MRI Gases

The basic principle of hyperpolarised gas MRI is similar to that of conventional MRI, which uses powerful magnetic fields and radio waves to create detailed images of the body’s internal structures. In conventional MRI, the magnetic moments of hydrogen atoms (protons) in the body’s water and fat molecules are aligned with the magnetic field and then subjected to a radiofrequency pulse. This causes the protons to absorb energy and become excited, and when the radiofrequency pulse is turned off, the protons relax and release their energy in the form of a detectable signal. This signal is used to construct an image of the body’s tissues.

Traditional MR imaging of the lungs is difficult because conventional scanners are designed to excite hydrogen protons, which are present in water molecules. However, the lungs have only a very low density of hydrogen protons compared to other structures, and their long relaxation time means that the signal available for imaging is minimal. In addition, the inhomogeneous magnetic environment of the lungs introduces susceptibility artifacts that further complicate MR acquisitions. These challenges are not faced by external gaseous contrast media like ³He or ¹²⁹Xe, which image the airways and airspaces within the lungs rather than the surrounding tissues. This greatly reduces the problems of unfavorable longitudinal and transverse relaxation times faced by hydrogen MRIs in the lung. However, MR imaging of a gas is challenging because its density is typically about 4 orders of magnitude lower than that of protons. To overcome this limitation, a process called hyperpolarization is used to increase the magnetization of these gases by about 5 orders of magnitude. This makes MR-based imaging of inhaled gases feasible within a single breath hold.