Contrast agents for magnetic resonance imaging

Contrast agents are deployed in medical imaging techniques for anatomical presentations as well as for functional investigations to increase the significance of the image. For instance, imaging the lungs by conventional magnetic resonance imaging (MRI) is difficult because the proton signal from lung tissue declines swiftly over time and because the air contained in the lungs does not yield any MRI signal. The stable rare gas isotopes (³He, ¹²⁹Xe) can be (hyper-) polarized (Hp) via optical pumping with laser radiation, i.e.: the nuclear spins can be aligned to a high degree. Rare gases prepared in that way (HpHe; HpXe) can be deployed as contrast agents for magnetic resonance imaging of the lungs and for functional diagnostics of the brain.

Several research groups in Europe (e.g.: at the University of Mainz) and America have already successfully imaged the lungs with magnetic resonance techniques by letting patients inhale HpHe. The groups have also carried out dynamic studies of pulmonary functions with the new technique. As a by-product of tritium production ³He is not a naturally occurring isotope on earth. ¹²⁹Xe on the other hand, has a naural apundance of 26 %. In addition, xenon is easily dissolved in blood and tissues, for instance, in brain tissue. This may open up further medical applications.

At PTB a new apparatus has been developed to produce sufficient amounts of HpXe (0,5 l/h) with polarization degrees of 10 % to 20 % (referred to ¹²⁹Xe) to image the lungs and to perform (time-resolved) spectroscopic imaging of the brain by means of MRI. Goals are to develop new fields of medical applications for HpXe as well as to compare quantitatively the images of lungs obtained with HpXe and HpHe. The latter activity aims at clarifying if HpXe can replace HpHe in lung imaging and distinguishing the respective medical applications. Furthermore, it is worthwhile to consider applications for HpHe and HpXe as contrast agents in SQUID measuring-techniques.