## 4 Discussion
A unique, nonmagnetic peristaltic pump is used both to accumulate the polarized gas and to transfer it to the one-liter administration box, which is visible on the left side of Figure 3a. In the second stage, the polarizer is removed from the magnet, making place for the patient, who was already trained. The only component that is left in the magnet is the administration box and some plastic tubes which are connected to the patient's mouthpiece when he is inside. Our design is significantly different from the Mainz system, which cannot cooperate with the high-field polarizer. The described ventilator is only the first step in making the imaging with HP \({}^{3}\)He a routine clinical procedure. The main goal was to design a system that would produce images of diagnostic value with high reproducibility and reliability, independent of the sex, age, and lung volume of the patient. Due to low density of water vapor in the lungs, the standard \({}^{1}\)H MR imaging does not provide any structural information, therefore a quantitative comparison of both methods is impossible. We present the best images obtained with the use of our ventilator up to now, and their quality is comparable with the ones reported in literature. According to medical doctors, they contain some diagnostic details that are not visible in the high-resolution computed tomography images, for example some dark areas in the peripheral region of the lung of the healthy heavy smoker. The problem of the initial flushing of the lungs with pure nitrogen gas is still controversial, especially in the case of a sick patient. Therefore, our software enables the omission of this step. Healthy volunteers did not report any discomfort when a single flushing was applied. However, it is known that several inspirations of pure nitrogen lead to a fast decrease of blood saturation and can be carried out only in the presence of a medical doctor. The blood oxygenation level was monitored by the pulse oximeter during the entire imaging procedure. At this stage, the comparison of images taken with and without nitrogen flushing was not made, but it is planned in the future. An additional limiting factor is the cost of \({}^{3}\)He gas, which in recent years has increased by two orders of magnitude. Some quantitative information about the oxygen-related relaxation of the HP helium gas is given in [24]. Computed tomography is the most widely used technique for lung imaging. However, it is sensitive to soft tissue only and cannot image the air spaces, which are dominant in the lungs. The only comparable method is MRI using hyperpolarized xenon, which is much less expensive. Its main disadvantage when compared to HP \({}^{3}\)He MRI is the long time necessary to produce a sufficient amount of hyperpolarized gas. Nevertheless, this direction is actively pursued in our lab [25]. Apart from standard imaging, the high solubility of xenon in blood makes it possible to apply the localized spectroscopy of \({}^{129}\)Xe to study perfusion in the lungs and metabolic processes in the brain. Bearing in mind the cost and time of examination, the application of our device will be probably limited to special cases, such as lung transplantology. It can also successfully replace CT when frequent examinations are necessary, in order to avoid ionizing radiation damages. The device is still a prototype and needs further improvements to be ready for wider distribution. Conceptualization, R.K., T.P., Z.O., W.F. and T.D. ; methodology and validation, R.K., B.G., L.M., T.P., and M.S. ; software, R.K. and B.G. ; writing--original draft preparation,--review and editing, R.K., M.K., B.G., Z.O., and M.S. ; visualization, T.P. and L.M. ; supervision, W.F. and T.D. ; project administration, M.K. ; funding acquisition, T.D. All authors have read and agreed to the published version of the manuscript. This research was funded by the Polish National Center for Research and Development within the PBS program, edition 3, number PBS3/A9/35/2015.