Investigations of signal-enhanced magnetic resonance contrast agents
by Sonja Katharina Sternkopf
Date of Examination:2023-12-15
Date of issue:2024-08-15
Advisor:Dr. Stefan Glöggler
Referee:Dr. Stefan Glöggler
Persistent Address:
http://dx.doi.org/10.53846/goediss-10668
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Abstract
English
Nuclear magnetic resonance spectroscopy (NMR spectroscopy) and its related technique, magnetic resonance imaging (MRI), enable scientists and doctors to make statements about the atomic structure of a sample and to produce anatomical images of the human body. The effect of magnetic resonance is used to manipulate magnetically active atomic nuclei. By applying an external, constant magnetic field in the NMR spectrometer or the MRI machine, it is possible to align the atomic nuclei either parallel or antiparallel to the external magnetic field, like small magnets. This produces the so-called thermal polarization, which describes the sum of the magnetization of a sample. It is roughly as if all the atomic nuclei, which we imagine as small magnets, had formed a large magnet together. The thermal polarization thus describes the strength of this entire magnet. With additional, short radio frequency (RF) pulses, it is possible to trigger the magnetic resonance and set the thermal polarization in motion. This is often done on hydrogen nuclei (1H), but investigations on so-called heteronuclei such as 13C are also possible. However, a major challenge of NMR and MRI is their sensitivity. Due to the aforementioned parallel or antiparallel alignment of the atomic nuclei, the magnetization vectors of a large part of the atomic nuclei cancel each other out. Only a fraction actually contributes to the detectable signal. This leads to the fact that in MRI, the signals of hydrogen nuclei in water and fat are detected for the most part, since they occur in high concentrations in the body. By developing special measurement programmes, the RF pulse sequences, it is nevertheless possible to produce anatomically high-resolution images. However, for special applications, such as the improved imaging of cancer, contrast agents are required. In this dissertation, contrast agents were investigated that achieve NMR or MRI signal enhancement with the help of hyperpolarization. Hyperpolarization refers to a state of increased magnetization of a sample compared to its thermal polarization. One of the methods of producing hyperpolarization is para-hydrogen induced polarization (PHIP). Here, para-hydrogen is used, an isomer of hydrogen, and its polarization is transferred to another target nucelus with pulse sequences. In the case of PHIP side-arm hydrogenation (PHIP-SAH), a modified molecule is used for this, which has a so-called side arm to which para-hydrogen is chemically added. After the pulse sequence and polarization transfer, this side arm is split off and the target molecule is present in its native form, with a strongly increased signal for NMR or MRI. One target molecule of particular interest is pyruvate, which plays a central role in human metabolism. In many types of cancer, for example, the metabolic conversion is greatly increased and altered compared to healthy tissue. By applying hyperpolarized pyruvate, it is then possible to better visualize cancer during an MRI examination, as well as to make additional statements about its metabolic activity. This could also be used for early detection or to assess the success of therapy. Other areas of application are neurodegeneration and cardiovascular diseases. There are already initial findings from clinical studies in humans that demonstrate the potential of hyperpolarized pyruvate. However, the pyruvate for these studies was generated using a different technique, Dynamic Nuclear Polarization (DNP). PHIP-SAH has several advantages over this technique. For example, the production of one dose of hyperpolarized pyruvate can be done much quicker, and with simpler and cheaper equipment. However, PHIP-SAH was developed later and is therefore not yet in clinical trials. This thesis demonstrates the process of biological application of hyperpolarized pyruvate using PHIP-SAH. First, the method was optimized so that physiologically compatible application solutions could be produced. These were then examined in a first application in cell culture samples (in vitro). Here, HEK298T cells (human embryonic kidney cells) were used, which contain a protein that occurs in neurodegenerative diseases in larger aggregates, either strongly increased or not at all. In addition, the cells were treated with a metabolic inhibitor. After examining these cells with hyperpolarized pyruvate, we were able to show the differences in the metabolic conversion of pyruvate of the different cell groups. This served as a feasibility study that we are able to visualize metabolic differences with PHIP-SAH pyruvate. In a second project, the hyperpolarized pyruvate was now tested in vivo. For this purpose, three mice each were examined that had human-derived pancreatic or colon tumors (xenografts). Pancreatic cancer is considered one of the most aggressive cancers and has a poor prognosis because it is often diagnosed at an advanced stage. Colorectal cancer has a higher survival rate, but due to its frequency it still causes a high number of deaths. For both types of cancer, therefore, an additional examination technique would be helpful to assist in early detection and therapy monitoring. After injection of hyperpolarized pyruvate, it has been possible to perform time- resolved, localized 13C spectroscopy and thus to visualize the pyruvate metabolism of the tumors. The metabolic conversion of pyruvate to its metabolites lactate and alanine was significantly higher in the pancreatic tumors than in the colon tumors. This made it possible to differentiate between these two types of cancer. Our study represents the first investigation of pancreatic and colorectal cancer in vivo with PHIP-SAH pyruvate and serves as a starting point for further research. In future, studies will be carried out with a larger number of mice and pulse sequences that allow better resolution and localization of the 13C signal will be used. In a third project, the method transfer from the previous preclinical experimental setup to a clinically used MRI device was started. The challenges here lie in the technical implementation. Hyperpolarization can now no longer be performed on high-field NMR spectrometers, but must be carried out with portable polarizers. Two different polarizers were tested for hyperpolarization in this project. In addition, different pulse sequences suitable for 13C and hyperpolarized pyruvate were obtained and optimized for this purpose. First experiments were carried out on thermally polarized standard samples, phantoms as well as with a mouse. These developments are the starting point for further studies on mice, which will lead to studies on large animals and finally to clinical application.
Keywords: Hyperpolarization; PHIP; MRI; NMR; Pyruvate; Metabolism
