MR imaging of mouse lung using hyperpolarized 3He: image acquisition and T1 estimation under spontaneous respiration

The purpose of this study was to acquire a hyperpolarized (HP) (3)He image of the mouse lung and to estimate 3He T1 in the lung from wash-out curve analysis under spontaneous respiration.

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.

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.

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 (pO2) in lungs of animals.