Library of Congress Cataloging-in-Publication Data Karzmark, C. J. Medical electron accelerators 1 C.J. Karzmark, Craig S. Nunan, and Eiji Tanabe. p. cm. : Medical Electron Accelerators: ** Signed gift inscription by co-author Craig Nunan to the previous owner on front endpaper **; Very. Medical Electron Accelerators by Karzmark, C. J.; Nunan, Craig S.; Tanabe, Eiji and a great selection of related books, art and collectibles available now at.
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MRI guided radiotherapy is a rapidly growing field; however, current electron accelerators are not designed to operate in the magnetic fringe fields of MRI scanners. As such, current MRI-Linac systems require magnetic shielding, which can degrade MR image quality and limit system flexibility. The purpose of this work karzmwrk to develop and test a novel medical electron accelerator concept which is inherently robust to operation within magnetic fields for in-line MRI-Linac systems.
Computational simulations were utilized to model the accelerator, including the thermionic emission process, the electromagnetic fields within the accelerating structure, and resulting particle trajectories through these fields.
The spatial and energy characteristics of the electron beam were quantified at the accelerator target and compared to published data for conventional accelerators. The model was then coupled to the fields from a simulated 1 T superconducting magnet and solved for cathode to isocenter distances between 1.
For the zero field solution, the average current at the target was Such an electron beam is suitable for therapy, comparing favorably to published data for conventional systems. Computational simulations suggest that replacing conventional DC electron sources with a RF based source could be used to develop medical electron accelerators which are robust to operation in in-line magnetic fields.
This would enable the development of MRI-Linac systems with no magnetic shielding around the Linac and reduce the requirements for optimization of magnetic fringe field, simplify design of the high-field magnet, and increase system flexibility.
Several research groups are developing coupled medical linear accelerator and magnet resonance imaging devices MRI-Linac. The goal of these efforts is to enable in-room MRI for anatomic and physiological treatment adaptation and response monitoring. The resultant electromagnetic coupling of the two devices results in many engineering and design challenges, one of which is the production of an acceptable treatment beam—the subject of this paper.
The accelerated electron beam is then typically collided with a tungsten target to produce a bremsstrahlung photon beam.
Medical Electron Accelerators
If not compensated for, this can cause severe aberrations in the Linac behavior, up to and including complete beam loss. As such, the orientation of the accelerator with respect to the MRI scanner becomes important. Two orientations are feasible; the in-line setup, in which electrons are accelerated in the same direction as the magnetic field of the MRI-scanner, and the perpendicular setup, in which the electrons are accelerated perpendicular to the magnetic field.
Each of these configurations has unique advantages and disadvantages associated with it which have been discussed elsewhere 5 —however, if one considers particle acceleration in isolation, then the in-line configuration is indisputably the superior option. This is because magnetic force on a charged particle is minimized when the particle is travelling in the same direction as the magnetic field lines to be precise, the magnitude of magnetic force is zero when a particle travels parallel to a magnetic field, and maximal when it travels perpendicular.
The effects of both in-line and perpendicular magnetic fields on linear accelerator operation have previously been studied via computational simulations.
This can be achieved by modifying the magnet and magnetically shielding the Linac Ref. Two solutions for operating the electron gun in in-line magnetic fields have been proposed.
The first is to redesign the optics of the electron gun taking the presence of in-line magnetic fields into account such that the modified gun functions within these fields.
Redesign of the gun optics requires a bespoke gun design for each different field it is to be used in. The published solution operates optimally only in the electrob high field karzmafk G or higher, and it is not clear if acceptable solutions of this nature exist at lower field strengths.
The alternative, ferromagnetic shielding, causes distortion in the imaging field of the MRI scanner. This distortion can be corrected up to a point, as evidenced by current first generation MRI-Linac systems which successfully utilize magnetic shielding for either the electron gun or the entire Linac.
It also limits the flexibility one has to compensate for other components which can cause magnetic distortion or require shielding, such as multileaf collimators. The ideal accelerator for in-line MRI-Linac systems would be robust to operation in a range of acccelerators strengths without magnetic shielding.
A solution which could meet these criteria and that has not previously been explored is a RF electron gun based system. As the name implies, instead of the steady state fields used to produce an electron beam in conventional systems, RF electron guns utilize Acceleratore fields Fig. RF guns are widely used in other particle accelerator fields, for instance, as injectors to synchrotron beams. A Conventional medical electron accelerator utilizing a steady state stream of electrons, however, B performance is compromised in in-line MRI-Linac systems.
C In this work, we are proposing a novel electron accelerator utilizing a RF electron source which is robust to operation in magnetic fields.
Computational simulations were utilized to investigate the behavior of a RF-gun based accelerator in in-line magnetic fields. In conventional medical DC electron guns, a relatively low kilovoltage kadzmark field is applied to a thermionic cathode, resulting in space charge limited thermionic emission the space charge of the beam limits the emitted current. In the karzmaek configuration, the megavoltage electric field at the cathode karzkark constantly changing as the electric field oscillates back and forth.
Half the time no current will be emitted at all as the electric medocal is pointing in the wrong direction. This emission model is appropriate as the electric karzmarm in the RF cavity is hundreds of times higher than that in a DC electron gun, and temperature limited emission will dominate over space charge limited emission.
An estimate of the total space charge limited contribution to the current ellectron be obtained as follows: The point at which the cathode becomes temperature limited is then calculated; i. The remainder of the pulse, which is considered space charge limited, is spilt into 10 bins, and in each bin the space charge limited current is calculated according to Eq. As will be seen in Sec. Higher fields further limit the impact of space charge.
Theoretical current extracted from a thermionic cathode versus potential difference. Note that the potential here includes the field of the extracted beam itself, and is assessed at a distance 0. The transverse RMS emittance of the beam is often used as a figure of merit to quantify electron beam quality.
The next step was to calculate the Acceleratods fields to which the electrons will be subject while in the accelerating structure. We have utilized the basic S band cavity design presented by St. The solver is based on the finite element method and utilizes a tetrahedral mesh with quadratic shape functions. An adaptive meshing strategy was utilized such that the discretization error in the frequency of the returned solution was less than ekectron. In order to minimize computational cost, a lossless eigenmode solver was utilized, which assumes perfect conductivity at all boundaries—a reasonable approximation for copper.
The losses in the waveguide were calculated as a postprocessing step, which uses perturbation theory based on the magnetic field distribution at the walls. A conductivity of 5. Further details on this method can be found in Ref. A small ring like structure was added around the cathode in order to increase the radial focusing fields at the point of emission.
A The accelerator structure used and the electric field eigenmode solution. B The axial electric field along electroon length of the accelerator. Electrn order to calculate the particle trajectories, the RF fields from Sec.
The electric fields in the central region where the radial coordinate is less than 5 mm were sampled on a 0. The PIC solver is a fully integrated solution which incorporates space charge and wake field effects of the electron beam based on the finite integration technique FITa formulation of the finite difference time domain FDTD method.
Exactly the same geometry as in Sec. The waveguide structure was discretized into 4. The electron source was defined based on the data from Sec. Particle tracking was carried acceleratorss over a time period of ps, accelertaors around six RF cycles and two full electron bunches at the target plane.
Medical Electron Accelerators : Clarence J. Karzmark :
Rather than explicitly simulate each individual electron trajectory, electrons are grouped into macro particles. Each macro particle in this work represented around electrons, and around three million macro particles reached the target in each simulation. In order to assess the performance of the RF gun based accelerator, a beam monitor was placed at the exit of the simulation; particle information was scored as it crossed this monitor and exported to matlab for further analysis.
The mean current, spatial, and energy distributions were evaluated to assess suitability of the beam for radiotherapy treatments. The normalized emittance at the target was calculated using Eq. An issue for all microwave accelerators which utilize a thermionic cathode is back-bombardment. This refers to electrons which are accelerated back toward the cathode, where they deposit unwanted power.
This has two effects—first, it can damage the cathode and reduce its lifetime. Conventional DC medical electron guns can be expected to have two advantages compared to the RF type cathode described here when considering back-bombardment. First, because they are operated in a space charge limited mode of emission as opposed to temperature limited the impact of additional heating during the pulse should be smaller. Second, any electron striking a DC cathode has to first navigate the anode drift tube and overcome the DC electric potential of the electron gun—meaning a cathode in a DC system has inherently greater protection from back-bombardment.
In order to quantify the extent of back-bombardment occurring in the proposed design, the back accelerated electrons striking the cathode plane in the simulation described in Sec. C were exported to matlab for further analysis.
Medical electron accelerators.
In order to compare karxmark to a conventional system, the RF source from Sec. Electorn was replaced with a DC source exported from an Opera electron gun simulation which has been previously described. Dit takes some time for back-bombardment power to reach a steady state, so the simulation time was extended to ps for these simulations.
In order to assess the performance of the RF gun accelerator in the presence of in-line magnetic fields, the particle in cell model from Sec. C was coupled to a previously published model of a 1 T MRI magnet. This field expansion is accurate to within a few gauss within the 2. CST contains a built-in interface for adding a magnetic field in this manner which was utilized. The simulation was repeated for cathode to isocenter distances from 1 to 2.
For each step, the beam assessment was repeated. Note that the SID is approximately mm smaller than the cathode to isocenter distance. These values are all well within the capabilities of modern tungsten dispenser cathodes. The conventional DC source modeled here is the acceleratods gun published by St.
Aubin 21 and frequently utilized in publications in this area. The mean target current of the RF based source is As such, we conclude medica a temperature limited RF cathode can easily generate the target currents required for radiotherapy. The expected thermal emittance of the electron beam on the cathode surface is 0.
Comparison of the electron current from a electro and RF electron source at the beginning source and end target of the accelerator.
Note that current is not a particularly well defined concept when dealing with electron bunches over short time scales, and this is the cause of the variability in the target current peaks. The electromagnetic field solution is shown in Fig. For comparison, previously published values for the same accelerating cavity are also included. Although further optimization of the RF structure elecfron be undertaken, we did not do this as our goal was to provide a proof of principle, and this was achieved with minimal modifications to the original geometry.