MRI hyperbole

First, watch the video in Kevin Clift’s post (via Irina Tcherednichenko) http://goo.gl/vgkIQ

Dr. Peter Wothers demonstrates how hyperpolarized xenon gas can be used to image your lungs. If you’ve been following my #CHMedicalImagingSeries then you know that MRI cannot image the air in your lungs. The air (lacking protons to spin) shows up as no signal/black on MRI.  So why not use CT instead of MRI? The density difference between tissue and air gives great contrast in CT. This isn’t my area but I’m guessing that some reasons to go through the trouble of using a hyperpolarized noble gas with MRI is that MRI is great with soft tissue and you can do functional imaging. Also, Xe129 is soluble in blood so that can make the imaging more interesting e.g., perfusion imaging.

What is hyperpolarization (HP)?

In the context of NMR/MRI, HP is the process of  transferring nuclear spin polarization from one material to another, taking advantage of the Overhauser effect. The hyperpolarized material can then be imaged with higher signal-to-noise ratio (SNR). Standard MRI works well because of the abundance of protons in the body (mostly water). However, the sensitivity of MRI is relatively weak compared to other methods, such as positron emission tomography (PET). There are two types of HP, spin exchange optical pumping with noble gases and dynamic nuclear polarization with C13 or N15.

http://en.wikipedia.org/wiki/Hyperpolarization_(physics)

Spin exchange optical pumping (SEOP)

The video shows hyperpolarized Xe. Helium (He3) can also be used. In both cases lasers are used with an alkali metal vapor, most often rubidium, where angular momentum is transferred from the metal electrons to the nuclei of the noble gas.  An overview of a device for SEOP is shown in the figures below.

Dynamic Nuclear Polarization (DNP)

Prior to seeing the video, I was only familiar with DNP even though SEOP has been around much longer. DNP utilizes a free radical to exchange polarization to either carbon (C13) or nitrogen (N15). An interesting advantage of this method is that the metabolites can be monitored as well. For example pyruvate with C13 and HP, can be   used to image changes in metabolism as the metabolites can be imaged too. An example of a DNP apparatus is shown in the figures below along with an example of the improved SNR. The other image is an example of the higher sensitivity of PET  vs. DNP. However, note the better soft tissue contrast in the underlying anatomic MRI vs. the CT with the PET image.

#ScienceSunday

References

MRI of the lungs using hyperpolarized noble gases.

Möller HE, Chen XJ, Saam B, Hagspiel KD, Johnson GA, Altes TA, de Lange EE, Kauczor HU.

Magn Reson Med. 2002 Jun;47(6):1029-51.

http://www.ncbi.nlm.nih.gov/pubmed/12111949

Hyperpolarized 13C MRI and PET: in vivo tumor biochemistry.

Gallagher FA, Bohndiek SE, Kettunen MI, Lewis DY, Soloviev D, Brindle KM.

J Nucl Med. 2011 Sep;52(9):1333-6. doi: 10.2967/jnumed.110.085258. 2011 Aug 17.

http://www.ncbi.nlm.nih.gov/pubmed/21849405

Spin-exchange optical pumping of noble-gas nuclei.

Walker TG, Happer W. 

Rev Mod Phys 1997; 69: 629–642.

http://rmp.aps.org/abstract/RMP/v69/i2/p629_1

Increase in signal-to-noise ratio of > 10,000 times in liquid-state NMR

Jan H. Ardenkjær-Larsen*, Björn Fridlund, Andreas Gram, Georg Hansson, Lennart Hansson, Mathilde H. Lerche, Rolf Servin, Mikkel Thaning, and Klaes Golman

http://www.pnas.org/content/100/18/10158.short