M Lamey

Radio frequency shielding for a linac-MRI system

The real-time operation of a linac-MRI system will require proper radio frequency (RF) shielding such that the MRI images can be acquired without extraneous RF noise from the linac. We report on the steps taken to successfully shield the linac from the MRI such that the two devices can operate independently of one another. RF power density levels are reported internally and externally to the RF cage which houses the linac and MRI. The shielding effectiveness of the RF cage has been measured in the frequency range 1-50 MHz and is presented. Lastly MRI images of two phantoms are presented during linac operation. This work illustrates that the accelerating structure of a linac and an MRI can be housed within the same RF cage. The 6 MV linac can be operated to produce radiation with no measurable degradation in image quality due to RF effects.

Radio frequency noise from an MLC: a feasibility study of the use of an MLC for linac-MR systems.

Currently several groups are actively researching the integration of a megavoltage teletherapy unit with magnetic resonance (MR) imaging for real-time image-guided radiotherapy. The use of a multileaf collimator (MLC) for intensity-modulated radiotherapy for linac-MR units must be investigated. The MLC itself will likely reside in the fringe field of the MR and the motors will produce radio frequency (RF) noise. The RF noise power spectral density from a Varian 52-leaf MLC motor, a Varian Millennium MLC motor and a brushless fan motor has been measured as a function of the applied magnetic field using a near field probe set. For the Varian 52-leaf MLC system, the RF noise produced by 13 of 52 motors is studied as a function of distance from the MLC. Data are reported in the frequency range suitable for 0.2-1.5 T linac-MR systems. Below 40 MHz the Millennium MLC motor tested showed more noise than the Varian 52-leaf motor or the brushless fan motor. The brushless motor showed a small dependence on the applied magnetic field. Images of a phantom were taken by the prototype linac-MR system with the MLC placed in close proximity to the magnet. Several orientations of the MLC in both shielded and non-shielded configurations were studied. For the case of a non-shielded MLC and associated cables, the signal-to-noise ratio (SNR) was reduced when 13 of 52 MLC leaves were moved during imaging. When the MLC and associated cables were shielded, the measured SNR of the images with 13 MLC leaves moving was experimentally the same as the SNR of the stationary MLC image. When the MLC and cables are shielded, subtraction images acquired with and without MLC motion contains no systematic signal. This study illustrates that the small RF noise produced by functioning MLC motors can be effectively shielded to avoid SNR degradation. A functioning MLC can be incorporated into a linac-MR unit.

Radio frequency noise from clinical linear accelerators

There is a great deal of interest in image-guided radiotherapy (IGRT), and to advance the state of IGRT, an integrated linear accelerator-magnetic resonance (linac-MR) system has been proposed. Knowledge of the radiofrequency (RF) emissions near a linac is important for the design of appropriate RF shielding to facilitate the successful integration of these two devices. The frequency spectra of both electric and magnetic fields of RF emission are measured using commercially available measurement probes near the treatment couch in three clinical linac vaults with distinct physical layouts. The magnitude spectrum of the RF power emitted from these three linacs is then estimated. The electric field spectrum was also measured at several distances from the linac modulator in order to assess the effects of variations in spatial location in the treatment vault. A large fraction of RF power is emitted at frequencies below 5 MHz. However, the measured RF power at the Larmor frequency (8.5 MHz) of the proposed 0.2 T MR in the linac-MR (0.4-14.6 microW m(-2)) is still large enough to cause artifacts in MR images. Magnetron-based linacs generally emit much larger RF power than klystron-based linacs. In the frequency range of 1-50 MHz, only slight variation in the measured electric field is observed as a function of measurement position. This study suggests that the RF emissions are strong enough to cause image artifacts in MRI systems.

Radio Frequency Noise From the Modulator of a Linac

A novel approach to image-guided radiotherapy being undertaken by a few groups involves the integration of a linac with a magnetic resonance imager (MRI). In order to successfully combine a linac with a MRI, it is important to understand the characteristics and major sources of radio frequency (RF) noise from the pulse power modulator of a linac since these may interfere with the operation of the MRI. The RF noise power spectral density from the modulator of a linac, loaded separately with a magnetron and a resistive load, is measured. The RF fields emitted by the pulse-forming network (PFN) were determined by simulation and compared with measurements. Saturable reactors were introduced in the trigger circuits of the thyratron to reduce the injected voltage spikes into the trigger circuit to assess the impact of these spikes on the RF noise measurements. The results illustrate that the major source of RF noise from the modulator of a linac is the operation of a magnetron. It also eliminates the PFN coil and the grid voltage spikes of thyratron as possible major sources of RF noise. For linac-MRI systems the modulator of a linac should be housed in a separate RF cage from the MRI.

Poster — Thur Eve — 44: Short Term Development Effects of Gafchromic EBT2 Film

Introduction: Gafchromic external beam therapy (EBT)2 is used in our clinic to verify the delivery of Tomotherapy patient plans. We have investigated the short term development effects of Gafchromic film up to 16 hours after irradiation. The importance of the scan position of the film along the active region of a Vidar Pro Dosimetryscanner is also studied. Materials/Methods: An EBT2 film is placed between two halves of a cylindrical phantom. A calibration plan which delivers dose in 13 predefined regions on the film is delivered on a Tomotherapy Hi‐ART 2 (Tomotherapy Inc., Madison WI) system. A point dose is taken within the phantom and used to determine the film irradiation response. The film was scanned using a Vidar Pro Dosimetry Advantage (Vidar, Herndon, VA, USA) scanner. The film was scanned after irradiation at times ranging from 5 minutes to 16 hours. The importance of the film position along the scanner active region was investigated by scanning the film under two conditions: at the edge of the scanner and at the middle of the scanner. Results/Discussion: One hour after irradiation the film had developed to 98.5 % of the measured OD at 16 hours, while after 2 hours, the film had developed to approximately 99 % of the measured OD at 16 hours. Specific to our calibration file setup, we measured a difference of 4.7 % in the 54 cGy region. This large difference could be important when using film to determine the acceptance of patient plans.

SU‐FF‐J‐84: Shielding of Radio Frequency Noise From Clinical Linear Accelerators for the Linac‐MR Project

Purpose: To determine the efficacy of shielding RFnoise generated by a linac during irradiation. Method and Materials: The electric (E) and magnetic (H) field strengths generated by a medicallinac during irradiation were measured using near field E and H probes connected with a 2 GHz digital oscilloscope and IEEE 488 data acquisition system. These measurements were performed both with and without our RF shield, which houses our 0.2 T MRI. These field measurements were then used to determine the power spectral density of RFnoise as a function of frequency. Results: The data measured without the RF shield shows several peaks in the power spectrum approaching levels of 1mW/m2 in the frequency range 1–20 MHz. Above 5 MHz the measured data with the RF shield shows significantly reduced power levels. For example at 35 MHz the attenuation is 38.5 dB. The RFnoise generated by a linac during the MR imaging k‐space acquisition must be less than μWs. In a 50 kHz bin, our data indicates power densities of ∼250, 69, 7.2 and 2.6 nW/m2 at 8.5, 21.25, 42.5 and 63.75 MHz, corresponding to 0.2, 0.5 1.0 and 1.5 T MR resonant frequencies, respectively. For a typical surface coil used for human imaging, the surface area might be 20×20 cm2. Thus the power available to a typical surface coil, according to our measurements, would be a maximum of 10, 2.76, 0.29 and 0.1 nW respectively. This does not include the coil efficiency which would further reduce the actual RFnoise reaching the acquisition system. Conclusion: The noise power levels measured using our RF shielding is much lower than those detected in typical human MR imaging.

SU-FF-J-76: Radio Frequency Noise From Clinical Linear Accelerators

Purpose: Several groups are actively pursuing the integration of an MRI with a megavoltage teletherapy treatment unit. Our system will use a 0.2 Tesla magnet for imaging and a 6MV linear accelerator(linac) for treatment. An important scientific aspect of this project is the possible radio frequency (RF) noise interference between the MRI and linac. The purpose of this work is to report on the measurement of the RF emissions in the treatment room of a clinical linac.Method and Materials: The electric (E) and magnetic (H) field components of the RF noise from a clinical linac were measured using commercially available field probes. Measurements were taken on a Varian 600C linac. The time response of the signal induced in the probes was captured by an appropriate oscilloscope. The discrete Fourier transforms of these time domain measurements were calculated to obtain the measured power as a function of frequency. A second set of measurements were taken to study the dependence of field strength on the location within the vault. Results: Our measurements indicate that the E field strength is on the order of V/m, around 8.5 MHz, and that most of the signal in the H field in concentrated at frequencies below 5 MHz. Besides a few peaks, in general toward higher frequencies both the E and H field strengths drop. We measured a power density of 14.6 μW/m2 at 8.5 MHz for a Varian 600C linac. Our measurements show that the measured field strength has little dependency on the measurement position within a treatment vault. Conclusion: Since an MRI coil receives power on the order of μWs during imaging, it will be important to properly shield or otherwise reduce this extraneous RF power in the linac‐MR unit.

Sci&—Wed PM: Delivery—12: The Radiofrequency Noise from MLCs for a Linac‐MR System

Introduction: This work reports on the feasibility of using an MLC for a linac‐MR system. The radiofrequency )RF(noise produced by a functioning MLC is a possible interference to an MR imaging system. The RFnoise spectral density produced by two brushed motors used to drive MLC leaves and one brushless fan motor has been investigated as a function of applied magnetic field.Materials/methods: An electromagnet was used to subject two brushed motors from two MLC assemblies )a Varian 52‐leaf and a Millennium 120 leaf assembly( and one brushless fan motor to a variable magnetic field. The RFnoise was measured while the motors moved. The RFnoise was measured with a set of commercially available near field electromagnetic probes. In a separate investigation the 0.22 T MR of our linac‐MR system was used to image a phantom. Images were taken with the MLC not present and then again with the MLC present and thirteen leaves moving. Results/Discussion: When viewing the RFnoise in the time domain we could see small spikes of measured noise when the two brushed MLC motors were running; no noise could be seen from the brushless fan motor. In the frequency domain, the Millennium MLC motor showed some noise above background. Images with a phantom showed no degradation from possible RFnoise interference when 13 MLC leaves from a Varian 52‐leaf MLC assembly were brought close to the MR coil and were moved.

Sci—Fri AM(2): Brachy—07: Investigation of the Sources of RF Noise from the Modulator of a Linac

Introduction: One of the possible complications associated with the integration of an MR with a linac is the RF noise interference. Significant work at our lab is underway to identify and separate various noise sources associated with a linac from the MRdata acquisition chain. This work reports upon the study of possible sources of RF noise in the modulator of a linac.Materials/Methods: A commercially available near field probe set was used to measure the RF noise generated from a modulator, loaded with either a magnetron or a high power rated 400 Ω resistive load. In a separate investigation MultiSIM was used to reproduce the currents and voltages in a modulator, the simulated PFN coil currents were then used as inputs into COMSOL Multiphysics to determine the electromagnetic fields produced around the coil during PFN discharge. For experimental comparison, a Hall probe was used to measure the low frequency response of the fields from a modulator of a linac and near field electromagnetic probes were used to measure the higher frequency response. Results/Discussion: Our measurements show a significant reduction in RF noise when the magnetron was replaced with a purely resistive load. These results suggest that it is particularly important for linac‐MR systems to shield the magnetron. Our simulations also suggest that the fields from the PFN coil during PFN discharge are only significant at lower frequencies. The significance of a large spike just before PFN discharge, due to the thyratron, is being investigated.

SU‐GG‐T‐186: Simulation of Room Distortion On Measurements of RF Noise Due to a Medical Linac

Purpose: The goal of radiotherapy is to sculpt radiationdose to tumor shape. The integration of an MRI with a linac for real time image‐guidedradiotherapy(IGRT) would allow clinicians to reduce treatment margins beyond current technological limits. A problem with the integration of an MRI and a linac is the radio frequency (RF) noise produced by a linac. Our measurements indicate this noise exists in the MHz range with wavelengths on the order of 10s of meters. The purpose of this project is to investigate the effects an enclosed room has on the electric (E) and magnetic (H) field measurements on this type of RF source. Purpose: A software programming environment (MultiStripes, version 7.5 Flomerics) has been utilized to investigate the E and H fields produced from a small dipole in free space and an enclosed room, by solving Maxwells equations using the transmission line matrix method. The results from these simulations elucidate the effects of room structure and contents on the radiation pattern of an RF source. Results: Our free space simulation demonstrates the theoretically expected falloff, of the E and H fields, as a function of distance from the dipole. When modeling the dipole confined in the laboratory, our results show perturbations from the free space solution. Our measurements also contain these perturbations. These results were found to depend strongly on the material properties used. Conclusion: Measurement of RF noise for MRI‐linac integration is dependant on the configuration of the laboratory. Simulation of electromagnetic wave propagation can determine the wave perturbation due to room effects. Such simulations allow determination of a “room factor” which can be applied to laboratory measurements to determine the true strength of the RF emissions. More work is needed to validate the model.