17O Magnetic Resonance Imaging (17O-MRI)

Oxygen plays a major role in many biochemical processes which makes it a potential marker for functional imaging applications. The energy consumed by the brain is predominantly provided through the consumption of oxygen, and therefore monitoring oxygen in vivo can provide unique information about tissue viability or tumor treatment response.

Oxygen-17 (¹⁷O) with a natural abundance of 0.038% is the only stable oxygen isotope that can be detected by nuclear magnetic resonance. The ¹⁷O isotope has a nuclear spin of I=5/2, creating relaxation times in the range of a few milliseconds, and requiring the use of special imaging techniques to account for the fast signal decay and low inherent sensitivity. Unlike the hydrogen protons (¹H) of water, ¹⁷O relaxation times are independent of the magnetic field strength. Thus the signal increase at higher magnetic fields is not compromised by variations in relaxation parameters. Another advantage that arises from the short relaxation times of ¹⁷O is the possibility to achieve a large signal-to-noise (SNR) increase by rapid signal averaging. Therefore ultra high fields (B0 > 3T) in addition to short-TE pulse sequences provide a large sensitivity gain for the in vivo application of direct ¹⁷O MR imaging.

At present, an effective oxygen delivery system for ¹⁷O-MRI is designed. In order to obtain functional information, the ¹⁷O concentration in vivo must be increased while not supplying an abundance of gas that is exhaled before it can be absorbed in the alveoli of the lungs, thereby wasting precious amounts of the expensive gas. Therefore, it is essential to ensure the accurate and efficient supply of oxygen to the patients while taking care of the patients comfort and ease of breathing throughout the imaging process. In parallel, ¹⁷O-MR imaging pulse sequences are optimized that utilize a radial k-space acquisition scheme.