Rainer Macdonald

Physikalisch-Technische Bundesanstalt, Brunswyck, Lower Saxony, Germany

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Publications (90)134.61 Total impact

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    ABSTRACT: The nEUROPt protocol is one of two new protocols developed within the European project nEUROPt to characterize the performances of time-domain systems for optical imaging of the brain. It was applied in joint measurement campaigns to compare the various instruments and to assess the impact of technical improvements. This protocol addresses the characteristic of optical brain imaging to detect, localize, and quantify absorption changes in the brain. It was implemented with two types of inhomogeneous liquid phantoms based on Intralipid and India ink with well-defined optical properties. First, small black inclusions were used to mimic localized changes of the absorption coefficient. The position of the inclusions was varied in depth and lateral direction to investigate contrast and spatial resolution. Second, two-layered liquid phantoms with variable absorption coefficients were employed to study the quantification of layer-wide changes and, in particular, to determine depth selectivity, i.e., the ratio of sensitivities for deep and superficial absorption changes. We introduce the tests of the nEUROPt protocol and present examples of results obtained with different instruments and methods of data analysis. This protocol could be a useful step toward performance tests for future standards in diffuse optical imaging.
    Journal of Biomedical Optics 08/2014; 19(8). · 2.75 Impact Factor
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    ABSTRACT: Performance assessment of instruments devised for clinical applications is of key importance for validation and quality assurance. Two new protocols were developed and applied to facilitate the design and optimization of instruments for time-domain optical brain imaging within the European project nEUROPt. Here, we present the “Basic Instrumental Performance” protocol for direct measurement of relevant characteristics. Two tests are discussed in detail. First, the responsivity of the detection system is a measure of the overall efficiency to detect light emerging from tissue. For the related test, dedicated solid slab phantoms were developed and quantitatively spectrally characterized to provide sources of known radiance with nearly Lambertian angular characteristics. The responsivity of four time-domain optical brain imagers was found to be of the order of 0.1 m^2 sr. The relevance of the responsivity measure is demonstrated by simulations of diffuse reflectance as a function of source-detector separation and optical properties. Second, the temporal instrument response function (IRF) is a critically important factor in determining the performance of time-domain systems. Measurements of the IRF for various instruments were combined with simulations to illustrate the impact of the width and shape of the IRF on contrast for a deep absorption change mimicking brain activation.
    Journal of Biomedical Optics 08/2014; 19(8):086010. · 2.75 Impact Factor
  • Journal of Biomedical Optics 07/2014; 19(7):76011. · 2.75 Impact Factor
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    ABSTRACT: In the present contribution we investigate the images of CW diffusely reflected light for a point-like source, registered by a CCD camera imaging a turbid medium containing an absorbing lesion. We show that detection of μa variations (absorption anomalies) is achieved if images are normalized to background intensity. A theoretical analysis based on the diffusion approximation is presented to investigate the sensitivity and the limitations of our proposal and a novel procedure to find the location of the inclusions in 3D is given and tested. An analysis of the noise and its influence on the detection capabilities of our proposal is provided. Experimental results on phantoms are also given, supporting the proposed approach.
    Biomedical Optics Express 05/2014; 5(5):1336-54. · 3.18 Impact Factor
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    ABSTRACT: We investigate the correction of fluorescence intensities in epi-illumination imaging by normalization to absorption images. Experiments on custom lymph node phantoms reveal improved estimation of dye concentration for superficial nodes.
    Biomedical Optics; 04/2014
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    ABSTRACT: Oxy- and deoxyhemoglobin concentration changes for two layers were reconstructed by a robust method based on moments of photon flight-time distributions. The method was validated on a phantom and applied to in-vivo functional activation studies.
    Biomedical Optics; 04/2014
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    ABSTRACT: The design of inhomogeneous phantoms for diffuse optical imaging purposes using totally absorbing objects embedded in a diffusive medium is proposed and validated. From time-resolved and continuous-wave Monte Carlo simulations, it is shown that a given or desired perturbation strength caused by a realistic absorbing inhomogeneity of a certain absorption and volume can be approximately mimicked by a small totally absorbing object of a so-called equivalent black volume (equivalence relation). This concept can be useful in two ways. First, it can be exploited to design realistic inhomogeneous phantoms with different perturbation strengths simply using a set of black objects with different volumes. Further, it permits one to grade physiological or pathological changes on a reproducible scale of perturbation strengths given as equivalent black volumes, thus facilitating the performance assessment of clinical instruments. A set of plots and interpolating functions to derive the equivalent black volume corresponding to a given absorption change is provided. The application of the equivalent black volume concept for grading different optical perturbations is demonstrated for some examples.
    Journal of Biomedical Optics 06/2013; 18(6):66014. · 2.75 Impact Factor
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    ABSTRACT: Multi-source operation in time-domain optical brain imaging often relies on the use of piezomechanical fiber switches which limit the speed when recording dynamic processes. The concept presented in this work overcomes this limitation by multiplexing on the nanosecond and microsecond time scales. In particular, the source positions were encoded by different delays on the nanosecond time scale. Multiplexing of wavelengths on the microsecond time scale (e.g. within 100 µs) was achieved by burst-mode operation of picosecond diode lasers in combination with addressing of different memory blocks in time-correlated single photon counting by means of routing inputs. This concept was implemented for 4 detectors and 5 source optodes yielding 12 measurement channels per hemisphere. In order to largely equalize the count rates for all source-detector pairs with minimal overall losses, a setup was developed that enabled the freely adjustable distribution of laser power to the various source optodes. It was based on polarization splitters and motorized broadband polarization rotators. The method was successfully demonstrated in an in vivo experiment employing two different types of motor activation of the brain.
    European Conference on Biomedical Optics; 05/2013
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    ABSTRACT: We present results of first in-vivo tests of an optical non-contact scanning imaging system, intended to study oxidative metabolism related processes in biological tissue by means of time-resolved near-infrared spectroscopy. Our method is a novel realization of the short source-detector separation approach and based on a fast-gated single-photon avalanche diode to detect late photons only. The scanning system is built in quasi-confocal configuration and utilizes polarizationsensitive detection. It scans an area of 4×4 cm2, recording images with 32×32 pixels, thus creating a high density of source-detector pairs. To test the system we performed a range of in vivo measurements of hemodynamic changes in several types of biological tissues, i.e. skin (Valsalva maneuver), muscle (venous and arterial occlusions) and brain (motor and cognitive tasks). Task-related changes in hemoglobin concentrations were clearly detected in skin and muscle. The brain activation shows weaker, but yet detectable changes. These changes were localized in pixels near the motor cortex area (C3). However, it was found that even very short hair substantially impairs the measurement. Thus the applicability of the scanner is limited to hairless parts of body. The results of our first in-vivo tests prove the feasibility of non-contact scanning imaging as a first step towards development of a prototype for biological tissue imaging for various medical applications.
    European Conference on Biomedical Optics; 05/2013
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    ABSTRACT: We have developed a method to quantify hemoglobin concentration and oxygen saturation within the renal cortex by near-infrared spectroscopy. A fiber optic probe was used to transmit the radiation of three semiconductor lasers at 690 nm, 800 nm and 830 nm to the tissue, and to collect diffusely remitted light at source-detector separations from 1 mm to 4 mm. To derive tissue hemoglobin concentration and oxygen saturation of hemoglobin the spatial dependence of the measured cw intensities was fitted by a Monte Carlo model. In this model the tissue was assumed to be homogeneous. The scaling factors between measured intensities and simulated photon flux were obtained by applying the same setup to a homogeneous semi-infinite phantom with known optical properties and by performing Monte Carlo simulations for this phantom. To accelerate the fit of the tissue optical properties a look-up table of the simulated reflected intensities was generated for the needed range of absorption and scattering coefficients. The intensities at the three wavelengths were fitted simultaneously using hemoglobin concentration, oxygen saturation, the reduced scattering coefficient at 800 nm and the scatter power coefficient as fit parameters. The method was employed to study the temporal changes of renal hemoglobin concentration and blood oxygenation on an anesthetized rat during a short period of renal ischemia induced by aortic occlusion and during subsequent reperfusion.
    03/2013
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    ABSTRACT: We propose and validate the design of inhomogeneous phantoms for diffuse optical imaging purposes using totally absorbing objects embedded in a diffusive medium. From Monte Carlo simulations, we show that a given or desired perturbation strength caused by an realistic absorbing inhomogeneity of a certain absorption and volume can be approximately mimicked by a small totally absorbing object of a so-called Equivalent Black Volume (Equivalence Relation). This concept can be useful to design realistic inhomogeneous phantoms using a set of black objects with different volumes. Further, it permits to grade physiological or pathological changes on a reproducible scale of equivalent black volumes, thus facilitating the performance assessment of clinical instruments. We have also provided a plot to derive the Equivalent Black Volume yielding the same effect of a realistic absorption object.
    Proc SPIE 02/2013;
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    ABSTRACT: Novel protocols were developed and applied in the European project "nEUROPt" to assess and compare the performance of instruments for time-domain optical brain imaging and of related methods of data analysis. The objective of the first protocol, "Basic Instrumental Performance", was to record relevant basic instrumental characteristics in a direct way. The present paper focuses on the second novel protocol ("nEUROPt" protocol) that was devoted to the assessment of sensitivity, spatial resolution and quantification of absorption changes within inhomogeneous media. It was implemented with liquid phantoms based on Intralipid and ink, with black inclusions and, alternatively, in two-layered geometry. Small black cylinders of various sizes were used to mimic small localized changes of the absorption coefficient. Their position was varied in depth and lateral direction to address contrast and spatial resolution. Two-layered liquid phantoms were used, in particular, to determine depth selectivity, i.e. the ratio of contrasts due to a deep and a superficial absorption change of the same magnitude. We introduce the tests of the "nEUROPt" protocol and present exemplary results obtained with various instruments. The results are related to measurements with both types of phantoms and to the analysis of measured time-resolved reflectance based on time windows and moments. Results are compared for the different instruments or instrumental configurations as well as for the methods of data analysis. The nEUROPt protocol is also applicable to cw or frequency-domain instruments and could be useful for designing performance tests in future standards in diffuse optical imaging.
    Proc SPIE 02/2013;
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    ABSTRACT: We report on the design and first in vivo tests of a novel non-contact scanning imaging system for time-domain near-infrared spectroscopy. Our system is based on a null source-detector separation approach and utilizes polarization-selective detection and a fast-gated single-photon avalanche diode to record late photons only. The in-vivo tests included the recording of hemodynamics during arm occlusion and two brain activation tasks. Localized and non-localized changes in oxy- and deoxyhemoglobin concentration were detected for motor and cognitive tasks, respectively. The tests demonstrate the feasibility of non-contact imaging of absorption changes in deeper tissues.
    Biomedical Optics Express 01/2013; 4(10):2257-68. · 3.18 Impact Factor
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    ABSTRACT: In functional near-infrared spectroscopy (fNIRS) superficial hemodynamics can mask optical signals related to brain activity. We present a method to separate superficial and cerebral absorption changes based on the analysis of changes in moments of time-of-flight distributions and a two-layered model. The related sensitivity factors were calculated from individual optical properties. The method was validated on a two-layer liquid phantom. Absorption changes in the lower layer were retrieved with an accuracy better than 20%. The method was successfully applied to in vivo data and compared to the reconstruction of homogeneous absorption changes.
    Biomedical Optics Express 01/2013; 5(5):1465-1482. · 3.18 Impact Factor
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    ABSTRACT: Using 15 rats with collagen-induced arthritis (30 joints) and 7 control rats (14 joints), we correlated the intensity of near-infrared fluorescence (NIRF) of the nonspecific dye tetrasulfocyanine (TSC) with magnetic resonance imaging (MRI), histopathology, and clinical score. Fluorescence images were obtained in reflection geometry using a NIRF camera system. Normalized fluorescence intensity (INF) was determined after intravenous dye administration on different time points up to 120 min. Contrast-enhanced MRI using gadodiamide was performed after NIRF imaging. Analyses were performed in a blinded fashion. Histopathological and clinical scores were determined for each ankle joint. INF of moderate and high-grade arthritic joints were significantly higher (p<0.005) than the values of control and low-grade arthritic joints between 5 and 30 min after TSC-injection. This result correlated well with post-contrast MRI signal intensities at about 5 min after gadodiamide administration. Furthermore, INF and signal increase on contrast-enhanced MRI showed high correlation with clinical and histopathological scores. Sensitivities and specificities for detection of moderate and high-grade arthritic joints were slightly lower for NIRF imaging (89%/81%) than for MRI (100%/91%). NIRF imaging using TSC, which is characterized by slower plasma clearance compared to indocyanine green (ICG), has the potential to improve monitoring of inflamed joints.
    Journal of Biomedical Optics 10/2012; 17(10):106008-1. · 2.75 Impact Factor
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    ABSTRACT: Non-invasive detection of fluorescence from the optical tracer indocyanine green is feasible in the adult human brain when employing a time-domain technique with picosecond resolution. A fluorescence-based assessment may offer higher signal-to-noise ratio when compared to bolus tracking relying on changes in time-resolved diffuse reflectance. The essential challenge is to discriminate the fluorescence originating from the brain from contamination by extracerebral fluorescence and hence to reconstruct the bolus kinetics; however, a method to reliably perform the necessary separation is missing. We present a novel approach for the decomposition of the fluorescence contributions from the two tissue compartments. The corresponding sensitivity functions pertaining to the brain and to the extracerebral compartment are directly derived from the in-vivo measurement. This is achieved by assuming that during the initial and the late phase of bolus transit the fluorescence signal originates largely from one of the compartments. Solving the system of linear equations allows one to approximate time courses of a bolus for each compartment. We applied this method to repetitive measurements on two healthy subjects with an overall 34 boluses. A reconstruction of the bolus kinetics was possible in 62% of all cases.
    Journal of Biomedical Optics 05/2012; 17(5):057003. · 2.75 Impact Factor
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    ABSTRACT: A novel protocol to determine sensitivity, spatial resolution and quantification of absorption changes in optical brain imaging was applied to assess time-domain instruments and methods of data analysis.
    Biomedical Optics; 04/2012
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    ABSTRACT: In the second part of this two-part series on the state-of-the-art comparability of corrected emission spectra, we have extended this assessment to the broader community of fluorescence spectroscopists by involving 12 field laboratories that were randomly selected on the basis of their fluorescence measuring equipment. These laboratories performed a reference material (RM)-based fluorometer calibration with commercially available spectral fluorescence standards following a standard operating procedure that involved routine measurement conditions and the data evaluation software LINKCORR developed and provided by the Federal Institute for Materials Research and Testing (BAM). This instrument-specific emission correction curve was subsequently used for the determination of the corrected emission spectra of three test dyes, X, QS, and Y, revealing an average accuracy of 6.8% for the corrected emission spectra. This compares well with the relative standard uncertainties of 4.2% for physical standard-based spectral corrections demonstrated in the first part of this study (previous paper in this issue) involving an international group of four expert laboratories. The excellent comparability of the measurements of the field laboratories also demonstrates the effectiveness of RM-based correction procedures.
    Analytical Chemistry 02/2012; 84(9):3899-907. · 5.82 Impact Factor
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    ABSTRACT: The development of fluorescence applications in the life and material sciences has proceeded largely without sufficient concern for the measurement uncertainties related to the characterization of fluorescence instruments. In this first part of a two-part series on the state-of-the-art comparability of corrected emission spectra, four National Metrology Institutes active in high-precision steady-state fluorometry performed a first comparison of fluorescence measurement capabilities by evaluating physical transfer standard (PTS)-based and reference material (RM)-based calibration methods. To identify achievable comparability and sources of error in instrument calibration, the emission spectra of three test dyes in the wavelength region from 300 to 770 nm were corrected and compared using both calibration methods. The results, obtained for typical spectrofluorometric (0°/90° transmitting) and colorimetric (45°/0° front-face) measurement geometries, demonstrated a comparability of corrected emission spectra within a relative standard uncertainty of 4.2% for PTS- and 2.4% for RM-based spectral correction when measurements and calibrations were performed under identical conditions. Moreover, the emission spectra of RMs F001 to F005, certified by BAM, Federal Institute for Materials Research and Testing, were confirmed. These RMs were subsequently used for the assessment of the comparability of RM-based corrected emission spectra of field laboratories using common commercial spectrofluorometers and routine measurement conditions in part 2 of this series (subsequent paper in this issue).
    Analytical Chemistry 02/2012; 84(9):3889-98. · 5.82 Impact Factor
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    ABSTRACT: We report results of the proof-of-principle tests of a novel non-contact tissue imaging system. The system utilizes a quasi-null source-detector separation approach for time-domain near-infrared spectroscopy, taking advantage of an innovative state-of-the-art fast-gated single photon counting detector. Measurements on phantoms demonstrate the feasibility of the non-contact approach for the detection of optically absorbing perturbations buried up to a few centimeters beneath the surface of a tissue-like turbid medium. The measured depth sensitivity and spatial resolution of the new system are close to the values predicted by Monte Carlo simulations for the inhomogeneous medium and an ideal fast-gated detector, thus proving the feasibility of the non-contact approach for high density diffuse reflectance measurements on tissue. Potential applications of the system are also discussed.
    Optics Express 01/2012; 20(1):283-90. · 3.55 Impact Factor