Quantitative MRI/PET bimodal pharmacokinetic modeling to improve diagnostic accuracy in medical imaging

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In the clinic, various imaging technologies are used to look inside patients and find out “what’s wrong”. PET/MRI is a newly available technology which combines two established imaging technologies, Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET). PET/MRI scanners are making an entry into Canadian hospitals, and several studies have presented some of the advantages of PET/MRI over separate PET and MRI scanners. However, PET/MRI scanners are very expensive, and to make sure Canadians get a high return from this investment, we must explore the full potential of this technology. Our research focuses on molecular imaging, a specific branch of imaging that quantifies physiological processes and characterizes tissues in very fine detail. To do this, we inject molecular “probes” that can be monitored during imaging. For example, PET can be used to detect the sugar consumption of cells using a traceable sugar, and MRI can be used to detect blood flow to a tumor using a contrast generating probe. This kind of information can be used in the clinic to plan and monitor anti-cancer therapy, and diagnose pathologies. However, molecular imaging is not easy. A difficulty is that distribution of an injected probe inside the body depends on both its intended target (what we are looking for) and any other interaction it has with the body. Imagine a building with many rooms where patients require treatment. Injecting a probe is like sending nurses and physicians inside whose goal is to attend to these patients. Imaging is like watching through the windows. How can an observer know whether a physician is inside a room because he/she has found a patient, or because he/she is looking for one? If a physician stops moving, he/she might simply be stuck in a room because the room is very busy. He/She might also be discussing with someone healthy. Molecular imaging is similar: We inject a probe to detect a tumor, and we detect the probe somewhere in the body. Yet, we can never be certain if the probe has indeed found its intended target, or if it is there simply because it is exploring the body “searching” for its target. The solution to this problem is fairly simple: We send another team, whose only goal is to walk inside the building and explore the rooms without stopping to attend to patients. At the end, we compare the location of both teams, and the difference indicates where the patients are. This is where PET/MRI comes in handy: we have two observers (imaging technologies) capable of detecting one probe each. The first probe is targeted (e.g., toward a tumor), and the second one is untargeted, and indicates where the first probe would be if it was just “exploring” the body. The difference between the two indicates where the target is located. As of now, the field of molecular imaging in the clinic is limited because of the problem described here. Our work will remove this limitation. Using PET/MRI, we will develop methods to monitor two probes. We will, among other things, be able to make the difference between untargeted (unspecific) and targeted (specific) signal in images, which will enable us to characterize tissues more accurately, and provide real, solid and quantitative information at the molecular level that clinicians can use to diagnose and monitor diseases.