Are there any magnetic particles that are safe to inhale? In medicine, there is an (old?) experiment where a subject is asked to inhale radioactive Xenon gas, and radiation counters are placed at different positions near the lungs. Using this experiment, it has been demonstrated that the (alveolar?) volume of the lungs is highest in the bottom parts, and smallest in the top parts.

I don't know if this experiment is still used today, since inhaling radioactive gas has the obvious radiation related risks.

I was wondering if there are some magnetic particles that would be safe to inhale? I know that for example helium does not diffuse into blood, and it does not permanently bind into anything. However, I think it does not have any suitable magnetic properties. Let's assume the detector is very very sensitive to any magnetic fields (so, a super-conducting quantum interference device).
$\ce{O_2}$ is paramagnetic.  



source: http://www.chem1.com/acad/webtext/chembond/CB-images/liquidoxygeninmagnet.jpg

The stable Xenon isotope Xe-129 has been used in MRI studies:

Hyperpolarized Xenon MR Imaging of the Brain 


  A radically new magnetic resonance imaging (MRI) technique, using hyperpolarized 3He and 129Xe, is being developed to produce high-contrast images of important body tissues that have resisted conventional MRI techniques. Hyperpolarization of the noble gas nuclei is achieved by spin exchange with rubidium atoms that have been optically pumped into one electronic spin state with the use of circularly polarized light from a high-power diode laser system. Hyperpolarization can increase the detectability of 3He and 129Xe up to a hundred thousand times. This technology is showing dramatic results for diagnostic imaging of the lungs, brain, and other parts of the body. Most notably, we have achieved high-resolution dynamic images of the lungs and airways using 3He, and measured cerebral perfusion in the brain using 129Xe. 


See video of lung being imaged: http://www.spl.harvard.edu/archive/HypX/about.html