The objective of this thesis was to investigate metabolic changes occurring in rat lungs during RP. As metabolic changes precede morphological and functional changes, assessment of metabolic profile could help in early diagnosis of RILI. Hyperpolarized 13C-MRI with its
ability to polarize variety of 13C-substrates was used to quantify metabolism during RP. 13C- substrates of 13C-pyruvate and 13C-bicarbonate were used to study enzyme-catalyzed reactions. We hypothesize that hyperpolarized 13C-MRI can be used to detect metabolic changes in early RP which correlate with the known histological transformations. The sub- objectives established for this purpose are included as part of the manuscripts and Appendices that follow.
This chapter has described the motivation for the study of unintended lung injury caused by therapeutic radiation of the lung cancer. An overview of lung cancer, radiation therapy and the pathophysiology of RILI have been described. Clinical diagnosis and emerging imaging modalities for detection of RILI have also been discussed. MRI physics of pulse sequences used for the research in this thesis such as Chemical Shift Imaging, gradient echo imaging and the IDEAL method were explained. Introduction to the DNP process used for polarization of 13C-substrate and the use of 13C-pyruvate and 13C-bicarbonate to probe particular metabolic pathways was described in a biological context.
Chapter 2 describes the validation of detection of metabolic signature in early RP using hyperpolarized 13C-pyruvate. Six rats were irradiated in the entire thorax region with a cobalt-60 source (dose of 14 Gy, 1 fraction) and imaged two weeks later along with healthy age-matched controls. Results were analyzed for change in lac/pyr in the irradiated cohort compared to the healthy cohort and histology was used as a confirmation for the presence of injury. Kundan Thind was responsible for construction of a 13C-birdcage coil and a 1H-
experimental data acquisition and analysis. Dr. Alexei Ouriadov provided assistance in optimization of the RF coils and the pulse sequences. Dr. Eugene Wong and Dr. Matthew Fox assisted with the irradiation procedure. Dr. Lanette Friesen-Waldner assisted with the animal preparation and in polarization of the 13C-pyruvate. Dr. Albert Chen was pivotal in providing numerous discussions and the technical guidance for the study. Dr. Eugene Wong and Dr. Jake Van Dyk provided meaningful discussion about the work. Dr. Giles Santyr provided overall guidance for the project and assistance with the manuscript presentation. This work has been published in Magnetic Resonance in Medicine. (DOI: 10.1002/mrm.24525)
Chapter 3 expands on the findings in Chapter 2. The objective for this Chapter was to capture metabolic profile in early RP in a regional and longitudinal manner and correlate the changes to histology. To capture regional metabolic changes, MRI hardware was improved by construction of a TORO coil system that enabled an increase in SNR of 3-4x over the transmit-receive RF coil configuration. This has been described in Appendix A-1. Twelve rats were irradiated in a conformal manner to the right lung using a micro-CT system. 13C- data was collected at day 5, 10, 15 and 25 post irradiation from three irradiated and three healthy age-matched control animals. Macrophage count from the histology of rat lungs was obtained and correlated with changes in lac/pyr. Kundan Thind was responsible for construction of the 13C-TORO coil system (Appendix A-1) in addition to the experimental design, experimental data acquisition and analysis. Michael Jensen assisted with the irradiation procedure using micro-CT. Dr. Francisco Martinez and Dr. Tim Scholl assisted in the polarization of 13C-pyruvate for the experiments. Elaine Hegarty performed the animal
preparation pre-MRI and obtained histology post-MRI. Dr. Albert Chen was pivotal in providing numerous discussions and the technical guidance for the study. Dr. Eugene Wong and Dr. Jake Van Dyk provided meaningful discussion about the work. Dr. Giles Santyr provided overall guidance for the project and assistance with the manuscript presentation. This work has been submitted to the journal of Radiotherapy and Oncology. (August 2013: RO-D-13-00907)
Chapter 4 describes a novel method to map pH using hyperpolarized 13C-bicarbonate with the IDEAL pulse sequence. Maintenance of pH balance is one of the most essential homeostatic processes in vivo, disruption of which could lead to severe adverse affects. The
disruption could be caused by onset of cancer, metabolic and respiratory acidosis and alkalosis. In particular, onset of severe RILI could lead to hypoxic tissue environment that would result in a build-up of lactic acid. Accumulation of lactic acid in the tissue results in an acidic tissue environment. Non-invasive pH mapping could potentially help with diagnosis of RILI and other diseases that result in pH imbalance. The IDEAL pulse sequence was used to map 13C-bicarbonate and its downstream metabolite of 13C-carbon dioxide. These form an acid-base pair and the ratio of 13C-bicarbonate to 13C-carbon dioxide aids in the construction of the pH map. Kundan Thind was responsible for construction of the phantoms, experimental design, experimental data acquisition and analysis. Dr. Francisco Martinez and Dr. Tim Scholl assisted in the polarization of 13C-bicarbonate for the experiments. Curtis Weins provided assistance with the IDEAL pulse sequence. Dr. Albert Chen was pivotal in providing numerous discussions and the technical guidance for the study. Dr. Giles Santyr provided overall guidance for the project and assistance with the manuscript presentation. This work will be submitted to the journal of Magnetic Resonance in Medicine.
Chapter 5 discusses the results from the three preceding manuscripts and their contribution towards metabolic imaging in the lung for early diagnosis of RP. The discussion includes the merits of early assessment, shortcomings of the studies and future work.
The Appendix A-1 provides additional detail on the design and construction of the TORO coil system and appendix A-2 provides details on statistical tests used in the thesis.