Evaluation of an active magnetic resonance tracking system for interstitial brachytherapy

Abstract

Purpose: In gynecologic cancers, magnetic resonance (MR) imaging is the modality of choice for visualizing tumors and their surroundings because of superior soft-tissue contrast. Real-time MR guidance of catheter placement in interstitial brachytherapy facilitates target coverage, and would be further improved by providing intraprocedural estimates of dosimetric coverage. A major obstacle to intraprocedural dosimetry is the time needed for catheter trajectory reconstruction. Herein the authors evaluate an active MR tracking (MRTR) system which provides rapid catheter tip localization and trajectory reconstruction. The authors assess the reliability and spatial accuracy of the MRTR system in comparison to standard catheter digitization using magnetic resonance imaging (MRI) and CT.

Methods: The MRTR system includes a stylet with microcoils mounted on its shaft, which can be inserted into brachytherapy catheters and tracked by a dedicated MRTR sequence. Catheter tip localization errors of the MRTR system and their dependence on catheter locations and orientation inside the MR scanner were quantified with a water phantom. The distances between the tracked tip positions of the MRTR stylet and the predefined ground-truth tip positions were calculated for measurements performed at seven locations and with nine orientations. To evaluate catheter trajectory reconstruction, fifteen brachytherapy catheters were placed into a gel phantom with an embedded catheter fixation framework, with parallel or crossed paths. The MRTR stylet was then inserted sequentially into each catheter. During the removal of the MRTR stylet from within each catheter, a MRTR measurement was performed at 40 Hz to acquire the instantaneous stylet tip position, resulting in a series of three-dimensional (3D) positions along the catheter's trajectory. A 3D polynomial curve was fit to the tracked positions for each catheter, and equally spaced dwell points were then generated along the curve. High-resolution 3D MRI of the phantom was performed followed by catheter digitization based on the catheter's imaging artifacts. The catheter trajectory error was characterized in terms of the mean distance between corresponding dwell points in MRTR-generated catheter trajectory and MRI-based catheter digitization. The MRTR-based catheter trajectory reconstruction process was also performed on three gynecologic cancer patients, and then compared with catheter digitization based on MRI and CT.

Results: The catheter tip localization error increased as the MRTR stylet moved further off-center and as the stylet's orientation deviated from the main magnetic field direction. Fifteen catheters' trajectories were reconstructed by MRTR. Compared with MRI-based digitization, the mean 3D error of MRTR-generated trajectories was 1.5 ± 0.5 mm with an in-plane error of 0.7 ± 0.2 mm and a tip error of 1.7 ± 0.5 mm. MRTR resolved ambiguity in catheter assignment due to crossed catheter paths, which is a common problem in image-based catheter digitization. In the patient studies, the MRTR-generated catheter trajectory was consistent with digitization based on both MRI and CT.

Conclusions: The MRTR system provides accurate catheter tip localization and trajectory reconstruction in the MR environment. Relative to the image-based methods, it improves the speed, safety, and reliability of the catheter trajectory reconstruction in interstitial brachytherapy. MRTR may enable in-procedural dosimetric evaluation of implant target coverage.

FIG. 1.

(a) Photograph of actively tracking brachytherapy catheter, composed of an active MRTR metallic stylet, enclosed by a plastic catheter; the inset figure shows the enlargement (blue dashed box at the tip of the catheter) of the distal portion of the active MRTR stylet. Two flexible printed circuit tracking coils (yellow arrows) were mounted onto the surface of the stylet. The black box at the proximal end was connected to the MR receiver box during MR tracking. (b) Cylindrical water phantom with a catheter fixed through its central axis for catheter tip localization measurements. A template block containing grids of embedded MR visible markers was placed underneath to serve as a coordinate map. (c) Cubic phantom with fifteen catheters inserted with conventional stylets for catheter trajectory reconstruction measurements (agar gel was filled in the phantom for the experiment but not shown in the figure). Thirteen stylets were placed parallel to each other, while one pair crossed each other.

FIG. 2.

Mean catheter tip localization error as function of (a) off-center distance along x-axis, (b) off-center distance along z-axis, (c) angle from the z-axis in the x–z plane, and (d) angle from the z-axis in the y–z plane. Two-side error bar represents the standard deviation of the distance error from 100 measurements at each location or orientation.

FIG. 3.

Fifteen catheter trajectories were acquired by continuously capturing the instantaneous stylet tip positions during active stylet pull-out. The inset figure shows an axial view of the phantom as seen in the brachytherapy treatment planning system with catheter numbering scheme overlaid. Nine catheters (#7–#15) were inserted in a 3 × 3 grid with a 4 mm spacing at the center, four catheters (#1–#4) close to the four corners of the phantom (with 10-, 15-, 20-, and 25-mm distance to the center, respectively), and a pair of catheters (#5 and #6) with crossed trajectories at approximately 10 mm from the center of template. In all cases, the active tracking system generated stable tracking signal profiles and all the catheter trajectories were resolved without ambiguity.

FIG. 4.

Left: average tip errors for the catheter trajectory reconstruction of the fifteen catheters. Right: average in-plane errors for the catheter trajectory reconstruction of the fifteen catheters. |d_x|, |d_y|, and |d_z| are one-dimensional errors along the x, y, and z axes, respectively. d_3D is the 3D distance error and d_2D is the in-plane distance error.

FIG. 5.

Catheter trajectory reconstruction on the treatment planning system (Oncentra, Elekta Bracytherapy, Stockholm, Sweden), using MRTR in a patient with recurrent endometrial cancer. [(a)–(c)] Screenshot of one MRTR-generated catheter trajectory (red dots) overlaid on the three orthogonal T2-weighted TSE MR images. Dose distribution is shown as isodose surfaces (solid lines). MRI-based contouring of the tumor and OARs is shown with dashed lines (red: tumor; yellow: bladder; blue: sigmoid; brown: rectum; green: bowel). (d) 3D view of catheter trajectories and geometric volumes representing the tumor and the OARs [organ color code is the same as in (a)–(c)]. The catheter trajectory acquired by MRTR is shown as a highlighted blue cylinder [same trajectory shown as red dots in (a)]. All the other catheters’ trajectories are from MRI-based digitization, shown in light blue color. [(e)–(g)] The MRTR generated catheter trajectory (red dot) overlaid on three orthogonal CT images, which was very consistent with the catheter location (bright signal) shown on CT. (h) 3D view of one catheter trajectory digitized from the CT images (light blue cylinder) and the same catheter trajectory extracted from MRTR (red dots).