Researchers and clinicians interested in minimally invasive MRI-guided interventions.The success rate of image guided interventions depends on many aspects including temporal and spatial resolution, accuracy and robustness of device visualization and the ability to simultaneously monitor the background anatomy. One way to accomplish this is by using active tracking coils^[1], incorporated in guidewires, catheters or needles^[2], however, this requires dedicated hardware and requires suitable device dimensions to be applicable. Additionally, in clinical practice intraprocedural visualization of the devices by susceptibility based passive tracking is highly desired as it helps to relate the device location to the anatomical background. Center-out Radial Sampling with Off-resonance Reconstruction (Co-RASOR), a robust positive contrast methodology based on the magnetic susceptibility of the marker, was recently shown to accurately and robustly depict small metal devices^[3,4]. To fully exploit the technique for MRI-guided interventions the method should be incorporated in a real-time visualization platform on the scanner. Therefore, the aim of this work was to implement Co-RASOR in the interventional Suite (iSuite, Philips Research Hamburg) software in a flexible way, and test its performance and robustness in phantoms.

Co-RASOR: The original co-RASOR imaging technique was a fully frequency encoded radial center-out 3D ultrashort TE (UTE) acquisition^[3]. By introducing a frequency offset during signal reception^[3] or retrospectively during reconstruction^[4], small paramagnetic objects (guidewires, markers, seeds, needles) can be visualized with positive contrast^[3,4]. To facilitate robust depiction of the device using a color overlay on top of the 3D roadmap, contrast and specificity needs to be maximized and image intensity variations minimized, which is accomplished by background suppression (off-resonance reconstruction minus on-resonance image).

iSuite implementation: Complex data was sent to the iSuite PC and reconstructed in real-time. Simultaneously a second image was reconstructed in the iSuite PC from the same data applying off-resonance. The frequency offset as well as the background suppression factor could be adapted real-time to enable intraprocedural optimization of device visualization and background suppression. This facilitated real-time monitoring of the device using a color overlay on the 3D roadmap.

Experimental setup and workflow: A gelatin phantom consisting of milk, yoghurt and several pieces of fruit was made. An MR compatible titanium puncture needle was used to perform MRI-guided needle targeting of a grape. First a 3D roadmap (3D-T2-TSE) was made and a needle trajectory was planned. The cross-section of two orthogonal planes was set to coincide with the planned needle trajectory. Then, inside the scanner room the finger-pointing technique^[5] was used to find the planned entry point prior to advancing the needle to the target under real-time image guidance using a SensaVue in-room display (in vivo Gainesville, USA).

Imaging parameters: To increase flexibility and temporal resolution crucial for fast multi-planar 2D monitoring of interventions, the hard excitation pulse and center-out sampling were omitted, creating a RASOR technique. This enables fast single slice radial scanning using the following imaging parameters: TE/TR=1.5/3.5ms, slice thickness=6mm, FOV=280x280mm², spatial resolution of 2x2mm² and frame rate of 2.5 fps.

The implementation of the 2D coRASOR reconstruction method enabled the generation of high positive contrast, allowing for robust visualization of the titanium needle. The possibility to adapt the frequency offset and background suppression factor made the procedure very flexible and visualization robust. The operator outside the MR suite can adapt these parameters real-time, even intra-procedurally. As the optimal off-resonance for reconstruction depends on the angle of the titanium needle with respect to B0, this is a crucial feature of the implementation. The intervention radiologist within the MRI suite can change between viewports, angulations and contrast real-time using a conventional foot paddle, which was experienced as very convenient by our intervention radiologists.iSuite allows to visualize the combination of real-time data with pre-acquired roadmaps. For real-time (Co-)RASOR, the processing of the images takes place on the iSuite PC and the processed images are shown directly or as color overlay on the 3D roadmap. The real-time adaption of off-resonance reconstruction parameters makes the overlay view very robust, as the optimal off-resonance depends on the needle orientation with B₀. This setup enables the clinician to combine a high quality roadmap with the preferred contrast (e.g. T₁W, T₂W or diffusion weighted) with real-time device tracking. Furthermore, iSuite facilitates fast switching between real-time contrasts and overlay views which allows to inspect both tissue and device in real-time.The current implementation of the coRASOR imaging technique for passive device visualization facilitates flexible and robust MRI-guided needle tracking by combining high temporal resolution color overlays on top of high spatial resolution 3D roadmaps.