Quantification of fiducial marker visibility for MRI-only prostate radiotherapy simulation

Abstract

To objectively compare the suitability of MRI pulse sequences and commercially available fiducial markers (FMs) for MRI-only prostate radiotherapy simulation. Most FMs appear as small signal voids in MRI images making them difficult to differentiate from tissue heterogeneities such as calcifications. In this study we use quantitative metrics to objectively evaluate the visibility of FMs in 27 patients and an anthropomorphic phantom with a variety of standard clinical MRI pulse sequences and commercially available FMs. FM visibility was quantified using the local contrast-to-noise-ratio (lCNR), the difference between the 80th and 20th percentile iso-intensity FM volumes (Vfall) and the largest iso-intensity volume that can be distinguished from background: apparent-marker-volume (AMV). A larger lCNR and AMV, and smaller Vfall represents a more easily identifiable FM. The number of non-marker objects visualized by each pulse sequence was calculated using FM-derived template-matching. The FM-based target-registration-error (TRE) between each MRI and the planning-CT image was calculated. Fiducial marker visibility was rated by two medical physicists with over three years of experience examining MRI-only prostate simulation images. The rater’s classification accuracy was quantified using the F1 score, which is the harmonic mean of the rater’s precision and recall. These quantitative metrics and human observer ratings were used to evaluate FM identifiability in images from nine subtypes of T1-weighted, T2-weighted and gradient echo (GRE) pulse sequences in a 27-patient study. A phantom study was conducted to quantify the visibility of 8 commercially available FMs. In the patient study, the largest mean lCNR and AMV and, smallest normalized Vfall were produced by the 3.0 T multiple-echo GRE pulse sequence (T1-VIBE, 2° flip angle, 1.23 ms and 2.45 ms echo-times). This pulse sequence produced no false marker detections and TREs less than 2 mm in the left–right, anterior–posterior and cranial–caudal directions, respectively. Human observers rated the 1.23 ms echo-time GRE images with the best average marker visibility score of 100% and an F1 score of 1. In the phantom study, the Gold-Anchor GA-200X-20-B (deployed in a folded configuration) produced the largest sequence averaged lCNR and AMV measurements at 16.1 and 16.7 mm3, respectively. Using quantitative visibility and distinguishability metrics and human observer ratings, the patient study demonstrated that multiple-echo GRE images produced the best gold FM visibility and distinguishability. The phantom study demonstrated that markers manufactured from platinum or iron-doped gold quantitatively produced superior visibility compared to their pure gold counterparts.

End‐to‐end validation of fiducial tracking accuracy in robotic radiosurgery using MRI‐only simulation imaging

“…A limitation of this study is that it relies on the assumption that fiducial markers can be identified with good accuracy in MRI images. Much work has been done on the selection of optimal MRI sequences, optimal fiducial markers and detection methods for fiducial markers in MRI 10,18,19,35–37 . The goal of many of these studies was to identify fiducial markers for identification at simulation with the eventual goal of voxel burning the fiducial locations in DRR images for pretreatment positioning and alignment using C‐arm linacs.…”

“…The fiducials were identified based on the distinctive signal void and artifact that they produce on MRI 38,39 . The use of T 1 ‐weighted gradient echo‐based sequences has been published since this sequence produces distinctive artifacts for fiducial marker identification using only MRI simulation images 18,19 . In this study we used a T 1 ‐weighted MRI sequence to identify fiducials.…”

“…Robotic radiosurgery platforms use fiducial tracking algorithms to detect fiducial markers for tumor localization on orthogonal X‐ray images for IGRT‐based tracking 14,17 . Fiducial markers appear as small dark signal voids on MRI and cannot be directly used with robotic radiosurgery fiducial marker tracking software 18–20 . For the clinical use of MRI‐only simulation with standard C‐arm linear accelerator (linac)‐based treatments, fiducials identified on MRI are voxel burned at the same location in their corresponding sCT images 21,22 .…”

A generative adversarial network‐based (GAN‐based) architecture for automatic fiducial marker detection in prostate MRI‐only radiotherapy simulation images

“…Images were acquired without contrast administration. This sequence was used in this study because it quantitatively produces good gold fiducial marker visibility and has been used in previous MRI‐only detection workflows 12,19 . Patient setup at the MRI was performed using a knee wedge on a curved couch.…”

“…This sequence was used in this study because it quantitatively produces good gold fiducial marker visibility and has been used in previous MRI-only detection workflows. 12,19 Patient setup at the MRI was performed using a knee wedge on a curved couch. Patients were scanned using 18-channel anterior phased array coils (18-channel body coil, Siemens, Erlangen, Germany).…”