Technical note: Institutional solution of clinical cine MRI for tumor motion evaluation in radiotherapy.

PURPOSE: Since 4D-MRI is inadequate to capture dynamic respiratory variations, real-time cinematographic (cine) MRI is actively used in MR-guided radiotherapy (MRgRT) for tumor motion evaluation, delineation, and tracking. However, most radiotherapy imaging platforms do not support the format of cine MRI from clinical MRI systems. This study developed an institutional solution of clinical cine MRI for tumor motion evaluation in radiotherapy applications. METHODS: Cine MRI manipulation software (called Cine Viewer) was developed within a commercial Treatment Planning System (TPS). It consists of (1) single/orthogonal viewers, (2) display controllers, (3) measurement grids/markers, and (4) manual contouring tools. RESULTS: The institutional solution of clinical cine MRI incorporated with radiotherapy application was assessed through case presentations (liver cancer). Cine Viewer loaded cine MRIs from 1.5T Philips Ingenia MRI, handling MRI DICOM format. The measurement grids and markers were used to quantify the displacement of anatomical structures in addition to the tumor. The contouring tool was utilized to localize the tumor and surrogates on the designated frame. The stacks of the contours were exhibited to present the ranges of tumor and surrogate motions. For example, the stacks of the tumor contours from case-1 were used to determine the ranges of tumor motions (∼8.17 mm on the x-direction [AP-direction] and ∼14 mm on the y-direction [SI-direction]). In addition, the patterns of the displacement of the contours over frames were analyzed and reported using in-house software. In the case-1 review, the tumor was displaced from +146.0 mm on the x-direction and +125.0 mm on the y-direction from the ROI of the abdominal surface. CONCLUSION: We demonstrated the institutional solution of clinical cine MRI in radiotherapy. The proposed tools can streamline the utilization of cine MRI for tumor motion evaluation using Eclipse for treatment planning.

Evaluation of diffusion-weighted MRI and geometric distortion on a 0.35T MR-LINAC at multiple gantry angles.

Diffusion-weighted imaging (DWI) provides a valuable diagnostic tool for tumor evaluation. Yet, it is difficult to acquire daily MRI data sets in the traditional radiotherapy clinical setting due to patient burden and limited resources. However, integrated MRI radiotherapy treatment systems facilitate daily functional MRI acquisitions like DWI during treatment exams. Before ADC values from MR-RT systems can be used clinically their reproducibility and accuracy must be quantified. This study used a NIST traceable DWI phantom to verify ADC values acquired on a 0.35 T MR-LINAC system at multiple gantry angles. A diffusion-weighted echo planar imaging sequence was used for all image acquisitions, with b-values of 0, 500, 900, 2000 s/mm2 for the 1.5 T and 3.0 T systems and 0, 200, 500, 800 s/mm2 for the 0.35 T system. Images were acquired at multiple gantry angles on the MR-LINAC system from 0° to 330° in 30° increments to assess the impact of gantry angle on geometric distortion and ADC values. CT images, and three fiducial markers were used as ground truth for geometric distortion measurements. The distance between fiducial markers increased by as much as 7.2 mm on the MR-LINAC at gantry angle 60°. ADC values of deionized water vials from the 1.5 T and 3.0 T systems were 8.30 × 10-6  mm2 /s and -0.85 × 10-6  mm2 /s off, respectively, from the expected value of 1127 × 10-6  mm2 /s. The MR-LINAC system provided an ADC value of the pure water vials that was -116.63 × 10-6  mm2 /s off from the expected value of 1127 × 10-6  mm2 /s. The MR-LINAC also showed a variation in ADC across all gantry angles of 33.72 × 10-6  mm2 /s and 20.41 × 10-6  mm2 /s for the vials with expected values of 1127 × 10-6  mm2 /s and 248 × 10-6  mm2 /s, respectively. This study showed that variation of the ADC values and geometric information on the 0.35 T MR-LINAC system was dependent on the gantry angle at acquisition.

Implementation of Magnetic Resonance Safety Program for Radiation Oncology.

During the last decade, radiation oncology departments have integrated magnetic resonance imaging (MRI) equipment, procedures, and expertise into their practices. MRI safety is an important consideration because a large percentage of patients receiving radiation therapy have histories of multiple surgeries and implanted devices. However, MRI safety guidelines and workflows were traditionally designed for radiology departments. This report presents an MR safety program designed for a radiation oncology department to address its specific needs.

MRI safety risks in the obese: The case of the disposable lighter stored in the pannus.

Obese patients constitute 40% of the adult population. MRIs of obese patients are typically challenging because of the effects of a large field of view on image quality and the increased risk of thermal burns from contact with the bore. In this case report, the impacts of obesity on MRI procedures and safety are introduced. Then a case is presented of a 30-year old female cervical cancer patient who received an MRI simulation to verify the placement of a titanium tandem and colpostats for brachytherapy. A large magnetic susceptibility artifact was detected near the right pelvis during the MRI scout indicating the presence of ferrous material. The source of the artifact turned out to be a disposable lighter that was stored inside the patient's pannus. The finding highlights an unanticipated risk to MRI safety and image quality associated with large body habitus.

MRI-based treatment planning and dose delivery verification for intraocular melanoma brachytherapy.

PURPOSE: Episcleral plaque brachytherapy (EPB) planning is conventionally based on approximations of the implant geometry with no volumetric imaging following plaque implantation. We have developed an MRI-based technique for EPB treatment planning and dose delivery verification based on the actual patient-specific geometry. METHODS AND MATERIALS: MR images of 6 patients, prescribed 85 Gy over 96 hours from Collaborative Ocular Melanoma Study-based EPB, were acquired before and after implantation. Preimplant and postimplant scans were used to generate "preplans" and "postplans", respectively. In the preplans, a digital plaque model was positioned relative to the tumor, sclera, and nerve. In the postplans, the same plaque model was positioned based on the imaged plaque. Plaque position, point doses, percentage of tumor volume receiving 85 Gy (V100), and dose to 100% of tumor volume (Dmin) were compared between preplans and postplans. All isodose plans were computed using TG-43 formalism with no heterogeneity corrections. RESULTS: Shifts and tilts of the plaque ranged from 1.4 to 8.6 mm and 1.0 to 3.8 mm, respectively. V100 was ≥97% for 4 patients. Dmin for preplans and postplans ranged from 83 to 118 Gy and 45 to 110 Gy, respectively. Point doses for tumor apex and base were all found to decrease from the preimplant to the postimplant plan, with mean differences of 16.7 ± 8.6% and 30.5 ± 11.3%, respectively. CONCLUSIONS: By implementing MRI for EPB, we eliminate reliance on approximations of the eye and tumor shape and the assumption of idealized plaque placement. With MRI, one can perform preimplant as well as postimplant imaging, facilitating EPB treatment planning based on the actual patient-specific geometry and dose-delivery verification based on the imaged plaque position.

Magnetic resonance imaging metal artifact reduction for eye plaque patient with dental braces.

PURPOSE: To determine if metal artifact reduction can minimize magnetic susceptibility artifacts in the orbits for an eye plaque brachytherapy patient with metallic dental braces. MATERIAL AND METHODS: A 62-year-old female patient with a choroidal melanoma in the right eye received a 1.5 T magnetic resonance imaging (MRI) simulation for 3D eye plaque brachytherapy planning. The protocol included conventional 3D T1-weighted and 2D T2-weighted MRIs. A vendor-supplied T2-weighted metal artifact reduction sequence was added to the protocol to reduce magnetic susceptibility artifacts from the metallic dental braces. The metal artifact reduction sequence combined turbo spin echo acquisitions, high RF excitation and readout bandwidths, and view angle tilting and slice encoding for metal artifact correction with z-shimming to correct in-plane and through-plane image distortions, respectively. RESULTS: Dental braces caused significant signal loss and image distortion in the orbits on the conventional T1-weighted and T2-weighted MRIs, and the MRIs were unusable for treatment planning. The metal artifact reduction sequence with 13 z-phase encodes minimized distortion and signal loss in the orbits, allowing the tumor to be clearly delineated. CONCLUSIONS: T2-weighted MRI with metal artifact reduction was successfully applied to minimize artifacts in the orbits resulting from the dental braces, thus allowing the MRIs to be used in 3D brachytherapy treatment planning.

Metal artifact reduction in MRI-based cervical cancer intracavitary brachytherapy.

Magnetic resonance imaging (MRI) plays an increasingly important role in brachytherapy planning for cervical cancer. Yet, metal tandem, ovoid intracavitary applicators, and fiducial markers used in brachytherapy cause magnetic susceptibility artifacts in standard MRI. These artifacts may impact the accuracy of brachytherapy treatment and the evaluation of tumor response by misrepresenting the size and location of the metal implant, and distorting the surrounding anatomy and tissue. Metal artifact reduction sequences (MARS) with high bandwidth RF selective excitations and turbo spin-echo readouts were developed for MRI of orthopedic implants. In this study, metal artifact reduction was applied to brachytherapy of cervical cancer using the orthopedic metal artifact reduction (O-MAR) sequence. O-MAR combined MARS features with view angle tilting and slice encoding for metal artifact correction (SEMAC) to minimize in-plane and through-plane susceptibility artifacts. O-MAR improved visualization of the tandem tip on T2 and proton density weighted (PDW) imaging in phantoms and accurately represented the diameter of the tandem. In a pilot group of cervical cancer patients (N = 7), O-MAR significantly minimized the blooming artifact at the tip of the tandem in PDW MRI. There was no significant difference observed in artifact reduction between the weak (5 kHz, 7 z-phase encodes) and medium (10 kHz, 13 z-phase encodes) SEMAC settings. However, the weak setting allowed a significantly shorter acquisition time than the medium setting. O-MAR also reduced susceptibility artifacts associated with metal fiducial markers so that they appeared on MRI at their true dimensions.