Medical Imaging: Using magnetic nanoparticles and negative contrast for quantitative perfusion imaging of the heart
Real-time visualization of blood flow in vessels (angiography) or organs such as the liver or brain is crucial for the diagnosis of various diseases. Currently, this is achieved through the use of x-ray based computed tomography (CT) or magnetic resonance imaging (MRI). The former poses a risk of direct exposure to harmful radiation while MRI suffers from inadequate temporal resolution and high financial costs. However, a new tracer-based imaging modality called Magnetic Particle Imaging (MPI) has recently emerged, offering high sensitivity and temporal resolution using electromagnetic fields in the low kHz region. MPI is still in the pre-clinical stages of development, and ongoing research projects are assessing its feasibility and potential in the future.
Researchers from the Institute for Biomedical Imaging (E-5) have investigated in an experimental flow phantom study, how the amount of applied tracer can be minimized while retaining images of high diagnostic value. Their study was presented at the International Workshop on Magnetic Particle imaging (IWMPI) and describes a modified perfusion imaging technique that is used to visualize the time to peak (TTP), mean transit time (MTT), blood flow (BF) and blood volume (BV) in a 3D printed phantom of a rat-sized heart. The research work is based on vascular phantoms that were designed and tested within the bachelor’s and master’s thesis of Miriam Exner, a former master’s student of Biomedical Engineering at TUHH. By means of 3D printed phantoms, many insights are gained at low effort without handling or sacrificing living animals. The research project was carried out in close cooperation with the Fraunhofer Research Institution for Individualized and Cell-Based Medical Engineering (IMTE) and is funded by the German Research Foundation (DFG, grant number KN 1108/7-1 and GR 5287/2-1).
Currently, MPI perfusion imaging relies on the use of a contrast agent (tracer) consisting of magnetic nanoparticles, which is administered as a bolus. The distribution and intensity of this bolus over time are measured to calculate the perfusion parameters mentioned earlier. However, in scenarios that require frequent imaging, such as stroke surveillance, the accumulation of tracer material in the bloodstream, liver and spleen may pose a yet unknown risk. Our proof-of-concept study demonstrates the potential to extend the total monitoring time and reduce the amount of tracer required for future surveillance scenarios. The idea of our work is to use the circulating tracer concentration after one or several positive boli and apply a negative bolus of neutral saline solution to displace the circulating tracer. By measuring the inverted dynamics, perfusion maps can be successfully derived and subtraction images calculated (as shown in the Figure on the right) for the negative bolus. Here, the steady state concentration is subtracted and only changes in the concentration are revealed. Processing of such subtraction images is done in real-time to monitor the bolus passage (e.g. in stent imaging). Our study involved a comparison between positive and negative boluses, from which we were able to generate perfusion maps of equal quality for both techniques. The proposed method allows for extended monitoring periods of future patients while maintaining a constant total iron dose, thereby potentially facilitating the application of MPI perfusion imaging in the near future.
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