Synopsis

Marker tracking with phase-only cross correlation (POCC) is an efficient technique to localize instruments during percutaneous MR-guided interventions in closed-bore MR systems. Unfortunately, POCC is time-consuming, as additional marker images need to be continuously acquired. Here, we present a drastically accelerated POCC sequence that uses radially undersampled images to detect the passive marker. With a modified POCC algorithm this sequence can detect marker movements within less than 108ms thus providing a very fast position feedback. The precision of the POCC sequence was evaluated in phantom measurements, and a possible in vivo targeting scenario was demonstrated in a volunteer.

Introduction

In MR-guided percutaneous biopsies, passive needle guides with markers are used for instrument visualization. These markers can be detected automatically and reliably with a phase-only cross-correlation (POCC) algorithm^1-3. With the POCC technique the marker is imaged in two tracking slices orthogonal to the marker’s symmetry axis and, subsequently, the position information is used to automatically align a targeting slice. This way, the sequence follows the marker and visualizes the theoretical needle trajectory so that it can be used for targeting of the needle pathway^4,5.

POCC tracking images do not contain clinically relevant information, but their acquisition is time-consuming which reduces the achievable temporal resolution. We have shown recently that the marker can be detected much faster in highly undersampled radial data⁶. Here, we demonstrate the integration of undersampled radial imaging into the tracking sequence to substantially reduce the duration of one tracking/targeting cycle to be able to visualize moving structures during the targeting process. The targeting accuracy and duration of the modified tracking sequence was evaluated in phantom measurements during needle insertions. Additionally, the sequence was tested in vivo in a volunteer mimicking a liver biopsy scenario.

Methods

The radial POCC tracking sequence automatically detects the position of a cylindrical marker with a central opening for needle insertion that is filled with a contrast agent. The two parallel radial tracking images (T1-weighted FLASH) are oriented perpendicular to the marker’s symmetry axis and acquired with an adjustable number of radial projections N_p. The images are reconstructed online with a Kaiser-Bessel regridding algorithm with a subsequent Fourier-Transform. In these images, the ring-like cross section of the marker is automatically detected with a modified POCC algorithm – the streaking artifacts in the highly undersampled images are accounted for by incorporating the radial point spread function of the marker⁶. The position information is then used to align a subsequent targeting image (bSSFP) in parallel to the marker axis, i.e. the theoretical needle trajectory. The acquisition of tracking and targeting images runs continuously to follow movements of the marker in real time (Fig. 1).

The tracking sequence with radially undersampled tracking images was implemented on a 1.5T whole body MR system (MAGNETOM Symphony, Siemens Healthcare, Erlangen, Germany). To assess the lateral needle targeting accuracy and duration, insertions were performed in a phantom with 15 embedded fiducial targets using the system’s loop coil for signal reception. The marker was manipulated with an in-bore assistance system (GantryMate, Interventional Systems, Kitzbühel, Austria) (cf. Fig. 2a) [7]. For each targeting and insertion maneuver, the marker was initially positioned in a neutral position. Then, the needle trajectory was aligned with the target under real time image guidance (TA_Cycle=0.5s) with the radial POCC tracking sequence with only N_p=15 projections. The other imaging parameters were: TR/TE=3.58/1.68ms, BW=965Hz/px, SL_bSSFP=6mm, SL_FLASH=10mm, FOV=300×243mm^², matrix = 156x192, PF=6/8 resulting in an acquisition time for the 2 tracking slices of 2 x 53.7ms. After alignment, a 16G needle (Somatex GmbH, Teltow, Germany) was inserted and the distance of the needle from the target center was determined in reformatted multi-planar views of a 3D bSSFP data set (resolution: (0.67mm)³, Fig. 2b).

To demonstrate the advantage of the accelerated marker tracking, the POCC sequence was tested in a healthy volunteer. To simulate a liver biopsy the volunteer was placed in supine position, with the assistance system on the liver region (Fig.3a). A cross section of a hepatic blood vessel was defined as a target (Fig.3b), and the POCC sequence was used to track the marker during the respiratory motion of the liver. During a breathhold the theoretical needle trajectory (green) was then aligned with the target (Fig. 4).

Result

Discussion & Conclusion

Acknowledgements

References

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[7] Krafft AJ, et al. An MR-compatible Assistance System for MR-guided Needle Interventions: Initial Phantom Evaluation. Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)