing
supportandkeeping upthespirit. Particularly, Iwould like tothank Dr. Baumgarten,
Mrs. Muhareb from the DAAD office in Jerusalem, Palestine, and Mrs. Hanzlik, Mrs.
B¨onninghausen, Mrs. Gerecke, and Mrs. Barth from the DAAD headquarter in Bonn,
Germany.
Coming to the work’s environment, I would like to deeply thank my first supervisor,
Prof. Abdelhak M. Zoubir, for giving me the great chance to work in the Signal
Processing Group besides invaluable support, significant guidance and encouragement.
I highly appreciate the warm-hearted brotherhood’s feelings he has shown and his to-
the-point advices. On the other side of the globe, in Australia, there has been Dr. D.
Robert Iskander, who has made the course of this work really fruitful and enjoyable.
I am very grateful for his unlimited assistance and great discussions, both technically
and personally. Also, many thanks go to Prof. Henryk Kasprzak for co-supervising
this thesis and the instructive comments and suggestions he has provided.
Having in mind the magnificent time I have spent in the Signal Processing Group, I
would like to sincerely thank ALL my great colleagues, both former and current ones,
and also the visitors, for their friendly feelings and the productive discussions we had.
Finally, I would like to dedicate this work to my family. For my father (may God
bless his soul) and my mother (God bless her), I would say that without your infinite
supportandlove fromchildhoodtonow, thisachievement wouldnothave cometrueat
all. Also, many thanks go formy brother and sister fortheir support and kind feelings.
Adjacently, I wish to express my love and gratitude to my wife and my two sons for
their patience, support, and understanding.
Darmstadt, July 2010
Weaam AlkhaldiIII
Kurzfassung
In dieser Dissertation betrachten wir zwei mit einander verknu¨pfte Probleme: Die
Verbesserung von videokeratoskopischen Bildern zur genaueren Sch¨atzung der Horn-
hauttopographie, sowie die Sch¨atzung der Modellordnung bei der Modellierung dieser
mittels orthogonaler Zernike-Polynome.
Die Sch¨atzung der Hornhauttopographie mit einem Videokeratoskop beruht auf dem
Placido-Scheiben-Prinzip. Vorraussetzungen fu¨r akkurate Sch¨atzungen sind eine hohe
Qualit¨at des pr¨aokul¨aren Tr¨anenfilms und ein hinreichend grosser Lidspalt zur Ver-
meidung von Augenlidreflektionen. Diese Bedingungen sind in der Praxis jedoch
nicht immer erfu¨llt, denn einerseits leiden einige Patienten an einer Austrocknung
des ¨auseren Auges, und andererseits haben, vor allem Patienten asiatischer Abstam-
mung, h¨aufig einen kleinen Lidspalt. Eine schlechte Qualit¨at des Tr¨anenfilms und
Reflektionen der Wimpern, fu¨hren zu Datenverlusten in bestimmten Regionen und
einem verminderten Erfassungsbereich durch die Placido-Scheibe. Dies resultiert in
einer schlechteren Sch¨atzung der Hornhauttopographie fu¨r diese Patienten. Unser Ziel
ist, durch eine Vergro¨sserung des Standarderfassungsbereichs von Placido-Scheiben-
Videokeratoskopen, auch in den oben genannten Fa¨llen, eine zuverl¨assige Sch¨atzung
der Hornhauttopgraphie zu ermo¨glichen. Dies erreichen wir durch die Einbindung von
lineareradaptiver Filterung(basierend aufderTheorie derWiener-Filter), sowie nicht-
linearer Filterung (basierend auf morphologischen Operationen) von Einzelbildern, in
denTopographiesch¨atzungs-Algorithmus des Instruments. DieErgebnisse derMessun-
gen von Referenzoberfl¨achen und echten Hornh¨auten belegen, dass die Einbeziehung
der vorgeschlagenen Techniken zu einer verbesserten Sch¨atzung der Hornhauttopogra-
phie, und in vielen F¨allen zu einer deutlichen Vergr¨osserung des Erfassungsbereiches,
und damit, zu einem verbesserten Videokeratoskop fu¨r die Facha¨rzte, fu¨hrt.
Die mathematische Modellierung der H¨oheninformation der Hornhauttopographie er-
folgt in der Regel durch eine Reihe von orthogonalen Zernike-Polynomen. Wir betra-
chten hier die Sch¨atzung der Anzahl der notwendigen Zernike-Polynome als Modell
Auswahl-Problem, welches in der linearen Regression formalisiert ist. Klassische in-
formationstheoretische Kriterien zur Modell Auswahl neigen dazu die Komplexit¨at der
Hornhautoberfl¨ache durch einen zu schwachen Strafterm zu u¨bersch¨atzen, w¨ahrend
Bootstrap-Techniken diese untersch¨atzen, oder einen hohen Verarbeitungsaufwand
ben¨otigen. Wir schlagen vor, das Effiziente Detektionskriterium (EDC), das die gle-
iche allgemeine Form wie die informationstheoretischen Kriterien hat, als alternative
Methode zur Modellordnungs-Sch¨atzung zu verwenden. Wir zeigen zun¨achst durch
Simulationen, dass das EDC einer grossen Anzahl von informationstheoretischen undIV
resampling-basierten Kriterien u¨berlegen ist. Wir veranschaulichen dann, anhand von
¨echten Hornhautdaten, dass unsere mit dem EDC erzielten Ergebnisse in enger Ubere-
instimmung mit klinischen Erwartungen sind, und somit das EDC ein Mittel fu¨r die
Unterscheidung zwischen normalen, astigmatischen und keratokonischen Hornhauto-
berfla¨chen dartellt.
Die beiden in dieser Dissertation behandelten Probleme sind dadurch miteinander
verknu¨pft,dassdiekorrekteModellierungderHornhauttopographie,mitwelcherFunk-
tionauchimmer, einepr¨aziseSch¨atzungdieservorraussetzt, welche wiederum aufeiner
ausreichenden Qualit¨at der videokeratoskopischen Bilder basiert.V
Abstract
In this thesis, we consider two interrelated problems, which are the enhancement of
videokeratoscopic images for more accurate corneal topography estimation and model-
order selection of corneal surfaces when expanded using orthogonal Zernike polynomi-
als.
Corneal topography estimation that is based on the Placido disk principle relies on
good quality of pre-corneal tear film and sufficiently wide eyelid (palpebral) aperture
to avoid reflections from eyelashes. However, in practice, these conditions are not al-
ways fulfilled resulting in missing regions, smaller corneal coverage, and subsequently
poorer estimates of corneal topography. Our aim is to enhance the standard operating
range of a Placido disk videokeratoscope to obtain reliable corneal topography esti-
mates in patients with poor tear film quality, such as encountered in those diagnosed
with dry eye, and with narrower palpebral apertures as in the case of Asian subjects.
This is achieved by incorporating in the instrument’s own topography estimation al-
gorithm an image processing technique that comprises of linear adaptive filter (based
on Wiener filtering theory) and non-linear filter (based on morphological operations).
The experimental results from measurements of test surfaces and real corneas show
that the incorporation of the proposed technique results in better estimates of corneal
topography and, in many cases, to asignificant increase in the estimated coverage area
making such an enhanced videokeratoscope a better tool for clinicians.
On the other hand, corneal height-data are typically modeled using a set of orthogonal
Zernike polynomials. Weaddress theestimation ofthenumber ofZernike polynomials,
which is formalized as a model-order selection problem in linear regression. Classical
informationtheoretic criteriatend tooverestimate the cornealsurface due totheweak-
nessoftheirpenaltyfunctions,whilebootstrap-basedtechniques tendtounderestimate
the surface or require extensive processing. We propose to use the Efficient Detection
Criterion (EDC), which has the same general form of information theoretic-based cri-
teria, as an alternative to estimating the optimal number of Zernike polynomials. We
first show, via simulations, that the EDC outperforms a large number of information
theoretic criteria and resampling-based techniques. We then illustrate that using the
EDCforreal corneas results inmodels that arein closer agreement with clinical expec-
tations and provides means for distinguishing normal corneal surfaces from astigmatic
and keratoconic surfaces.
The two problems are interrelated in the sense that appropriate modeling for corneal
surfaces, regardless of the used functions, requires accurate corneal topography that
canbeefficientlyestimatedprovidedthatthevideokeratoscopicimageisnotdegraded.VII
Publications
Internationally Refereed Journal Articles
• W.Alkhaldi,D.R.Iskander,andA.M.Zoubir,“Model-orderselectioninZernike
polynomial expansion of corneal surfaces using the efficient detection criterion,”
IEEE Transactions on Biomedical Engineering, 2010, in press.
• W. Alkhaldi, D. R. Iskander, A. M. Zoubir, and M. J. Collins, “Enhancing the
standard operating range of a placido disk videokeratoscope for corneal surface
estimation,” IEEE Transactions on Biomedical Engineering, vol. 56, no. 3, pp.
800–809, March 2009.
Internationally Refereed Conference Papers
• W. Alkhaldi, D. R. Iskander, and A. M. Zoubir, “Improving the performance of
model-order selection criteria by partial-model selection search,” in The Proceed-
ings of The 35th IEEE International Conference on Acoustics, Speech and Signal
Processing (ICASSP), Dallas, Texas, USA, March 2010, pp. 4130–4133.
• W. Alkhaldi, D.R. Iskander, and A. M. Zoubir, “The applicability of biased esti-
mationinmodelandmodelorderselection,”inTheProceedingsofThe34thIEEE
International Conference on Acoustics, Speech and Signal Processing (ICASSP),
Taipei, Taiwan, April 2009, pp. 3461–3464.
• D. R. Iskander, W. Alkhaldi, and A. M. Zoubir, “On the computer intensive
methods in model selection,” in The Proceedings of The 33rd IEEE International
Conference on Acoustics, Speech and Signal Processing (ICASSP), Las Vegas,
USA, March 2008, pp. 3461–3464.
• D. R. Iskander and W. Alkhaldi, “The “hook and loop” resampling plane,” in
The Proceedings of The 2nd IEEE International Workshop on Computational
Advances in Multi-Sensor Adaptive Processing (CAMSAP), Virgin Islands, USA,
December 2007, pp. 65–68.