## 4 Discussion  
The ventilation system for breath monitoring and control of the HP \({}^{3}\)He dosing for MR imaging of human lungs has been designed and constructed. It adapts to the patient's lung volume and his breath cycle, providing maximum achievable comfort during the lung imaging procedure, and making the intubation unnecessary. The lung imaging procedure was tested in the John Paul II Hospital in Krakow on a group of healthy volunteers of both sexes, different age and body mass. Good quality images were obtained and the usability of the ventilator for the HP \({}^{3}\)He lung imaging was confirmed. The presence of the ventilator in the medical scanner room did not interfere with its normal operation, as no artifacts were observed in the MR images. Although different groups of people were imaged, the total number of successful results was about ten, which is not sufficient for any meaningful statistical analysis. At this stage our work was focused on the optimization of the performance of the polarizer-ventilator system and the MR image was presented as the proof of its successful operation. In the view of a very high cost of the \({}^{3}\)He isotope, the collection and recycling of the exhaled gas mixture is an important factor. The design of a certified recycling unit for the extraction of helium from the gas mixture and removal of any chemical and biological contaminants is planned in the future. Figure 5: MR lung images of a healthy volunteer. Five coronal slices and one transversal slice are shown. The described ventilator can be also used, after small modifications, for the HP \({}^{129}\)Xe lung imaging, and this direction is being currently explored in our group [26]. Our ventilator compares favorably with similar systems that were reported in the literature. [PERSON] et al. describe the ventilator for animal applications, which impose different requirements [21]. In contrast to humans, the animals to be imaged do not cooperate and have to be anesthetized. In the case of small animals, like mice or rats, the intubation is not used, and the breathing and heart rate need to be monitored continuously. Stable breathing is achieved by supplying a controlled mixture of air, oxygen, and anesthetic. Moreover, the animal is kept in constant temperature, because the temperature self-control is not active in the sleep phase. The above procedure has to be carried out by a trained personnel, which is not necessary in imaging humans, who can control breathing by themselves. Nevertheless, it is of utmost importance for both the patient and the personnel to get familiar with the ventilator operation before the actual experiment. During initial training, the software automatically records the breathing parameters of the patient, using air in the proper sequence. Therefore, the patient feels comfortable during the actual imaging procedure, when hyperpolarized helium mixed with nitrogen is applied. When compared to self-controlled breathing of the prepared in advance gas mixture, our automatic system is more reliable and reproducible. [PERSON] et al. for the first time presented the operation of the ventilator for humans and the resulted MR images of human lungs [22]. The authors used the medical scanner and the RF coil that was produced by the same company, and we analyzed and utilized their experiences to a large extent. Compared to their design, our system is of significantly smaller size. It cooperates with the high-field optical polarizer, which was designed and built in our lab [23] and uses the magnet of the medical scanner as a source of high magnetic field. The critical components of the ventilator are also located in the imaging room and the imaging procedure includes the preparation of the hyperpolarized helium gas. Therefore, there is no need for an external production, transport, and delivery unit, as it is in the Mainz design. In the first stage, the polarizer is placed in the magnet and the sufficient amount of the polarized helium is produced in about 20 min.