Reduced k-space acquisition to accelerate MR imaging of moving interventional instruments: a phantom study.

PURPOSE

The goal of this study was to investigate the impact of reduced k-space sampling rates on the visualization of a moving MR-compatible puncture needle and to demonstrate the feasibility of keyhole imaging in interventional magnetic resonance imaging (MRI).

MATERIAL AND METHODS

All experiments were performed in an open 1.0 Tesla MRI. MR images of a moving puncture needle were taken with different keyhole sampling rates from 15-100%, in 10% increments. The needle was submerged in a water-filled basin and was imaged in motion with a T1-weighted gradient-echo sequence with an initial acquisition rate of 1.4 s per image. An apparatus operated by a compressor unit enabled needle rotation and ensured reproducible needle movements. The median forward velocity of the needle tip was 2 cm/s. To evaluate the depiction of the needle, artifact diameter of the needle, contrast-to-noise ratio (CNR), and needle tip profiles (delineation) were measured.

RESULTS

The needle position was determined with an longitudinal error of 3 mm and a transverse error of 0.8 mm with respect to the needle's orientation and the theoretically calculated trajectory. No significant correlation was found between the CNR and velocity. A reduction of k-space update rates caused neither a significant reduction of CNR nor a significant increase in artifact diameter or blurring of the needle profile.

CONCLUSION

The application of keyhole imaging with update rates of greater than 15% is sufficient for the MR guidance of interventions with an signal-to-noise ratio >9 of the surrounding tissue and a target accuracy of >1 mm. Keyhole imaging can increase temporal resolution while ensuring unimpaired spatial resolution and image quality of the depicted instrument.