Design and Testing an Open, Human MRI system for Orientational Lung Study

Considerable anecdotal evidence suggests gravity plays a significant role in lung development, and that adaptive mechanisms contribute to lung structure and function. Laser-polarized He MRI provides a powerful method to study lung function and inhalation, but in traditional MRI systems, patients are restricted to lying in a horizontal position. We have designed an open access, human MRI system that operates at very low applied magnetic fields and allows for complete two-dimensional rotation of subjects within the applied field. Demonstration two-dimensional images, H and He, of phantoms similar in size to human lungs have been obtained from the MRI system. Introduction A current subject of much debate in the lung physiology community concerns the role of gravitational effects on lung inhalation and function, such as the role of gravity in fundamental cardiopulmonary physiology, e.g., the origin of perfusion heterogeneity in the lung [1]. The recent advances in spin-exchange optical pumping [2] have made laser-polarized He MRI a powerful method for studying lung structure and function. However, in conventional MRI systems, patients are restricted to lying in a horizontal position. However, as the laser-polarization is produced without the aid of a large applied magnetic field, we can benefit from novel magnet design that does not restrict the patient, while still permitting high-quality laserpolarized gas MRI. We have previously demonstrated very low field (~20 gauss) imaging of small samples of laser-polarized He gas [3,4] and shown the effects of background field gradients are greatly suppressed. We have now developed and tested an open-access human-scale imaging system to operate at similarly low magnetic fields in which it will be possible to vary the orientation of the subject in a two-dimensional plane. Methods The basic design of the open-access human scale MRI system was reported previously [5]. The B0 field of 20 70 Gauss is created by two pairs of Helmholtz coils, with custom-designed planar gradients allowing complete orientation of subjects in a plane ~ 75cm wide. Audio-frequency control is provided by a SMIS console, modified to produce kHz frequencies, and interfaced to an Outlaw home-theater amplifier. We have recently employed and tested human-sized B1 transmit and receive coils. The transmit coil is a ~ 50 cm ID Helmholtz pair with a bandwidth of 2.8 kHz, while the receive coil is a modified Bedstead coil of ~ 30 cm ID, with an operating bandwidth of 810 Hz (See Fig. 1a). Such narrow bandwidth coils require the images to the post-processed by dividing through by the frequency response of the coil. Significant reductions in noise from the gradient amplifiers (Techron) were obtained by employing passive inductors on the gradient lines making it possible to acquire 2-D H images at 127 kHz. Results Images of water (H) and laser-polarized He phantoms were acquired with gradient and spin echo pulse sequences. Figure 1(b) shows a 2-D image of three laser-polarized He phantoms with different levels of polarization, exhibiting SNR comparable to that of high-field commercial instruments. Figure 1(c) shows a 2-D image of a large tub of water with dimensions similar to human lungs, and resolution comparable to human scanners.