Static MR Imaging of the Airways Using Hyperpolarized ³He and ¹²⁹Xe: The University of Virginia Experience

K. D. Hagspiel¹, J. P. Mugler III^1,2, T. A. Altes^1,2, E. E. de Lange¹, J. Knight-Scott¹, T. M. Munger¹, S.S. Berr^1,2, V. M. Mai², T. M. Daniel³, M. J. Spellman Jr.¹, J. F. Mata¹, P.L. Bogorad⁴ , B. Dreihuys⁴, T.R. Gentile⁵, G.L.Jones⁵ , A.K.Thompson⁵, J. R. Brookeman^1,2

Departments of ¹Radiology, ²Biomedical Engineering and ³Surgery, University of Virginia Health Sciences Center, Charlottesville,VA 22908 USA, kdh2n@virginia.edu; ⁴MITI,Durham, NC 27713 USA, ⁵NIST,Gaithersburg, MD 20899 USA

Introduction: Imaging of the airways with MRI is difficult due to low proton densities and field inhomogeneities secondary to magnetic susceptibility interfaces. Hyperpolarized gas MRI helps overcome these problems and opens exciting new diagnostic possibilities.

Materials and Methods: Animal and human experiments were performed on a 1.5T MRI system (Siemens Magnetom Vision, Iselin, NJ, USA). ³He and ¹²⁹Xe were laser polarized on site by the spin-exchange method (Model 9600 Polarizer, MITI, Durham, NC, USA). For some studies, ³He was polarized by the metastability-exchange method (National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA) and transported 200 km to the MR site. Polarizations of 2-20% were achieved. Human experiments were performed to assess the lungs with hyperpolarized xenon¹ and helium². For the ³He study 60 healthy volunteers and patients with airway disease were studied. A rabbit model also was used to compare ¹²⁹Xe and ³He MRI in normal lungs and in lungs with ventilation defects created by temporary balloon occlusion of the main bronchus. ³He MRI of the lung was combined with a non-contrast-agent enhanced MR perfusion sequence (Flow-sensitive Alternating Inversion Recovery with an Extra Radiofrequency pulse (FAIRER))³ for the diagnosis of pulmonary embolism in a rabbit model using temporary balloon occlusion of the pulmonary artery.

Results: Diagnostic-quality lung images were routinely obtained with ³He, both in normal volunteers and patients with chronic lung disease². Subjects with seasonal allergies (Figs. 1 and 2, images taken 1 week apart) were found to have small peripheral transient ventilation defects (arrow, fig 2), while patchy and wedge-shaped defects were seen in patients with COPD. Ventilatory changes after lung reduction surgery could be detected. No adverse side-effects occurred in either patients or volunteers after the inhalation of ³He. Both ¹²⁹Xe and ³He allowed the acquisition of images of the lungs in animals both with and without ventilation defects. All ventilation images were of diagnostic quality, with lower resolution on the ¹²⁹Xe images. Combination of ³He MRI (Fig. 3) and ¹H FAIRER (Fig. 4) in the rabbit enabled the detection of PE presenting as a mismatched defect with perfusion deficit but normal ventilation, thus representing the equivalent to the nuclear medicine V/Q scan.

Discussion: Static imaging of the upper and lower respiratory tract is possible with hyperpolarized gas MRI. Both ³He and ¹²⁹Xe allow the acquisition of diagnostic quality images. Potential applications include the evaluation of the lungs in patients with multiple pulmonary pathologies, especially emphysema. Differences in regional pulmonary ventilation can be easily detected with both gases and the combination with a perfusion technique might allow screening for the presence of pulmonary embolism.

References:

1. Mugler J.P. et al (1997), MR imaging and spectroscopy using hyperpolarized ¹²⁹Xe gas: preliminary human results, Magn Reson Med. 37: 809-817.
2. De Lange E.E. et al (1999), Lung airspaces: MR imaging evaluation with hyperpolarized helium-3 gas, Radiology. 210:851-857.
3. Mai V.M, Berr S.S.et al (1999) MR Perfusion Imaging of Pulmonary Parenchyma Using FAIRER. JMRI, 9(3):483-487.