Matthias Klemm

Independent component analysis: comparison of algorithms for the investigation of surface electrical brain activity.

We compared the performance of 22 algorithms for independent component analysis with the aim to find suitable algorithms for applications in the field of surface electrical brain activity analysis. The quality of the separation is assessed with four performance measures: a correlation coefficient based index, a signal-to-interference ratio, a signal-to-distortion-ratio and the computational demand. Artificial data are used consisting of typical electroencephalogram and evoked potentials signal patterns, e.g. spikes, polyspikes, sharp waves and spindles. We evaluate different noise scenarios and the influence of pre-whitening. The comparisons reveal considerable differences between the algorithms, especially concerning the computational load. Algorithms based on the time structure of the data set seem to have advantages in separation quality especially for sine-shaped signals. Derivates of FastICA and Infomax also attain good results. Our results can serve as a reference for selecting a task-specific algorithm to analyze a large number of signal patterns occurring in the surface electrical brain activity.

Fluorescence lifetime imaging ophthalmoscopy in type 2 diabetic patients who have no signs of diabetic retinopathy

Abstract. The time-resolved autofluorescence of the eye is used for the detection of metabolic alteration in diabetic patients who have no signs of diabetic retinopathy. One eye from 37 phakic and 11 pseudophakic patients with type 2 diabetes, and one eye from 25 phakic and 23 pseudophakic healthy subjects were included in the study. After a three-exponential fit of the decay of autofluorescence, histograms of lifetimes τi, amplitudes αi, and relative contributions Qi were statistically compared between corresponding groups in two spectral channels (490<ch1<560  nm, 560<ch2<700  nm). The change in single fluorophores was estimated by applying the Holm–Bonferroni method and by calculating differences in the sum histograms of lifetimes. Median and mean of the histograms of τ2, τ3, and α3 in ch1 show the greatest differences between phakic diabetic patients and age-matched controls (p<0.000004). The lack of pixels with a τ2 of ∼360  ps, the increased number of pixels with τ2>450  ps, and the shift of τ3 from ∼3000 to 3700 ps in ch1 of diabetic patients when compared with healthy subjects indicate an increased production of free flavin adenine dinucleotide, accumulation of advanced glycation end products (AGE), and, probably, a change from free to protein-bound reduced nicotinamide adenine dinucleotide at the fundus. AGE also accumulated in the crystalline lens.

Repeatability of Autofluorescence Lifetime Imaging at the Human Fundus in Healthy Volunteers

Abstract Purpose: We aim to evaluate the repeatability of a new fluorescence lifetime imaging (FLIM) technique which measures time-resolved autofluorescence to assess metabolism of the retina. Materials and methods: We performed FLIM with two spectral channels (channel 1: 490–560 nm and channel 2: 560–700 nm) on 10 healthy volunteers, with 10 replicates per volunteer. From the 30° fundus FLIM images, we selected three regions: the fovea, the optic disc and the papillo-macular bundle. For each channel in these regions, we determined an average multi-exponential approximation with three components, and the six resulting parameters, α1–α3 (amplitudes) and τ1–τ3 (fluorescence lifetimes), were analyzed in terms of the coefficient of variation (CV). Results: Repeatability was highest in the papillo-macular bundle, followed by the fovea and the optic disc. Repeatability was higher in channel 1 (mean CV of 7.9%) than in channel 2 (mean CV of 17.7%). The average CV for the diagnostically most relevant channel 1 and the most relevant parameters was as follows: τ1 (5.5%) and τ2 (4.7%) in the papillo-macular bundle, and τ1 (6.8%) and τ2 (6.9%) in the fovea. Conclusions: We demonstrated repeatability of FLIM measurement results within acceptable ranges of variation. Based on the detailed coefficients of variation, we derived recommendations for parameter ranges suitable for diagnostic applications.

Vergleich von Parametern der zeitaufgelösten Autofluoreszenz bei Gesunden und Patienten mit früher AMD

BACKGROUND A fluorescence lifetime mapper (FLM) was tested for quantitative estimation of early alterations in age-related macular degeneration (AMD) which are assumed to be in cellular metabolism. METHOD In FLM time-resolved autofluorescence of the fundus is excited by picosecond (ps) laser impulses at 448 nm and detected in 2 spectral ranges (K1=490-560 nm and K2=560-700 nm) by time-correlated single photon counting. The time-dependent decrease in fluorescence intensity was approximated using 3 decay rates. The calculated lifetimes allow a comparison with endogenous fluorophores of cellular metabolism. RESULTS Initially mean lifetimes were determined for 8 healthy subjects (K1: tau1=118 ps, tau2=584 ps, tau3=2826 ps, K2: tau1=104 ps, tau2=477 ps, tau3=1623 ps). In 15 AMD patients (AREDS categories I and II) the lifetimes were longer (K1: tau1=166 ps, tau2=986 ps, tau3=3309 ps, K2: tau1=137 ps, tau2=583 ps, tau3=1924 ps). The best separation between healthy subjects and patients with early AMD was possible by parameters 1 and 2 in the short-wave channel. Fluorophore-specific alterations in the macula could be demonstrated in isolated cases with advanced AMD. CONCLUSION Measurements in the 30 degrees fundus field demonstrated that specific alterations were already present even in early AMD and also outside the macula. These act in the neuronal retina, because parameter tau2 is related to this layer. Increases in the lifetime of parameter tau2 in the short wave channel could at least partially be determined by an increase of protein bound NADH, the content of which increases with reduced cellular respiration.

[Comparison of parameters of time-resolved autofluorescence between healthy subjects and patients suffering from early AMD].

BACKGROUND A fluorescence lifetime mapper (FLM) was tested for quantitative estimation of early alterations in age-related macular degeneration (AMD) which are assumed to be in cellular metabolism. METHOD In FLM time-resolved autofluorescence of the fundus is excited by picosecond (ps) laser impulses at 448 nm and detected in 2 spectral ranges (K1=490-560 nm and K2=560-700 nm) by time-correlated single photon counting. The time-dependent decrease in fluorescence intensity was approximated using 3 decay rates. The calculated lifetimes allow a comparison with endogenous fluorophores of cellular metabolism. RESULTS Initially mean lifetimes were determined for 8 healthy subjects (K1: tau1=118 ps, tau2=584 ps, tau3=2826 ps, K2: tau1=104 ps, tau2=477 ps, tau3=1623 ps). In 15 AMD patients (AREDS categories I and II) the lifetimes were longer (K1: tau1=166 ps, tau2=986 ps, tau3=3309 ps, K2: tau1=137 ps, tau2=583 ps, tau3=1924 ps). The best separation between healthy subjects and patients with early AMD was possible by parameters 1 and 2 in the short-wave channel. Fluorophore-specific alterations in the macula could be demonstrated in isolated cases with advanced AMD. CONCLUSION Measurements in the 30 degrees fundus field demonstrated that specific alterations were already present even in early AMD and also outside the macula. These act in the neuronal retina, because parameter tau2 is related to this layer. Increases in the lifetime of parameter tau2 in the short wave channel could at least partially be determined by an increase of protein bound NADH, the content of which increases with reduced cellular respiration.

FLIMX: A Software Package to Determine and Analyze the Fluorescence Lifetime in Time-Resolved Fluorescence Data from the Human Eye.

Fluorescence lifetime imaging ophthalmoscopy (FLIO) is a new technique for measuring the in vivo autofluorescence intensity decays generated by endogenous fluorophores in the ocular fundus. Here, we present a software package called FLIM eXplorer (FLIMX) for analyzing FLIO data. Specifically, we introduce a new adaptive binning approach as an optimal tradeoff between the spatial resolution and the number of photons required per pixel. We also expand existing decay models (multi-exponential, stretched exponential, spectral global analysis, incomplete decay) to account for the layered structure of the eye and present a method to correct for the influence of the crystalline lens fluorescence on the retina fluorescence. Subsequently, the Holm-Bonferroni method is applied to FLIO measurements to allow for group comparisons between patients and controls on the basis of fluorescence lifetime parameters. The performance of the new approaches was evaluated in five experiments. Specifically, we evaluated static and adaptive binning in a diabetes mellitus patient, we compared the different decay models in a healthy volunteer and performed a group comparison between diabetes patients and controls. An overview of the visualization capabilities and a comparison of static and adaptive binning is shown for a patient with macular hole. FLIMX’s applicability to fluorescence lifetime imaging microscopy is shown in the ganglion cell layer of a porcine retina sample, obtained by a laser scanning microscope using two-photon excitation.

Fundus autofluorescence lifetimes are increased in non‐proliferative diabetic retinopathy

To discriminate non‐proliferative diabetic retinopathy (NPDR) patients from healthy controls by fluorescence lifetime imaging ophthalmoscopy (FLIO).

Monitoring macular pigment changes in macular holes using fluorescence lifetime imaging ophthalmoscopy

To investigate the impact of macular pigment (MP) on fundus autofluorescence (FAF) lifetimes in vivo by characterizing full‐thickness idiopathic macular holes (MH) and macular pseudo‐holes (MPH).

Monitoring foveal sparing in geographic atrophy with fluorescence lifetime imaging ophthalmoscopy – a novel approach

To investigate fundus autofluorescence (FAF) lifetimes in geographic atrophy (GA) with a focus on macular pigment (MP) and foveal sparing.

Effects of excitation coil configurations in magnetorelaxometry imaging of magnetic nanoparticles

Magnetorelaxometry imaging of magnetic nanoparticles is a promising technique for detecting the distribution of these particles in order to monitor novel cancer therapy approaches. The imaging is based on the reconstruction of the particle distribution from signals measured with several SQUID sensors. For a practical application, suitable configurations of the sensor and excitation setup need to be defined. In this paper, the influence of the excitation coil configuration on the imaging quality is investigated in simulation studies. In this respect, the number, diameter, arrangement, and shape of the coils are regarded. The results show that increasing the number of coils will improve the reconstruction result. Simulations with different coil diameters show an improvement with smaller coil diameters. Our simulations reveal a minor influence of the arrangement and the shape of the coils.