Radio frequency shielding for a linac-MRI system

Compact bunker shielding assessment for 1.5 T MR-Linac

This study evaluated the effect of the 1.5 T magnetic field of the magnetic resonance-guided linear accelerator (MR-Linac) on the radiation leakage doses penetrating the bunker radiation shielding wall. The evaluated 1.5 T MR-Linac Unity system has a bunker of the minimum recommended size. Unlike a conventional Linac, both primary beam transmission and secondary beam leakage were considered independently in the design and defined at the machine boundary away from the isocenter. Moreover, additional shielding was designed considering the numerous ducts between the treatment room and other rooms. The Linac shielding was evaluated by measuring the leakage doses at several locations. The intrinsic vibration and magnetic field were inspected at the proposed isocenter of the system. For verification, leakage doses were measured before and after applying the magnetic field. The intrinsic vibration and magnetic field readings were below the permitted limit. The leakage dose (0.05–12.2 µSv/week) also complied with internationally stipulated limits. The special shielding achieved a five-fold reduction in leakage dose. Applying the magnetic field increased the leakage dose by 0.12 to 4.56 µSv/week in several measurement points, although these values fall within experimental uncertainty. Thus, the effect of the magnetic field on the leakage dose could not be ascertained.

Performance investigation of LabPET II detector technology in an MRI-like environment

The EMI-compatibility of the LabPET II detection module (DM) to develop a high-resolution simultaneous PET/MRI system is investigated. The experimental set-up evaluates the performance of two LabPET II DMs in close proximity to RF coils excited at three different frequencies mimicking the electromagnetic environments of 3 T, 7 T, and 9.4 T MRI scanners. A gradient coil, with switching frequency from 10 kHz to 100 kHz, also surrounds one of the DMs to investigate the effects of the gradient field on the individual detector performance, such as the baseline of the DC-voltage and noise level along with both the energy and coincidence time resolutions. Measurements demonstrate a position shift of the energy photopeaks (⩽9%) and a slight deterioration of the energy and coincidence time resolutions in the presence of electromagnetic interferences from the gradient and RF coils. The electromagnetic interferences cause an average degradation of up to ~50% of the energy resolution (in time-over-threshold spectra) and up to 18% of the timing resolution. Based on these results, a modified version of the DM, including a composite shielding as well as an improved heat pipe-based cooling mechanism, capable of stabilizing the temperature of the DM at ~40 °C, is proposed and investigated. This shielded version shows no evidence of performance degradation inside an MRI-like environment. The experimental results demonstrate that a properly shielded version of the LabPET II DM is a viable candidate for an MR-compatible PET scanner.

MR-guided radiation therapy: transformative technology and its role in the central nervous system

This review article describes advancement of magnetic resonance imaging technologies in radiation therapy planning, guidance, and adaptation of brain tumors. The potential for MR-guided radiation therapy to improve outcomes and the challenges in its adoption are discussed.

Feasibility of magnetic resonance imaging-guided liver stereotactic body radiation therapy: A comparison between modulated tri-cobalt-60 teletherapy and linear accelerator-based intensity modulated radiation therapy

PURPOSE

The purpose of this study was to investigate the dosimetric feasibility of liver stereotactic body radiation therapy (SBRT) using a teletherapy system equipped with 3 rotating (60)Co sources (tri-(60)Co system) and a built-in magnetic resonance imager (MRI). We hypothesized tumor size and location would be predictive of favorable dosimetry with tri-(60)Co SBRT.

METHODS AND MATERIALS

The primary study population consisted of 11 patients treated with SBRT for malignant hepatic lesions whose linear accelerator (LINAC)-based SBRT plans met all mandatory Radiation Therapy Oncology Group (RTOG) 1112 organ-at-risk (OAR) constraints. The secondary study population included 5 additional patients whose plans did not meet the mandatory constraints. Patients received 36 to 60 Gy in 3 to 5 fractions. Tri-(60)Co system SBRT plans were planned with ViewRay system software.

RESULTS

All patients in the primary study population had tri-(60)Co SBRT plans that passed all RTOG constraints, with similar planning target volume coverage and OAR doses to LINAC plans. Mean liver doses and V10Gy to the liver, although easily meeting RTOG 1112 guidelines, were significantly higher with tri-(60)Co plans. When the 5 additional patients were included in a univariate analysis, the tri-(60)Co SBRT plans were still equally able to pass RTOG constraints, although they did have inferior ability to pass more stringent liver and kidney constraints (P < .05). A multivariate analysis found the ability of a tri-(60)Co SBRT plan to meet these constraints depended on lesion location and size. Patients with smaller or more peripheral lesions (as defined by distance from the aorta, chest wall, liver dome, and relative lesion volume) were significantly more likely to have tri-(60)Co plans that spared the liver and kidney as well as LINAC plans did (P < .05).

CONCLUSIONS

It is dosimetrically feasible to perform liver SBRT with a tri-(60)Co system with a built-in MRI. Patients with smaller or more peripheral lesions are more likely to have optimal liver and kidney sparing, with the added benefit of MRI guidance, when receiving tri-(60)Co-based SBRT.

The rotating biplanar linac-magnetic resonance imaging system

We have successfully built linac-magnetic resonance imaging (MR) systems based on a linac waveguide placed between open MR planes (perpendicular) or through the central opening of one of the planes (parallel) to improve dosimetric properties. It rotates on a gantry to irradiate at any angle. Irradiation during MR imaging and automatic 2-dimensional MR image-based target tracking and automatic beam steering to the moving target have been demonstrated with our systems. The functioning whole-body system (0.6-T MR and 6-MV linac) has been installed in an existing clinical vault without removing the walls or the ceiling and without the need of a helium exhaust vent.

Phase errors in FSE signals due to low frequency electromagnetic interference

OBJECTIVE

To quantitatively evaluate induced phase errors in fast spin echo (FSE) signals due to low frequency electromagnetic inference (EMI).

METHODS

Specific form of Bloch equation is numerically solved in time domain for two different FSE pulse sequences (ETL=8) with two different bandwidths. A single spin is modeled at x=10cm, EMI frequencies are simulated from 1 to 1000Hz and phase errors at different echo times are calculated.

RESULTS

Phase errors in the received echo signals induced by EMI are significantly higher at low frequencies (<200Hz) than at high frequencies and the phase errors at low frequencies can be effectively reduced by using high receiving bandwidth.

CONCLUSION

Pulse sequence bandwidth can be used to control the phase errors in the FSE signals due to low frequency EMI.

Consistency evaluation between EGSnrc and Geant4 charged particle transport in an equilibrium magnetic field

Following the proposal by several groups to integrate magnetic resonance imaging (MRI) with radiation therapy, much attention has been afforded to examining the impact of strong (on the order of a Tesla) transverse magnetic fields on photon dose distributions. The effect of the magnetic field on dose distributions must be considered in order to take full advantage of the benefits of real-time intra-fraction imaging. In this investigation, we compared the handling of particle transport in magnetic fields between two Monte Carlo codes, EGSnrc and Geant4, to analyze various aspects of their electromagnetic transport algorithms; both codes are well-benchmarked for medical physics applications in the absence of magnetic fields. A water-air-water slab phantom and a water-lung-water slab phantom were used to highlight dose perturbations near high- and low-density interfaces. We have implemented a method of calculating the Lorentz force in EGSnrc based on theoretical models in literature, and show very good consistency between the two Monte Carlo codes. This investigation further demonstrates the importance of accurate dosimetry for MRI-guided radiation therapy (MRIgRT), and facilitates the integration of a ViewRay MRIgRT system in the University of Wisconsin-Madison's Radiation Oncology Department.