Vratislav Harabis

Hybrid retinal image registration using phase correlation

Abstract This paper deals with registration of retinal images, which were taken by high-resolution digital colour fundus cameras. The proposed method describes successful application of phase correlation method. It combines several basic steps — global correction of shift, rotation and scaling, detection of landmarks, their correspondences and finally image registration using second-order polynomial model. The method is tested on two sets of images. The first set contains images from the diabetic patients where many retinal pathologies can disturb the registration process. The second set contains images from healthy subjects, which were acquired by different illumination conditions. The method was evaluated using four different criteria - tree objective and one subjective. These criteria are also compared. The achieved registration accuracy of the landmarks position error is 1·13 and 0·93 pixels for respective image sets. Finally, the simple scheme for retinal pathology visualisation of registered fundus pairs is presented.

Ultrasound perfusion analysis combining bolus-tracking and burst-replenishment

A new signal model and processing method for quantitative ultrasound perfusion analysis is presented, called bolus-and-burst. The method has the potential to provide absolute values of blood flow, blood volume, and mean transit time. Furthermore, it provides an estimate of the local arterial input function which characterizes the arterial tree, allowing accurate estimation of the bolus arrival time. The method combines two approaches to ultrasound perfusion analysis: bolus-tracking and burst-replenishment. A pharmacokinetic model based on the concept of arterial input functions and tissue residue functions is used to model both the bolus and replenishment parts of the recording. The pharmacokinetic model is fitted to the data using blind deconvolution. A preliminary assessment of the new perfusion-analysis method is presented on clinical recordings.

Parametric ultrasound perfusion analysis combining bolus tracking and replenishment

The paper presents a new perfusion analysis method using ultrasound which combines burst-replenishment and bolustracking acquisition methods. It allows absolute quantification of the mean transit time, blood flow and blood volume. It is based on the concept of arterial input function and tissue residue function and is formulated as a blind-deconvolution problem. It is illustrated on recordings from Crohns disease patients.

Comparison and evaluation of indicator dilution models for bolus of ultrasound contrast agents.

Dynamic contrast-enhanced ultrasound (DCE-US) imaging is a promising diagnostic method, which enables the evaluation of tissue perfusion via different parameters. The mean transit time and time-to-peak parameters are the main time parameters and their values depend on the model used for the approximation of the noisy perfusion curves. In this paper, we described a new comparison of different perfusion models using a tissue mimicking phantom. The following models were compared: log-normal, lagged, Erlang, Gamma and the local density random walk model. We discovered that the mean-square error is not the best criterion for model evaluation. More important is the comparison between the estimated time perfusion parameters and the physical parameters of the developed tissue mimicking phantom. Based on the statistical analysis, we can suggest that for the DCE-US perfusion analysis more models should be used, excluding the log-normal model, which gives the highest error of mean transit time value.

Automatic Rigid Registration and Analysis of Colour Fundus Image in Patients with Diabetic Retinopathy

This paper describes a new method for detection biffurcation points of retinal vascular tree. This approach uses Hessian matrix for blood vessel segmentation, followed by morphological thinning and binary matched filtering. These bifurcation points are consequently used as landmarks for rigid image registration based on the iterative closest point method. Registered images were used for simple analysis with the aim to enhance the new pathologies in analyzed image sequence.

Absolute ultrasound perfusion parameter quantification of a tissue-mimicking phantom using bolus tracking [Correspondence]

This study presents three methods for absolute quantification in ultrasound perfusion analysis based on bolus tracking. The first two methods deconvolve the perfusion time sequence with a measured AIF, using a nonparametric or a parametric model of the tissue residue function, respectively. The third method is a simplified approach avoiding deconvolution by assuming a narrow AIF. A phantom with a dialyzer filter as a tissue-mimicking model was used for evaluation. Estimated mean transit times and blood volumes were compared with the theoretical values. A match with a maximum error of 12% was achieved.

Nakagami model of scattering in ultrasound contrast media

This paper describes an experimental evaluation of the statistical properties of microbubbles by the means of Nakagami probability distribution. The first and second harmonic images are used for statistical analysis of backscattered signals from saline with different concentration of microbubbles (SonoVue). There were no differences between the shape parameters m from Nakagami distribution obtained from the first and second harmonic images, which was confirmed by t-test (p<;0.001). The results also show the scattering conditions for each concentration described by the values of this shape parameter.

Influence of injection speed of contrast agent on perfusion analysis using fast bolus method

Dynamic contrast enhanced ultrasound (DCE-US) imaging is a promising modality for noninvasive assessment of various tissue kinetic parameters (e.g. time-to-peak, mean transit time (MTT), blood flow etc.). These parameters can be used for diagnosis of pathological tissue functions caused by different diseases. Fast bolus method is based on the fast injection of bolus of contrast agent and repeated imaging with low MI to track contrast agent in tissue of interest (TOI) in time. Mathematical model fitting is an important part of this analysis, which usually presumes that injected bolus is infinitely short, but in practice this is impossible. The main aim of this paper is an analysis of influence of bolus injection rate on MTT estimated by different fitting model. We discover that using a finite-width bolus causes systematic error in MTT estimation.

Rician inverse Gaussian model of scattering in ultrasound contrast media

A statistical analysis of 1st and 2nd harmonic echoes backscattered from contrast media using Rician inverse Gaussian (RiIG) envelope model is described. Knowledge of statistics of the scattered signal may contribute to perfusion analysis. A range of ultrasound contrast agent (UCA) concentrations is examined. The RiIG distribution was fitted to image histograms, using Matlab Global optimization toolbox. Influence of simulated attenuation was also tested. Analysis of resulting RiIG distribution parameters shows that while its parameters α and β do not follow any properties of the changing concentration, parameter δ shows logarithmic dependence on concentration in range of 3.125 - 50 mg/l. Taking into account simulated attenuation, we observed that the range and shape of the dependency remain stable, although the actual range of parameter δ decreases with increasing attenuation. This analysis contributes to our previous results, where the Nakagami distribution showed similar dependence of the shape parameter on UCA concentration in a smaller, partially overlapping, concentration range.