Mainz concept of central production of hyperpolarized ³He
A high-power polarization apparatus has been developed in Mainz. Our concept of production of HP 3He includes a remote type of operation, where the 3He is spin-polarized in a central production facility from where it is transported to users. After usage the gas can be recovered, the transport vessels are refilled with freshly polarized gas and the cycle starts again (Fig.1).
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Fig.1: Preparation cycle of the hyperpolarized 3He
Current 3He Polarizing Facility in Mainz
The polarization apparatus in Mainz is based on the method of optical pumping of metastable 3He atoms in a weak gas discharge at a gas pressure of about 1 mbar . The whole equipment is located in a homogeneous magnetic field B0 of 1 mT, which serves as a holding field and as quantization axis for the polarized 3He-nuclei. The polarizing and compressing system consists of three parts  (Fig.2): The first part contains the 3He reservoir and getters for gas purification. The second part consists of the optical pumping volume. The optical pumping itself is done by two 15 W fibre lasers at 1083 nm. The five OP-cells have a length of 2.20 m. In order to maximize the light absorption the circularly polarized laser light is back-reflected at dichroic mirrors. The nuclear polarization of the ³He gas can be monitored during the OP-process by measuring the circular polarization of the 668 nm-light emitted by the discharge. The third part contains a mechanical polarization-conserving compressor driven by hydraulics.
Fig.2: Schematic of the present 3He-polarization facility
The polarization of 75 % to 78 % can be obtained at a production rate of about 1 bar*liter per hour, this is the regime we have to work in for fundamental science. For the medical application, where higher production rates are required and lower polarization values around 65 % are sufficient, we can work in a regime between 2 and 3 bar*liters per hour.
For the production of the HP 3He for human studies the Manufacturing Authorization has been given. In the last fifteen years Mainz polarization equipment has made feasible many measurements in medical research with the HP 3He. So far per year several hundreds bar*liters of spin polarized Helium are prepared in Mainz only for the medical applications. HP gas production and management played and plays an important role in the the European Research and Training "Polarized Helium Lung Imaging Network" and in the German network “Magnetic Resonance Imaging for Diagnosis and Monitoring of COPD and Asthma”.
Storage and transport of spin polarized 3He
When the gas is being used at a time t after having been polarized the polarization will have been decayed exponentially to a value
with a total relaxation rate
Here 1/T1dd accounts for the relaxation due to the dipolar coupling of 3He spins during collisions. It scales with the 3He pressure p and amounts at 2.7 bar and room temperature to
1/T1dd ≈ 1/300 h.
T1wall characterizes wall relaxation of the glass vessels. After polarization the hyperpolarized 3He can be stored and transported in relaxation-poor vessels (Fig.3), which are blown from iron free glass. Without internal coating, wall relaxation times of up to 250 h are observed .
Fig.3: 1.1 Litre iron-free aluminosilikate-glass (GE-180) vessel on special demand from Schott Duran Group Mainz for the transport of the hyperpolarized gases
This is sufficient for shipping the HP gas all around the world  using specially designed transport units, which provide a homogeneously magnetic field to guide the spins. T1G describes the relaxation induced by the diffusive motion of the gas atoms through inhomogeneous magnetic field. The corresponding relaxation rate is given by the product of the diffusion coefficient D times the square of the relative gradient of the transverse field components Bx, By with respect to the central field value B0 in z-direction :
The average is taken over the gas containing vessel. For 3He at room temperature the relaxation time takes then the value as function of pressure:
The new developed method for the homogenisation of the magnetic field on the inside of the transport unit made it possible to construct compact shielded boxes for the transport of three cells with hyperpolarized gases (Fig.4) with minimization of losses of the polarization (gradient relaxation time is about 400 h) .
Fig.4: Magnetically shielded transport box
In order to recover the long relaxation time of the storage cells after they have been exposed to high magnetic fields, e.g., 1,5 T scanner, a degaussing setup essentially consisting of a solenoid which can provide a magnetic field strength up to 0,14 T has been developed. Applying an oscillating magnetic field (1-3 Hz) with linearly decreasing amplitude in time, the embedded glass cell or more precise, the magnetized dust or pieces on its inner surface, are degaussed due to the cyclic passage through their hysteresis loops from saturation (B = 0,14 T) to the demagnetised state at the end of the demagnetization procedure (B = 0).
Recycling of 3He
After usage the 3He can be recovered in the recycling unit composed of zeolith- and cold traps (Fig.5).
Fig.5: Schematic of the cryogenic separator: B1, B2, B3 - gas reservoirs, PR - pressure reducer, F1-F6 - filter, G1-G6 - gauges, T1-T4 - cold and zeolith traps, S1-S2 - safety valves, P1-P3 - pumps, CH - cold head, 1-17 - valves
The washout of 3He of the lung requires at least 15 breathing cycles. For patients with strong disease and flat breathing the number of required breathing cycles could increase up to 30. Hence the concentration of the Helium in the collected gas mixture is quite low (only about 1%). Nearly 500 bar*liters of He-3/Air mixture can be processed in the recycling unit per day. With typical amounts of between 15 and 20 bar*liters per one 3He-inhalation and ten images taken per patient, 500 bar*liters are the collected gas mixture from three patients. The recovery efficiency is about 95% and the purity is comparable with that of the fresh gas . The recycling is recently included to our Manufacturing Authorization , so one is allowed to use recovered gas for clinical trials.
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8. Manufacturing authorisation No. 2010/128/55/M bythe State Office for Social Matters, Youth and Pensions,Rhineland Palatinate, Germany.
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