Prospective Clinical Implementation of a Novel Magnetic Resonance Tracking Device for Real-Time Brachytherapy Catheter Positioning

Abstract

Purpose: We designed and built dedicated active magnetic resonance (MR)-tracked (MRTR) stylets. We explored the role of MRTR in a prospective clinical trial.

Methods and materials: Eleven gynecologic cancer patients underwent MRTR to rapidly optimize interstitial catheter placement. MRTR catheter tip location and orientation were computed and overlaid on images displayed on in-room monitors at rates of 6 to 16 frames per second. Three modes of actively tracked navigation were analyzed: coarse navigation to the approximate region around the tumor; fine-tuning, bringing the stylets to the desired location; and pullback, with MRTR stylets rapidly withdrawn from within the catheters, providing catheter trajectories for radiation treatment planning (RTP). Catheters with conventional stylets were inserted, forming baseline locations. MRTR stylets were substituted, and catheter navigation was performed by a clinician working inside the MRI bore, using monitor feedback.

Results: Coarse navigation allowed repositioning of the MRTR catheters tips by 16 mm (mean), relative to baseline, in 14 ± 5 s/catheter (mean ± standard deviation [SD]). The fine-tuning mode repositioned the catheter tips by a further 12 mm, in 24 ± 17 s/catheter. Pullback mode provided catheter trajectories with RTP point resolution of ∼1.5 mm, in 1 to 9 s/catheter.

Conclusions: MRTR-based navigation resulted in rapid and optimal placement of interstitial brachytherapy catheters. Catheters were repositioned compared with the initial insertion without tracking. In pullback mode, catheter trajectories matched computed tomographic precision, enabling their use for RTP.

Figure 1. MRTR-guided HDR gynecologic-cancer brachytherapy setup

(A) Physician placing the stylets inside the MRI during the procedure. (B) Actively tracked stylet. Inset: Active tracking micro-coils in the stylet’s tip. (C) Dedicated 8-channel MRI preamplifier unit. (D) Patient isolation unit, placed between the MRTR stylet and the MRI preamplifier unit, which prevents electrical currents, if they propagate on the electrical lines, from reaching the patient’s body.

Figure 2

Axial (A), sagittal (B) and coronal (C) MRI images from a case where two catheters were pulled back with real-time MR monitoring during the catheter adjustment. The images are overlaid with clinician-segmented areas of CTV (Orange), sigmoid (Blue), bladder (Yellow), vagina (Pink) and rectum (Brown). The location of each MRTR-tracked catheter over time, during manipulation by the clinician, was superimposed using green and red dots. Cross and star signs mark the starting and ending points of the motion, respectively. Continuous trajectories of the green and red catheters are displayed in panels D and E, respectively.

Figure 3

Axial (A), sagittal (B) and coronal (C) MRI images from a case where a catheter was pulled-back and then re-inserted, obtaining an improved location. The images are overlaid with clinician-segmented areas of CTV (Orange), bladder (Yellow), vagina (Pink) and rectum (Brown). The location of each MRTR-tracked catheter over time, during manipulation by the clinician, was superimposed using red dots. Cross and star signs mark the starting and ending points of the motion, respectively. A continuous trajectory of the catheter is displayed in panel D.

Figure 4

Axial (A, D), sagittal (B, E) and coronal (C, F) MRI images from a case where four catheters were repositioned. The images are overlaid with clinician-segmented areas of CTV (Orange), bladder (Yellow), vagina (Pink) and rectum (Brown). The tracking data is displayed on duplicate MRI images to facilitate easier visualization. Each tracked catheter was superimposed using yellow, red, green and blue dots. Cross and star signs mark the starting and ending point of the motion, respectively. Continuous trajectories of the green, blue, yellow and red catheters are displayed in panels G, H, I and J, respectively. Although motion was smooth, some trajectories may appear jagged due to noise.