Update to Pulse Sequences for Interventional MR Imaging

Abstract

The motivations for developing MR-guided minimally invasive therapy include its excellent soft tissue contrast, tomographic imaging in any direction (as opposed to projection imaging as in fluoroscopy), the absence of ionizing radiation,the abundance of contrast mechanisms (including bright blood pulse sequences that lead to excellent vessel conspicuity without exogenous contrast agent injection), the ability to obtain physiologic information such as perfusion, and an overall excellent safety profile. The main pulse sequences used today for interventional MR imaging are T1/T2-weighted FISP and TrueFISP, T2-weighted turbo spin-echo, and T1-weighted FLASH. The specific clinical question, the underlying pathophysiology,and the procedure to be performed dictate which sequence is used. Each of these sequences has been written to acquire data in conventional rectilinear trajectories, radial k-space paths, or even spirals. In many ways, the questions being researched in interventional MR imaging have been dictated by the primary issues in greatest need of resolution or that most directly facilitate new clinical development. A decade ago, research focused on exploration of new scan strategies for contrast and temporal resolution. Advancements in the last decade have made it possible to acquire and display greater than 10 images per second in realtime with millimeter resolution in all three directions. This temporal and spatial resolution is considered high enough to guide most interventions. With this capability, other research has focused on instrument tracking. The field has gone from the capability to track a single coil and superimpose it on a previously acquired roadmap to systems that follow, adapt, and provide high-resolution images due to the advent of multichannel receiver systems, improved graphics, higher processor speeds, and increases in speed and quantity of memory. Hence, instruments can be reliably identified and tracked and the information can be used to update pulse sequence parameters in real time, thereby opening new opportunities for interventional MR imaging that extend from biopsy and thermal therapy to image-guided vascular and cardiac procedures. Today, we see such issues as RF heating of wires used for device localization and the noise generated by rapid switching of MR gradients being significant obstacles yet to overcome to allow the full strength of MR-guided interventions to be realized clinically. It is anticipated that these topics will emerge as critical concepts in the next decade of interventional MR imaging research.