Real-time active MR-tracking of metallic stylets in MR-guided radiation therapy

Abstract

Purpose To develop an active MR-tracking system to guide placement of metallic devices for radiation therapy. Methods An actively tracked metallic stylet for brachytherapy was constructed by adding printed-circuit micro-coils to a commercial stylet. The coil design was optimized by electromagnetic simulation, and has a radio-frequency lobe pattern extending ~5 mm beyond the strong B0 inhomogeneity region near the metal surface. An MR-tracking sequence with phase-field dithering was used to overcome residual effects of B0 and B1 inhomogeneities caused by the metal, as well as from inductive coupling to surrounding metallic stylets. The tracking system was integrated with a graphical workstation for real-time visualization. 3T MRI catheter-insertion procedures were tested in phantoms and ex-vivo animal tissue, and then performed in three patients during interstitial brachytherapy. Results The tracking system provided high-resolution (0.6 × 0.6 × 0.6 mm3) and rapid (16 to 40 frames per second, with three to one phase-field dithering directions) catheter localization in phantoms, animals, and three gynecologic cancer patients. Conclusion This is the first demonstration of active tracking of the shaft of metallic stylet in MR-guided brachytherapy. It holds the promise of assisting physicians to achieve better targeting and improving outcomes in interstitial brachytherapy.

Evaluation of an active magnetic resonance tracking system for interstitial brachytherapy

“…Moreover, our MRTR sequence has a built-in mechanism to correct for the static magnetic field B 0 and radio-frequency magnetic field B 1 inhomogeneities. 16 Hence, this technique is relatively insensitive to systematic error or patient-related errors. In template-based interstitial brachytherapy where placement of a large amount of catheters is required, MRTR can be especially useful in resolving catheters in the setting of ambiguous imaging artifacts, such as when catheters approach or cross each other, because each catheter is individually connected to an independent receiver channel.…”

“…16 Each active MRTR stylet consists of two radiofrequency (RF) microcoils attached to a custom-machined ProGuide metallic stylet (Elekta Brachytherapy, Stockholm, Sweden) [ Fig. 1(a)].…”

“…[11][12][13][14][15] Recently, an active MR tracking (MRTR) system was developed by our group to provide accurate and rapid real-time localization of interstitial brachytherapy catheters. 16 It integrates multiple (≥2) receive microcoils into the metallic stylets that are used to advance the catheters into tissue. The microcoil positions are tracked with a dedicated MR sequence.…”

Magnetic Resonance–Guided Gynecologic Brachytherapy

“…A third approach is the use of active tracking of needles using smart stylets containing one or more coils relaying their location in the MR scanner frame of reference to an external computer system. This approach was recently described 47 and has the dual advantages of allowing the automatic identification of the coordinates of the needles in the image and increased visibility of the needles. This approach is similar to the recent proposal to use an RF coil to identify brachytherapy needles in ultrasound-and CTbased insertions.…”

Magnetic Resonance Sequences and Rapid Acquisition for MR-Guided Interventions

“…This unique signature can be overlaid in color onto anatomical images in real-time 63 . Alternatively, device tracking uses fast device localization and graphically represents the device position on previously acquired images 64 (Figure 6). Point source coils can be localized rapidly using three orthogonal echoes 65 whereas spatially extensive coils can be localized using three orthogonal projection images (non-slice-selective).…”