Author: Ivan M. Buzurovic, Evangelia Kaza 👨🔬
Affiliation: Department of Radiation Oncology, Dana-Farber/Brigham and Women’s Cancer Center, Harvard Medical School, Brigham and Women's Hospital, Harvard Medial School, Dana-Farber Cancer Institute, Department of Radiation Oncology 🌍
Purpose: Recent advances allow for depiction of surface brachytherapy flap applicators using MRI, with empty catheters detected by signal intensity reduction. As manual catheter detection is subjective and time-consuming, we aimed to develop an algorithm for automatic detection of Freiburg Flap (FF) applicator spheres and catheters on 3D MR images without manual selection of regions of interest.
Methods: A coronally oriented flat FF applicator (31 empty catheters, 21 spherical beads each), topped with signal producing materials, was imaged in a 3T MAGNETOM Vida using a 3D T1-weighted Dixon Volumetric interpolated breath-hold examination (VIBE) sequence with 0.9x0.9x1.2 mm3 voxels. Novel code written in MATLAB R2024b detected circles on the axial, coronal and sagittal views of the 3D opposed-phase series. The closest regional minima to the circle centers in the axial direction perpendicular to the catheters were selected as possible catheter locations. Common points in circles detected in all 3 orientations were considered as applicator bead locations. Signal intensity thresholding and distance criteria using applicator geometry knowledge were employed to reduce spurious points.
Results: Spurious circles and signal minima outside the applicator area were eliminated well by the employed conditions. All 31 catheters were detected as central minima on axial slices intersecting >40% of the bead diameter. For these slices, average distance between adjacent catheters on the same slice was (10.037±0.006) mm, in excellent agreement with the known 10mm sphere diameter.
Catheters could be reconstructed by interpolating between accepted minima locations on subsequent axial slices. The intersection of accepted circles in three orientations corresponded to the spherical shape of 649/651 applicator beads, with the remaining 2 beads being partially reconstructed.
Conclusion: The developed fully automated algorithm detected well the FF applicator spheres and empty catheters tunneling them on 3D VIBE MR images of a phantom and bears the potential for clinical application.