9 research outputs found
Spin echo formation in the presence of stochastic dynamics
Spin echo formation in magnetic field gradients in the presence of fast stochastic motion is studied for hyperpolarized He3 gas at different diffusivities. The fast translational motion leads to frequency shifts already during echo formation, which can be described analytically for a linear gradient. Despite complete signal loss at the position of the spin echo itself, considerable intensity can be preserved at an earlier time (2Ï rather than 2Ï, where Ï is the pulse delay). Hence, the phenomenon is designated as a pseudo spin echo. © 2007 The American Physical Society.Fil: ZĂ€nker, Paul P.. Max Planck Institute For Polymer Research; AlemaniaFil: Schmidt, Jochen. Max Planck Institute For Polymer Research; AlemaniaFil: Schmiedeskamp, Jörg. Max Planck Institute For Polymer Research; AlemaniaFil: Acosta, Rodolfo HĂ©ctor. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - CĂłrdoba. Instituto de FĂsica Enrique Gaviola. Universidad Nacional de CĂłrdoba. Instituto de FĂsica Enrique Gaviola; ArgentinaFil: Spiess, Hans W.. Max Planck Institute For Polymer Research; Alemani
Oxygen-sensitive 3He-MRI in bronchiolitis obliterans after lung transplantation
Oxygen-sensitive 3He-MRI was studied for the detection of differences in intrapulmonary oxygen partial pressure (pO2) between patients with normal lung transplants and those with bronchiolitis obliterans syndrome (BOS). Using software developed in-house, oxygen-sensitive 3He-MRI datasets from patients with normal lung grafts (nâ=â8) and with BOS (nâ=â6) were evaluated quantitatively. Datasets were acqiured on a 1.5-T system using a spoiled gradient echo pulse sequence. Underlying diseases were pulmonary emphysema (nâ=â10 datasets) and fibrosis (nâ=â4). BOS status was verified by pulmonary function tests. Additionally, 3He-MRI was assessed blindedly for ventilation defects. Median intrapulmonary pO2 in patients with normal lung grafts was 146Â mbar compared with 108 mbar in patients with BOS. Homogeneity of pO2 distribution was greater in normal grafts (standard deviation pO2 34 versus 43Â mbar). Median oxygen decrease rate during breath hold was higher in unaffected patients (â1.75Â mbar/s versus â0.38Â mbar/s). Normal grafts showed fewer ventilation defects (5% versus 28%, medians). Oxygen-sensitive 3He-MRI appears capable of demonstrating differences of intrapulmonary pO2 between normal lung grafts and grafts affected by BOS. Oxygen-sensitive 3He-MRI may add helpful regional information to other diagnostic techniques for the assessment and follow-up of lung transplant recipients
Weiterentwicklung einer Produktionsanlage und der Speicherungs- bzw. Transportkonzepte fĂŒr hochpolarisiertes 3He
Der Bedarf an hyperpolarisiertem 3He in Medizin und physikalischer Grundlagenforschung ist in den letzten ca. 10-15 Jahren sowohl in Bezug auf die zu VerfĂŒgung stehende Menge, als auch auf den benötigten Grad der Kernspinpolarisation stetig gestiegen.
Gleichzeitig muĂten Lösungen fĂŒr die polarisationserhaltende Speicherung und den Transport gefunden werden, die je nach Anwendung anzupassen waren. Als Ergebnis kann mit dieser Arbeit ein in sich geschlossenes Gesamtkonzept vorgestellt werden, daĂ sowohl die entsprechenden Mengen fĂŒr klinische Anwendungen, als auch höchste Polarisation fĂŒr physikalische Grundlagenfor-schung zur VerfĂŒgung stellen kann.
Verschiedene unabhĂ€ngige Polarimetriemethoden zeigten in sich konsistente Ergebnisse und konnten, neben ihrer eigenen Weiterentwicklung, zu einer verlĂ€Ălichen Charakterisierung des neuen Systems und auch der Transportzellen und âboxen eingesetzt werden.
Die Polarisation wird mittels âMetastabilem Optischen Pumpenâ bei einem Druck von 1 mbar erzeugt. Dabei werden ohne GasfluĂ Werte von P = 84% erreicht. Im FluĂbetrieb sinkt die erreichbare Polarisation auf P â 77%. Das 3He kann dann weitgehend ohne Polarisationsver-luste auf mehrere bar komprimiert und zu den jeweiligen Experimenten transportiert werden.
Durch konsequente Weiterentwicklung der vorgestellten Polarisationseinheit an fast allen Komponenten kann somit jetzt bei einem FluĂ von 0,8 barl/h eine Polarisation von Pmax = 77% am AuslaĂ der Apparatur erreicht werden. Diese skaliert linear mit dem FluĂ, sodaĂ bei 3 barl/h die Polarisation immer noch bei ca. 60% liegt. Dabei waren die im Rahmen dieser Arbeit durchgefĂŒhrten Verbesserungen an den Lasern, der Optik, der Kompressionseinheit, dem Zwischenspeicher und der Gasreinigung wesentlich fĂŒr das Erreichen dieser Polarisatio-nen. Neben dem Einsatz eines neuen Faserlasersystems ist die hohe Gasreinheit und die lang-lebige Kompressionseinheit ein SchlĂŒssel fĂŒr diese LeistungsfĂ€higkeit. Seit Herbst 2001 er-zeugte das System bereits ĂŒber 2000 barl hochpolarisiertes 3He und ermöglichte damit zahl-reiche interdisziplinĂ€re Experimente und Untersuchungen.
Durch Verbesserungen an als Prototypen bereits vorhandenen Transportboxen und durch weitgehende UnterdrĂŒckung der Wandrelaxation in den TransportgefĂ€Ăen
aufgrund neuer Erkenntnisse ĂŒber deren Ursachen stellen auch polarisationserhaltende Transporte ĂŒber groĂe Strecken kein Problem mehr dar. In unbeschichteten 1 Liter Kolben aus AluminosilikatglĂ€-sern werden nun problemlos Speicherzeiten von T1 > 200h erreicht. Im Rahmen des europĂ€i-schen Forschungsprojektes âPolarized Helium to Image the Lungâ wurden wĂ€hrend 19 Liefe-rungen 70barl 3He nach Sheffield (UK) und bei 13 Transporten 100 barl nach Kopenhagen (DK) per Flugzeug transportiert.
Zusammenfassend konnte gezeigt werden, daĂ die Problematik der Kernspinpolarisationser-zeugung von 3He, die Speicherung, der Transport und die Verwendung des polarisierten Ga-ses in klinischer Diagnostik und physikalischen Grundlagenexperimenten weitgehend gelöst ist und das Gesamtkonzept die Voraussetzungen fĂŒr allgemeine Anwendungen auf diesen Gebieten geschaffen hat.The use of
hyperpolarized 3He in medical applications and physics raised during the last 10-15 years constantly. In MRI a bigger amount was needed and in fundamental physics one asked for highest polarization. Parallel to the development of a production unit one had to solve the problems of storage and transport the gas with negligible polarization losses. The conclusion of this Thesis is that one can introduce an over all concept, that is able to produce big amounts of 3He at moderate polarization degrees for medical applications as well as highest polarization to be used at fundamental physics experiments.
Different independent methods of polarization measurements were further developed. They show consistent results and made it possible to characterise the system including storage and transport reliably.
The polarization is produced by âMetastable Optical Pumpingâ at pressures of about 1 mbar. At a steady state a maximum polarization of P = 84% is reached. Due to the gasflux this maximum value is reduced to P
â 77%. After polarization the gas can be compressed up to several bars and transported to the experiment with negligible losses. Because of consequent further enhancements Pmax = 77% at 0,8 barl/h is possible. The polarization scales linear with the flux and 60% are still reached at 3 barl/h. Essential for the improvement of the system were the implementation of a new lasersystem and optics as well as the new long living compression-unit, the puffer cell and the enhanced purity of the gas. Since autumn 2001 more than 2000 barl of highly polarized 3He were produced to serve numerous interdisciplinary experiments and investigations.
By improving the transportboxes, which were already existing as prototypes and by suspending wallrelaxation effects inside the transportcontainers because of new findings of their sources, polarization conserving transports over long distances are no problem any more. Noncoated containers made of aluminosilicateglases with a volume of about one litre now reach storage times
better than 200h. In the context of the european project âPolarized Helium to Image the Lungâ 19 deliveries with a total amount of 70barl were send to Sheffield (UK) and 13 deliveries with a total amount of 100 barl were send to Copenhagen (DK) via airplane.
As a conclusion of this thesis one can show, that the problems in producing, storage and transport of hyperpolarized 3He and its use in medical applications and fundamental physics are solved. The conditions for further investigations in this areas are now given
Paramagnetic relaxation of spin polarized He-3 at bare glass surfaces. Part I
In this first in a series of three papers on wall relaxation of spin
polarized, gaseous 3He we investigate both by theory and by experiment
surface-induced spin relaxation due to paramagnetic sites in the containing
glass. We present experimental and theoretical evidence that â contrary to
the traditional opinion â distant dipolar coupling to paramagnetic
impurities in the glass, in particular iron ions, cannot be the dominant
relaxation mechanism of 3He-spins, although iron dominates the bulk
static permeability. Instead dangling-bond type defects in the glass matrix
are found to interact much stronger via the isotropic Fermi contact
interaction. A model of paramagnetic site controlled 3He relaxation
including the Fermi contact interaction is presented. With reasonable
semi-empirical assumptions our model allows to describe satisfactorily the
measured relaxivities, both in the dissolution-dominated regime of fused
silica or borosilicate glasses of the Pyrex type as well as in the surface
dominated situation of aluminosilicate glasses which have only a low
permeability for He atoms. In a large sample of 1.1 litre cells, built from
various aluminosilicate glasses, an average relaxation time of 150 h is
reached in case contaminant ferromagnetic particles have been demagnetized
beforehand. From the maximum observed value of 250Â h we derive after
subtraction of dipolar relaxation in the gas phase a paramagnetic surface
relaxivity of Ï<0.005Â cm/h at room temperature
Intrapulmonary 3He gas distribution depending on bolus size and temporal bolus placement
OBJECTIVE: Dynamic ventilation (3)He-MRI is a new method to assess pulmonary gas inflow. As differing airway diameters throughout the ventilatory cycle can influence gas inflow this study intends to investigate the influence of volume and timing of a He gas bolus with respect to the beginning of the tidal volume on inspiratory gas distribution. MATERIALS AND METHODS: An ultrafast 2-dimensional spoiled gradient echo sequence (temporal resolution 100 milliseconds) was used for dynamic ventilation (3)He-MRI of 11 anesthetized and mechanically ventilated pigs. The applied (3)He gas bolus was varied in volume between 100 and 200 mL. A 150-mL bolus was varied in its application time after the beginning of the tidal volume between 0 and 1200 milliseconds. Signal kinetics were evaluated using an in-house developed software after definition of parameters for the quantitative description of (3)He gas inflow. RESULTS: The signal rise time (time interval between signal in the parenchyma reaches 10% and 90% of its maximum) was prolonged with increasing bolus volume. The parameter was shortened with increasing delay of (3)He application after the beginning of the tidal volume. Timing variation as well as volume variation showed no clear interrelation to the signal delay time 10 (time interval between signal in the trachea reaches 50% of its maximum and signal in the parenchyma reaches 10% of its maximum). CONCLUSIONS: Dynamic ventilation (3)He-MRI is able to detect differences in bolus geometry performed by volume variation. Pulmonary gas inflow as investigated by dynamic ventilation (3)He-MRI tends to be accelerated by an increasing application delay of a (3)He gas bolus after the beginning of the tidal volume
Relaxation of spin polarized He-3 by magnetized ferromagnetic contaminants. Part III
In the first in a series of three papers on wall relaxation of spin
polarized 3He we have reported on a breakdown of relaxation times which
is observed after exposing the 3He containing glass cells to a strong
magnetizing field. In this third paper we give a quantitative analysis of
this phenomenon, based on magnetic signal detection by means of SQUIDs, on
the pressure dependence of relaxation times in magnetized cells, as well as
on Monte Carlo simulations of 3He-relaxation in a macroscopic dipole
field. Our analysis allows to identify the contaminants as being aggregates
of dust-like Fe3O4 particles (magnetite) with a radius m
and a remanent magnetic moment of the order of m âO(10A m2). The particles are located at or close to the inner glass surface