MR Imaging of Mouse Lung Using Hyperpolarized ³He: Image Acquisition and T₁ Estimation under Spontaneous Respiration

Abstract

Purpose: The purpose of this study was to acquire a hyperpolarized (HP) ³He image of the mouse lung and to estimate ³He T₁ in the lung from wash-out curve analysis under spontaneous respiration.
Materials and Methods: We first tested the K-Rb hybrid method for the spin-exchange optical pumping (SEOP) of ³He using a home-built noble gas polarizer operated at atmospheric pressure and then applied it to MR imaging and spectroscopy of the mouse lung. The longitudinal relaxation time (T₁) of ³He in the mouse lung was estimated under spontaneous respiration by exploiting a novel method in which SF₆ gas at thermal equilibrium was utilized in combination with the HP ³He gas in the quantitative wash-out curve analysis. This method utilizes the difference in the profile of the wash-out curve of HP ³He and SF₆ at thermal equilibrium. That is, the slope of the ³He wash-out curve in the semi-logarithmic plots is affected by 3 factors, including RF pulse angle, respiration, and T₁, whereas the slope of the SF₆ wash-out curve is only the function of respiratory term.
Results: A ³He lung image was obtained successfully, and we were able to estimate successfully ³He T₁ in the mouse lung under spontaneous respiration using a novel method; the estimated T₁ value was 68±25 s, which was reasonable compared with the value calculated from the literature data measured during breath-hold.
Conclusion: We succeeded in acquiring the first ³He image of mouse lung in vivo in this country, and our proposed method of estimating ³He T₁ in the lung under spontaneous respiration is noninvasive and readily applied to animals and would be useful to evaluate the alveolar gas exchange function as well as oxygen partial pressure (p_O2) in lungs of animals.