Leif Schröder

Publications (33) View all

• Article: Sensitivity Enhancement of (Hyper-)CEST Image Series by Exploiting Redundancies in the Spectral Domain

  Jörg Döpfert, Christopher Witte, Martin Kunth, Leif Schröder
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  ABSTRACT: CEST has proven to be a valuable technique for the detection of hyperpolarized xenon-based functionalized contrast agents. Additional information can be encoded in the spectral dimension, allowing the simultaneous detection of multiple different biosensors. However, due to the low concentration of dissolved xenon in biological tissue, the signal to noise ratio (SNR) of Hyper-CEST data is still a critical issue. In this work, we present two techniques aiming to increase SNR by exploiting the typically high redundancy in spectral CEST image series: PCA-based post-processing and sub-sampled acquisition with low-rank reconstruction. Each of them yields a significant SNR enhancement, demonstrating the feasibility of the two approaches. While the first method is directly applicable to proton CEST experiments as well, the second one is particularly beneficial when dealing with hyperpolarized nuclei, since it distributes the non-renewable initial polarization more efficiently over the sampling points. The results obtained are a further step towards the detection of xenon biosensors with spectral Hyper-CEST imaging in vivo.
  05/2013;
• Article: Optimized use of reversible binding for fast and selective NMR localization of caged xenon.

  Martin Kunth, Jörg Döpfert, Christopher Witte, Federica Rossella, Leif Schröder
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  ABSTRACT: The NMR signal of hyperpolarized (129) Xe trapped in cryptophane cages in different solvents experiences different chemical shifts. An encoding method is presented that involves the optimal use of reversible Xe binding and efficiently uses hyperpolarization. This method is utilized in nanomolar imaging, subsecond imaging, and time-resolved studies while maintaining high spectral selectivity.
  Angewandte Chemie International Edition 07/2012; 51(33):8217-20. · 13.45 Impact Factor
• Article: Hyperpolarized xenon for NMR and MRI applications.

  Christopher Witte, Martin Kunth, Jörg Döpfert, Federica Rossella, Leif Schröder
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  ABSTRACT: Nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) suffer from intrinsic low sensitivity because even strong external magnetic fields of ~10 T generate only a small detectable net-magnetization of the sample at room temperature (1). Hence, most NMR and MRI applications rely on the detection of molecules at relative high concentration (e.g., water for imaging of biological tissue) or require excessive acquisition times. This limits our ability to exploit the very useful molecular specificity of NMR signals for many biochemical and medical applications. However, novel approaches have emerged in the past few years: Manipulation of the detected spin species prior to detection inside the NMR/MRI magnet can dramatically increase the magnetization and therefore allows detection of molecules at much lower concentration (2). Here, we present a method for polarization of a xenon gas mixture (2-5% Xe, 10% N2, He balance) in a compact setup with a ca. 16000-fold signal enhancement. Modern line-narrowed diode lasers allow efficient polarization (7) and immediate use of gas mixture even if the noble gas is not separated from the other components. The SEOP apparatus is explained and determination of the achieved spin polarization is demonstrated for performance control of the method. The hyperpolarized gas can be used for void space imaging, including gas flow imaging or diffusion studies at the interfaces with other materials (8,9). Moreover, the Xe NMR signal is extremely sensitive to its molecular environment (6). This enables the option to use it as an NMR/MRI contrast agent when dissolved in aqueous solution with functionalized molecular hosts that temporarily trap the gas (10,11). Direct detection and high-sensitivity indirect detection of such constructs is demonstrated in both spectroscopic and imaging mode.
  Journal of Visualized Experiments 01/2012;
• Source

  Available from: Thomas Stach

  Article: Evolution of a novel muscle design in sea urchins (Echinodermata: Echinoidea).

  Alexander Ziegler, Leif Schröder, Malte Ogurreck, Cornelius Faber, Thomas Stach
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  ABSTRACT: The sea urchin (Echinodermata: Echinoidea) masticatory apparatus, or Aristotle's lantern, is a complex structure composed of numerous hard and soft components. The lantern is powered by various paired and unpaired muscle groups. We describe how one set of these muscles, the lantern protractor muscles, has evolved a specialized morphology. This morphology is characterized by the formation of adaxially-facing lobes perpendicular to the main orientation of the muscle, giving the protractor a frilled aspect in horizontal section. Histological and ultrastructural analyses show that the microstructure of frilled muscles is largely identical to that of conventional, flat muscles. Measurements of muscle dimensions in equally-sized specimens demonstrate that the frilled muscle design, in comparison to that of the flat muscle type, considerably increases muscle volume as well as the muscle's surface directed towards the interradial cavity, a compartment of the peripharyngeal coelom. Scanning electron microscopical observations reveal that the insertions of frilled and flat protractor muscles result in characteristic muscle scars on the stereom, reflecting the shapes of individual muscles. Our comparative study of 49 derived "regular" echinoid species using magnetic resonance imaging (MRI) shows that frilled protractor muscles are found only in taxa belonging to the families Toxopneustidae, Echinometridae, and Strongylocentrotidae. The onset of lobe formation during ontogenesis varies between species of these three families. Because frilled protractor muscles are best observed in situ, the application of a non-invasive imaging technique was crucial for the unequivocal identification of this morphological character on a large scale. Although it is currently possible only to speculate on the functional advantages which the frilled muscle morphology might confer, our study forms the anatomical and evolutionary framework for future analyses of this unusual muscle design among sea urchins.
  PLoS ONE 01/2012; 7(5):e37520. · 4.09 Impact Factor
• Source

  Available from: Leif Schröder

  Chapter: Xenon Biosensors for Multi-Purpose Molecular Imaging

  Leif Schröder, Tyler Meldrum, Monica Smith, Franz Schilling, Philipp Denger, Sina Zapf, David Wemmer, Alexander Pines
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  ABSTRACT: Hyperpolarized xenon is an exquisite NMR probe for sensing molecular environments of the noble gas in solution. By trapping it in molecular cages like cryptophane-A, 129Xe can report information about molecular-specific binding events or resolve multiple signals simultaneously from different micro-environments in a lipid emulsion-a macroscopically-homogeneous phase that mimics properties of biological relevance. The Hyper-CEST detection scheme can be used in this context to pair significant signal enhancement with high specificity of xenon NMR resonances. Hyper-CEST can reduce the measurement time by a factor of up to 16 million and is currently able to detect biosensor concentrations as low as 1.4 nM. When combined with highly frequency-selective pulses, it also allows for demonstration of multiplexing potential using a single cage type as contrast agent for different environments in NMR imaging. This molecular imaging approach enables a switchable contrast that includes also temperature-sensitive imaging with molecular sensors that can be functionalized with various targeting molecules to bind, e.g., specifically to receptors of cancer cells. KeywordsBiosensors–Hyper-CEST–xenon–hyperpolarization–molecular imaging
  01/2010: pages 176-179;