X-Nuclei Imaging in Brain Tumors
Tanja Platt1
1German Cancer Research Center, Germany

Synopsis

Physiologically relevant nuclei that enable MR applications ('X-nuclei') in tumor imaging in addition to hydrogen (1H) will be presented and the special MR characteristics of these nuclei will be explained. X-nuclei MR applications offer a wide variety of applications in science and translational research. Here, an overview of the physical MR properties of these nuclei and clinical research applications in brain tumor imaging will be given.

Target Audience

Scientists, clinicians, and technicians interested in brain tumor imaging with nuclei other than hydrogen (e.g., 2H, 17O, 23Na, 31P) that provide further insights into physiology.

Objectives

  • Physiologically relevant nuclei that enable MR applications ('X-nuclei') in tumor imaging in addition to hydrogen (1H) will be presented and the special MR characteristics of these nuclei will be explained
  • Furthermore, additional insights into brain tumor physiology and viability that can be gained with X-nuclei imaging will be highlighted

Scientific Background

About 40 years ago, Hilal et al. published the first in vivo MR sodium images [1]. Since then, higher magnetic field strengths B0, improved hardware (coils, gradients), and further developments of pulse sequences and reconstructions have steadily improved the quality and also enabled MR studies of further X-nuclei [2, 3].

Nuclei that have a non-vanishing nuclear spin (I ≠ 0) can be examined by means of magnetic resonance. These include nuclei with a high natural abundance (NA), such as 23Na and 31P, but also nuclei with a low natural abundance, such as 2H and 17O [4]. The latter can therefore also be used as tracers. Compared to hydrogen, the in vivo concentrations (NA ∙ c) of all of these X-nuclei are typically three to four orders of magnitude lower.

The resonance frequency depends on the gyromagnetic ratio (𝛾) of the nuclei as well as on the field strength B0. Thus, a different resonance frequency results for each combination of field strength and nucleus. The MR system must support these resonance frequencies, and specific MR coils are needed for different nuclei and applications [5].

Here, the MR signal S depends on the mentioned quantities as follows:
$$ 𝑆 \ \ α \ \ 𝐼 (𝐼 + 1) · 𝛾^3 · NA · 𝑐 $$
Low in vivo concentrations and/or low natural abundances result in poorer spatial/temporal resolution and/or longer acquisition times compared to 1H morphological imaging. High magnetic field strengths, such as 7 Tesla or higher, can help to increase the signal-to-noise ratio (SNR) [6]: SNR ~ B0 (for resonance frequencies << 300 MHz). Using either internal or external references, concentrations can be estimated, e.g. the tissue sodium concentration (TSC).

Research Applications in Tumor Tissue

The distinguishing feature of these X-nuclei MR methods is that they can provide information that cannot be obtained with standard proton MRI. In tumor tissue, MR imaging of X-nuclei can e.g. provide additional insights into
  • the glucose and energy metabolism via 2H and 31P MR applications
  • the ion physiology and tissue viability by means of 23Na and 35Cl MRI
  • the oxygen consumption via 17O inhalation during an MR examination.
Feasibility studies in brain tumors have often been performed first in animal models or in single patients or small cohorts (e.g., [7-10]), followed by studies in 10-30 tumor patients, investigating different tumor grades or assessing the response to therapies (e.g., [11-14]). Large patient studies confirming clinical efficacy are still missing, however.

Conclusion

X-nuclei MR applications offer a wide variety of applications in science and translational research. In this lecture, an overview of the MR properties and possible applications of these nuclei in brain tumors will be given.

Acknowledgements

No acknowledgement found.

References

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Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)