Title:
Single shot MR tagging to quantify local tissue deformation during MRI-guided needle interventions: a feasibility study.

Abstract:
In MRI-guided needle interventions, such as biopsies and brachytherapy, tissue deformation caused by needle movement may result in localization errors and thus hamper the outcome of the procedure. Monitoring the local tissue deformation provides the ability to compensate for it, e.g., by increasing the needle insertion depth. Fast MR scans are useful to track the needle, but cannot be used to quantify local tissue deformation, in case anatomical landmarks are missing. Artificial landmarks can be created by MR tagging. This method provides a spatial saturation pattern (tag) in the tissue. Deformation of this pattern reflects the tissue motion between tag creation and tag imaging. As the needle movement is nonperiodic, k-space cannot be acquired with a multishot approach, like is usually done for cardiac imaging. Hence, a single shot MR tagging sequence is needed, which entails tag creation, needle movement and tag acquisition. In this study, the feasibility of single shot MR tagging for MRI-guided needle interventions in phantom and volunteer experiments is shown.
Four different experiments were performed on a 1.5 T MR scanner: the first to quantify translations, the second to quantify rotations, the third to mimic a needle intervention, and the fourth to investigate the tag persistence in a volunteer. The tag pattern is created by a 1331 composite pulse. A balanced steady state free precession sequence is used for imaging. To minimize undesired changes in contrast or sharpness of the tag pattern, we chose a relatively small flip angle and a short imaging time in all experiments. In the volunteer experiments, we modified the sequence to also be able to inspect the influence of the used k-space sampling profile and the flip angle on the temporal persistence of image contrast and tag pattern. In all scans, head or surface coils were used for signal reception.
In all experiments, the tag pattern was clearly visible and could be used to quantify the local tissue deformation caused by (needle) movement. Strong correlations between the actual and measured (angular) phantom motions were obtained. In the needle intervention experiment, the tag lines were perfectly horizontal when there was no needle movement. With needle movement, local tissue displacements up to 5 mm were observed. Volunteer's anatomy could be discriminated, despite the tag pattern. The tag pattern in the prostate, for example, could still be read in all tagging images acquired 2 s after creating the tag pattern. With optimized scan parameters the tag persistence was even longer. The best image tag contrast was obtained using a large flip angle and the profile order low-high, although the image was slightly blurred.
This study demonstrates that single shot MR tagging can be used to quantify tissue deformation caused by needle movement. The in-vivo tag persistence is sufficient to enable the application of the tagging sequence during MRI-guided needle interventions in patients.