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Abstract—We present the experimental results of generation

and of detection of hyperpolarized xenon obtained by 4.7 T, 200 MHz magnetic resonance (MR) system. Xenon 129 was hyperpolarized by the spin-exchange with Rubidium pumped first by Ti:Sa laser and also by extended cavity laser system based on a high-power laser diode optimized for maximum efficiency of the optical pumping process. The application of the arrangement with semiconductor laser system is oriented medicine, especially to continuous production of hyperpolarized Xenon (HpXe) which serves as an contrast media in MR imaging systems.

Index Terms—Semiconductor lasers, Laser stability, Laser biomedical applications, Optical pumping, Nuclear magnetic resonance, Magnetic resonance imaging.

I. INTRODUCTION YPERPOLARIZED noble gasses (HpG) have found a steadily increasing range of applications in nuclear

magnetic resonance imaging. They can be regarded as a new class of magnetic resonance contrast media or as a way of great enhancement of the temporal resolution in measurements of processes relevant to areas such as material science and biomedicine [1]. The hyperpolarized Xenon (129Xe) plays a major role in the study of microporous materials like zeolites. The study and application is based on the chemical shift in Xenon gas. At very low pressures of the hyperpolarized Xenon and at the room temperature, the chemical shift is sensitive to the interaction of Xenon atoms with the walls.

The production of HpG, predominantly the 129Xe became attractive for its potential applications in medicine as well. The powerful technique of magnetic resonance imaging of human body has a limited ability to examine organs with low water content and/or with air spaces, such as colon or lungs. An introduction of a highly contrasting ingredient would significantly extend its potential. Gaseous Xenon is normally not present in the body, so the experiments do not suffer from unwanted background signals. More, the hyperpolarized Xenon acts as a source of very strong nuclear magnetic

Manuscript received March 5, 2007. This work was supported by projects:

GAČR: GA102/04/2109, MŠMT: LC06007 and 2C06012, AVČR: AV0 Z20650511 and GAAV: IAA200650504 and IAA1065303.

All authors are with the Institute of Scientific Instruments of the ASCR, v.v.i., Královopolská 147, 61264 Brno, Czech Republic, (corresponding author Zdeněk Buchta - phone: +420 541 514 255, fax: +420 541 514 402, e-mail: buchta@isibrno.cz).

resonance (NMR) signal which can even lead to introduction of specialized simple MRI apparatuses with significantly less powerful magnets.

The phenomenon of angular momentum transfer by the exchange of spin between optically pumped Rb atoms and nuclear spins of 3He was first reported by [2] and theoretically described in [3]. With the higher efficiency of the polarization process that the spin-exchange technique namely of 129Xe can offer, this technique became the mainstream in HpG production and experiments [4].

The output radiation of most lasers is linearly polarized and this is absorbed by both magnetic substates m=±1/2 of the Rb 5s 2S1/2 ground state. Unlike for the production of hyperpolarized noble gasses (namely 129Xe) the light absorption by the one magnetic substate is necessary. This is ensured by the circularly polarized light which can be generated by introduction of a quarter-wave plate. For example, right circularly polarized light selectively excites population from the m=-1/2 ground state to the m=1/2 excited state. Relaxation of the exited state brings population down to both ground states, m=1/2 and m=-1/2. Under continuous radiation, the m=-1/2 ground state will be depleted and population will subsequently accumulate in the m=1/2 ground state, leaving the Rb electrons spin-polarized. The Rb polarization is subsequently transferred to 129Xe nuclear spins through spin-exchange collisions. For Zeeman splitting of ground state to substates m=±1/2 is necessary to apply a homogenous magnetic field, which is generated by a set of Helmholtz coils.

II. OPTICAL PUMPING OF RUBIDIUM WITH TI:SA LASER The first experiments with optical pumping of Rb vapor

employed a Rb lamp or dye lasers. When the first commercially available Titanium-Sapphire (Ti:Sa) lasers appeared, they proved to be a useful replacement for dye lasers in many spectroscopic applications. We employed a commercial Ti:Sa laser for our experiments for its easy wavelength tuning and narrow linewidth.

We used a manually tunable cw Ti:Sa laser for the first set of experiments [5]. A small amount of the output radiation was coupled into a wavemeter for coarse adjustment of the laser wavelength. The diameter of the beam is expanded by a telescope and a circular polarization of the light is generated by a retardation quarter-wave plate. The output laser beam

Generation and Detection of Hyperpolarized Xenon

Zdeněk Buchta, Jan Rychnovský, Zenon Starčuk, and Josef Lazar

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width of approximately 0.3 mm (FWHM) was expanded to the width matching the diameter of the absorption cell (35 mm) by a telescope (Fig. 1).

Fig. 1 Experimental set-up for optical pumping of rubidium. SM – semireflecting mirror; PS – periscope; λ/4 - quarter-wave retardation plate; L1, L2 – lenses of the telescope; C - Helmholz coils; A – aperture; PD – photodetector; WM – wavemeter; AD/DA – analog/digital and digital/analog converter unit; HV – high-voltage amplifier; PC – personal computer.

The Ti:Sa laser, is in a ring cavity configuration pumped by solid-state diode-pumped, frequency-doubled Nd:Vanadate (Nd:YVO4) Coherent Verdi V-6 laser Wavelength of the Ti:Sa laser is tunable in a wide range (680 nm - 1025 nm) by birefringent filter while the useful output varies with the wave length and is close to 1 W. The birefringent filter is controlled manually by micrometer adjusting screw. To avoid drift of the operating wavelength the laser had to be frequency stabilized to the maximum absorption of the Rb D1 line. We extended the tuning screw by piezoelectric element which makes possible the laser to be tuned electronically over 0.25 nm which exceeds thermal wavelength drifts under laboratory conditions.

The target cell used for measurement was a cylinder 9.2 cm long. The cell was made of borosilicate (Pyrex) glass with flat windows. It was enclosed in a teflon box to allow thermal control by a hot-air flow. The box with target cell was placed in a pair of Helmholtz coils designed to generate a homogenous 10 mT magnetic field with homogeneity 2.18*10-4. The cell is attached to a vacuum manifold by non-magnetic vacuum components. A small amount of Rb was distilled into the cell under vacuum conditions. The experiments were performed with a set of cells filled and vacuum sealed before placing into the setup. The level of vacuum achieved before filling was on the level of 10-5 Pa

Tuning and wavelength control of the Ti:Sa laser in a laboratory arrangement was running on a PC computer and performed via external AD/DA board and high voltage amplifier driving piezo transducer. The control software was operates in LabVIEW environment. The program works in two different modes. The first allows manual search of the desired absorption line of rubidium by manual coarse screw tuning. Than the ramp-signal generator is used for matching of the line into the center of tuning range and the stabilization loop is activated. The second mode allows automatic detection of the absorption line. In the second regime after a manual coarse tuning the program monitors variations in detected signal and if the absorption line is found out, the stabilization

loop is activated automatically. In both of this cases the Ti:Sa laser wavelength is stabilized on the center of the D1 absorption line of rubidium at the wavelength 794.76 nm to achieve the maximum efficiency of the optical pumping process. For the frequency locking of the Ti:Sa laser, we employed a technique with wavelength modulation and the first harmonics detection.

Technical linewidth (bandwidth) of the Ti:Sa laser with wavelength selection only by means of the birefringent filter was measured through coherence length to be 25 GHz. To achieve optimum energy transfer and the best efficiency of the pumping process, the laser emission line should fit to the absorption spectral profile. The linewidth of the CW Ti:Sa laser could be further reduced by inserting Fabry-Perot etalons into the cavity which results in discontinuities in tuning and automatic laser frequency control becomes impossible. Spectral profile of the Rb absorption line can be on the other hand increased by pressure broadening when the gas mixture in the cell is heated or by introduction of buffer gasses.

We measured the linewidth of the Rb absorption in a sealed cell with about 2 mg of liquid Rb filled by Helium in the role of buffer gas close to the atmospheric pressure at room temperature. The whole cell was heated by a flux of a hot air which increased the pressure of the mixture of Rb vapor and He. In this configuration the line broadening of Rb is dominated by the pressure of the buffer gas and the level of absorption is given predominantly by the partial pressure of Rb vapor [6]. The detected absorption line for a set of temperatures and corresponding pressures is in Fig. 2 and the measured values in Table 1.

Fig. 2 Detected absorptions in the Rb vapor under presence of He buffer gas for various temperatures of the mixture.

TABLE 1 Values of He and Rb pressure in the cell and corresponding FWHM

linewidth and absorption of the Rb absorption line

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The optimum value of the He pressure of approximately

1.83*105 Pa was extrapolated to match the emission linewidth of 25 GHz of the laser. Suitable partial pressure of Rb vapor which corresponds to nearly 100% absorption of the incident laser radiation in our cell is at the temperature 373 K.

III. EXPERIMENTAL ARRANGEMENT WITH SEMICONDUCTOR LASER

A. Vacuum Part The main part of the arrangement for the target cell

evacuation is the Turbomolecular Drag Pumping Station TSH 071 E which consists of diaphragm Pump MVP 015-2 (≤ 400 Pa) and Turbomolecular Drag Pump TMH 071 P (< 1*10-5 Pa). Single Gauge Control Unit TPG 261 and combination of cold cathode and Pirani gauge - Vacuum Gauge PKR 251 (10 Pa – 5*10-7 Pa) are used for vacuum measurement. The minimum pressure in the vacuum manifold could be achieved approximately at the 10-5 Pa level. The problem of relaxation of the hyperpolarized Xenon in a continuous production process led to introduction of plastic (teflon) set of valves and tubing. This resulted in a small leakage of the manifold and vacuum level only 10-3 Pa was achieved. A target cell was made again of borosilicate (Pyrex) glass with Borofloat windows. The cell is a 100 mm long cylinder with the inner diameter of 35 mm. The cell was filled with a small amount of rubidium, 200 kPa of N2 and 100 kPa of natural mixture of Xenon isotopes.

B. Laser System and Optical Part Following experiments with optical pumping of Rb were

performed with semiconductor lasers. For medical and industrial applications, a simple high-power laser system with narrow linewidth and wide wavelength tuning range is definitely easier to operate and handle. we concentrated on the design of the external cavity laser (ECL) system based on a high-power laser diode (LD) [7]. The ECL laser is based on the broad-stripe laser diode S-λ-3000C-200-H. Its maximum output CW power is approximately 2.5 W, the selected central wavelength is 797 nm at the temperature 25°C and the LD emission linewidth is about 700 GHz. The original emission linewidth is reduced by diffraction grating used in the Littrow configuration as a wavelength selective feedback. The operation of the ECL was optimized to achieve the maximum power spectral density at the center of the Rubidium D1 line. We were able to achieve the power spectral density enhancement about 4 times compared to the free running LD (Figure 3). Then the ECL emission linewidth was approximately 40 GHz and the power loss is about 40%, from 2.49 W to 1.48 W. The central laser wavelength is stabilized to the D1 Rubidium absorption line (794.76 nm) to eliminate the ECL thermal drift. The diffraction grating holder was designed to cover the thermal drift of the ambient temperature in the range ±5°C by the piezoelectric transducer. In result it is

be able to cover the tuning range up to 232 GHz (±8°C). The experimental setup is shown in Figure 4.

Fig. 3 The emission spectral profile of the free running broad-stripe power laser diode and extended-cavity laser. Amplitude represents normalized power spectral density related to the maximum achieved by the LD.

Fig. 4 . Experimental arrangement for Xenon hyperpolarization. LD – high-power laser diode. LC – aspheric collimating lens, λ/2 is retardation half-wave plate, BG – diffraction grating 2400 lines/mm in Littrow configuration, M – mirror, λ/4 – retardation quarter-wave which transforms the linear polarization of the laser radiation to the circular one, L – lens, HC – Helmholtz coils, DC – detection coils, A – an aperture and PD – photodetector.

C. NMR Part and Experimental Procedure Detection of the hyperpolarized Xenon was performed in

magnetic resonance (MR) system of 4.7 T, 200 MHz. The detection part of the MR system was tuned to Xenon resonance frequency 54.2691 MHz. The target cell was placed into the 16 mT scattered field of the MR system. The scattered magnetic field of the 4.7 T magnet proved to be comparable in homogeneity to the set of Helmholtz coils and in the further experiments we exploited the simplicity of the setup. The cell was enclosed into a plastic (Delrin) box and heated by hot-air flow up to 110°C. After one period of the optical pumping process, the RF pulse is applied and the MR image is constructed from the signal induced in receiver coil.

IV. EXPERIMENTAL RESULTS Figure 5 shows the dependence of spectral line amplitude of

hyperpolarized Xenon on duration of laser optical pumping. The optimal duration of the optical pumping process is about

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15 minutes. In the second experiment, we investigated the influence of

laser emission linewidth, resp. it’s power spectral density to the Xenon spectral line amplitudes. The experimental results are shown in Table 2 and Figure 5. In the first column is the value related to thermally polarized Xenon. The cell was placed in the magnet of the NMR system for about 24 hours. The values in the next 3 columns are related to hyperpolarized Xenon with free-running laser diode with output power 2.49 W. The last four columns shows the experimental results for hyperpolarized Xenon obtain by ECL system with lower overall output power 1.48 W, but with enhanced power spectral density.

0 20 40 60 80 100 1200

10

20

30

40

50

60

Time [min]

Am

plit

ud

e []

Fig. 5 Dependence of amplitude of hyperpolarized xenon spectral line on the optical pumping duration

TABLE 2. Data comparison of thermally polarized xenon 129, and hyperpolarized xenon 129 by free running LD and ECL system.

Fig. 6 Spectral line amplitude comparison of thermally (symbol o), hyperpolarized by spin exchange with Rb pumped by free running laser diode (symbol +) and by ECL laser (symbol ×).

V. CONCLUSION We realized the experimental arrangement for Xenon

hyperpolarization based on 4.7 T NMR system and high-power pumping laser system. To achieve Xenon hyperpolarization, the free running laser diode and the ECL system was used and their efficiency compared and evaluated. We found out that the efficiency of the optical pumping process is significantly higher for the ECL system because of the higher power spectral density at the wavelength of desire. The influence of the optical pumping duration to the 129Xe spectral line amplitude was investigated as well.

ACKNOWLEDGMENT The authors wish to express their thanks to the grant

agencies (GAČR) and ministries (MŠMT, MPO) of the Czech Republic for financial support through grant projects mentioned above.

REFERENCES [1] A. M. Oros and N. J. Shah, Hyperpolarized Xenon in NMR a MRI,

Physics in Medicine and Biology, vol. 49, pp. 105-153, 2004 [2] M. A. Bouchiat, T. R. Carver, and C. M. Varnum, Nuclear polarization

in 3He gas induced by optical pumping and dipolar exchange, Phys. Rev. Lett., vol. 5, 8, pp. 373-375, 1960

[3] R. M. Herman, Theory of spin exchange between optically pumped Rubidium and foreign gas nuclei, Phys. Rev., vol. 137, 4A, pp. 1062-1065, 1965

[4] T. G. Walker and W. Happer, Spin-exchange optical pumping of noble-gas nuclei, Rew. of Mod. Phys. vol. 69, 2, pp.629-642, 1997

[5] Z. Buchta, J. Rychnovský, J. Lazar, Optical pumping of Rb by Ti:Sa laser and high-power LD, Journal of materials science: Materials in electronics Iss. 1, vol. 8, p. 350-354, 2006

[6] M. V. Romalis, E. Miron, and G. D. Cates, Pressure broadening of Rb D1 and D2 lines by 3He, 4He, N2, and Xe: Line cores and near wings, Phys. Rev. Lett., vol. 56, numb. 6, pp. 4569-4578, 1997

[7] Buchta Z, Rychnovský J and Lazar J, High-power laser diode system for optical pumping of Rb, Photonics Europe 2006 Proceedings of SPIE, vol. 6184 pp. 68141T1-68141T6, 2006

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