10 research outputs found
Possible depth-resolved reconstruction of shear moduli in the cornea following collagen crosslinking (CXL) with optical coherence tomography and elastography
Corneal collagen crosslinking (CXL) is commonly used to prevent or treat
keratoconus. Although changes in corneal stiffness induced by CXL surgery can
be monitored with non-contact dynamic optical coherence elastography (OCE) by
tracking mechanical wave propagation, depth dependent changes are still unclear
if the cornea is not crosslinked through the whole depth. Here,
phase-decorrelation measurements on optical coherence tomography (OCT)
structural images are combined with acoustic micro-tapping (AT) OCE to
explore possible reconstruction of depth-dependent stiffness within crosslinked
corneas in an ex vivo human cornea sample. Experimental OCT images are analyzed
to define the penetration depth of CXL into the cornea. In a representative ex
vivo human cornea sample, crosslinking depth varied from in the
periphery to in the cornea center and exhibited a sharp
in-depth transition between crosslinked and untreated areas. This information
was used in an analytical two-layer guided wave propagation model to quantify
the stiffness of the treated layer. We also discuss how the elastic moduli of
partially CXL-treated cornea layers reflect the effective engineering stiffness
of the entire cornea to properly quantify corneal deformation.Comment: Submitted to Biomedical Optics Express on June 13th 2023, Manuscript
ID: 497970 - Under Review. Manuscript, 10 pages / 6 figures / 2 tables.
Supplementary, 7 pages / 4 figure
Possible depth-resolved reconstruction of shear moduli in the cornea following collagen crosslinking (CXL) with optical coherence tomography and elastography
Collagen crosslinking of the cornea (CXL) is commonly employed to prevent or
treat keratoconus. Although the change of corneal stiffness induced by CXL
surgery can be monitored with non-contact dynamic Optical Coherence
Elastography (OCE) by tracking mechanical wave propagation, the depth
dependence of this change is still unclear if the cornea is not crosslinked
through the whole depth. Here we propose to combine phase-decorrelation
measurement applied to OCT structural images and acoustic micro-tapping
(AT) OCE to explore possible depth reconstruction of stiffness within
crosslinked corneas in an ex vivo human cornea sample. The analysis of
experimental OCT images is used to define the penetration depth of CXL into the
cornea, which varies from 100 in the periphery to 150
in the central area and exhibits a sharp transition between areas. This
information was used in a two-layer analytical model to quantify the stiffness
of the treated layer. We also discuss how the elastic moduli of partially
CXL-treated cornea layers reconstructed from OCE measurements reflect the
effective mechanical stiffness of the entire cornea to properly quantify
surgical outcome.Comment: Main: 10 Pages, 6 Figures Supplemental: 12 Pages, 3 Figure
Kardiale Ultraschall-Elastographie zur Bestimmung der linksventrikulÀren Wandspannung vor und nach transfemoralem Aortenklappenersatz
Introduction
Heart failure is associated with a high mortality and morbidity rate, which imposes a huge financial burden on the health system1. Among patients with heart failure, about 50 % show normal systolic function while their diastolic function is impaired2. Diastolic function is commonly measured with echocardiography by estimation of left ventricular diastolic filling pressures. While this is only an indirect estimate of a tissueâs mechanical properties, cardiac ultrasound-elastography (USE) was recently introduced to measure the fundamental mechanism that enables ventricular dilatation: namely, relaxation of muscle tissue that leads to sufficient reduction of myocardial tension (shear modulus) within the filling phase. As cardiac function is determined by the alteration of myocardial elasticity during the cardiac cycle, measurement of myocardial elasticity changes with USE may support the diagnosis of relaxation abnormalities.
Materials and Methods
USE examination was performed on 103 subjects, grouped into healthy young volunteers (n = 20, 20 - 45 years), healthy older individuals (n = 23, > 45 years), patients suffering from diastolic dysfunction (DD) (n = 39) and patients with both DD and severe aortic valve stenosis (n = 21), who were examined both before and one week after valve replacement. Swinging of the patientâs heart was externally induced via low-frequency acoustic vibrations produced by a loudspeaker, which was built into the medical examination table. Myocardial elasticity changes were assessed by evaluating the delay between wave amplitude alteration and cardiac wall motion for the measurement of elasticity alteration particularly during relaxation (ÏR). Cardiac tissue motion and shear wave amplitude alteration were analysed in the infero-lateral wall of the heart by M-mode echocardiography.
Results
On average, wave amplitudes changed between systole and diastole by a factor of 1.75 ± 0.41. The change in wave amplitude preceded wall motion. Among healthy young volunteers ÏR was 82 ± 13 ms, which is significantly longer (p = 0,021) than in all other groups. However, no statistical discrepancy of ÏR was detected between patients with DD or those who suffer additionally from severe aortic stenosis, and healthy older individuals (> 45 years). A negative correlation between the subjectsâ age and the parameter ÏR was observed (p = 0,002).
Conclusion
As shear wave amplitudes increase and decrease throughout the cardiac cycle, and are measured by USE, it is possible to determine the systolic and diastolic phase of the heart. The elastic time-intervals provide sufficient information to specify that myocardial elasticity changes occur ahead of actual heart motion during systole and diastole. Although the feasibility of USE has been proven in this study, at this point of development, this method does not seem capable of identifying differences between patients with DD and age-matched controls.
Therefore, cardiac USE should be further developed towards a quantitative, spatially resolved elastography modality.Einleitung
Herzinsuffizienz ist mit einer hohen MortalitĂ€t und MorbiditĂ€t assoziiert und stellt eine groĂe finanzielle Belastung fĂŒr das Gesundheitssystem dar1. Nur bei 50 % aller Herzinsuffizienz-Patienten lĂ€sst sich eine erniedrigte linksventrikulĂ€re Ejektionsfraktion feststellen. Bei allen anderen Patienten sind Anomalien der diastolischen Funktion fĂŒr das Herzversagen verantwortlich2. WĂ€hrend die Echokardiografie als klinischer Standard nur eine indirekte Charakterisierung der diastolischen Funktion ĂŒber die AbschĂ€tzung der ventrikulĂ€ren FĂŒllungsdrĂŒcke ermöglicht, erlaubt die kardiale Ultraschallelastographie (USE) direkten Fokus auf den grundlegenden Mechanismus der Dilatation des linken Ventrikels in der Diastole zu legen; die Relaxation von Muskelgewebe, die unmittelbar zu einer Abnahme von Ventrikelsteifigkeit im Myokard fĂŒhrt3. Weil die Funktion des Herzens maĂgeblich durch Ănderungen der GewebeelastizitĂ€t bestimmt wird, könnte die Messung dieser VerĂ€nderungen ĂŒber die Bestimmung der Scherwellenamplitude mithilfe der USE die Diagnose der diastolischen Dysfunktion unterstĂŒtzen, vereinfachen und ausbauen.
Materialien und Methoden
103 Patienten und Probanden wurden in der Klinik mit Schwerpunkt Kardiologie der CharitĂ© âBerlin am Campus Mitte in die dieser Arbeit zugrundeliegende Studie, 21 davon prospektiv, eingeschlossen. Diese wurden unterteilt in junge, gesunde Kontrollen (n = 20, †45 Jahre), Ă€ltere gesunde Kontrollen (n = 23, > 45 Jahre), Patienten mit diastolischer Dysfunktion (DD) (n = 39) und Patienten, die zusĂ€tzlich noch eine hochgradige Aortenklappenstenose aufwiesen (n = 21), welche vor und eine Woche nach transfemoralem Aortenklappenersatz untersucht wurden. Durch eine extern angelegte Vibrationsquelle wurde das Herz in Schwingung versetzt. Ănderungen der GewebeelastizitĂ€t wurden mithilfe M-Mode gestĂŒtzter USE der inferolateralen Herzwand durch Bestimmung der Zeitspanne, die von Ănderung der Scherwellenamplitude bis zur Gewebebewegung vergeht (Ï), gemessen. Im Fokus stand das diastolische Zeitintervall (ÏR), welches Hinweise auf eine bestehende DD geben soll.
Ergebnisse
Die Scherwellenamplituden wĂ€hrend der Diastole waren im Vergleich zu den systolischen Amplituden im Durchschnitt um den Faktor 1.75 ± 0.41 erhöht. Bei allen Untersuchungen wurde die Ănderung der Vibrationsamplitude zeitlich vor dem Beginn der Gewebebewegung beobachtet.
FĂŒr die junge, gesunde Kontrollgruppe ergab sich ein signifikant höherer ÏR-Wert von 82 ± 13 ms (p = 0,021) im Vergleich zu allen anderen Gruppen. Allerdings lieĂ sich kein statistisch relevanter Unterschied zwischen Patienten mit DD zu Aortenstenose-Patienten mit DD sowie zu gesunden, Ă€lteren Probanden (â„ 45 Jahre) bezĂŒglich ÏR feststellen. Zudem zeigte sich eine signifikante Abnahme des Parameters ÏR mit dem Alter der Patienten (p = 0,002).
Zusammenfassung
Die einzelnen Phasen des kardialen Zyklus konnten mittels USE gezeigt werden. Die elastographischen Zeitintervalle zeigen, dass der Gewebebewegung immer eine Ănderung der ElastizitĂ€t vorausgeht. Somit ist die prinzipielle DurchfĂŒhrbarkeit dieser Technik demonstriert worden. Allerdings scheint die USE auf der Grundlage von Scherwellenamplituden zum jetzigen Zeitpunkt unfĂ€hig zu sein, Unterschiede zwischen Patienten mit DD und gesunden Probanden aufzuzeigen. Aus diesem Grund sollte auf diesem Gebiet kĂŒnftig weiter geforscht werden, um die USE zu einer quantitativen, ortsaufgelösten Methode zu entwickeln
Viscoelasticity Imaging of Biological Tissues and Single Cells Using Shear Wave Propagation
Changes in biomechanical properties of biological soft tissues are often associated with physiological dysfunctions. Since biological soft tissues are hydrated, viscoelasticity is likely suitable to represent its solid-like behavior using elasticity and fluid-like behavior using viscosity. Shear wave elastography is a non-invasive imaging technology invented for clinical applications that has shown promise to characterize various tissue viscoelasticity. It is based on measuring and analyzing velocities and attenuations of propagated shear waves. In this review, principles and technical developments of shear wave elastography for viscoelasticity characterization from organ to cellular levels are presented, and different imaging modalities used to track shear wave propagation are described. At a macroscopic scale, techniques for inducing shear waves using an external mechanical vibration, an acoustic radiation pressure or a Lorentz force are reviewed along with imaging approaches proposed to track shear wave propagation, namely ultrasound, magnetic resonance, optical, and photoacoustic means. Then, approaches for theoretical modeling and tracking of shear waves are detailed. Following it, some examples of applications to characterize the viscoelasticity of various organs are given. At a microscopic scale, a novel cellular shear wave elastography method using an external vibration and optical microscopy is illustrated. Finally, current limitations and future directions in shear wave elastography are presented
Shear wave echocardiography
In this thesis we demonstrate that the assessment of the diastolic function of the left ventricle withclassical echocardiography remain
Ultrasound shear wave imaging for diagnosis of nonalcoholic fatty liver disease
Pour le diagnostic et la stratification de la fibrose hépatique, la rigidité du foie est un biomarqueur quantitatif estimé par des méthodes d'élastographie. L'élastographie par ondes de cisaillement (« shear wave », SW) utilise des ultrasons médicaux non invasifs pour évaluer les propriétés mécaniques du foie sur la base des propriétés de propagation des ondes de cisaillement. La vitesse des ondes de cisaillement (« shear wave speed », SWS) et l'atténuation des ondes de cisaillement (« shear wave attenuation », SWA) peuvent fournir une estimation de la viscoélasticité des tissus. Les tissus biologiques sont intrinsÚquement viscoélastiques et un modÚle mathématique complexe est généralement nécessaire pour calculer la viscoélasticité en imagerie SW. Le calcul précis de l'atténuation est essentiel, en particulier pour une estimation précise du module de perte et de la viscosité. Des études récentes ont tenté d'augmenter la précision de l'estimation du SWA, mais elles présentent encore certaines limites.
Comme premier objectif de cette thĂšse, une mĂ©thode de dĂ©calage de frĂ©quence revisitĂ©e a Ă©tĂ© dĂ©veloppĂ©e pour amĂ©liorer les estimations fournies par la mĂ©thode originale de dĂ©calage en frĂ©quence [Bernard et al 2017]. Dans la nouvelle mĂ©thode, l'hypothĂšse d'un paramĂštre de forme dĂ©crivant les caractĂ©ristiques spectrales des ondes de cisaillement, et assumĂ© initialement constant pour tous les emplacements latĂ©raux, a Ă©tĂ© abandonnĂ©e permettant un meilleur ajustement de la fonction gamma du spectre d'amplitude. En second lieu, un algorithme de consensus d'Ă©chantillons alĂ©atoires adaptatifs (« adaptive random sample consensus », A-RANSAC) a Ă©tĂ© mis en Ćuvre pour estimer la pente du paramĂštre de taux variable de la distribution gamma afin dâamĂ©liorer la prĂ©cision de la mĂ©thode. Pour valider ces changements algorithmiques, la mĂ©thode proposĂ©e a Ă©tĂ© comparĂ©e Ă trois mĂ©thodes rĂ©centes permettant dâestimer Ă©galement lâattĂ©nuation des ondes de cisaillements (mĂ©thodes de dĂ©calage en frĂ©quence, de dĂ©calage en frĂ©quence en deux points et une mĂ©thode ayant comme acronyme anglophone AMUSE) Ă l'aide de donnĂ©es de simulations ou fantĂŽmes numĂ©riques. Ăgalement, des fantĂŽmes de gels homogĂšnes in vitro et des donnĂ©es in vivo acquises sur le foie de canards ont Ă©tĂ© traitĂ©s.
Comme deuxiĂšme objectif, cette thĂšse porte Ă©galement sur le diagnostic prĂ©coce de la stĂ©atose hĂ©patique non alcoolique (NAFLD) qui est nĂ©cessaire pour prĂ©venir sa progression et rĂ©duire la mortalitĂ© globale. Ă cet effet, la mĂ©thode de dĂ©calage en frĂ©quence revisitĂ©e a Ă©tĂ© testĂ©e sur des foies humains in vivo. La performance diagnostique de la nouvelle mĂ©thode a Ă©tĂ© Ă©tudiĂ©e sur des foies humains sains et atteints de la maladie du foie gras non alcoolique. Pour minimiser les sources de variabilitĂ©, une mĂ©thode d'analyse automatisĂ©e faisant la moyenne des mesures prises sous plusieurs angles a Ă©tĂ© mise au point. Les rĂ©sultats de cette mĂ©thode ont Ă©tĂ© comparĂ©s Ă la fraction de graisse Ă densitĂ© de protons obtenue de l'imagerie par rĂ©sonance magnĂ©tique (« magnetic resonance imaging proton density fat fraction », MRI-PDFF) et Ă la biopsie du foie. En outre, lâimagerie SWA a Ă©tĂ© utilisĂ©e pour classer la stĂ©atose et des seuils de dĂ©cision ont Ă©tĂ© Ă©tablis pour la dichotomisation des diffĂ©rents grades de stĂ©atose.
Finalement, le dernier objectif de la thÚse consiste en une étude de reproductibilité de six paramÚtres basés sur la technologie SW (vitesse, atténuation, dispersion, module de Young, viscosité et module de cisaillement). Cette étude a été réalisée chez des volontaires sains et des patients atteints de NAFLD à partir de données acquises lors de deux visites distinctes. En conclusion, une méthode robuste de calcul du SWA du foie a été développée et validée pour fournir une méthode de diagnostic de la NAFLD.For diagnosis and staging of liver fibrosis, liver stiffness is a quantitative biomarker estimated by elastography methods. Ultrasound shear wave (SW) elastography utilizes noninvasive medical ultrasound to assess the mechanical properties of the liver based on the monitoring of the SW propagation. SW speed (SWS) and SW attenuation (SWA) can provide an estimation of tissue viscoelasticity. Biological tissues are inherently viscoelastic in nature and a complex mathematical model is usually required to compute viscoelasticity in SW imaging. Accurate computation of attenuation is critical, especially for accurate loss modulus and viscosity estimation. Recent studies have made attempts to increase the precision of SWA estimation, but they still face some limitations.
As a first objective of this thesis, a revisited frequency-shift method was developed to improve the estimates provided by the original implementation of the frequency-shift method [Bernard et al 2017]. In the new method, the assumption of a constant shape parameter of the gamma function describing the SW magnitude spectrum has been dropped for all lateral locations, allowing a better gamma fitting. Secondly, an adaptive random sample consensus algorithm (A-RANSAC) was implemented to estimate the slope of the varying rate parameter of the gamma distribution to improve the accuracy of the method. For the validation of these algorithmic changes, the proposed method was compared with three recent methods proposed to estimate SWA (frequency-shift, two-point frequency-shift and AMUSE methods) using simulation data or numerical phantoms. In addition, in vitro homogenous gel phantoms and in vivo animal (duck) liver data were processed.
As a second objective, this thesis also aimed at improving the early diagnosis of nonalcoholic fatty liver disease (NAFLD), which is necessary to prevent its progression and decrease the overall mortality. For this purpose, the revisited frequency-shift method was tested on in vivo human livers. The new method's diagnosis performance was investigated with healthy and NAFLD human livers. To minimize sources of variability, an automated analysis method averaging measurements from several angles has been developed. The results of this method were compared to the magnetic resonance imaging proton density fat fraction (MRI-PDFF) and to liver biopsy. SWA imaging was used for grading steatosis and cut-off decision thresholds were established for dichotomization of different steatosis grades.
As a third objective, this thesis is proposing a reproducibility study of six SW-based parameters (speed, attenuation, dispersion, Youngâs modulus, viscosity and shear modulus). The assessment was performed in healthy volunteers and NAFLD patients using data acquired at two separate visits. In conclusion, a robust method for computing the liverâs SWA was developed and validated to provide a diagnostic method for NAFLD
Cardiac Shear Wave Elastography
This dissertation focusses on âshear wave elastographyâ, a non-invasive technique that can potentially be used for the early detection of an increased stiffness of the myocardium in people with (an increased risk on) heart failure. The accurate measurement and interpretation of natural shear waves after valve closure are focused on in particular. The results show that the propagation speeds of these natural shear waves are not only affected by intrinsic characteristics of the myocardium (passive myocardial stiffness, relaxation and contraction), but also by the hemodynamic load
Thérapies ultrasonores cardiaques guidées par élastographie et échographie ultrarapides
Atrial fibrillation (AF) affects 2-3% of the European and North-American population, whereas ventricular tachyarrhythmia (VT) is related to an important risk of sudden death. AF and VT originate from dysfunctional electrical activity in cardiac tissues. Minimally-invasive approaches such as Radio-Frequency Catheter Ablation (RFCA) have revolutionized the treatment of these diseases; however the success rate of RFCA is currently limited by the lack of monitoring techniques to precisely control the extent of thermally ablated tissue.The aim of this thesis is to propose novel ultrasound-based approaches for minimally invasive cardiac ablation under guidance of ultrasound imaging. For this, first, we validated the accuracy and clinical viability of Shear-Wave Elastography (SWE) as a real-time quantitative imaging modality for thermal ablation monitoring in vivo. Second we implemented SWE on an intracardiac transducer and validated the feasibility of evaluating thermal ablation in vitro and in vivo on beating hearts of a large animal model. Third, a dual-mode intracardiac transducer was developed to perform both ultrasound therapy and imaging with the same elements, on the same device. SWE-controlled High-Intensity-Focused-Ultrasound thermal lesions were successfully performed in vivo in the atria and the ventricles of a large animal model. At last, SWE was implemented on a transesophageal ultrasound imaging and therapy device and the feasibility of transesophageal approach was demonstrated in vitro and in vivo. These novel approaches may lead to new clinical devices for a safer and controlled treatment of a wide variety of cardiac arrhythmias and diseases.La fibrillation atriale affecte 2-3% des europĂ©ens et nord-amĂ©ricains, les tachycardies ventriculaires sont liĂ©es Ă un risque important de mort subite. Les approches minimalement invasives comme lâAblation par CathĂ©ter RadiofrĂ©quence (RFCA) ont rĂ©volutionnĂ© le traitement de ces maladies, mais le taux de rĂ©ussite de la RFCA est limitĂ© par le manque de techniques dâimagerie pour contrĂŽler cette ablation thermique.Le but de cette thĂšse est de proposer de nouvelles approches ultrasonores pour des traitements cardiaques minimalement invasifs guidĂ©s par Ă©chographie.Pour cela nous avons dâabord validĂ© la prĂ©cision et la viabilitĂ© clinique de lâĂlastographie par Ondes de Cisaillement (SWE) en tant que modalitĂ© dâimagerie quantitative et temps rĂ©el pour lâablation thermique in vivo. Ensuite nous avons implĂ©mentĂ© la SWE sur un transducteur intracardiaque et validĂ© la faisabilitĂ© dâĂ©valuer lâablation thermique in vitro et in vivo sur cĆur battant de gros animal. Puis nous avons dĂ©veloppĂ© un transducteur intracardiaque dual-mode pour effectuer lâablation et lâimagerie ultrasonores avec les mĂȘmes Ă©lĂ©ments, sur le mĂȘme dispositif. Les lĂ©sions thermiques induites par Ultrasons FocalisĂ©s de Haute IntensitĂ© (HIFU) et contrĂŽlĂ©es par la SWE ont Ă©tĂ© rĂ©alisĂ©es avec succĂšs in vivo dans les oreillettes et les ventricules chez le gros animal. Finalement la SWE a Ă©tĂ© implĂ©mentĂ©e sur un dispositif dâimagerie et thĂ©rapie ultrasonores transĆsophagien et la faisabilitĂ© de cette approche a Ă©tĂ© dĂ©montrĂ©e in vitro et in vivo. Ces approches originales pourraient conduire Ă de nouveaux dispositifs cliniques pour des traitements plus sĂ»rs et contrĂŽlĂ©s dâun large Ă©ventail dâarythmies et maladies cardiaques