Medical applications of spin polarized gases
Our group provides hyperpolarized gases for clinics and other users. These gases are clinically certified and were already transported all over the globe. Our support also includes design and supply of transport vessels, clinical application devices, MRI sequence development and recycling of the expensive gases.
MRI of the lung
The lung is difficult to investigate with tomographic methods. X-ray (CT) suffers from low contrast and gamma-ray scintigraphy from poor resolution. While 1H-MRI provides good contrast and sufficient resolution for most soft tissues, the lung is almost invisible in conventional 1H MRI (see Fig. 1). This is due to its low density and many structural inhomogeneity (large magnetic susceptibility changes). 3He and 129Xe are noble gases which are chemically inert, harmless and possess a nuclear spin, which can be hyperpolarized and detected by MRI. Hence they are ideal for diagnosis of the lung’s gas space (see Fig. 1).
Fig. 1: Left: 1H-MRI of the chest. The black area is the lung, which hardly gives a signal. Right: Lung after inhalation of HP-3He. Now only the lung is visible. (courtesy of Universitätsmedizin Mainz)
Already straight-forward imaging of inhaled HP-3He can be used to detect obstructions, ventilation deficits and other severe defects which attend almost all diseases of the lung (see Fig. 2).
Fig. 2: Images of the lungs of two healthy volunteers: Non-smoker (left) and a smoker (right). The arrows indicate ventilation defects.
Ultrafast MRI sequences also allow to follow the inhalation process with a time resolution of a few milliseconds (see Fig. 3). Such “movies” can be used to study the dynamics of the ventilation in detail. It is hoped that this allows for early recognition of the onset of chronic lung diseases like COPD (chronic obstructive lung disease) or asthma.
Fig. 3: Time sequence of MR images monitoring the inhalation and exhalation of 3He over roughly 8 seconds. (courtesy of Universitätsmedizin Mainz)
Although the spatial resolution of MRI is too low to image most structures of the lung (except for the major air conducts like trachea and bronchi) there are certain MRI-techniques that can diagnose void sizes even below this limit. This trick is achieved by measuring the motion of the gas, which is restricted in the lung by the size of the voids. Certain diseases (e. g. emphysema) cause destruction of cell boundaries and hence enlarge the voids and therewith the measured diffusion coefficient (see Fig. 4).
Fig. 4: Image of the 3He diffusion coefficient in the lung of a patient with a diseased lung wing on the left and a transplanted wing on the right. The red to yellow colors indicate high diffusion rates due to fibrosis. The yellow colors in the center belong to the trachea where diffusion is unrestricted. (courtesy of Universitätsmedizin Mainz)
Because molecular oxygen destroys the polarization of HP-3He and the rate of destruction depends on its concentration. Exact determination of this decay rate allows therefore the quantification of the local oxygen concentration. Even the consumption rate of oxygen can be determined this way and allows a non-invasive, spatially resolved study of lung function.
129Xenon is another highly interesting contrast gas for MRI lung diagnosis, which can be also be hyperpolarized. In difference to 3He xenon is not biologically inactive. It dissolves in the blood and has anesthetic properties. Therefore it can serve as a tool to study ventilation as well as perfusion. Its huge chemical shift can also be utilized for this purpose, because the NMR-signal is strongly different for Xenon gas and Xe dissolved in liquids. However it must be polarized via spin exchange optical pumping.
Medical administration and gas application
Originally HP-gases were filled in Tedlarbags and directly inhaled by the patients. However for follow-up studies a better reproducibility and monitoring is desirable. Therefore, an administration unit was built respectively to the Medical Devices Law to administer well defined gas boli (3He,129Xe) in defined quantities (accuracy < 3%) and at a predefined time during inspiration with high reproducibility and reliability without reducing MR-quality (see Fig. 5. The patients’ airflows are monitored and recorded and the exhaled gas is collected for later recycling. Furthermore, it is possible to use gas admixtures and the polarization is measured on line (error < 7%). First images with healthy volunteers were taken with this setup in a clinical study.
Fig. 5: Application unit in clinical environment. Left : MRI scanner with patient and inhalation unit. Right: controls.