The lungs are difficult to image with conventional proton magnetic resonance imaging (MRI) because of their low tissue density and the existence of high solid-gas magnetic susceptibility gradients. Laser-polarized 3He overcomes these barriers by providing an exogenous gas-phase-only signal source whose magnetization is independent of magnet field strength. We are currently developing 3He MRI as a non-invasive, in-vivo method of studying pulmonary function on a regional level. Our efforts include the development of low-field imaging systems for orientation-dependent studies as well as imaging protocols that can quantify sub-clinical changes in respiratory diseases such as asthma.
We have obtained initial laser-polarized 3He human lung images using a very-low field (<50 Gauss) MRI system. The open-access design allows for changes in subject orientation and provides a gas-phase imaging resolution comparable to that of high field clinical scanners. Preliminary images of the lungs with the subject in supine and upright positions show clear changes in ventilation and lung position. Similar data will be used to study the effects of gravity on local pulmonary ventilation and function. To improve our measurements, we are currently constructing a second-generation, 100-Gauss magnet that will have 1000 times the field uniformity and nearly twice the imaging area as the older system.
We are also developing 3He imaging techniques in clinical systems (0.2 Tesla and 1.5 Tesla) to explore sub-clinical changes in asthmatic lungs. Preliminary 3He MRI studies have shown dramatic ventilation defects in asymptomatic asthmatics. We will investigate the temporal and spatial nature of these defects in greater detail using spin-density mapping. We also plan to measure the 3He apparent diffusion coefficient (ADC) within the peripheral air-spaces of the lung to resolve regional structural changes due to chronic inflammatory airway remodeling.
