Outer volume suppression in steady state sequences (OVSuSS) for percutaneous interventions

Abstract

Magnetic resonance-guided percutaneous interventions with needles require fast pulse sequences with acquisition times less than 1 s to image the needle trajectory within moving organs. To guide the movement of a rigid instrument with high sampling rate, an magnetic resonance imaging method was developed that reduces the acquisition time down to a few hundred milliseconds by restricting the field of view to a small stripe around the instrument trajectory. To maintain the dynamic steady state, saturation pulses for outer volume suppression were inserted into additional repetition time-intervals. These saturation intervals were combined with three sequence variants: a spoiled gradient echo sequence, an echo-shifted steady state free precession and a balanced steady state free precession sequence. The magnetization dynamics were analyzed by means of numerical optimized simulations. Results were compared with phantom experiments and an average signal-to-suppression-ratio of 15.5 could be achieved. With a field of view reduction of up to 12.5% an update rate of six images per second could be achieved. Finally, animal experiments demonstrated the fast and reliable needle tip visualization during percutaneous magnetic resonance-guided interventions with the help of a robotic assistance system.