Evaluation of Cardiovascular and Pulmonary Functions using MRI

Ludovic de Rochefort
Cornell University

There is a great interest in understanding and evaluating the cardiovascular and pulmonary systems. While performing their transport functions, these systems are subject to various physiological and physiopathological stresses induced by flow such as pressure, deformation, shear stress or particle deposition. Detailed knowledge of the fluid dynamics in biological systems is required to understand cardiovascular and pulmonary function as well as dysfunction resulting from diseases. Magnetic Resonance Imaging (MRI) offers a variety of non-invasive tools for anatomic and functional assessments. From the basic principle of measuring nuclear magnetism through radio-frequency field excitation and by varying the receive frequency with spatially varying magnetic fields, spatial encoding became possible (1,2). While imaging of hydrogen atom at thermal polarization has seen a tremendous number of applications (3), recent advances in hyperpolarization techniques have allowed imaging gases (4) and the development of new lung imaging applications (5). In MRI, the time needed to obtained spatial information is large (6) necessitating the development of techniques that deal with respiratory and cardiac motion to accurately measure moving or otherwise changing anatomy. Improving fast and dynamic imaging is a topic of major ongoing research (7,8). For cardiovascular imaging, a breakthrough also occurred with the use of contrast agents that drastically enhance detection by modifying the nearby blood signal (9). They are now commonly used for angiography and organ perfusion imaging. Once again, fast imaging is needed to measure uptake and quantification to extract circulation parameters remains an issue. In addition to faster imaging with increased signal to noise, quantitative motion characterization is possible. Velocity mapping techniques based on the direct measurement by means of motion-encoded gradients (10) is used clinically today to characterize blood flow issues. Phase contrast MRI allows full in vivo flow characterization in the cardiovascular network, and recent applications to gas flow (11,12) open a way to the experimental study of gas dynamics in the lungs.
In this talk, the MRI methods to characterize motion in the human body will be presented. The use of these principles will be illustrated by applications ranging from 1) fast and dynamic imaging for cardiac motion characterization and gas dynamics in the lungs, through 2) the use of contrast agents for blood and tissue signal enhancement and for quantification techniques allowing the extraction of circulation parameters, to finally 3) direct motion encoding using phase-contrast methods to map blood and gas velocities.

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