We applied a recently developed method of following the time course of the intrapulmonary oxygen partial pressure pO2(t) during apnea by 3He MRI to healthy volunteers. Using two imaging series with different interscan times during two breathholds (double acquisition technique), relaxation of 3He due to paramagnetic oxygen and depolarization by RF pulses were discriminated. In all four subjects, the temporal evolution of pO2 was found to be linear, and was described by an initial partial pressure p0 and a decrease rate R. Also, regional differences of both p0 and R were observed. A correlation between p0 and R was apparent. Finally, we discuss limitations of the double acquisition approach. Copyright © 2000 John Wiley & Sons, Ltd.

Inhalation of hyperpolarized (3)He allows magnetic resonance imaging (MRI) of ventilated airspaces. (3)He hyperpolarization decays more rapidly when interacting with paramagnetic O(2). We describe a method for in vivo determination of intrapulmonary O(2) concentrations ([O(2)]) based on MRI analysis of the fate of measured amounts of inhaled hyperpolarized (3)He in imaged regions of the lung. Anesthetized pigs underwent controlled normoventilation in a 1.5-T MRI unit. The inspired O(2) fraction was varied to achieve different end-tidal [O(2)] fractions (FET(O(2))). With the use of a specifically designed applicator, (3)He (100 ml, 35-45% polarized) was administered at a predefined time within single tidal volumes. During subsequent inspiratory apnea, serial two-dimensional images of airways and lungs were acquired. At least once in each animal studied, the radio-frequency excitation used for imaging was doubled at constant FET(O(2)). Signal intensity measurements in regions of interest of the animals' lungs (volume range, 54-294 cm(3)), taken at two different radio-frequency excitations, permitted calculation of [O(2)] in these regions of interest. The [O(2)] fractions in the regions of interest correlated closely with FET(O(2)) (R = 0.879; P < 0.0001). O(2)-sensitive (3)He-MRI may allow noninvasive study of regional distribution of ventilation and alveolar PO(2) in the lung.

We present a new method to determine in vivo the temporal evolution of intrapulmonary oxygen concentrations by functional lung imaging with hyperpolarized 3Helium (3He→). Single-breath, single-bolus visualization of 3He→ administered to the airspaces is used to analyze nuclear spin relaxation caused by the local oxygen partial pressure pO2(t). We model the dynamics of hyperpolarization in the lung by rate equations. Based hereupon, a double acquisition technique is presented to separate depolarization by RF pulses and oxygen induced relaxation. It permits the determination of pO2 with a high accuracy of up to 3% with simultaneous flip angle calibration using no additional input parameters. The time course of pO2 during short periods of breathholding is found to be linear in a pig as well as in a human volunteer. We also measured the wall relaxation time in the lung and deduced a lower limit of 4.3 min.

The electric form factor of the neutron GEn has been determined in double polarized exclusive 3He(e,e'n) scattering in quasi–elastic kinematics by measuring asymmetries A⊥, A∥ of the cross section with respect to helicity reversal of the electron, with the nuclear spin being oriented perpendicular to the momentum transfer q in case of A⊥ and parallel in case of A∥. The experiment was performed at the 855 MeV c. w. microtron MAMI at Mainz. The degree of polarization of the electron beam and of the gaseous 3He target were each about 50%. Scattered electrons and neutrons were detected in coincidence by detector arrays covering large solid angles. Quasi–elastic scattering events were reconstructed from the measured electron scattering angles ϑe, φe and the neutron momentum vector pn′ in the plane wave impulse approximation. We obtain the result (0.27 < Q2c2/GeV2 < 0.5)= 0.0334 ± 0.0033stat± 0.0028syst which is averaged over the indicated range of Q2, the squared momentum transfer. This GEn value is significantly smaller than measured from the D(e,e'n) reaction under similar kinematical conditions. To what extent final state interactions in 3He quench the GEn result is subject of calculations currently in progress elsewhere.

Optical pumping of metastable 3He atoms is a very efficient method to produce large quantities of nuclear spin-polarized 3He. Recent developments in mechanical compression of the gas, its storage and transport allow for its flexible use in different fields of physics and applied science. Among these are (1) scattering experiments of polarized beams from polarized 3He-targets, (2) 3He as neutron spin filter to polarize neutron beams at research reactors, and (3) polarized 3He gas inhaled into the lungs to perform magnetic resonance imaging. The paper discusses the different topics along with results obtained in a first round of experiments.

Magnetic Resonance Imaging (MRI) usually relies on magnetization of hydrogen nuclei (protons) in water or molecules in tissue as source of the signal. Biological environments with low proton content, notably the lungs, are difficult to image. Inhaling of hyperpolarized 3He gas opens the possibility to investigate ventilated spaces by MRI. To overcome the loss in signal due to the low density of the gas the nuclear polarization of the 3He spins is greatly enhanced by laser Optical Pumping. For more than three decades Optical Pumping of noble gases has been investigated, using spin exchange scattering (SE) or metastability exchange scattering (ME). Since powerful resonant laser light is available for Optical Pumping, large quantities of 3He gas can be operated. The original interest was the development of dense spin polarized targets for fundamental research in physics. As a spin off, the possibility of MRI of lung tissue filled with hyperpolarized 129Xenon was demonstrated in 1994. Later 3He was used for MRI in a guineapig. While these authors have used the SE method to polarize noble gases, more recently 3He MRI in human lungs was reported by our group where the ME method is in use.

The neutron electric form factor G(En) has been measured at the cw electron accelerator MAMI in the double polarized exclusive reactions 3 (H) over right arrow e((e) over right arrow,e'n) [1-3] and D((e) over right arrow,e'(n) over right arrow) [4-7] in quasi elastic kinematics with one common detector system. Experimental set-up and analysis of a (3) (H) over right arrow e((e) over right arrow e'n)-measurement in the range of momentum transfer Q(2)=0.27-0.5 (GeV/c)(2) are discussed in this article. For events with low missing momentum (\(P) over right arrow(m)\less than or equal to 100 MeV/c) the PWIA analysis yields G(En)=0.0352+/-0.0033+/-0.0024 with statistical and systematical error, corresponding to a quadratically added total error of 11.6% relativ. This result will be compared with preliminary results obtained from the D((e) over right arrow, e'(n) over right arrow)-reaction and with data from elastic D(e, e') scattering [8].

In the frame of the Al-Collaboration at the Mainz Microtron a test measurement of doubly polarized (3) (H) over right arrow e((e) over right arrow, e'n) scattering from a high pressure target was performed in July aiming for the determination of the neutron electric form factor G(en) at high momentum transfer (Q(2) = 0.7(GeV/c)(2)) [1]. Due to the small value of G(en) compared to G(mn) a preferred procedure is to determine the asymmetry in the exclusive quasi elastic scattering of polarized electrons (P approximate to 70%, I greater than or equal to 2 mu A) from polarized (3) (H) over right arrow e. The scattered electrons are detected in a high resolution magnetic spectrometer while the scattering angles of the outgoing neutrons are measured in a plastic scintillator. In this reaction the polarized (3) (H) over right arrow e nucleus serves as an effective polarized neutron target.Because of the large magnetic field gradients caused by the spectrometer and limited space at the target place, the (3) (H) over right arrow e gas is polarized elsewhere and transported to the target place in specially prepared glass cells [2]. The glass cells are designed for high pressure (up to 10 bar) and with thin windows to prevent background. To reduce the relaxation due to the magnetic field gradients a mu-metal shielded guiding field of 4 Gauss is used. The guiding field is generated bu three independent coils which permit the rotation of the target spin in any direction desired, especially perpendicular and parallel with respect to (q) over right arrow. So we are able to take the ratio of the asymmetries A(perpendicular to)/A(parallel to), which in first order depends only on kinematical factors and the ratio G(en)/G(mn).The method of metastable optical pumping is used to polarize the (3) (H) over right arrow e at about 1 mbar with a LNA laser (approximate to 6W). Subsequently the gas is compressed by means of a two-stage-piston compressor up to 6 bar with a polarization of roughly 45% [3].

Magnetic Resonance Imaging (MRI) usually relies on magnetization of hydrogen nuclei (protons) in water or molecules in tissue as source of the signal. Biological environments with low proton content, notably the lungs, are difficult to image. Inhaling of hyperpolarized He-3 gas opens the possibility to investigate ventilated spaces by MRI. To overcome the loss in signal due to the low density of the gas the nuclear polarization of the He-3 spins is greatly enhanced by laser Optical Pumping.For more than three decades Optical Pumping of noble gases has been investigated, using spin exchange scattering (SE) [1] or metastability exchange scattering (ME) [2]. Since powerful resonant laser light is available for Optical Pumping, large quantities of He-3 gas can be operated [3,4]. The original interest was the development of dense spin polarized targets for fundamental research in physics [5-8]. As a spin off, the possibility of MRI of lung tissue filled with hyperpolarized (129)Xenon was demonstrated in 1994 [9]. Later He-3 was used for MRI in a guineapig [10]. While these authors have used the SE method to polarize noble gases, more recently He-3 MRI in human lungs was reported by our group [11] where the ME methods is in use.

The strongly spin dependent absorption of neutrons in nuclear spin polarized 3He opens the possibility to polarize beams of thermal and epithermal neutrons. An effective 3He neutron spin filter (NSF) requires high 3He nuclear polarization as well as a filter thickness corresponding to a gas amount of the order of 1 barl. We realized such a filter using direct optical pumping of metastable 3He∗ atoms in a 3He plasma at 1 mbar. Metastable exchange scattering transfers the angular momentum to the whole ensemble of 3He atoms. At present 3 × 1018 3He-atoms/s are polarized up to 64%. Subsequent polarization preserving compression by a two stage compressor system enables to prepare NSF cells of about 300 cm3 volume with 3 bar of polarized 3He within 2 h. 3He polarizations up to 53% were measured in a cell with a filter length of about 15 cm. By this cell a thermal neutron beam from the Mainz TRIGA reactor was polarized. A wavelength selective polarization analysis by means of Bragg scattering revealed a neutron polarization of 84% at a total transmission of 12% for a neutron wavelength of 1 Å.

PURPOSE of the study is the visualisation of normal pulmonary ventilation in healthy volunteers and the evaluation of abnormalities in patients with different lung diseases using 3He magnetic resonance imaging (3He-MRI). MATERIAL AND METHODS Hyperpolarized 3He gas (V = 300 ml, p = 3 x 10(5) Pa, polarised to 35-45% by optical pumping, provided in special glass cells) was inhaled by eight healthy volunteers and ten patients with different lung diseases. A 3D FLASH sequence (TR = 11.8 ms; TE = 5 ms; matrix 144 x 256, FOV 350 mm, section thickness 7-10 mm, coronal orientation) was performed in a single breath-hold (22-42 s). Clinical and radiological examinations were available for correlation. RESULTS The studies were successfully carried out in 8/8 volunteers and in 8/10 patients. The central airways were constantly visualised with intermediate to high signal intensity. The lung parenchyma of volunteers with normal ventilatory function showed rather homogeneous intermediate to high signal, whereas patients with chronic obstructive lung disease and/or pneumonia presented severe signal inhomogeneities. Space-occupying lesions and pleural effusion caused large areas with little or no signal. The represented the lesion and adjacent ventilatory disturbances whose extent had not been presumed from chest x-ray or CT. The spatial resolution was higher than in ventilation scintigraphy. CONCLUSION 3He MRI is a promising new modality for the assessment of pulmonary ventilation and its anomalies.

The purpose of this study was to describe the 3He MRI findings of normal pulmonary ventilation in healthy volunteers and to evaluate abnormalities in patients with different lung diseases. Hyperpolarized 3He gas (300 ml, 3 × 10 5 Pa, polarized to 35–45% by optical pumping, provided in special glass cells) was inhaled by 8 healthy volunteers and 10 patients with different lung diseases. Imaging was performed with a three‐dimensional fast low‐angle shot (FLASH) sequence (TR = 11.8 msec; TE = 5 msec; transmitter amplitude, 5–8 V; corresponding flip angle, <5°) in a single breath‐hold (22–42 seconds). Clinical and radiologic examinations were available for correlation. The studies were performed successfully in eight of eight volunteers and in 8 of 10 patients. The lung parenchyma of volunteers with normal ventilatory function exhibited rather homogeneous intermediate to high signal, whereas patients with chronic obstructive lung disease or bronchiectasis presented with severe signal inhomogeneities with patchy or wedge‐shaped defects. The mass effect of bronchogenic carcinoma, chronic empyema, lymphadenopathy, or pleural effusion caused large signal defects, representing the lesion and adjacent hypoventilation, the extent of which had not been presumed from chest x‐ray or CT. 3He MRI is a promising new modality for the assessment of pulmonary ventilation and its abnormalities. Additional studies are needed to determine its potential clinical role.

The nuclear spin polarization of noble gases can be strongly enhanced by laser optical pumping followed by electron-nuclear polarization transfer. Direct optical pumping of metastable 3He atoms has been shown to produce enormous polarization in the order of 0.4-0.6. This is about 10(5) times greater than the polarization of water protons at thermal equilibrium used at conventional MR imaging. We demonstrate that hyperpolarized 3He gas can be applied to nuclear MR imaging of human organs with air-filled spaces. In vivo 3He MR experiments were performed in a whole-body MR scanner with a superconducting magnet ramped down to 0.8 T and at 1.5 T using a double resonant Helmholtz coil operating at 63.6 and 48.6 MHz for 1H and 3He, respectively. Anatomical details of the lungs of a volunteer were visualized with the FLASH technique demonstrating the potential of the method for fast imaging of airways in the human body and for pulmonary ventilation studies.