[Magnetic resonance tomography with inhalation of polarized noble gases: new perspectives in functional imaging diagnosis of emphysema].

PURPOSE

Based on a review of the background of MRI using inhaled hyperpolarized noble gases first experiences and perspectives for functional imaging in emphysema patients are presented.

MATERIAL AND METHODS

In vonventional MRI, the spin density of protons, which is defined by the Boltzmann equilibrium, is the source of the imaging signal. Since proton density in the lungs is low and multiple air-tissue interfaces exist, MRI of the lung parenchyma is unsatisfactory. The possibility to artificially enrich the spin density (hyperpolarization) in noble gases (H3-3, Xe-129) by optical pumping results in a non-equilibrium polarization five orders of magnitude higher than the Boltzmann equilibrium. Hyperpolarized noble gases can then be applied as "inhaled contrast media" using a dedicated application device. At the MR unit several prerequisites must also be fulfilled: transmit-receive coil, boradband amplifier and fast sequences with low flip angles. These are essential for dynamic scans in breath-hold tecnique of the highly diffusible He-3 or the well soluble Xe-129.

RESULTS

He-3 and Xe-129 have been successfully applied for imaging of the ventilated airspaces. Besides the well-known narcotic effects of Xenon no adverse effects were observed. A homogeneous distribution of signal intensity can be regarded as a normal findings in people without lung disease. Obstructive diseases and emphysematous changes lead to generalized or localized signal inhomogeneities. Most likely they are caused by disorders of the distribution of ventilation bases on a different functional vehavior of different alveolar regions. By making use of the paramagenetic properties of oxygen, He-3 can also be used for local measurements of oxygen partial pressure in the lung. Xe-129 exhibits a different chemical shift within alveoli, interstitial space and vessels which can be measured by MRI.

CONCLUSIONS

MRI using inhaled hyperpolarized noble gases is a functional imaging modality with high spatial and/or temporal resolution. First studies for early detection of obstructive lung diseases and disorders of distribution of ventilation in emphysema are promising.