The polarization build-up of noble gases is maintained in the polarizer using an external holding magnetic field (typically a few tens to hundreds of Gauss) and by ensuring a highly clean storage environment (e.g. a baked and evacuated glass cell in the case of 3He or a cryogenic trap in the case of 129Xe). However, the moment HP gas is dispensed from the polarizer, its polarization starts decaying (i.e. relaxes) with a time constant in the order of a few tens of minutes, and therefore it needs to administered and imaged almost immediately. Therefore in order to preserve the polarization of HP gas for much longer time, an auxiliary holding field is employed to provide the means to hold the dispensed dose of HP gas largely unaffected until the time of delivery to the subject inside the MRI scanner.
When the HP gas dose is ready for delivery, it is essential to know its polarization level before releasing to the patient. Accurate knowledge of polarization level is essential because it indicates the activity level of the HP gas administered to the subject. Moreover polarization levels need to be standardized among different clinical sites when conducting clinical trials. Finally, real-time availability of polarization level makes it possible to titrate the amount of HP gas to the minimum concentration necessary to achieve diagnostically useful images, which in turn minimizes the amount of HP gas entering the subject’s lungs as well as the associated cost.
All the above objectives are achievable using a polarization measurement station. This instrument consists of an electromagnetic holding field and a dedicated polarimetry circuit. The field is typically stronger and more homogenous than that of the polarizer in order to enhance the longevity of the polarization level and its measurement accuracy. The polarimeters are calibrated by standard NMR procedures using standard water samples, hyperpolarized gas samples and an active circuit designed to closely emulate the signal generated from a HP gas sample with well-known properties.
A schematic diagram of an actual Polarean 2881 polarization measurement station is shown below showing the major system components. Please refer to technical specifications for more details.