Daniel Grucker

Brain dysmyelination and recovery assessment by noninvasive in vivo diffusion tensor magnetic resonance imaging.

Diffusion tensor magnetic resonance imaging (DT‐MRI) was applied for in vivo quantification of myelin loss and regeneration. A transgenic mouse line (Oligo‐TTK) expressing a truncated form of the herpes simplex virus 1 thymidine kinase gene (hsv1‐tk) in oligodendrocytes was studied along with two induced phenotypes of myelin pathology. Myelin loss and axonal abnormalities differentially affect values of DT‐MRI parameters in the brain of transgenic mice. Changes in the anisotropy of the white matter were assessed by calculating and mapping the radial (D⊥) and axial (D∥) water diffusion to axonal tracts and fractional anisotropy (FA). A significant increase in D⊥ attributed to the lack of myelin was observed in all selected brain white matter tracts in dysmyelinated mice. Lower D∥ values were consistent with the histological observation of axonal modifications, including reduced axonal caliber and overexpression of neurofilaments and III β‐tubulin. We show clearly that myelination and axonal changes play a role in the degree of diffusion anisotropy, because FA was significantly decreased in dysmyelinated brain. Importantly, myelin reparation during brain postnatal development induced a decrease in the magnitude of D⊥ and an increase in FA compared with the same brain before recovery. The progressive increase in D∥ values was attributed to the gain in normal axonal morphology. This regeneration was confirmed by the detection of enlarged oligodendrocyte population, newly formed myelin sheaths around additional axons, and a gradual increase in axonal caliber.

Recovery from Chronic Demyelination by Thyroid Hormone Therapy: Myelinogenesis Induction and Assessment by Diffusion Tensor Magnetic Resonance Imaging

The failure of the remyelination processes in multiple sclerosis contributes to the formation of chronic demyelinated plaques that lead to severe neurological deficits. Long-term cuprizone treatment of C57BL/6 mice resulted in pronounced white matter pathology characterized by oligodendrocyte depletion, irreversible demyelination and persistent functional deficits after cuprizone withdrawal. The use of a combination of in vivo diffusion tensor magnetic resonance imaging (DT-MRI) and histological analyses allowed for an accurate longitudinal assessment of demyelination. Injection of triiodothyronine (T3) hormone over a 3 week interval after cuprizone withdrawal progressively restored the normal DT-MRI phenotype accompanied by an improvement of clinical signs and remyelination. The effects of T3 were not restricted to the later stages of remyelination but increased the expression of sonic hedgehog and the numbers of Olig2+ and PSA-NCAM+ precursors and proliferative cells. Our findings establish a role for T3 as an inducer of oligodendrocyte progenitor cells in adult mouse brain following chronic demyelination.

Registration of MR/MR and MR/SPECT Brain Images by Fast Stochastic Optimization of Robust Voxel Similarity Measures

This paper describes a robust, fully automated algorithm to register intrasubject 3D single and multimodal images of the human brain. The proposed technique accounts for the major limitations of the existing voxel similarity-based methods: sensitivity of the registration to local minima of the similarity function and inability to cope with gross dissimilarities in the two images to be registered. Local minima are avoided by the implementation of a stochastic iterative optimization technique (fast simulated annealing). In addition, robust estimation is applied to reject outliers in case the images show significant differences (due to lesion evolution, incomplete acquisition, non-Gaussian noise, etc.). In order to evaluate the performance of this technique, 2D and 3D MR and SPECT human brain images were artificially rotated, translated, and corrupted by noise. A test object was acquired under different angles and positions for evaluating the accuracy of the registration. The approach has also been validated on real multiple sclerosis MR images of the same patient taken at different times. Furthermore, robust MR/SPECT image registration has permitted the representation of functional features for patients with partially complex seizures. The fast simulated annealing algorithm combined with robust estimation yields registration errors that are less than 1 degree in rotation and less than 1 voxel in translation (image dimensions of 128(3)). It compares favorably with other standard voxel similarity-based approaches.

In vitro validation of MR measurements of arterial pulse-wave velocity in the presence of reflected waves.

A magnetic resonance imaging projective velocity encoding sequence was used to determine the pulse‐wave velocity in an artery model. To this end, a well‐defined flow phantom simulating flow propagation in large arteries was used. In order to validate the measurement method in the presence of large reflected waves, these were deliberately created in the phantom. The projective sequence was applied to two measurement sites and the wave velocity was determined from the spatial and temporal separations of the foot of the velocity waveform. A theoretical model describing reflection and attenuation phenomena was compared with experimental velocity waveforms. The model showed that reflections and attenuation can explain the important changes in velocity waveforms. The model also confirmed that in the presence of reflecting waves, the foot of the waveform can be used as a characteristic point for measurements through changes in the waveform. J. Magn. Reson. Imaging 2001;14:120–127.

Oxygen imaging in perfused hearts by dynamic nuclear polarization

The feasibility of localized oximetry by dynamic nuclear polarization (DNP) imaging is demonstrated on perfused sheep hearts. DNPI is a magnetic, double resonance technique, in which the electron paramagnetic resonance (EPR) of a free radical dissolved in a perfusion medium is saturated, while the nuclear magnetic resonance (NMR) of water protons is used to perform conventional NMR imaging. The presence of oxygen reduces the enhancement of the NMR signal induced by DNP. The oxygen content of sheep heart tissues was detected by the subtraction of the DNP image of the heart, perfused with a nitrogen-equilibrated solution, from an image obtained when the heart was perfused with an oxygen-equilibrated solution. This result was obtained with extreme oxygen partial pressure, and the discussion presents physical and chemical means for improving the DNP imaging method. Physical means include field cycling, electron paramagnetic rotary saturation, and the use of a 180 degrees NMR pulse before EPR irradiation. The chemical means discussed are deuterium substitution in nitroxides and the potential use of solid, free radical probes. It is suggested to use the perfused heart model for comparing the numerous methods available to measure the oxygen content of tissues.

Contribution of T2Relaxation Time Mapping in the Evaluation of Cryptogenic Temporal Lobe Epilepsy

In this study we compared the results of visual analysis of MR imaging with T2 relaxation time mapping of the mesial structures in a group of 97 patients with cryptogenic temporal lobe epilepsy. All patients underwent a clinical neurological examination, neuropsychological investigation, prolonged video-EEG monitoring, SPECT imaging, MR imaging, and T2 relaxation time mapping. T2 relaxation times were estimated with a Carr‐Purcell‐Meiboom‐Gill pulse sequence with 48 echoes (15 to 720 ms). The mean T2 relaxation time value was 118.5 6 2 ms in the hippocampi and 120.3 6 1.9 ms in the amygdala of 21 healthy subjects used as controls. T2 relaxation mapping revealed mesial temporal sclerosis in 91.8% of the patients (often involving both the hippocampus and the amygdala) and evidenced bilateral involvement in 44.6% of the patients against 72.2 and 6.2%, respectively, for MR imaging. The ipsilateral and contralateral hippocampal T2 relaxation time values did significantly correlate with seizure frequency and the contralateral hippocampal T2 relaxation time value with the duration of epilepsy. In conclusion, this quantitative method is highly sensitive for the detection of mesial temporal sclerosis and permits a better evaluation of the apparently normal contralateral mesial structures. r 1998 Academic Press

Recovery of Myelin after Induction of Oligodendrocyte Cell Death in Postnatal Brain

A transgenic mouse line (Oligo-TTK) was established to monitor oligodendrocyte cell death and myelin formation in the CNS. The expression of a conditionally toxic gene, the herpes simplex virus-1 thymidine kinase (HSV1-TK), was made under control of the MBP (myelin basic protein) gene promoter. A truncated form of the HSV1-TK (TTK) gene was used to avoid both bystander effect resulting from leaking in thymidine kinase activity and sterility in transgenic males observed in previous transgenic mice. The transgene was expressed in the CNS with a restricted localization in oligodendrocytes. Oligodendrocyte proliferation and myelin formation are therefore tightly controlled experimentally by administration of ganciclovir (GCV) via the induction of oligodendrocyte cell death. The most severe and irreversible hypomyelination was obtained when GCV was given daily from postnatal day 1 (P1) to P30. Oligodendrocyte plasticity and myelin recovery were analyzed in another phenotype generated by GCV treatment from P1 to P15. In this model, after dysmyelination, an apparent normal behavior was restored with no visible pathological symptoms by P30. Proliferating cells, which may be implicated in myelin repair in this model, are detected primarily in myelin tracts expressing the oligodendrocyte phenotype. Therefore, the endogenous potential of oligodendrocytes to remyelinate was clearly demonstrated in the mice of this study.

Multi-exponential analysis of T2 images

Clinical images show that T2 contrast allows in certain cases a better discrimination between the healthy and pathological tissues. For instance, in Fig. 1 it can be seen that multiple sclerosis’ lesions are more clearly visible on the images obtained with late echoes than those obtained with the early echoes. On the other hand in vitro studies on perfused organs’ have shown that the T2 decay curve depends on the interval between 180” pulses: The decay curve is mono-exponential for 4-ms time intervals between 180” pulses, and bi-exponential for 0.4-ms time intervals (Fig. 2). For these reasons, we have tried to analyze the T2 decay curve by multi-exponential decomposition in order to improve tissue characterization. To obtain the T2 decay curve, we used the classical CPMG pulse sequence. By adding linear magnetic field gradients to this sequence it is possible to reconstruct a sample image for each echo. The T2 decay curve is constructed by taking the corresponding pixel from the echo image series, for instance, the first point corresponds to the first image, the second point to the second images, etc. Thus we obtain the T2 decay curve of the elementary volume defined by the pixel whose dimensions are the slice thickness (7 mm) and the spatial resolution (2-2.5 mm). It suffices, therefore, to repeat this operation pixel by pixel in order to obtain parametric T2 decay curves over the whole slice. A slice of the head is represented by about 6000 pixels, which means that we need to repeat the decomposition operation 6000 times. We therefore had to find an algorithm sufficiently rapid and not too memory consuming. The algorithm chosen was of the noniterative, Prony (algebraic) type. 3 It was then necessary to verify this method of analysis using first simulated image signals and thereafter simple test objects. Classically, the multi-exponential decomposition is an ill-posed problem. The “solution” is obtained by nonlinear least-squares optimization. With a noniterative algorithm we transform the nonlinear leastsquares optimization into two linear least-squares problems. The decomposition method may be presented in four steps:

Improved Specificity of a New Direct Assay for Urinary Cortisol: Application in Corticoid Treated Patients

Abstract Twenty four hours urinary free cortisol (UFC) excretion has been determined in 35 eucortisolic control patients, in seven of them before and after tetracosactide (Synacthen®) stimulation and in 18 patients treated by anti-inflammatory steroids. Results of the new direct Immunotech RIA (DIm) were compared to those of the INCSTAR RIA kit with (ECA) or without methylene chloride extraction (DCA). In controls DIm UFC (106.2±45.8 nmol/24h) was significantly lower than DCA UFC (397±119 nmol/24h) and than ECA UFC (127±49 nmol/24h). After tetracosactide stimulation, median of DCA/DIm ratio decreased from 3.61 to 1.88 whereas ECA/DIm ratio did not change significantly (1.31 to 1.06). In treated patients most DCA and ECA results were over the upper limit of controls but only 5 DIm results were increased. DIm assay showed good specificity and practicability and may be used with benefit in the evaluation of the adrenal gland function.

Use of insulin immunoassays in clinical studies involving rapid-acting insulin analogues : Bi-insulin IRMA preliminary assessment

Abstract Background: In clinical studies involving rapid-acting analogues (RAAs), insulin immunoreactivity is frequently measured, including endogenous, regular insulin (RI) and RAA immunoreactivities. Such a procedure implies equivalent cross-reactivities of all insulins present in serum. Commercially available human insulin immunoassays have been widely used, but their limitations (including hemolysis and anti-insulin antibodies) were not fully investigated. The aims of our study were to compare cross-reactivities of RI and RAAs in buffer and in serum and to investigate insulin immunoassay pitfalls. Methods: Cross-reactivities were assessed using Bi-insulin IRMA (Schering Cis-Bio International) in phosphate-buffered saline (PBS)-1% bovine serum albumin (BSA) and in pools of sera spiked with RI and RAAs (lispro and aspart). To investigate the influence of hemolysis, a pool of sera spiked with RAA was mixed with a concentrated hemolysate (final hemoglobin concentration 10g/L) and incubated for 3h at room temperature. To determine interference by anti-insulin antibodies, insulin was removed using charcoal from 18 sera with anti-insulin antibodies and from 17 sera without detectable anti-insulin antibodies. These insulin-free samples were then spiked with RI and RAAs and the immunoreactivity was determined. Results: Compared with buffer, cross-reactivity in serum for RI, lispro and aspart was lower (35%, 29% and 26% lower, respectively). Hemolysis degraded almost all RI and RAAs contained in the serum (≥95%). Anti-insulin antibody interference was significant for RI and RAAs (p≤0.004) and correlated with anti-insulin antibody level in the serum (p≤0.001). Conclusions: In serum, RI and RAA cross-reactivities are slightly lower than in buffer. For RAA assessment, hemolysed samples should be discarded and anti-insulin antibodies should be removed from samples before immunoreactivity measurements. Clin Chem Lab Med 2006;44:1379–82.