Cardiovascular Interventional Magnetic Resonance Imaging

Abstract

Interventional cardiovascular magnetic resonance imaging (iCMR) refers to catheter-based therapeutic procedures using MRI rather than conventional radiographic guidance. iCMR promises to further blur the distinction between medical and surgical therapeutics by permitting surgical-quality “exposure” in minimally invasive procedures. Catheter-based procedures conducted without x-rays may be useful in avoiding or reducing radiation exposure to children1 and clinical staff,2 in avoiding nephrotoxic radiocontrast,3 and in reducing staff musculoskeletal injury4 from x-ray–protective lead aprons. More important, iCMR is exciting because it should permit an entirely new range of procedures otherwise attainable only with open surgical exposure. iCMR requires “real-time” imaging, which for our purposes means image acquisition and display to clinicians, completely refreshed 1 to 10 frames per second, depending on the application, within a short delay (approximately 250 ms). Clinical investigational procedures have begun at several centers. The chief limitation to clinical translation, at present, appears to be the availability of clinical-grade catheter devices. This brief review will survey interventional cardiovascular imaging, treatment, and patient handling considerations; unique iCMR catheter design requirements; proof-of-concept animal and clinical experiments conducted to date; and novel applications we can expect in the near future. MRI is possible because water protons, ubiquitous in tissue and blood, have magnetic moments (or “spins”) that align in a magnetic field like compass needles. These spins have a characteristic resonance frequency at which they absorb electromagnetic radiation, a frequency that varies with the intensity of the surrounding magnetic field. Exposed to radio waves, spins become energized at these characteristic frequencies and afterward emit radio signals (“relax”), a process that can be detected with sensitive radio hardware. Pictures of tissues inside the magnet bore are created by “encoding” the position of proton spins in space by using small magnetic field changes (gradients). The positions correspond to known emitted …